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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)

T1 Terry 29-08-2011 17:08

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
As long as the voltage difference isn't too extreme cable conductor diameter will control current flow. A balanced 4 cell Li battery will be fully charged at 13.8v (3.45v per cell) so this is the finishing voltage. The cells can handle a higher voltage but there is some evidence that this reduces their cycle life, Jay Whitacre from the Carnegie University briefly touched on this in his LiFeP04 over view.
An alternator charge voltage of 14.4v would equal 3.6v per cell in a well balanced battery, within the capacity these cells can handle, so if 14.4v or lower was set as the alternator control voltage and this was sensed at the alternator end of the cable then basic cable voltage drop tables can be used to determine the most suitable cable conductor diam determined by length and the max amps you want the alternator to put out.
If you use something like this calculator chart (halfway down the page) add in the cable length, say 12ft, 2/3rds the alternator output, say 80 amps, calculate and look down the chart, 0.6 voltage drop would create 14.4v at the alternator but still provide 13.8v at the battery end and this lines up with 8awg cable. Keep in mind DC is a continuous loop circuit so if both cables are the same conductor diameter then the length both ways needs to be added together. If you charge to your start batteries at 14.4v and connect the house batteries via a suitable switch or solenoid you won't risk disconnecting a load from the alternator at any time so diode damage can be avoided.
Hope this helped and didn't just add confusion.

T1 Terry

witzgall 29-08-2011 17:29

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I would be much more concerned about monitoring alternator temperature than battery temp. The internal resistance of LIFEPO4 is low, so in my mind this means that the battery temp will rise much less in them when being charged.

However, an alternator that is being asked to pump out max amps for potentially hours at a time may have a short lifespan, if the regulator does not have a way to monitor this and dial back when needed. This is why an external regulator and temp sensor is often recommended. I asked the Balmar folks if their alternator and regulator (612?) would work with LIFEPO4. They were not very interested in saying anything about "fancy batteries" as the guy put it, but he did recommend using an alternator temp sensor.

Chris


Quote:

Originally Posted by romanystar (Post 762257)
I'm new here, as a member, but have read most all the threads on this subject. I've seen mention in other places that the ability of these batteries to accept a high charge rate can lead to melting down an alternator that does not have temperature protection in the controller.

I am close to spending the money to change out my tired T145's to 8 CALB 180's. I'm trying to determine which other pieces of the puzzle will need to be upgraded. I already have a programmable shore power source, and an adjustable MPPT solar controller.

The Ample Power alternator controller is nearly 20 years old, but does a fine job of 3 step regulation. Should I be replacing it to get something that will monitor battery temperature? Any suggested products?

Paul

OBTW: Welding cable is not usually tinned and is not a great idea in the marine environment.


OceanPlanet 29-08-2011 20:13

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

Originally Posted by romanystar (Post 762257)
I'm new here, as a member, but have read most all the threads on this subject. I've seen mention in other places that the ability of these batteries to accept a high charge rate can lead to melting down an alternator that does not have temperature protection in the controller.

I am close to spending the money to change out my tired T145's to 8 CALB 180's. I'm trying to determine which other pieces of the puzzle will need to be upgraded. I already have a programmable shore power source, and an adjustable MPPT solar controller.

The Ample Power alternator controller is nearly 20 years old, but does a fine job of 3 step regulation. Should I be replacing it to get something that will monitor battery temperature? Any suggested products?

Paul

OBTW: Welding cable is not usually tinned and is not a great idea in the marine environment.

I'm a long-time user of Ample Power alts & alt regs, but have had bad luck trying to use their alt regs for lithium. Tried once again on a recent job where there were two SAR-V3's for two big new alts (one American Power Systems 42i-250-14V alt on each engine). They kept having problems...won't go into details here but switched to Genasun lithium-specific alt regs and problems were solved.

The Genasun alt regs and APS alts seem to be a good combo for lithium. Also recently set up dual 42i-160-28V alts & regs to charge a 920Ah x 24V lithium system on the new Telefonica Volvo 70. Combined, the two alts put out well over 400A x 27V when cold and can charge indefinitely at 300+A x 27V. Takes a lot of engine power though. The Volvo D1-30 driving them has to work pretty hard.

