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-   -   LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks (https://www.cruisersforum.com/forums/f166/lifepo4-batteries-discussion-thread-for-those-using-them-as-house-banks-65069.html)

sytaniwha 28-07-2011 01:12

LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Hi,

There are two main threads which have been used for discussions about LiFePO4 batteries for use in sailing applications:

https://www.cruisersforum.com/forums/...art-36530.html

started by Jallum back in Jan 2010, and:

https://www.cruisersforum.com/forums/...bms-40389.html

started by electric1 in mid-2010.

They are both wide ranging threads, with a lot of useful information in them, but they also have a lot of posts debating the merits of Lithium based technology.

I am most interested in discussions with actual users of LiFePO4 cells in real sailing situations (I am using 4 * 400Ahr CALB cells), exchanging useful real information and data, but I've found wading through some of the general debate a bit tedious.

I think there may be now enough of us actually using this technology (and some for a few years now) to start this thread for a meaningful exchange of ideas and data.

If you want to debate the merits (or not) of the technology, please don't do it on this thread: please use one of the other threads. Also, if you're a cell manufacturer just trying to push your cells vs another brand, please also don't hijack this thread.

I look forward to using this thread to increase my knowledge, and share my findings. I hope enough of you also do to make it successful.

Thanks,
Paul.

sytaniwha 28-07-2011 01:14

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
To kick this thread off, below is a copy of a post I made on the other 2 threads describing the system I have set up on our yacht.

Hi,

Below is a description of my 400Ahr LiFePO4 bank using 4 * 400Ahr CALB cells for the house bank of our 46ft cruising yacht, plus a link to an interesting video on LiFePO4 charge/discharge characteristics.

McG you may find this link interesting of a video by a Carnegie Mellon Robotics Institute PhD talking to a group of electric car enthusiasts about LiFePO4 charging and conditioning.

Robotics Institute: Videos

He recommends an initial charge/discharge cycle with your cells all in parallel, and stopping at a well defined curve point (e.g. very near full charge or full discharge), to initially ensure all your cells are at a very similar SOC. This is a step further than the good advice offered to you by Electric1.

Also, some of the earlier posts recommend matching your cells for capacity and internal impedance very closely at the factory, and I also think that is critical.

I purchased 4 * 400AHr cells off CALB direct earlier this year, and asked them to match as closely as possible. When they arrived, with the equipment I have I couldn't measure a difference in capacity or impedance, and parallel charge/discharge showed no imbalance (by monitoring individual cell current).

I was also very impressed with CALB's customer service. The sales person still contacts me every few months to see how the cells are, and answers tech questions very quickly.

I run the cells as a 12V (nominal), 400 Ahr bank. They replaced a 600Ahr Trojan SLA pack, and provide more useful energy per discharge. Another side benefit is that I don't have to worry about self-discharge ruining the cells when I leave the boat for an extended period.

I'm currently charging using a 120A alternator through a Sterling Power regulator, and a Victron 50A galvanically isolated mains charger. Both can have their charge parameters changed to fit the ideal charge curve for whatever brand of LiFePO you're using (I note that different brands have some quite significantly different charging parameters).

I use a Vetus Battery Watch unit which controls low voltage cutoff at the pack level to the consumers. Combined with the high voltage cutoff provided by the charge sources, this produces the sensible separation between charge and discharge as suggested by OceanPlanet (i.e. after a low voltage cutoff, the cells can still be charged, whilst a high voltage cutoff doesn't remove the ability for the consumers to draw power from the battery).

I was originally going to add a cell-level BMS, but agreed with GoBoatingNow's comments back in Mar/Apr 2010 that cell level probably isn't necessary for a relatively low current situation (like a typical cruising house bank), especially when there are only a few closely matched cells (4 in my case). My usage thus far supports that view, so we'll see how it goes in future.

I figured that as worst case I might have to do a parallel charge/discharge cycle manually on an annual basis to realign the cells, but I can't detect any drift thus far, so that may not even be necessary.

Anyway, I hope that this info helps some people.

McG 28-07-2011 04:19

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 739448)
To kick this thread off, below is a copy of a post I made on the other 2 threads describing the system I have set up on our yacht.

Hi,

Below is a description of my 400Ahr LiFePO4 bank using 4 * 400Ahr CALB cells for the house bank of our 46ft cruising yacht, plus a link to an interesting video on LiFePO4 charge/discharge characteristics.

McG you may find this link interesting of a video by a Carnegie Mellon Robotics Institute PhD talking to a group of electric car enthusiasts about LiFePO4 charging and conditioning.

Robotics Institute: Videos

He recommends an initial charge/discharge cycle with your cells all in parallel, and stopping at a well defined curve point (e.g. very near full charge or full discharge), to initially ensure all your cells are at a very similar SOC. This is a step further than the good advice offered to you by Electric1.

Also, some of the earlier posts recommend matching your cells for capacity and internal impedance very closely at the factory, and I also think that is critical.

I purchased 4 * 400AHr cells off CALB direct earlier this year, and asked them to match as closely as possible. When they arrived, with the equipment I have I couldn't measure a difference in capacity or impedance, and parallel charge/discharge showed no imbalance (by monitoring individual cell current).

I was also very impressed with CALB's customer service. The sales person still contacts me every few months to see how the cells are, and answers tech questions very quickly.

I run the cells as a 12V (nominal), 400 Ahr bank. They replaced a 600Ahr Trojan SLA pack, and provide more useful energy per discharge. Another side benefit is that I don't have to worry about self-discharge ruining the cells when I leave the boat for an extended period.

I'm currently charging using a 120A alternator through a Sterling Power regulator, and a Victron 50A galvanically isolated mains charger. Both can have their charge parameters changed to fit the ideal charge curve for whatever brand of LiFePO you're using (I note that different brands have some quite significantly different charging parameters).

I use a Vetus Battery Watch unit which controls low voltage cutoff at the pack level to the consumers. Combined with the high voltage cutoff provided by the charge sources, this produces the sensible separation between charge and discharge as suggested by OceanPlanet (i.e. after a low voltage cutoff, the cells can still be charged, whilst a high voltage cutoff doesn't remove the ability for the consumers to draw power from the battery).

