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Old 23-12-2012, 17:14   #826
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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How do I get these cells in pack #2 balanced without a BMS? As soon as I turned the charger back on, it starts at 50 amps and within 15 seconds the #2 cell was fast approaching 4.0 volts from the 3.51 volts resting.
I think you want to discharge cell #2 a little bit with a suitable resistor before proceeding with charging.
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Old 23-12-2012, 17:27   #827
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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How do I get these cells in pack #2 balanced without a BMS? As soon as I turned the charger back on, it starts at 50 amps and within 15 seconds the #2 cell was fast approaching 4.0 volts from the 3.51 volts resting.
discharge them all, they are now all balanced at full discharge.
discharge the series string until one goes empty, then manually do the other cells individually. Now charge them all serially and wait until you find the one that fills up first, that is the weakest cell, and once it fills that is all you can get from the pack.

The problem is most chargers use the full string voltage to determine charge cutoff,
so that poor weak cell gets overcharged until the rest fillup, making it worse the next time, until it gets killed completely. Lead acid can overcharge without damage, but not lithium.
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Old 23-12-2012, 17:32   #828
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Are you sure that complete discharge does no harm to lithium phosphate cells?
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Old 23-12-2012, 18:13   #829

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

Bob-
You could always do it the old fasioned way. You hook up a 4-volt battery charger, and charge each cell until it hits 4v. Now you've got 'em balanced. At whatever voltage you prefer.

In a real BMS, that's what actually happens. Each cell is monitored and charged individually, or in parallel pairs.

Be very careful about "complete" discharges. If, for instance, you have a 3.6V cell and a couple of 4 volt cells in the same string, as you discharge the strong the weak cell hits zero, the stronger cells are still +0.4volts, and now they actually REVERSE THE POLARITY of the weak cell. That's the problem with running complicated batteries without complicated battery charging systems, You can reverse polarize some batteries, and wind up with all sorts of damage.
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Old 23-12-2012, 18:23   #830
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Are you sure that complete discharge does no harm to lithium phosphate cells?
Discharge them down to 2.5volts does them no harm, that is empty.
The cycle life numbers are based on full to empty cycles (of course, if we believe them, I think the chinese make them up, generally battery makers are big liars cuz it is very hard to check their claims)

However, research shows they last longer if you don't do that, use only 80%,
Hard to do that if you just use voltage to measure the state of charge. You need to count amps instead.
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Old 23-12-2012, 18:30   #831
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

This is why you bottom balance the pack (start with them all discharged), so they all go empty at the same time. If you top balance them, the weakest goes empty before the others.

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Bob-
..
Be very careful about "complete" discharges. If, for instance, you have a 3.6V cell and a couple of 4 volt cells in the same string, as you discharge the strong the weak cell hits zero, the stronger cells are still +0.4volts, and now they actually REVERSE THE POLARITY of the weak cell. That's the problem with running complicated batteries without complicated battery charging systems, You can reverse polarize some batteries, and wind up with all sorts of damage.
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Old 23-12-2012, 18:32   #832
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

My charger is constant voltage, so unless I can get pack #2 balanced, I'll have to watch it as it charges. I've been pulling that cell down with a resistor to below the other cells but it still reaches 4.0 volts before the others hit 3.7. I will start cycling the pack as soon as my "fun" load arrives. Pack #1, the charger stops at 14.8 volts with pretty even voltages across the cells, < 0.07 volts between them.
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Old 23-12-2012, 18:33   #833
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks for the info about the UPS tracking.

Bob, when one cell's voltage rapidly rises, that means it's fully charged. If you are doing top balancing, remove it from the set of four, set it aside, and repeat the process on the remaining three cells in series, until the next one rises above 3.45V. Repeat with two in series, then finally, charge the last one until it reaches 3.45V. Now you have four fully charged cells---top balanced.