CharlieJ 30-08-2011 01:27

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I have installed Balmar alternators and regulators on TPPL AGM and LFP banks. Both of these battery chemistries are very, very hard on alternators w/LFPs being the most demanding.

In all cases, I have taken two actions to treat the alternators well:
1. Spec and install the largest case alternator possible and then use the alt regulator to regulate the alt output to about 80% of the its max rated output.
2. Install the alternator temperature monitor to cutback field current, and thus alt output, if the alt temperature exceeds a preset limit (about 220F).

Charlie

OceanPlanet 30-08-2011 01:44

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Balmar alts are notorious for overheating. On the plus side they have the temp sense/cutoff in their alt regs to compensate for this...;-)

For TPPL or lithium I like to know the alts can run long and hard without overheating.

downunder 03-11-2011 17:56

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

Originally Posted by OceanPlanet (Post 762656)
Balmar alts are notorious for overheating. On the plus side they have the temp sense/cutoff in their alt regs to compensate for this...;-)

For TPPL or lithium I like to know the alts can run long and hard without overheating.

I noted an article of Nigle Calder's where he had to modify his brothers Balmer to stop it overheating.

Seems a good reason to stay from Balmar.

Overlord 06-11-2011 22:00

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I have just bought twelve 200ah lithium ion cells from witzgall and a BMS from cleanpower auto.I have just removed my three 4D lifelines and will start the installation soon

senormechanico 06-11-2011 22:41

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I purchased four "tall cells" from Chris as well.
Will install a BMS as well.
Can't wait!

bill good 07-11-2011 00:45

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
The point is normal belt type of car alternators are not designed for continuous charging but to use them on LIFEPO4 will need to (a) d-rate the alternator (b) use different regulators with constant current / constant voltage design to the requirements of LIFEPO4 batteries. Flywheel alternators will takeover this job in the future. It is the same as an electric motor has a continuous rating like industrial motors but will have a 5 min rating up to 3 times that figure. Heat removal is the limit in design. Automotive & the like normally recharge the starting battery in minutes then their job is to run all the electrical needs of the motor while the battery just floats. It maybe hard work for the engine driven alternator to ever get pass the constant current phase of the curve as required for LIFEPO4.
Regards Bill

T1 Terry 07-11-2011 01:56

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Li batteries are energy hungry, they have very little internal resistance so current wil flow into them very quickly. Some form of current limiting would be required if a low otput alternator was used. Possibly even charging via the start batteries instead of direct from the alternator and limiting the voltage to 13.8v would enough. Alternators will run for a long time without overheating at 50% to 60% of the their rated output.
Li batteries wil not slow their acceptance rate like lead acid batteries so another possible method is using a DC to DC charger with a capacity low enough to protect the alternator from overheating.

T1 Terry

goboatingnow 07-11-2011 03:19

Quote:

Originally Posted by T1 Terry
Li batteries are energy hungry, they have very little internal resistance so current wil flow into them very quickly. Some form of current limiting would be required if a low otput alternator was used. Possibly even charging via the start batteries instead of direct from the alternator and limiting the voltage to 13.8v would enough. Alternators will run for a long time without overheating at 50% to 60% of the their rated output.
Li batteries wil not slow their acceptance rate like lead acid batteries so another possible method is using a DC to DC charger with a capacity low enough to protect the alternator from overheating.

T1 Terry

An alternator with a proper regulator can be set so the alternator is producing as much power as it's cooling system can withstand. Like all electronics that's the defining characteristics.

Acceptance rates etc are irrelevant. All large battery capacity systems of any chemistry have acceptance rates that will destroy charging devices without control systems. Alternators with standard LA orientated regulators can charge Li tech. But you have to understand it's a sub-optimal situation.

Terry all LA chemistries also have low internal resistance too especially AGMs and Gels.

Dave

OceanPlanet 07-11-2011 03:55

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
TRUE that belt-driven alts will eventually be replaced for efficient & fast charging.

However there are certainly alts available that have much better cooling than others and can run at high output without an alt reg de-rating it. They just cost more.