I was originally going to add a cell-level BMS, but agreed with GoBoatingNow's comments back in Mar/Apr 2010 that cell level probably isn't necessary for a relatively low current situation (like a typical cruising house bank), especially when there are only a few closely matched cells (4 in my case). My usage thus far supports that view, so we'll see how it goes in future.

I figured that as worst case I might have to do a parallel charge/discharge cycle manually on an annual basis to realign the cells, but I can't detect any drift thus far, so that may not even be necessary.

Anyway, I hope that this info helps some people.

Well now that was a GREAT idea to start this thread. Let me not waste any time:

I have just fitted my Lifepo pack today. See pics attached.

This is a ThunderSky pack consisting 8 x 400AH cells in 2P4S configuration.
I connected a Meanwell 100A mains charger. I also have a HousePower BMS from CleanPowerAuto installed.
These cells were in transit for approx 2 months. First thing I did was to check the voltage.
Findings:
All 8 cells were sitting at 3.27V each and the datasheet stated that it is shipped with apporx 50% charge.

I thought it meant they had an equal SOC.
So I switched on the charger.
It took around 4 hours to fill up which seems right for 800AH pack at 50%.
Every now and then, I took cell voltages. A strange thing happened. As the ampscame down when the battery neared its top level, the parallel pair connected to the + terminal showed 4.23V, the next pair 3.43V, then 3.58V and last pair 3.65V.

The LEDs in the BMS looked as per attachment. 4 x Green and 3 x Red. I am not exactly sure what it means even after carefully reading the User Guide
I left the charger on and after about 30minutes, here's the readings:
4.21v, 3.41v, 3.58v, 3.69v
Readings after another 2 hours:
4.18v, 3.47v, 3.58v, 3.67
At this stage I swithed off the charger and BMS.
After approx 45min took readings again:
4.04, 3.47, 3.58, 3.67

My question: What is happening now?
Should I restart the charger and BMS?
Should I get a cell charger to fix it?
Should I connect all cells in parallel and stand it for a day to fix it?
Should I manually discharge the high cell a bit?:eek::sprint:

electric1 28-07-2011 08:10

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Your bank has some imbalance at the top, nothing unusual, especially for TS cells. You can take some corrective action to speed up balancing process.
According to your last report, your second cell pair is just below the upper “knee”, first one is at max voltage, just about to trip BMS relay, and remaining 2 are at the upper “knee”.
I would recommend to drain the 1st pair a little, to bring it under 3.8V, then charge for 15-20 minutes at a time, repeating drain/charge cycle until 2nd pair reaches the “knee”. It seems to be close enough, so it should not take too many cycles. Also watch remaining 2 pairs so they don’t go over 4.0V, if they do, then drain them same way as the first one.

To drain the cell(s) without going crazy on equipment, I use a large spool of insulated solid copper wire ( same one used in house wiring and sold at any hardware store ), a 250ft spool of AWG14 for example has enough resistance to produce limited current ( less than 20-30 Amp at 4V ). The spool will get warm, so I use it in 10-20 seconds bursts, just touching stripped wire ends to battery terminals, while watching voltage on DVM connected to same terminals. You can even drain while charging, so more current is bypassed around cell(s) being drained, while others are catching charge. Don’t bypass more than one segment at a time, since higher voltage will make the wire too hot and may melt insulation. Once wire gets noticeably warm, pause and let it cool down.

Based on your voltages you are very close to a good balance. Don’t go over 14.4V on the charging source , there is no point in raising voltage above the “knee”, you aren’t getting much more energy in the bank anyway.

electric1 28-07-2011 08:15

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Another way to drain cells a little is to use battery testers made for 12V, which produce ton of heat at 12V and only meant to be used in 2-3 second bursts, but at single cell voltage around 4V it will produce 3 times less current, so you can do longer bursts. Some testers use heater element as load and some use carbon pile or carbon block, those can handle higher loads.

sytaniwha 28-07-2011 18:55

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Hi McG,

I don't have the depth of knowledge and experience that Electric1 has with these cells since I have only started using them at the beginning of this year, but this is how I did the first couple of charge/discharge cycles with the cells parallel to make sure they were at the same level (as per the video posted).

For the discharge I purchased some high power wire-wound resistors, which I had calculated would result in a drain current of about 120A. These are common resistors, and cost about US$30. I mounted them on a steel plate for heat dissipation (120A @ 3.6V is 432W, so they get hot). Make sure you do your power calculations for the resistors properly, because you need them to drain continuously for a long time so you need them to be properly spec'd (in my case 4 * 400Ahr cells in parallel = 1600 Ahr, / 120A = 13 hours, so I paralleled 4 200W resistors). Two short battery cables manually attached works well for the connections.

For the charge (try finding an off-the-shelf 3.6V charger that can supply enough current to charge 1600Ahr of battery in a decent time), I purchased a dc:dc converter chip that takes the 12V output from my normal battery charger and can output 50A at 3.6V. I had to build the circuit, but it's simple, and it provides a good stable charge current. It was also a lot cheaper than a 50A bench power supply: it cost me about $50, and it uses the 12V power supply which I already had. Of course, it still takes about 30 hours to charge the cells, but that's ok.

It sounds like quite a lot of work, but it wasn't actually very hard or expensive to assemble the parts, and it did a great job of allowing me to see that the cells were in fact on the same part of the curve. I can also use it in future if I find the cells are drifting, so I think it was a useful exercise.

It sounds like the TS cells came to you not particularly closely matched, and from Electric1's comment, this sounds to be normal for TS cells. As I mentioned in an earlier post, my CALB cells arrived very closely matched, although I did specifically ask for that from the factory (and for them to be from the same manufacturing run).

A question for Electric1: as I understand it, a BMS with top balancing ability should be able to bring the cells together if left on charge long enough. Is that a true statement? I assume that the solutions you suggested for McG are just to speed up the process. Or is there a limit to how much imbalance a BMS can cater for?