It helps to have a device that shows all the cells' voltages at once. Otherwise you'll invariably overcharge one.
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Old 23-12-2012, 18:42   #834
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Bottom balancing and top balancing are just two ways to skin the same cat. One is not better than the other. Series charge should stop when any cell reaches 3.45V. Low voltage disconnect should occur when any cell reaches 3.10V. Check the cell voltages when either occur. If the balance is out of whack, perform whichever balance is easier at the time.
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Old 23-12-2012, 18:45   #835

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

nimble, I don't argue one way or the other matters, so long as you don't reverse cells. You want to discharge each cell to a set point somewhere ABOVE zero volts, great. Start 'em all at zero, I won't argue there either. I don't think it really matters when every batter maker claims there's only one contradictory was to manage any type of pack. And of course, a real BMS would just do all this without making the ewtware crawl around reading and balancing cells manually.

I look forward to seeing 80% of the technologies go off the market, 80% of the vendors die with them, and in maybe ten years, we'll finally get some standards. No doubt imposed by the Navy or the SAE or someone who will put out an edict that says "Look, if they don't work this way, you can peddle them for clown cars."

I can wait. It's the vulture DNA that got mixed in when I was young.<G>
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Old 23-12-2012, 18:51   #836
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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My charger is constant voltage, so unless I can get pack #2 balanced, I'll have to watch it as it charges. I've been pulling that cell down with a resistor to below the other cells but it still reaches 4.0 volts before the others hit 3.7. I will start cycling the pack as soon as my "fun" load arrives. Pack #1, the charger stops at 14.8 volts with pretty even voltages across the cells, < 0.07 volts between them.
Deck,

Get yourself a bench top switch mode power supply with separate adjustable voltage and current. With one of these you can individually balance each cell.

The Mastech 3030EX is a good choice (30V & 30A) but there are many others. Extech also makes some decent ones. I like the EX series because it has an "over voltage" dial specifically intended for charging expensive batteries.

Mastech 3030EX

I use mine for charging and equalizing batteries and when I get my 4 400Ah Winston cells I will use it for balancing them too... A real handy tool...

When balancing lithium cells I would carefully adjust the voltage and current before connecting the power supply to the battery. I would suggest limiting the current to about 25A on a 30A model as that will allow it to run cooler. They also make a 3050EX but it is a lot more money.. I don't know for sure but I think the Winston cells can be charged at 3.75V if done individually or in parallel.. Always check with the manufacturer of the cells on this...
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Old 23-12-2012, 19:02   #837
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks Maine Sail. I wish I hadn't sold most all of my electronic test gear 20 years ago. Charging each cell separately for the first couple of cycles with a power supply would be the way to go to get a balance. The light weight switching power supply charger I have was set up for lithium and is 50 amp, and I chose it for both the 50 amp charge and it being 8 lbs, as it will be packed in the kayak for long trips.

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Old 23-12-2012, 19:24   #838
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

This shows the charger better. I wanted the highest wattage per lb. Instead of depending on massive heat sinks, it has a fan on a thermostat. 2 minutes of charging at 50 amps will bring the fan on, but as rate tapers to 30 amps, the fan cycles off.
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Old 23-12-2012, 19:27   #839
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I think you want to discharge cell #2 a little bit with a suitable resistor before proceeding with charging.
When I installed mine, I used an old 12 volt headlight with both high and low beams connected.
I just clipped alligator jumpers across the light to the cell needing to be discharged to the level of the others.
I wound up getting all of them pretty close by discharging first one, then another for about five or ten minutes and then starting the charger while monitoring the individual cell voltages.
It got down to needing only a minute or two with the headlight load to achieve balance.
A MiniBMS attached showed me when shunting started by green LEDS turning red.
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Old 23-12-2012, 19:47   #840
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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A MiniBMS attached showed me when shunting started by green LEDS turning red.
Who did you buy your miniBMS through? I see a few folks selling them like CleanPowweAuto.com.....

I am thinking four of these: MiniBMS Cel Module

and one of these: HousePower BMS
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