T1 Terry 07-11-2011 03:58

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

Terry all LA chemistries also have low internal resistance too especially AGMs and Gels.
Until you have actually worked with them you can't appreciate just how fast they will take on a charge. The prismatic cells that are best suied to house battery applications will handle up to 3C without much terminal voltage rise, that 600amps into a 200Ah battery pack. I have witness then accept 5C but it was a 40Ah cell so charging only lasted for about 10 mins before the terminal voltage started to rise rapidly. At 5C (200amps) over 3Ah were being pumped into the battery every minute, over 30Ah in the 10 mins, no comparison to lead acid batteries.
i posted this on another thread but I'll post it here too, this graph shows 50 amps being charged into a heavily discharged 90Ah battery, the charging curve can be clearly seen and the rapid terminal voltage rise once the cells are full. This was to demonstrate how quickly these batteries can go into run away if the charge voltage is set too high, 3.45v per cell is the max safe limit, anything above that you are playing Russian Roulette with rather expensive batteries.

T1 Terry

goboatingnow 07-11-2011 05:25

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

Originally Posted by T1 Terry (Post 812380)
Until you have actually worked with them you can't appreciate just how fast they will take on a charge. The prismatic cells that are best suied to house battery applications will handle up to 3C without much terminal voltage rise,

T1 Terry


I know , I have a set that I am experimenting with ( BMS, integrated charging etc). In fact I've seen 30C into Li technology!!.

As to runaway, I have not seen evidence of it in LIfepo4. But it is possible to heat damage the casing significantly. IN fact the case suffers from movement during any aggressive charging.

There no problem in charging LIfePO4 batteries from alternators. The regulator needs modification to ensure that the alternator remains within its cooling capacity.( but thats a common problem anyway for any high current charge source).

In practice, however this argument is somewhat theorectical. With large prismatic cells, the typical bank is in the 400-1000Ah range and greater then 0.5C is actually very difficult to generate.

Dave

Captain Bill 07-11-2011 08:20

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
I am giving LiFePo4 cells a serious look for replacing my house bank. Early in this thread there was a dicussion on equalizing the SOC on new cells. Various contributers suggested a number of ways to drain the high cells and bring up the low cells. Maybe I'm being a bit simplistic, but would not connecting the cells in parallel for a period of time bring them to the same state of charge. It would seem to me that the higher voltage cells would drain into the lower voltage cells until they reached equalibrium. I'm looking at 8 cells for my house bank and it just seems that hooking them up in parallel for a day or two after I get them would be much simpler than chasing small voltage differences. Can any of you experts comment on why this would not be a reasonable idea?

OceanPlanet 07-11-2011 08:31

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
While the DIY approaches discussed on this forum are great for some, you might consider a full BMS system where all that balancing and HV and LV cutoffs are all taken care of. Mastervolt does current redirecting from cell to cell, however I'm convinced it's not worth all the effort they put into it. They also stuck with a single positive buss. I think the Genasun BMS is the best though-out and safest for marine house banks. The energy lost in shunt/balancing is next to nothing with matched cells.

senormechanico 07-11-2011 10:44

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
The cutoff voltage for the HiPower cells I will be installing is 3.85 Volts.

https://www.evsource.com/datasheets/B...wer%20cell.pdf

Does that mean I should set my programmable MPPT controller to 15.4 volts, or somewhere less?
I can generate around 22 amps in full sun in the summer.
Nothing on my boat would be damaged by that voltage.

Obviously, I'll set my 100 amp alternator to a setting where it won't burn up the belt.

goboatingnow 07-11-2011 16:58

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

Originally Posted by Captain Bill (Post 812524)
I am giving LiFePo4 cells a serious look for replacing my house bank. Early in this thread there was a dicussion on equalizing the SOC on new cells. Various contributers suggested a number of ways to drain the high cells and bring up the low cells. Maybe I'm being a bit simplistic, but would not connecting the cells in parallel for a period of time bring them to the same state of charge. It would seem to me that the higher voltage cells would drain into the lower voltage cells until they reached equalibrium. I'm looking at 8 cells for my house bank and it just seems that hooking them up in parallel for a day or two after I get them would be much simpler than chasing small voltage differences. Can any of you experts comment on why this would not be a reasonable idea?


The problem with LI cell balancing is that the one cell reaches it "terminal voltage" before the others, if all are parallel then the charged cell will be damaged by continuing to charge it in order to bring the other cells to their terminal voltage. Without cell balancing with a parallel system you have to stop charging once the bank reaches the terminal voltage , that leaves certain cells with less then full SOC, sometimes alot. Again these cells will hit the LVC quicker then those with full SOC, so in effect a large loss of capacity in a severe out of balance system can occur.

