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Old 28-07-2011, 02:12   #1
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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:

LiFePO4 Batteries - Okay Tear Me Apart ;-)

started by Jallum back in Jan 2010, and:

LiFePo4 House Bank Using SmartHousePower BMS

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.
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Old 28-07-2011, 02:14   #2
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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.
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Old 28-07-2011, 05:19   #3
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
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?
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Old 28-07-2011, 09:10   #4
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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.
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Old 28-07-2011, 09:15   #5
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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.
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Old 28-07-2011, 19:55   #6
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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
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Old 28-07-2011, 19:56   #7
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Electric1,

That's your BMS McG is using, right?
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Old 28-07-2011, 23:38   #8
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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
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Old 29-07-2011, 02:56   #9
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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.
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Old 29-07-2011, 03:09   #10
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
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.

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.
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Old 29-07-2011, 03:12   #11
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
Electric1,

That's your BMS McG is using, right?
I'll jump the gun and answer on behalf.... Yip, bought one from him.
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Old 29-07-2011, 04:20   #12
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
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
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Old 29-07-2011, 06:00   #13
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by sytaniwha View Post
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.
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Old 29-07-2011, 07:35   #14
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Re: LiFePO4 Batteries for sailing house banks users discussion forum

Quote:
Originally Posted by T1 Terry View Post
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
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.
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Old 29-07-2011, 08:10   #15
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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.
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