Cheers,
Paul

sytaniwha 28-07-2011 18:56

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Electric1,

That's your BMS McG is using, right?

T1 Terry 28-07-2011 22:38

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
What great idea, people who are actually using these cells as house batteries talking about their experiences, well done.
I have only recently started experimenting with these cells for motorhome hose batteries so a similar role.
I have done quite a few tests recording cell voltages using a cell logger and I discovered that stopping the charge at 3.45v per cell worked the best. Doing this they virtually self balance, these cells are fully charged at 3.4v The 0.5v cell over charge has no real capacity behind it (no idea what the technical term is) so any discharge load blows the 0.5v over charge away very quickly but it's much slower to burn the charge away once below 3.4v. Even if the cells are out of balance initially after the first few light cycles they come back together. My max. discharge current is 1C so load imbalance seems to be limited to around 20 millivolts. I'll try to upload a graph showing the cells coming together after only 1 discharge.

T1 Terry

sytaniwha 29-07-2011 01:56

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Hi Terry,

Nice graph. What cells are you using - manufacturer, size, etc (I think I saw on another thread but can you please confirm)?

Also, what device did you use to get those graphs? (A data logger, your BMS, a digital scope?)

Thanks,
Paul.

McG 29-07-2011 02:09

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 740131)
Hi McG,

I don't have the depth of knowledge and experience that Electric1 has with these cells since I have only started using them at the beginning of this year, but this is how I did the first couple of charge/discharge cycles with the cells parallel to make sure they were at the same level (as per the video posted).

Every bit helps, just let the details roll in. This is fast becoming the most comprehensive and practical LiFePO4 forum thread on the Net.:thumb:

Quote:

For the discharge I purchased some high power wire-wound resistors, which I had calculated would result in a drain current of about 120A. These are common resistors, and cost about US$30. I mounted them on a steel plate for heat dissipation (120A @ 3.6V is 432W, so they get hot). Make sure you do your power calculations for the resistors properly, because you need them to drain continuously for a long time so you need them to be properly spec'd (in my case 4 * 400Ahr cells in parallel = 1600 Ahr, / 120A = 13 hours, so I paralleled 4 200W resistors). Two short battery cables manually attached works well for the connections.
Oops, I have 8 x 400AH cells which means the equipment will cost me double! Anyway, what I did to drain the "high" cell was to connect 2 x PMGs to the one cell. The motors ran for approx 1.5hrs and dropped the voltage from 4.21 to 3.45 which equates to a VERY LOW C rate - less than 0.1C I recon. I then connected all the cells in parallel which I'm leaving for 36Hrs. The cell readings after 5hrs rest period just before connecting in parallel :
3.45v
3.39v
3.51v
3.54v

I'll post the "after" readings on Sunday.


Quote:

For the charge (try finding an off-the-shelf 3.6V charger that can supply enough current to charge 1600Ahr of battery in a decent time), I purchased a dc:dc converter chip that takes the 12V output from my normal battery charger and can output 50A at 3.6V. I had to build the circuit, but it's simple, and it provides a good stable charge current. It was also a lot cheaper than a 50A bench power supply: it cost me about $50, and it uses the 12V power supply which I already had. Of course, it still takes about 30 hours to charge the cells, but that's ok.
Very nice if you can afford, but instead of charging single cells, is it not the same as charging all together and discharging seperately to reach the same SOC?

Quote:

It sounds like the TS cells came to you not particularly closely matched, and from Electric1's comment, this sounds to be normal for TS cells. As I mentioned in an earlier post, my CALB cells arrived very closely matched, although I did specifically ask for that from the factory (and for them to be from the same manufacturing run).
When I ordered my TS cells, I had to wait 20days. They said it is because they have to manufacture it in one batch to get them "equal". I hope that is what they do.

McG 29-07-2011 02:12

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 740133)
Electric1,

That's your BMS McG is using, right?

I'll jump the gun and answer on behalf.... Yip, bought one from him.

T1 Terry 29-07-2011 03:20

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 740295)
Hi Terry,

Nice graph. What cells are you using - manufacturer, size, etc (I think I saw on another thread but can you please confirm)?

Also, what device did you use to get those graphs? (A data logger, your BMS, a digital scope?)

Thanks,
Paul.

Hi Paul,
The logger is a Cell Log 8 by Junsi, I bought it through Hobby King, about AU$28 from memory. The cells are Winston LYP 90ah units. I am in the process of setting up a bank of 2 cells in parallel by 8 sets for 180ah 24v nom. for a recent motorhome fit out. I'm experimenting with matching a high voltage and a low cell together in an attempt to balance the pack. As something completely different I'm measuring the voltage under load rather than resting voltage to match up the sets, the results should be interesting

T1 Terry

electric1 29-07-2011 05:00

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 740131)
A question for Electric1: as I understand it, a BMS with top balancing ability should be able to bring the cells together if left on charge long enough. Is that a true statement? I assume that the solutions you suggested for McG are just to speed up the process. Or is there a limit to how much imbalance a BMS can cater for?

Cheers,
Paul

Yes, BMS balancing is limited due to amount of heat generated by shunting resistors. BMS balancing is meant to keep the pack balanced over long time ( many cycles ). Initial balancing when you assemble the pack is better done manually to save time.

What you did was ideal scientific approach, but not really necessary for average consumer. You don't need 120A load since imbalance is typically small and easy to correct with smaller loads. Having single cell voltage power source is also ideal, but I don't recommend it to people usually since I don't expect them to pay $$$ for bench power supply they will only use once. So there is a practicality aspect of doing this thing, you can go all out and have fun, or you can just do it with stuff laying around. You get there either way.

electric1 29-07-2011 06:35

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by T1 Terry (Post 740260)
What great idea, people who are actually using these cells as house batteries talking about their experiences, well done.
I have only recently started experimenting with these cells for motorhome hose batteries so a similar role.
I have done quite a few tests recording cell voltages using a cell logger and I discovered that stopping the charge at 3.45v per cell worked the best. Doing this they virtually self balance, these cells are fully charged at 3.4v The 0.5v cell over charge has no real capacity behind it (no idea what the technical term is) so any discharge load blows the 0.5v over charge away very quickly but it's much slower to burn the charge away once below 3.4v. Even if the cells are out of balance initially after the first few light cycles they come back together. My max. discharge current is 1C so load imbalance seems to be limited to around 20 millivolts. I'll try to upload a graph showing the cells coming together after only 1 discharge.