You cant solve them by paralleling, if you let the HVC occur the damage is done, its too late to then reduce the terminal voltage by bleeding charge.

You can top balance, ie equalise individual cells to the same fully charged terminal voltage or bottom balance, ie run each cell down to the same low voltage discharged voltage . The consensus is that top balancing is better as it can be done on each charge cycle , whereas bottom balancing might not occur for a long time.

However in fractional C charge regimes and where peak power draw remains a fraction of C, cells do not tend to get out of balance in my experience. Cell Balance is a holy grail in EV development, but there the requirements are high multiple C charges, multiple C peak drain and requirements to run the battery right to the discharge limit to grant the best range. Thats a very different arrangement to the domestic requirements on a boat

Dave

T1 Terry 07-11-2011 17:25

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Hi Dave,
I've got to disagree with you on this one. If all the cells are in parallel and charged to 3.45v per cell then as soon at a cell reaches full copacity it can't accept any more charge without the terminal voltage climbing higher than the 3.45v setting. The charge current will simple flow across the link of that cell to the next cell. Charge voltage limiting is the key, constant current charging is for EV's constant voltage charging is for house batteries and 3.45v is a safe voltage. Even in series a 4 cell 12v battery can't have a single cell go over the deadly 4v mark without the oher 3 cells all being below 3.3v. The way the charging voltage curve maps out, that would be a huge out of balance.
Steve B,
that 3.85v per cell is the upper limit, this is used for constant current charging as a point to drop back to constant voltage charging. Constant current is only used in fast charging, 3C to 5C in these type prismatic cells, in house battery banks sticking to constant voltage and allowing them drop into float voltage of 3.4v per cell will ensure long life, these cells will overheat if held at 3.85v for an extended period, I don't even like to see them reach 3.8v, they are in runaway by that stage.

I posted this on another thread but it won't hurt posting it here too, it shows roughly what the charging curve looks like, the 4 coloured lines running in close proximity to each other are the individual cell voltages, the chart was generated using the $28 Junsi Cell Logger 8 available from hobby shops, comes with a free program for the computer to do all sorts of stuff, be even better if you can read German :lol:.
The lines I've added are roughies so don't that them too literally but it gives a good guide

T1 Terry

Sparohok 07-11-2011 17:44

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

Originally Posted by goboatingnow (Post 812915)
The problem with LI cell balancing is that the one cell reaches it "terminal voltage" before the others, if all are parallel then the charged cell will be damaged by continuing to charge it in order to bring the other cells to their terminal voltage.

I wish I had a better understanding of Li cell physics. However when I hear things like this I start to wonder if the problem isn't my understanding of batteries, but rather others understanding of electricity.

If all cells are in parallel, all cells are at the same voltage. How can one cell in a parallel configuration be at a dangerous voltage while the others aren't?

Also, I thought "terminal voltage" was defined as the voltage potential between positive and negative terminals of a battery. By definition a battery is "at" its terminal voltage; it doesn't "reach" its terminal voltage.

You seem to describing the risks of cells being charged in series, not parallel.

Help?

Martin

bill good 07-11-2011 18:03

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Martin, I would think provided you have the correct charger with the constant current/constant voltage selected for the cells you have then you achieve top balancing by paralleling. It appears if less than .5C be used then cell balance in the14v (4 cell) battery the balance may not even be needed. The low voltage cut off & high voltage cut off would be still needed as min. The monitoring of each cell with a logger could show if/when the balancing was needed. I think if you look into the auto 14v batteries being offered you will find they may not have all the bells & whistles.

Bill

Viking Sailor 07-11-2011 20:48

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Capt. Bill,

You are correct. Suggest you do this at a low SOC.

