Battery cells draining / dying after couple weeks unused

andyshinn

100 µW
Joined
Apr 12, 2021
Messages
8
I built a 20S2P Headway 38120HP battery pack for an electric kart project. The pack works and performs well. I've bursted up to about 200 amps briefly but generally stay at 100 amps and 10-30 when cruising.

I let the pack sit charged for a couple weeks unused and when I came back to it the BMS showed some cell groups at 0 volts and others less than 2 volts. I ordered replacement cells for the cells which appeared bad (under 2.5 volts) and rebuilt the pack. It holds charge and performed well and the cell groups would stay within 50mV of each other for a couple days.

Again, I left it sitting with BMS and everything off and when I came back it had the same problem. A bunch of cell groups were under 2 volts and some at 0 volts.

Is this because the cell pairs are varying in capacity too much? Is this just how LiFePO4 cells work? Am I supposed to keep the pack on a trickle charger when not in use? Or am I just unlucky with a bunch of bad cells twice in a row?

Help! I want to rebuild my pack but want to keep the battery healthy when unused long term.

PXL_20220326_211418802.jpeg
 
Are they the *same* cell groups at the *same* voltages?

If not, then it's probably the cells. That is best tested by taking all the cells, individually, when you get them and charge them up, then just let them all sit untouched and unconnected to anything, for about as long as the pack sat when the problem occurred. If the cells don't all stay at the voltages you charged them to, then those that don't are problematic in one way or another. If you need to charge them as a pack (no single-cell charger available) then you can do that, then unbolt them all and separate to individual cells and let them sit. If the self-drain rate is high, you won't actually have to wait long to see voltages drop. :/


If yes, then if it's a balancing BMS, it may have stuck-on balancers, which will continue draining, always, at the rate they're designed to; typically this is a max of around 50mA when cells are full, dropping as they empty.

Some BMS have lights to indicate balancing, most don't; in those cases you'd need to either measure current in the balance wire, or voltage across the balance resistor, to tell whether each balancer is off or on.

If the balancers aren't *fully* stuck on, they might only drain a cell group a little bit, compared to others that *are* fully stuck on that drain at higher currents.
 
andyshinn said:
Again, I left it sitting with BMS and everything off and when I came back it had the same problem. A bunch of cell groups were under 2 volts and some at 0 volts.
HOW LONG did you let it sit (5 days, 10 days, 15 days)?

Being the BMS was disconnected (OFF) then the problem with some of the cells is most likely high self-discharge (self-drain). Even if these "USED HEADWAY" cells are halfway as 'good' as BH claims the cells still may show signs of self-drain after 10-15 days.

Don't charge more than 80% (2.92V) before storage. May have to recharge certain individual p-groups via the disconnected BMS balance lead wire connector as often as every two weeks. If self-drain of a p-group is more than 100 mV after 7-10 days suggest you replace that 2p group or the 'weak' cell in that 2p-group.

Call BH explaining high self-drain problem with some of their cells. Ask to speak to their Tech Rep to explain how you should pre-test the cells (putting them on the spot so as not to just give you some BS) before a DIY build.

Pre-test the next LFP HEADWAY cells by connecting them in parallel with low amp charge to 80%-85% (2.92V-3.10V). Let them sit for at least 7-10 days. Disconnect and record individual cell voltages, let sit for another 7-10 days. Again record individual voltages and discard the cells that have high self-drain. If you would have done this the first time (before DIY build) you could have saved yourself some of the frustration to come.

Whenever buying used salvaged cells always pre-test before DIY build. The way i figure 80% and 90% LFP charge is from 3.65V FULL to 2.65V LVC. That's a diffrence of 1.00V which makes it easier to figure 10% decreases in voltage from FULL. Same for 1865 or 2170 (FULL 4.20V to LVC 3.20V ... difference of 1.00V ... so easy to figure 90%, 80%, etc.) My Controllers' LVC is set at 3.20V for my 18650 DIY build. Only charge my 18650 p-groups 90% (4.10V) for longer cycle life. I bottom balance the p-groups within 5-10mV of each other before 90% bulk charge.

Your LFP Bluetooth will allow you to check your p-group voltages after discharge to see if the p-groups are all within say 30mV to 50mV of each other before bulk charging. You could add/use two 10s Jst-xh balance wire connections coming off same p-group cell connection point as the BMS leads. Thus, allowing you to bottom balance before bulk charging. When done correctly you won't short-circuit any of the 20s2p groups as each of the two 10s Jst-xh are independant of each other ... you'll figure it out if you find it necessary to bottom balance before or top balance after bulk charging :) ... the BMS by itself may not be able to adequately balance the 'used' Headway cells.

Great you have Bluetooth BMS to keep track of pack balance. Probably always disconnect BMS when pack in storage?
 
amberwolf said:
Are they the *same* cell groups at the *same* voltages?

I am not sure but I wrote down the previous group numbers that failed and would compare. If they _are_ the same failed groups would this indicate the BMS is probably draining them?

amberwolf said:
If you need to charge them as a pack (no single-cell charger available) then you can do that, then unbolt them all and separate to individual cells and let them sit. If the self-drain rate is high, you won't actually have to wait long to see voltages drop. :/

Thanks, I did not think of this but I will do this the next pack rebuild.

eMark said:
HOW LONG did you let it sit (5 days, 10 days, 15 days)?

Over 15 days. I would say about 3 weeks. The BMS is a Chargery BMS24T. The balance wires are connected. But the power to the BMS is switched off from an external DC-DC 24V isolated converter.

The initial 40 cells were from https://www.evcomponents.com/ and then the replacements I got from https://batteryhookup.com/. Both were sold as new cells (as far as I could tell). I didn't want to gamble with used cells.

It sounds like I was a bit naive for using the cells without testing them in some way first. At the time I couldn't think of a good way to test and match groups of 40 cells. But I will try charging the pack and then disconnecting all the bus bars and take readings every couple days to compare.

Thank you both for the input. It is helpful and I have some good things to test now!
 
andyshinn said:
The initial 40 cells were from https://www.evcomponents.com/ and then the replacements I got from https://batteryhookup.com/. Both were sold as new cells (as far as I could tell). I didn't want to gamble with used cells.
Battery Hookup states they are USED HEADWAY cells ... https://batteryhookup.com/products/used-headway-38120-hp-3-2v-8ah-lifepo4-battery

When BH marketeers of these 38120HP Headway cells say: "THESE CELLS HAVE ALL BEEN VOLTAGE TESTED AND INSPECTED TO BE IN PERFECT WORKING CONDITION" by no way means they are new Grade A cells. What they define as "PERFECT WORKING CONDITION" gives new meaning to the word "PERFECT" ... kind of like when a used car salesman says: "This 2011 Honda with only 135K has passed our 21 inspection checklist in perfect working condition".
 
andyshinn said:
amberwolf said:
Are they the *same* cell groups at the *same* voltages?

I am not sure but I wrote down the previous group numbers that failed and would compare. If they _are_ the same failed groups would this indicate the BMS is probably draining them?
Yes, this is where AW was suggesting the BMS "stuck-on balancers" failure mode.
 
andyshinn said:
Again, I left it sitting with BMS and everything off and when I came back it had the same problem. A bunch of cell groups were under 2 volts and some at 0 volts.
Does "off" also include disconnecting the 20s BMS Harness Connector from the BMS?? If not then everything isn't OFF. Meaning the 'self-drain' could still be due in part or fully ... due to a faulty BMS contribution ... :(

Any possibility they were the same p-groups both times?? ... "when I came back it had the same problem" ... "Or am I just unlucky with a bunch of bad cells twice in a row?"
 
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