T1 Terry

Terry,

that is a very nice test graph, but its not matching your story and I'm afraid you misunderstand some important factors. I will paste my reply to you from another post, so we can keep it all together.

----------------------------------

What makes discussions about Li batteries confusing is that people refer to voltages without relating to state of the battery at the time voltage is measured. Such states are - charging ( at what C rate? ), discharging ( at what C rate ? ), resting ( for how long since last charge/discharge? ). Without answering these questions your voltage reading is utterly useless.

For example, its true that 3.4V is a fully charged cell, but only if its resting voltage, several hours after charge is done and no load has been applied. Same 3.4v while charging indicates that cell is far from full, how far depends on C rate of charging.
----------------------------------
First off, there are 2 basic flavors of LiFePO4 cells on the market, which doesn't help in trying to understand them :rolleyes:
There is yellow cell ( ThunderSky aka Winston aka Sinopoly, last one is a split off company, but appears to be same product ) which has a little different electrolyte composition ( likely to avoid patent wars ) which makes a small difference in charging profile.
Then, there is everyone else, classic LiFePO4, blue cells ( CALB aka SkyEnergy ), white cells ( HiPower ), etc etc.

So, when talking about specific details of charging profile, you need to specify the cell make/size and relate voltages to the state of the battery.

Your graph shows that you had all cells above 3.55V when test started, yet you claim 3.4V being full.

When you charge the battery you must apply higher voltage, to allow current flow into the battery and 3.4V is not enough to charge it. Don't confuse resting voltage with charge voltage.

Please don't take it personally, I am not picking on you at all, just trying to sort out some confusing data for everyone :)

BTW, your graph reveals that one cell has significantly different IR from the rest since it sags less under load. Also, the sag is pretty heavy, indicating high C rate. What is the make and size of your cells? Typical house bank will see very small C rate and will not sag this much under load. In order to make sense from your graph we need to translate 1050W load into C rate, so we need to know size of your cells.

electric1 29-07-2011 07:10

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Sorry, I missed that Terry reported cell type/size, which is Winston ( yellow cell ) at 90AH. So, now we know that 1050W load at 12V nominal means approximately 90Amps, which is 1C for 90AH cells.

Now your graph is making more sense and it reveals a sizable mismatch in IR among 4 cells under 1C load, which is not unusual for yellow cells in my experience, but not a big deal for low C applications. What would be nice to have is the timeline of the test, i.e. how long the 1C load was applied for. I suppose it doesn't take more than 10 min to boil water in the coffeemaker, so there is still much charge left in the bank, hence very flat voltage line under load. If the load continues, voltages under 1C will slowly drop under 3.0V, then keep dropping faster as it gets emptier , then drop fast at the end.

These experiments reveal the need to keep C rate low, which is obvious in house bank application since you want your charge to last for many hours.

electric1 29-07-2011 07:32

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Another apology to Terry, you stated 1C load in your post. I need to stop posting on forums before my morning coffee :(

However, I still need to say that resting voltage is meaningless (with some exceptions) and the fact that cells have same resting voltage tells you virtually nothing about them, as your test clearly reveals. You have one cell with significantly different IR, yet you can never tell it from resting voltage. You also can't tell SOC from resting voltage unless its at either end of SOC range.
You can't judge balance by this test at all since top balance can only be observed at the end of charge, not discharge. Perhaps we need to define meaning of top balance. In my mind, top balance is when all cells in the bank reach upper knee region together, hence raising total bank voltage equally and causing charge current to stop. If one cell reaches top sooner, then others have no chance to get a full charge and first one is risking overcharge. So, top balance is all about lining up voltages at the end of charge, once they reach upper knee where you can have meaningful interpretation of the voltage.

Saying that you cells are all lined up at 3.2V or 3.3V or even 3.4V is meaningless because at that voltage they are not in the upper knee region, hence you can't tell SOC level. The key is to observe end of charge, when voltages start to climb at different rates, which reveals their relative SOC.

T1 Terry 29-07-2011 16:43

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Hi Electric1,
A couple of points you are missing for some reason,
You can't have resting voltages above 3.4v unless the batteries are fully charged, no matter how high the C charge rate is.

Prismatic cell manufacturers rate their cycle life at 0.5C discharge and recharge

If the charge is voltage controlled then a voltage of 3.45v per cell or 13.8v across a 4 cell battery the current will taper off as the cells reach fully charged. Once 13.8v is reached the current flow stops, it doesn't continue to trickle charge like AGM lead acid batteries, it just stops.

The 3.45v is just at the knee of the charge curve, this gives lots of scope for cells to be out of balance, 0.55v before the 4.0v limit is reached. If one cell is a long way behind the other 3 it's charging cycle will stop before it reaches 3.45v, the other 3 cells will be above 3.45v, as soon as a load is applied the 3 cells above 3.45v will rapidly loose voltage till the 3.4v threshold is reached but the low cell will hold virtually the same voltage till nearly fully discharged.
A theoretical example:
Cell 1, 3 & 4 are closer to fully charged than cell 2, let's do an extreme of 0.5v. Charge stops at 13.8v, cell 1/ 3.5v, cell2/ 3.335v , cell3/ 3.48v, cell4/ 3.485v. When the load is applied the voltage on cell 1 will rapidly drop to 3.4v, the voltage on cell 2 will remain fairly close to 3.3v because it is in the flat part of the curve, cell 3 will rapidly drop to 3.4v and cell 4 will rapidly drop to 3.4v. Now the terminal voltage is 13.5v, as little as 1 ah would have been drawn from the battery, cell 1, 3 & 4 are still fully charged and cell 2 has only lost 1ah. On the next recharge cycle cell 2 is 1ah closer to being at the same SOC as the other 3 cells, as cycling continues this gap will continually reduce.