:viking:

Captain Bill 07-11-2011 21:48

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Goboatingnow, Perhaps I did not make my proposal clear and maybe I do not understand the original problem. It seemed to me that people were reporting that their newly acquired cells were arriving in a significantly different state of charge and that some cells tended to charge at slightly slower rated due to variability in internal resistance. The rsult was that after hooking the cells to a nominal 12 volt charger that some cells reached their peak voltage before others had completed charging. My proposal was not to charge them in parallel, but to charge them until at least one cell reached it's peak voltage, then disconnect the cells and reconnect them in parallel. In short order the high cells should drain into the low cells and all voltages will come to equalibrium at a voltage slightly less than fully charged. In a parallel arrangement electrons will; move from the higher voltage cells to the lower voltage cells until they are equal. Given an assumed difference of only a few tenths of a volt the amps flowing between them should not be too high. In the earlier posts it seemed to me that people were just trying to drain the high cells back to the level of the low cells by throwing the electrons through a resistor. The amps wre relatively high because one was working with 3.4+ volts instead of a few tenths of a volt so one needed a big resistor.

VikingSailor, why at a low state of charge? I was under the impression that this was more of a problem at nearly a full SOC because one cell in a bank reaches the top of its charge curve first and then the voltage rises dramatically at that one cell, even to the point of causing damage. I can certainly see that they would in fact reach equalibrium at any state of charge, though the amps might cause issues if there were dramatic differences n the SOC. I could even see using small resistors between the terminals istead of wires to limit the amperage, though one might be pretty hard pressed to find one small enough at a .2 volt delta. Even a 1 ohm would only let .2 amps pass through and it would take a very long time for them to reach equalibrium.

T1 Terry 07-11-2011 22:06

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Won't work Capt Bill, the difference in voltage is in the hundreths of volts not tenths of volts. The out of balance only really appears at the end of charging, as soon as the cell voltage drops below 3.9v the cells are all within 10 to 20 millivolts. If you charge them all together and hold at the 3.45v no cell will get cooked from over charging
The easiest way to top balance a set in series on the conditioning charge is stop when a cell goes high to 3.8v, attach a light bulb or my favourite, a 12v coffee heater element, the cigarette lighter plug removed and alligator clips fitted. Drag the high cell back down to 3.4v and then with the load still attached turn the charger on again. Takes maybe an hr at most for the conditioning charge, once all the cells are brought up to 4v together turn the charger off and let them sit, if any go below 4.45v those cells aren’t fully charged, repeat the process at a much lower charge rate, they stay fairly close in balance after that and never need to be charged to 4v again.

T1 Terry

Viking Sailor 07-11-2011 23:20

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Capt. Bill,

The goal is to bring all the cells to the same state of charge. That way when charged all of the cells will end up at the same SOC. The lower the cell's SOC, the lower the energy stored in the cell. Thus, the lower the energy that needs to be transferred between cells.

:viking:

goboatingnow 08-11-2011 02:08

Quote:

Originally Posted by T1 Terry
Hi Dave,
I've got to disagree with you on this one. If all the cells are in parallel and charged to 3.45v per cell then as soon at a cell reaches full copacity it can't accept any more charge without the terminal voltage climbing higher than the 3.45v setting. The charge current will simple flow across the link of that cell to the next cell. Charge voltage limiting is the key, constant current charging is for EV's constant voltage charging is for house batteries and 3.45v is a safe voltage. Even in series a 4 cell 12v battery can't have a single cell go over the deadly 4v mark without the oher 3 cells all being below 3.3v. The way the charging voltage curve maps out, that would be a huge out of balance.


T1 Terry

Terry I'm agreeing you. I agree that for safety, long cell life and practically speaking at fractional C charging, constant voltage charging. is the way to go. ( you must be using yittrium cells btw).

I was making the point that BMs arnt really necessary at fractional C charging and discharging

Dave

goboatingnow 08-11-2011 02:27

Quote:

Originally Posted by Sparohok

I wish I had a better understanding of Li cell physics. However when I hear things like this I start to wonder if the problem isn't my understanding of batteries, but rather others understanding of electricity.

If all cells are in parallel, all cells are at the same voltage. How can one cell in a parallel configuration be at a dangerous voltage while the others aren't?

Also, I thought "terminal voltage" was defined as the voltage potential between positive and negative terminals of a battery. By definition a battery is "at" its terminal voltage; it doesn't "reach" its terminal voltage.

You seem to describing the risks of cells being charged in series, not parallel.

Help?

Martin

This is just terminology. What I mean is the by terminal is the voltage the cell is at at it's full SOC.

