Lifepo4 won't stay balanced while sitting in storage

auraslip

auraslip

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2+ year old 48v20ah ping. Over 10,000 miles. May have been overcharged to 4v a few times. Capacity un-tested in the last 12 months, but previously it was holding up fine.

NO BMS ON THIS BATTERY.

Sitting on my power supplies in storage, as in ready to ride when ever, at 3.6v per cell It goes out of balance after two weeks. 150mv deviation according to the cell logs. Some cells at 3.45v some at 3.65.

What causes this?
 
i am of the opinion that excursions to the dark side of 2V lead to a reduced terminal voltage after charging.

the cell holds the same amount of charge it appears, but the resting voltage drops to the 3.37V level on the ping pouches i have seen with over discharge histories.
 
You had the celllogs always connected to them? There's a "bug" on some cellogs that will drain differente amount of current from each cell.
 
Sounds to me like the battery is acting normal for one with that much wear on it. Time to save for a christmas present to yourself.
 
The absolute top of the charge is more variable than the middle. The capacity could be off by a meager 0.5AH. You can find the truth of the state of this pack with a discharge test.
 
Does each cell have its own charger? Is that how you are balancing initially?

Assuming each cell was at the same voltage when you pulled off the charge power, and if after two weeks at rest the lowest you have is 3.45 volts, then I'd say that all the cells are still essentially "full". When they are still on the steep part (or end point) of the charge curve like that (3.45 and higher), then I would consider that pack as still "balanced". Pretty remarkable if you ask me how long they all stay above that 3.45 level. That's my perspective anyway.
 
Hopefully it is just a loss of surface charge, and not much self discharge. Remember how voltage drops after less than 1 minute of use? My A123 pack does that within one day. After 6-8 weeks one group has only enough self discharge to drift out of balance 400-500mah of the total capacity of the original 18ah total capacity of the 8p. That's less than 3%. All I do is bulk charge the pack first, and then top that one group off using the 1s setting of one of my RC chargers about once every 2 months.

No cause for big concern, but you do need to keep your eye on it. When it's charging, if it affect too many groups, then some cells can get charged too high. Mine is 23s and my cutoff is 83V, so for me it's near zero risk, since I don't deep discharge and my charge is conservative.

John
 
dnum - I don't think the cells have been over discharged ever.

The celllogs are not permanently connected. Believe me, I am well aware of the problem cellogs have with unbalancing a battery pack!

I use a 48v powersupply to charge them and store them.

The power supply is ALWAYS CONNECTED AND ALWAYS POWERED UP. This is why this problem is strange. The cells should be holding fast at 3.6v per cell.
 
I think you're not suppose to "trickle charge" lithium batteries, I've read around somewhere by a manufacturer I think...
 
I think the recommended float charge is 3.45v per cell for lifepo4, but that doesn't answer your question. I'm guessing you have different IRs causing the cells to drift and there is no bms to "butt up against" as it were. I assume it's a 16s pack? On 48v float?
 
Alan, that's a good point, and it's the reason I'm keeping them at such low charging voltage. IIRC ping charges to 3.7v

I charge to 3.6v. Maybe not too different.

If I don't keep it on the charger, the controller will discharge the pack till it's dead. I could discconect the controller, but all the connectors are water proof. It'd be a pain to plug it in every time I ride it.

Anyways, the ping charger keeps the battery at 3.7v while it's on the charger, and it's recommended to keep it on the charger since the bms self discharges the battery.
 
Old cells. They want to self discharge to 3.4 or perhaps even less I bet. Maybe your trickle is just now unable to keep up with it enough to keep some of them topped up anymore?
 
You might consider a timer for standby charging. Charge for a brief period, just enough to offset the controller. Or consider removing the drain resistor from the controller. I leave mine connected and it would take 3.8 months to discharge the pack with the controller turned off and just the standard bleeder resistor inside the controller draining the battery. Charging once or twice a month is plenty to keep it ready.
 
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