This self balancing works far better with solar and/or wind/water turbine charging included as part of the charging regime because the charge cycles are multiple while the sun is out or the turbine is spinning, a high output isn’t required as it’s only a balancing current.
I can post another graph to show that even with an available charge voltage as low as 13.8v the cells will fully charge, a little slower granted but they still fully charge.
With alternator charging the charge voltage potential will likely be over 15v for a nom. 12v alternator, pulse width modulation charging allows this 15v pulse to impact the battery until the terminal voltage rises to the cut off point, in my case I'm suggesting 13.8v so charging is still quick, much faster than lead acid.

Another advantage LiFeP04 cells have is they are not harmed by sitting partially charge or rarely being fully charged to the 3.4v point so the fact that not every cell reached the same point on the knee curve isn't really that critical is it.

T1 Terry

PS. The time scale is across the bottom of the charts

electric1 29-07-2011 18:04

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Terry, I am sorry, but your entire theory is 100% wrong. It simply does not work as you describe. There is no such thing as self balance. If you apply any amount of charge or any amount of load to a number of series connected cells, each cell will contribute exactly the same amount of charge/discharge and they will stay at the same level of imbalance, unless something intervenes on a cell level, like BMS shunting or manual charge/discharge of individual cell to bring it in line with others.

I don't see a point of discussing ways to better use imbalanced pack, by intentionally reducing charge limit, when you can simply balance it and get on with your life, fully using the battery. Once initial balance is done it tends to stay very well balanced, especially in low C rate application like house bank. Initial balance is so simple that I don't see why we are discussing ways around it.

T1 Terry 30-07-2011 15:32

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
I guess time will tell, at the moment I am fitting up a house battery pack with daily cell monitoring that records back to a hard drive so the self balancing question will be answered one way or the other. This unit runs a 240Vac 250ltr fridge, a 2500w inverter, lights, water pump, computer, router, mobile phone chargers along with all the usual house stuff and never connects to shore power so it will be a fair dinkum house battery test rig. Charging is via solar and alternator. These are the same cells as used in the load test, the ones you believe "reveals a sizable mismatch in IR among 4 cells under 1C load" so we will see if miss match is an issue also.

T1 Terry

OceanPlanet 31-07-2011 16:49

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by sytaniwha (Post 739448)
To kick this thread off, below is a copy of a post I made on the other 2 threads describing the system I have set up on our yacht.

Hi,

Below is a description of my 400Ahr LiFePO4 bank using 4 * 400Ahr CALB cells for the house bank of our 46ft cruising yacht, plus a link to an interesting video on LiFePO4 charge/discharge characteristics.

McG you may find this link interesting of a video by a Carnegie Mellon Robotics Institute PhD talking to a group of electric car enthusiasts about LiFePO4 charging and conditioning.

Robotics Institute: Videos

He recommends an initial charge/discharge cycle with your cells all in parallel, and stopping at a well defined curve point (e.g. very near full charge or full discharge), to initially ensure all your cells are at a very similar SOC. This is a step further than the good advice offered to you by Electric1.

Also, some of the earlier posts recommend matching your cells for capacity and internal impedance very closely at the factory, and I also think that is critical.

I purchased 4 * 400AHr cells off CALB direct earlier this year, and asked them to match as closely as possible. When they arrived, with the equipment I have I couldn't measure a difference in capacity or impedance, and parallel charge/discharge showed no imbalance (by monitoring individual cell current).

I was also very impressed with CALB's customer service. The sales person still contacts me every few months to see how the cells are, and answers tech questions very quickly.

I run the cells as a 12V (nominal), 400 Ahr bank. They replaced a 600Ahr Trojan SLA pack, and provide more useful energy per discharge. Another side benefit is that I don't have to worry about self-discharge ruining the cells when I leave the boat for an extended period.

I'm currently charging using a 120A alternator through a Sterling Power regulator, and a Victron 50A galvanically isolated mains charger. Both can have their charge parameters changed to fit the ideal charge curve for whatever brand of LiFePO you're using (I note that different brands have some quite significantly different charging parameters).

I use a Vetus Battery Watch unit which controls low voltage cutoff at the pack level to the consumers. Combined with the high voltage cutoff provided by the charge sources, this produces the sensible separation between charge and discharge as suggested by OceanPlanet (i.e. after a low voltage cutoff, the cells can still be charged, whilst a high voltage cutoff doesn't remove the ability for the consumers to draw power from the battery).

I was originally going to add a cell-level BMS, but agreed with GoBoatingNow's comments back in Mar/Apr 2010 that cell level probably isn't necessary for a relatively low current situation (like a typical cruising house bank), especially when there are only a few closely matched cells (4 in my case). My usage thus far supports that view, so we'll see how it goes in future.

I figured that as worst case I might have to do a parallel charge/discharge cycle manually on an annual basis to realign the cells, but I can't detect any drift thus far, so that may not even be necessary.

Anyway, I hope that this info helps some people.

Sitaniwha,

Some things to double check about your HVC and LVC configuration:
1) Are the loads and the charging sources on two separate busses?

2) If so, does each bus should have it's own relay so the either the loads or the charge sources can be isolated from the battery?

3) From what I understand, you only have a LVC (a relay controlled by the Vetus Battery Watch?), but are depending only on the REGULATION of the charge sources to prevent over charge.

Regulation and a HVC are two very different things. If any of your charge sources (solar, alternator, shore charger, inverter/charger, hydro, whatever) has a regulation problem then you could overcharge your batteries. Unless you have them all on a bus that can be isolated from the battery by a BMS (or whatever) that is monitoring cell (not just pack) voltage.

My 2 cents.

T1 Terry 31-07-2011 17:38

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by OceanPlanet (Post 742043)
Sitaniwha,

Quote:

Originally Posted by OceanPlanet (Post 742043)

Some things to double check about your HVC and LVC configuration:
1) Are the loads and the charging sources on two separate busses?

2) If so, does each bus should have it's own relay so the either the loads or the charge sources can be isolated from the battery?