The problem with Li is that in dome cases you really need cell charging rather then bank charging. In boats you will in effect have a series parallel battery bank in most cases.

What I was saying that in this situation ie the common " bank" charging regime , some cells reach the terminal voltage first. Ie they exhibit full SOC in parallel charging the charger must now be disconnected hence other cells can get left undercharged. bMS system balance at a cell level by allowing the charging process to continue by controlling at the cell level. ( sorry this all easier to show on my Fluke Scopemeter them write it down)

What I was trying to convey. as opposed to Genasuns very over the top BMS. In current boat environments, with typically banks in the 400-10000 Ah range. It's very difficult to charge at greater then 0.5 to 0.7 C.

Equally discharge currents are typical 0.1 to 0.5 C. In this environment cells dp not get put of balance. ( other then initially from the manufacturers. ) cell balancing in this environment could be done maybe annually or monthly using a separate system.

Anyway small imbalances just result in a slight loss of total " theoretical " capacity. In practice it's irrelevant. High voltage cutoff is best as I aggreed with T1 done by constant voltage charging. ( or more correctly voltage limited charging ), low voltage cutoff is not needed but simple to rig up . Mainly because in a boat there will be monitoring of batteries anyway. ( low voltage cutover are available commercially anyway ).

There's no mystery to charging Li. In fact it's vastly simpler then LA as there's no reason to try and achieve 100% SOC, which is the main reason for complex IUI or IUU charging in LAs. HVC and LVC protection is easy to arrange or design out of a boat system.

Dave

Sparohok 08-11-2011 11:24

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

Originally Posted by goboatingnow (Post 813131)
The problem with Li is that in dome cases you really need cell charging rather then bank charging. In boats you will in effect have a series parallel battery bank in most cases.

Yes, but Bill was specifically asking about parallel balancing... not series/parallel or series. In parallel, by definition, all cells are at the same voltage.

I just wanted to verify that you are in fact talking about series banks, not parallel.

Martin

Marqus 08-11-2011 17:26

Simple form of one-off or occasional balancing
 
Quote:

Originally Posted by Sparohok (Post 813462)
Yes, but Bill was specifically asking about parallel balancing... not series/parallel or series. In parallel, by definition, all cells are at the same voltage.

I just wanted to verify that you are in fact talking about series banks, not parallel.

Martin

Martin (Sporahok), hope you don't mind if I elaborate on a related question:

- To get 12v from LifePO, we always connect 4 cells in series (each cell around 3.4v) when in use or being charged.

- If you have a simple 4-cell battery, and want to balance these 4 cells (once-off, or occasionally) without using a cell-balancing BMS, can you simply charge them up, then connect these 4 cells in parallel with zero load, let them settle. Then disconnect and measure individual cell voltages and compare. Then maybe charge a bit more (in series) if necessary, and re-connect in parallel to let settle again until individual cell voltages come up close enough?

Question is whether such a 'primitive' approach is viable for fraction-of-C load usage, or whether it does not make sense for LifePO?

Secondly, if it is viable, what would "close enough" be when comparing individual cell voltages? For example, would readings of 3.45v, 3.35v, 3.30v and 3.40v for the four cells be Ok?

Lastly, when comparing cell voltages, do these cells first need to be rested for 24h after charging like Lead-Acid types, or is that not relevant?

Thanks.

Sparohok 08-11-2011 18:53

Re: Simple form of one-off or occasional balancing
 
Quote:

Originally Posted by Marqus (Post 813789)
- If you have a simple 4-cell battery, and want to balance these 4 cells (once-off, or occasionally) without using a cell-balancing BMS, can you simply charge them up, then connect these 4 cells in parallel with zero load, let them settle.

One thing I would be concerned about here is current limiting. If the individual cells are at different state of charge, how much current will flow as the cells balance? Will that exceed the charging current limits of the batteries? I guess there are a few approaches. One way is to do the math, calculate the current using the voltage difference and the batteries internal resistance. Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt). But I wouldn't mess with something like this without making sure you know exactly what you are doing.

As for your questions, hopefully the LiFePO4 experts here can comment, I don't know.

Martin

Marqus 08-11-2011 19:14

Re: Simple form of one-off or occasional balancing
 
Quote:

Originally Posted by Sparohok (Post 813843)
Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt).
Martin

Excellent point - when paralleling the cells to balance, inter-cell current surges can be avoided via small ballast loads (12v globes, wirewound resistors, etc.).

