Cell drift in commercial batteries?

[LVLAaron]

I've never purchased a commercial LifePo4 battery, so I'm curious about cell drift.

I've built several DIY packs. Some VERY diy, and recently a Sun Fun Kit with quality eve 280ah cells.

I notice that no matter what balancing method used (actual proper top balance with all cells in parallel, just build it and let an active balancer sort out the differences, etc)

I can get cells within ~ .006v when the packs are at 14.5 volts.

Inevitably, the cells will drift. Maybe because I suck at building things (I don't) maybe because I run them hard (Yes, I do)


So I'm curious. If I ran an SOK 100ah 12v battery for a year and then took it apart and measured the cells - how close would they be?


I ask because I see so many lithium chargers that go all the way to 14.6v and I just can't imagine a scenario where that isn't going to trip overvoltage protection.

 

[upnorthandpersonal]

The SOK has a BMS with balancer, so they should be pretty close. Even with DIY packs with less than perfect cells, if you have a decent balancing BMS, they should be kept pretty close to one another. There is not need to charge them to 14.6V though. It will most likely cause a high voltage disconnect at times, and it doesn't give you any more usable energy in the first place.

 

[LVLAaron]

Yes, they have "balancers" in them, something like 30-50ma which is nothing.

I know we don't need to charge to 14.6 - But many commercial chargers DO charge to 14.6 and are not adjustable.

And to clarify here.... I'm not trying to solve any of my own problems... just having a conversation.

 

[RCinFLA]

Sun Fun Kits are used cells.
Sometimes good, sometimes bad. Their higher priced 'HP' versions get the better condition cells.

In general, the more mis-matched the cells, the tougher it is to keep cells balanced, particularly when discharged at moderate load currents above 0.25 CA rate.

Matching is not just AH cell capacity matching. As cells age their internal impedance gets greater so with moderate load current, the higher impedance cells have greater internal losses contributing to faster state of charge balancing degradation.

 

[LVLAaron]

Sun Fun Kits are used cells.
Sometimes good, sometimes bad. Their higher priced 'HP' versions get the better condition cells.

In general, the more mis-matched the cells, the tougher it is to keep cells balanced, particularly when discharged at moderate load currents above 0.25 CA rate.

Matching is not just AH cell capacity matching. As cells age their internal impedance gets greater so with moderate load current, the higher impedance cells have greater internal losses contributing to faster state of charge balancing degradation.

SunFunKits.com - Certified Automotive Grade A Genuine EVE Lithium LF280 Lithium Iron Phosphate Battery 280AH 4 pack - Sun fun kits

SunFunKits.com - Certified Automotive Grade A Genuine EVE Lithium LF280 Lithium Iron Phosphate Battery 280AH 4 pack - Sun fun kits

www.sunfunkits.com

These are used?


Regardless, that isn't my point or question.

 

[Aly_Oops]

Normally the bulk is set at 13.8 volts, after that it goes into absorption which is set for 14.4 to 14.6 and the current is very low. Some BMS do balancing during charge and other top balancing. Some have argued balancing during charge creates wider cell differences. For top balancing, absorption mode is where it starts. It seems cell differences is usually best. It seems BMS that can balance at 5vdc verses 0.20 - 0.40 work better and more consistent.

Unfortuantely, some BMS batches have bad firmware/hardware, bugs and won't allow absoption and shut off the charging at 13.8 vdc and if one cell hits 3.65 it shut down all cell balancing. Your charger will be putting out 14.6, but the BMS hold it at 13.8 vdc internally. One then can expect 300mv to 400mv cell differences. Probably, only 90 to 98 % charged. Often the balancing is at it worse at 13.8 volts just before it switch to aborption.

Those who have such unit, usually not able to return it. So, some are ordering just BMS unit from other brands, such as Sun Fun and replacing their garbage BMS.

One way to assume your BMS is on the low end, when it cannot be discharge and/or charge at 100 amps, especially for batteries that are over 200amp lifepo4 single batteries. This would indicate they are using BMS designed for 100 amp on their larger packages. One expensive fix is to have two or more units in parallel.

Keep in mind, this is observations/testing seen and varies with product brands and batches of one brand. One would need to check each battery to see how it works. Sometimes, it can be a bad cell (expect that from Questionable Seller) or the BMS poor design. Many brands have switch from proven BMS (looking to increase profits, I guess) and made their own. (their expertise may be lacking)