3) From what I understand, you only have a LVC (a relay controlled by the Vetus Battery Watch?), but are depending only on the REGULATION of the charge sources to prevent over charge.

Regulation and a HVC are two very different things. If any of your charge sources (solar, alternator, shore charger, inverter/charger, hydro, whatever) has a regulation problem then you could overcharge your batteries. Unless you have them all on a bus that can be isolated from the battery by a BMS (or whatever) that is monitoring cell (not just pack) voltage.

My 2 cents.

Good point, the cell logger I am using has an alarm setting for individual cell LVC, HVC and cell differential so they can't drift too far apart. The problem is that it's only one alarm output. To safely protect cell over voltage separately from cell under voltage 2 Cell Logger unit would be required, each controlling a relay. I guess only one unit needs to be data logging so the cheaper unit that is just a monitor would do the job, I think they were around AU$12. I have wired the cells via a DB9 female 9 pin data plug at the battery pack. That way any 9 pin data cable will plug in and the Cell Logger can be mounted in a convenient position for easy viewing and cable replacement is an off the shelf item. A piggy back plug or splitter cable would make connecting 2 units easy enough with a separate relay connected to each unit. The alarm setting can be toggled on or off on each unit so they could be set up so one monitors cell LVC and cell differential and the other HVC and cell differential. A passive BMS for under $50 plus relays.
It’s a work in progress so a few more mods to the current set up isn’t a biggy, thanks for that :cheers:

T1 Terry

McG 01-08-2011 06:10

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by T1 Terry (Post 742056)
Good point, the cell logger I am using has an alarm setting for individual cell LVC, HVC and cell differential so they can't drift too far apart. The problem is that it's only one alarm output. To safely protect cell over voltage separately from cell under voltage 2 Cell Logger unit would be required, each controlling a relay. I guess only one unit needs to be data logging so the cheaper unit that is just a monitor would do the job, I think they were around AU$12. I have wired the cells via a DB9 female 9 pin data plug at the battery pack. That way any 9 pin data cable will plug in and the Cell Logger can be mounted in a convenient position for easy viewing and cable replacement is an off the shelf item. A piggy back plug or splitter cable would make connecting 2 units easy enough with a separate relay connected to each unit. The alarm setting can be toggled on or off on each unit so they could be set up so one monitors cell LVC and cell differential and the other HVC and cell differential. A passive BMS for under $50 plus relays.
It’s a work in progress so a few more mods to the current set up isn’t a biggy, thanks for that :cheers:

T1 Terry

One thing to keep in mind: My housebus solar controller was wired by the bus builder. He connected the 12v load and solar charge cables into the same lug and bolted it to the battery + terminal. All good when nothing goes wrong. I had no idea that he did it this way so I installed the BMS with the same lug now connected to the relay. With the first cell HVC, the relay opened and the solar power pumped straight into the load cable frying my Espar heater controller and almost set my bus on fire :banghead:. I suggest having one on the load and one on the solar side and only cut the load on LVC.
On HVC, only cut the Solar.

Also, thus far I tend to agree with Electric1 on the SOC.
No matter what the SOC of the cell is, when load is applied, it uses equal amounts of amps from all cells. So the out-of-sync will stay. When charging, equal amounts of amps are sucked up by the cells, thus the imbalance should stay. A great influence seems to be the connections between the parallel pairs. If you have a slightly imperfect connection or inferior (too thin) cable, it can mess up the balance. If the cables get warm while charging, get thicker ones. This is just my humble opinion thru experience thus far and I'm open to correction if I'm talking bolony - so have a go.

Curious to see the outcome of your experiment though Terry.:popcorn:

OceanPlanet 01-08-2011 07:59

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by McG (Post 742288)
One thing to keep in mind: My housebus solar controller was wired by the bus builder. He connected the 12v load and solar charge cables into the same lug and bolted it to the battery + terminal. All good when nothing goes wrong. I had no idea that he did it this way so I installed the BMS with the same lug now connected to the relay. With the first cell HVC, the relay opened and the solar power pumped straight into the load cable frying my Espar heater controller and almost set my bus on fire :banghead:. I suggest having one on the load and one on the solar side and only cut the load on LVC.
On HVC, only cut the Solar.

Also, thus far I tend to agree with Electric1 on the SOC.
No matter what the SOC of the cell is, when load is applied, it uses equal amounts of amps from all cells. So the out-of-sync will stay. When charging, equal amounts of amps are sucked up by the cells, thus the imbalance should stay. A great influence seems to be the connections between the parallel pairs. If you have a slightly imperfect connection or inferior (too thin) cable, it can mess up the balance. If the cables get warm while charging, get thicker ones. This is just my humble opinion thru experience thus far and I'm open to correction if I'm talking bolony - so have a go.

Curious to see the outcome of your experiment though Terry.:popcorn:

Egads; what voltage was/is your solar controller set to? Why would it go overvoltage and fry other devices on the load bus? Or perhaps the controller is too slow to handle the voltage spike when a relay opens? Just wondering.

bene505 01-08-2011 10:27

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
An automated balancer would be easy to build. It would be clipped onto the batteries of an existing bank. (No need to drian them down of connect them in parallel.) Currents would be minimal and over a short-enough time it would balance the batteries.

Regards,
Brad

McG 01-08-2011 15:21

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Quote:

Originally Posted by bene505 (Post 742473)
An automated balancer would be easy to build. It would be clipped onto the batteries of an existing bank. (No need to drian them down of connect them in parallel.) Currents would be minimal and over a short-enough time it would balance the batteries.

Regards,
Brad

Hi Brad, can you give a little more detail please? How will the balancing happen without draining the high cell? Curious to know your view.....

McG 01-08-2011 15:28

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by OceanPlanet (Post 742343)
Egads; what voltage was/is your solar controller set to? Why would it go overvoltage and fry other devices on the load bus? Or perhaps the controller is too slow to handle the voltage spike when a relay opens? Just wondering.

It's an Outback Flexmax 80 set to 14.4V. The Outback went into "limp" and started flashing its display after frying the heater control unit. I also thought it to be strange but obviously the Outback does not like being connected to a batteryless circuit. I think what happened is the controller did its sensing routine to determine SOC. It probably found the circuit to be "uncharged" and opened full throttled bulk charge into the circuit. Does that sound like a possibility?