Given that sensible precaution, how about the rest of the question?

goboatingnow 08-11-2011 19:39

Quote:

Originally Posted by Sparohok

One thing I would be concerned about here is current limiting. If the individual cells are at different state of charge, how much current will flow as the cells balance? Will that exceed the charging current limits of the batteries? I guess there are a few approaches. One way is to do the math, calculate the current using the voltage difference and the batteries internal resistance. Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt). But I wouldn't mess with something like this without making sure you know exactly what you are doing.

As for your questions, hopefully the LiFePO4 experts here can comment, I don't know.

Martin

You cant actually cell balance by paralleling the cells.

Let's look at what happens. Cells in parallel are being charged by a common charger. When the cell limit voltage is reached ( ie less the HVC ) the charger must disconnect. At that point the common terminal voltage will be basically the limit voltage but individual cells may have different SOCs. Leaving them sit will in fact do nothing. ( this is due to differences in internal impedances)

Breaking the parallel set apart you will then see that each cell has slight ( ie 0.01 v) differences. Hence each cell that's below the original limit needs individual charging.

This problem also occurs in LAs, but because we seek 100% and in doing so we drive excessive voltage and drive into absorption ,we mask the problem as LAs tolerate the overvoltage.

In Li we can't do that. So in practice cell balancing can only be sensed on series sets. Hence for large prismatics that you wish to balance the battery should be built from series configurations only.

Dave

goboatingnow 08-11-2011 19:47

Quote:

Originally Posted by Marqus

Excellent point - when paralleling the cells to balance, inter-cell current surges can be avoided via small ballast loads (12v globes, wirewound resistors, etc.).

Given that sensible precaution, how about the rest of the question?

No none of this works Li terminal voltage remains almost flat until near full SOC is reached. Hence there's little or no voltage difference , it's only when you teach near the end that you get a cell voltage rise and then you stop charging.

Connecting up a Li cell with say 80% charge in parallel with one with 90 % will not result in both cells arriving at 85% SOC. All that happens is both cells basically sit at there respective SOCs.

Dave.

T1 Terry 08-11-2011 20:26

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
As Dave has said, parallel balancing can only occur while charging but the voltage must be limited to 3.45v otherwise the cells that fill first will overheat and die.
Simple connecting the cells together when at rest will achieve very little, there is only0.4v difference between 1o% charged and 100% charged when the cells are at rest, that 0.4v adds up to 13.6volts fully charged and 12.2 volts near flat, enough difference there but not enough at individual cell level.
I have a heap of appointments this afternoon but this evening I'll post a few charts and a bit of an explination regarding the importance or lack of it when it comes to cell balance and just where is the balance measured.

T1 Terry

Marqus 08-11-2011 20:32

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

Originally Posted by goboatingnow (Post 813897)
Connecting up a Li cell with say 80% charge in parallel with one with 90 % will not result in both cells arriving at 85% SOC. All that happens is both cells basically sit at there respective SOCs.

Dave.

I guess that's a point I've been missing. You're saying that we can have two cells with identical voltages, but they can harbor a 10% difference in SOC. Quite a significant point.

Given that's the case, how does a BMS then equalize SOC across multiple cells? The BMS cannot rely on cell voltage measurement, because (per above) it cannot rely on a cell's voltage to reflect it's SOC. So what does a BMS measure in order to be able to equalize a set of 4 cells?

goboatingnow 08-11-2011 20:39

Quote:

Originally Posted by Marqus

I guess that's a point I've been missing. You're saying that we can have two cells with identical voltages, but they can harbor a 10% difference in SOC. Quite a significant point.

Given that's the case, how does a BMS then equalize SOC across multiple cells? The BMS cannot rely on cell voltage measurement, because (per above) it cannot rely on a cell's voltage to reflect it's SOC. So what does a BMS measure in order to be able to equalize a set of 4 cells?

When the cells are in series or isolated from each cell imbalance results in small differences in terminal voltage. Hence good BMS top balancers divert charge to the lower SoC cell to bring it to the Setpoint voltage. In some cases a higher voltage right at the HVC point had to be used to cure large imbalances. In this case you charge and then bleed off back to the safe set point voltage.