T1 Terry 01-08-2011 15:36

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
The main problem with active cell balancing units is have a bad history of turning on to discharge a high voltage cell but not turning back off again because of a component failure. If you are constantly watch you battery pack then I guess you would notice the light on far longer than expected but if you are like most people there are better things to do than watch batteries, isn't that why you bought a BMS in the first place. The problem doesn't show up until the cell is completely flattened because the voltage remains virtually the same between 98% charged and 5% charged if the load is only small. It shows up just at the worst time, when you really need the stored energy in the battery pack, suddenly it's not there and then things can go wrong big time.
A method of monitoring the cell voltages will soon tell you that a particular cell is falling behind the rest indicating further attention is required, this is referred to as passive BMS where you are the intelligence not an electronic circuit.
I don't want to start the great BMS debate over here, it would wreck a good thread and never ends in a clear cut result. Each to their own. I don't sell anything so I have no interests one way or the other, just a user that doesn't want to spend money unnecessarily, I'm yet to be convinced cell balancing is needed in such a small pack.

T1 Terry

T1 Terry 01-08-2011 15:45

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by McG (Post 742637)
It's an Outback Flexmax 80 set to 14.4V. The Outback went into "limp" and started flashing its display after frying the heater control unit. I also thought it to be strange but obviously the Outback does not like being connected to a batteryless circuit. I think what happened is the controller did its sensing routine to determine SOC. It probably found the circuit to be "uncharged" and opened full throttled bulk charge into the circuit. Does that sound like a possibility?

They are a very nice unit, I agree that the relay for HVC should be in the solar panel to regulator circuit. A quick question, are your panels connected in parallel to give a 12v nom output or in series to give a much higher output? High voltages require special relays to open a loaded circuit because the arc on a dc switch isn't self extinguishing like on an ac switch where the voltage passes through zero thousands of time a sec.

T1 Terry

McG 02-08-2011 02:02

Re: LiFePO4 Batteries for sailing house banks users discussion forum
 
Quote:

Originally Posted by T1 Terry (Post 742648)
They are a very nice unit, I agree that the relay for HVC should be in the solar panel to regulator circuit. A quick question, are your panels connected in parallel to give a 12v nom output or in series to give a much higher output? High voltages require special relays to open a loaded circuit because the arc on a dc switch isn't self extinguishing like on an ac switch where the voltage passes through zero thousands of time a sec.

T1 Terry

My panels are in parallel.
What I did as a safety measure meanwhile is I fitted a manually operated relay switch. So now I can disconnect the output from the controller before disconnecting the battery pack. I need to still change the wiring so that the controller output goes directly to the battery and not to the shunt where the loads are connected. That way it will be safe when the LVC opens the relay to the loads. The BMS does not distinguish between LVC and HVC. It triggers the same relay in both cases, so I will rely on the charger and the Controller and the current state of balance to keep from over voltage although I think it is necessary to have 2 seperate triggers.

Oh BTW, AC does only 50 to 60 zeros per second, not thousands.......:D

bene505 02-08-2011 07:13

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Quote:

Originally Posted by McG (Post 742630)
Hi Brad, can you give a little more detail please? How will the balancing happen without draining the high cell? Curious to know your view.....

It would drain the high cell(s) very slightly. IMHO (and I'm new to these batteries), that would be the way to go, using voltage comparitors, and only when above the knee.

Regards,
Brad

T1 Terry 02-08-2011 10:25

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Quote:

Oh BTW, AC does only 50 to 60 zeros per second, not thousands.......:D
Hmmm... seems I'm not immune to the odd dumb statement either :lol: (or dumb act it appears, wrote this last night and didn't hit "submit reply" still here on the computer this morning):rolleyes:

With the cell logger alarm circuit it can be wired for normally open or normally closed but I can't find a hysteresis setting so I'm guessing it's an alarm while the problem exists only. If I just wired a relay directly the alarm circuit to open a relay between the solar panels and the regulator as soon as the charge stopped the over voltage problem would probably stop too, resulting in the relay resetting etc
A few possible solutions come to mind, a ratcheting relay so a manual reset would be required or a timer circuit to hold the relay open for say 30 mins and the rest. In a HVC situation the load would still be active so the over voltage would soon be burnt off and if the self balancing things works like I think it will then the differential between the cells should be reduced after each single cell HVC till the problem self corrected. With the LVC but the solar still charging the 30 mins without load could be enough to carry the system through for a while longer before another single cell LVC occurred, this would at least save the stuff in the fridge/freezer.

T1 Terry
For those not up wit the jargon- HVC = High Voltage Cut-out, LCV = Low Voltage Cut-out

electric1 02-08-2011 10:40

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Quote:

Originally Posted by bene505 (Post 743060)
It would drain the high cell(s) very slightly. IMHO (and I'm new to these batteries), that would be the way to go, using voltage comparitors, and only when above the knee.

Regards,
Brad

This is precisely what BMS does. It just takes too long on large house banks, so I recommend simple manual intervention when assembling a new pack. You only need to do it once and it doesn't need to be perfect, just close enough so all 4 segments are in the upper knee when charge voltage goes over 14V, to prevent BMS cutoff. Then you can run your new bank as usual and BMS will even out remaining imbalance over time.

electric1 02-08-2011 10:45

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I recommend to wire solar ( or wind ) circuits directly to the bank, bypassing BMS cutoff relay, such that renewable charge is not lost in the LVC event. But only do this after initial balancing is done. The reason is that overcharge of balanced LiFePO4 bank with sources limited to 14.4V ( charge controllers/regulators ) is impossible, so no point in letting BMS cutoff renewable sources, ever.

s/v Jedi 02-08-2011 16:23

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Quote:

Originally Posted by T1 Terry (Post 743204)
Hmmm... seems I'm not immune to the odd dumb statement either :lol:

Let me correct that once more: it's 100-120 times a second that we pass 0, so twice each period.

ciao!
Nick.

bill good 03-08-2011 16:47

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
OK Here is a question on making a "hybrid" installation because the house batteries I have are expected to have more than 5yrs left. I am running 2X 200 watt each 37.5V solar panels in series to feed a MX-60. Standby current for the MX-60 is quoted as 1watt. So without doing anything much it appears I could put 20 cells in series directly feed from the panels & accept the inverter losses. Since the currents thru the input circuit involved are so low balance circuits may not even be needed. It would be like a collection dam for electrical storage with years of storage from the power being not used. How does one know which is going to be the best LYP or LFP or is this marketing only. I also think the prismatic construction could not be as reliable as a round form but I not in the position to have any experience to collect that type of data.

Regards Bill Goodward

Whimsical 03-08-2011 17:50

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Solar panels output voltage can vary quite a lot depending on load and illuminence, would need it's own regulator. In a mixed system it would be better to have 2 independent systems and a batt selector switch.
If your batts are working well and u don't need more capacity then i would wait for the 5 years and the lifepo's will be a lot better then.

bill good 03-08-2011 18:41

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Ths Whimsical, The batteries are good but I could use more capacity!! So rather that add to the 12v circuit I was thinking "out side of the box". My thought was if the supply voltage was below the low voltage cutoff settings of the cells then the MTTP MX-60 would still be working for the present setup. (no need for other regulator) The batteries ahead would charge only when the voltage was high enough. Their job is to collect the excess power available.Given 40a/hr cells X 20 is 2.8kw fully charged pack (charged when necessary from shore or alternator power) would seem to give a very low thru current of the cells in a house battery case & with low discharge a very long backup. The cost of 20X40a/h is cheaper than the present 250ahr 12v batteries as well. There appears that cell balancing may only need to be done if out of balance is detected in low current applications by what is being expressed by people using lifepo4 in low current circuits. So what is really aimed at is the present 12v bank would be more the backup & that series string be the working system. That other question on LPY/LFE any one going to help here?

The number of 3.6v cells was based on the panels 37.5V X 2 = 20.8 cells for lifepo4 & maybe less for LYP?

T1 Terry 03-08-2011 19:58

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Hi Bill,
I'll try to answer this one the best I can. LYP/LFP have a different charging voltage/ resting voltage to lead acid batteries.
Lead acid like some where around 14.4v to 14.7v or even 14.9v for flooded cells in bulk charging mode, once the this terminal voltage is reached the charger should drop back to absorption mode of around 14.4v till the charge rate drops to roughly 2amps, this is a formula of amps multiplied by ah capacity but 2 amps will do for a rough figure. Once this charge rate is reached the regulator should drop back into float at around 13.6v to 13.8v to maintain the battery charge.
LYP/LFP cells need the charge voltage to be limited, some say high cell voltage while charging won't harm but the Carnegie professor says different and I tend to believe him. With LYP/LFP cells there is no need for a boost/bulk charge rate that drops back to and absorption rate, they just want to be charged to 3.4v per cell and left at that. It takes a little higher voltage to get the 3.4v in there but not much, I use 4.45v per cell and that works out to 13.8v for a 12v nom. battery. Set the regulator to 13.8v and it will charge the cells as fast as they will take it till they are full, no need to stop at a certain amp acceptance rate like is required at higher cell voltages.
When the cells are full the terminal voltage will be 13.6v or a little higher and it will stay there till the batteries are discharged. The terminal voltage will remain higher than 13.v till the batteries are virtually completely flat.
Now for lead acid, up to 14.9v charging but standing voltage is only 12.8v fully charged and as low as 12v discharged.
To combine the 2 sets the charging voltage for lead acid would damage LYP/LFP cells, the resting voltage would result in the LYP/LFP cells trying to recharge the lead acid battery till they were completely flattened and the lead acid batteries would never assist in any loads applied as the LPY/LFP cells are completely flat before the fully charged lead acid terminal voltage is reached.

It could be achieved by using a Voltage Sensing relay between the 2 battery banks, reducing charging voltage to 13.8v and drawing load off the 2 banks separately, the LYP/LFP first as they can handle being left partly discharged where lead acid battery sulphate is left partly charged.
I guess there are those who will disagree with my figures recharging voltages and the point LYP/LFP cells will be damaged but I think the bulk of what I’ve said is correct.

As far as one cell manufacturer against another, there is a lot of it is hype/spin, if the cycle life quoted by the manufacturers can be believed the Winston LYP cells have the highest cycle life, prismatic V cylindrical….. for house battery stuff the cylindricals don’t offer anything more but are more complex to build into banks, cycle life ????? good cells like A123 are very expensive, headway have a lot of failures but are cheap. For house batteries prismatic cells are easy to work with so I tend to think they would be the better choice but there are a lot of followers for each different camp, mostly with electric vehicles though and their use is very different to house battery use.

Did that help or just add to the confusion?

T1 Terry

Whimsical 03-08-2011 20:45

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Bill
I see better now what u are trying to achieve. 75V would be your open circuit voltage but it will fall as u draw more current from the panels. That gives u a max of 3.75V per cell, well below the danger point and above the nominal cell voltage so they should take some current. A lot will depend on how much the panel voltage drops as u take current, do u know the MPPT voltage for your panels. I have been trying to find info on charge acceptance rates of lifepo's at other than the usually stated voltage of about 4.? . I have read several times where the resellers claim a 4 cell pack is a drop in replacement for a battery to be charged off an alternator at 14 to 14.4V so about 3.5 to 3.6V per cell and you would be right in the ball park, maybe a 21 cell pack to get u back to that range. I would only worry if that long string wasn't well balanced as u have a bigger potential to take a single cell way over the safe limit but that could be checked occasionally.
Where are u getting cells that are cheaper than L/A?

Whimsical 03-08-2011 21:05

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
T1 Terry
13.8V Thanks for that, never seen anyone state what is the lowest voltage they need to accept a full charge rate.
I think Bill wants leave his existing L/A as they are and use the lifepo's to charge them through the MX60. Not sure if there are wet or AGM so maybe he needs the higher voltages for equalisation.


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