It's also a factor that Li SOC can only really be determined at the near the full SOC. Ie they exhibit ( after some initial charge) an almost flat constant voltage throughout the bulk of the SOC range. As they approach 80 + point there is a noticeable rise in terminal voltage. Thus is a great feature as it enables easy charge cutoff ( unlike LAs)

This means that balancing can only be at near the top or near the bottom. These are the only events you can detect You simply can't detect a 50% SOC as against a 60% SOC Li as the voltages are virtually identical. ( leaving aside cell manufacturing issues)

Dave

Marqus 08-11-2011 23:30

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

Originally Posted by goboatingnow (Post 813945)
When the cells are in series or isolated from each cell imbalance results in small differences in terminal voltage. Hence good BMS top balancers divert charge to the lower SoC cell to bring it to the Setpoint voltage. In some cases a higher voltage right at the HVC point had to be used to cute large imbalances. In this case you charge and then bleed off back to the safe set point voltage.

Dave

The above says that, ultimately, the BMS relies on cell voltage differences to equalize, even though these voltage deltas are small.

So, if our 4-cell battery measures around 13.6v or above (ie. region nearing full charge), the voltage deltas will be enough to make the BMS do some balancing. At the same time, why would those voltage deltas not be enough to force current flow between paralleled cells (eg. 3.45v to 3.43v)?

Could it be a matter of taking too long?

Saw a figure of 6 - 10 milliOhm on the web for the internal resistance of a LifePo cell (more reliable figure avail?).

For argument's sake, if we assume 10mOhm, and assume a voltage delta of 0.05v between two cells, that would mean an initial equalizing current of 5Amp, reduced by adding a ballast resistor (or lamp).

That doesn't sound too slow. However, the internal resistance may change as SOC increases and that could reduce the current to lower levels. If it rose by ten times, the current would only be 0.5A, and that could be too slow. Or would it? Is that the real obstacle? Possibly not; anyone know?

senormechanico 08-11-2011 23:32

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
As an experienced electronic technician but a noob to these batteries, I really appreciate all these posts !

Thanks guys.

Viking Sailor 09-11-2011 01:28

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
Internal resistance increases as SOC decreases. Thus, when two cells with unequal SOC's are connected in parallel, there will be less surge current with two low SOC cells then when the cells have a high SOC.

:viking:

T1 Terry 09-11-2011 01:48

Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks
 
A Li cell is fully charged at 3.4v rested but it requires 0.05v over voltage to create a charging condition. A cell at 3.35v rested is somewhere between 80% and 90% charged, a cell at 3.0v could be anywhere between 100% discharged and 70% discharged. State of charge can be measured by cell volage to a degree but it's in the single milli volt range, a large imbalance has occured by the time there is sufficient difference between cell voltages for it to tranfer. I remember reading some where on the EV DIY forum where someone linked all their cells together and left them for 48hrs, then connected them is series and charged them, they were still all over the place when the 3.45v mark was passed for each cell. This is the method the majority of BMS active cell balancers use, when a cell reaches say 3.8v a controlled rate short circuit discharge across a resistor in switched on and burns the excess voltage down and turns off at 3.5v. Unfortunately manufacturing being what it is some will turn on at between 3.85v and 3.75v and turn off between 3.55v and 3.45v. some drift into the higher end and cells are damaged, some drift under the 3.45v mark and then the balancer is trying to discharge the real charge capacity, this overheats the little units either burning them out or causing them to latch on gradually dragging the cell dead flat.
As balance is really only an issue at the the very top of charging, a cell running away and overheating, or at the bottom and a cell being dragged into reverse voltage. The second one is extremely unlikely to happen in a hose battery situation so top balancing is the choice of methods for house batteries.
Now, if the cells are dragged up to 100% full the over voltage doesn't happen, if the 100% charged point and top end balancing is left to a time the operator can watch what's going on then if a cell goes high a load can be applied to that cell to bring it back down, once all cells are 3.45v to 3.5v and the charge rate has dropped to zero the cells are considerd fully charged, the 100% is reset on the SOC meter and the system returns to being charged to 98% each time till anoher top balance is required. If the cells stay within 50 milli volts while working then a top balance isn't required.

T1 Terry


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