# Determining Faulty Lithium Batteries in Battery Pack

I am having some difficulties with a 72V 200aH LiFePo4 battery pack that I am using in conjunction with an EMUS BMS. The BMS indicated some of the batteries were not charging properly, I corresponded with the BMS manufacturer and he was able to ascertain from the CAN messages that some batteries were indicating higher than normal resistance values, but couldn't tell which ones.

I have disconnected the battery pack and want to go through each of the 24 cells and make sure there are no other faulty batteries in the pack (besides the 4 very swollen ones).

Am I correct in understanding that I would need to charge each individual cells (separately with a PSU, not the BMS), attach a resistor to the fully charged battery and then measure the voltage drop over time?

Should I attempt this on the swollen batteries or is it too dangerous to bother trying to charge? Would I be better off just swapping the physically altered cells with replacement, reconnecting the pack and using the BMS to determine if there are any further issues?

• Don't use a PSU to charge the cells. LiFePo4 don't behave well with just a constant voltage. – Brian Carlton Aug 31 '17 at 16:46
• How fast many cycles did these last before failure? What is xy array size? 24S30P ? – Sunnyskyguy EE75 Aug 31 '17 at 17:28
• For such elaborate system as EMUS BMS, with individual controllers per each cell, it is very strange that the system can 't identify which cells have problem. Specially in 24S configuration. Strange. – Ale..chenski Aug 31 '17 at 18:40

## 2 Answers

I don't know array size or serviceability of cells, but I would remove all swollen cells first.

"Good" Batteries are balanced for mAh <1% when new. This imbalance obviously increases with aging such that the weakest cell dies 1st due to more rapid aging from rising ESR during charge and discharge and more from undercharge condition and cold temps.

After-charging, ESR values rises very slightly with each charge cycle then rapidly towards End of Life EOL, causing excess heat and bulging. This depends on CC rates used and expected life cycles, that have a wide value depending on Depth of discharge , DoD i.e. 200 to 2000 cycles or more.

The job of the BMS is to extend the string battery life by preventing overcharge or undercharge during use (if capable).

Low power BMS products may not be able to bypass all the potential power during max current in the field with rapid charging i.e. V*I=Pd bypass, during CC mode and may rely on balancing during CV mode when current is declining.

e.g. If the BMS uses passive balance, and limited to 5W load resistor or 1.5A balance load and you are still charging in CC mode, > 1.5A , you are going to have infant failure mode.

State of the art () BMS designs use patented SMPS methods with dual bridges and series choke to bypass cell charging current without much loss or have greater Amp bypass capacity for rapid charge.

## Test Method

I would suggest the way to identify the weak link is a low duty cycle high pulse current and store peak-peak voltage swings of each cell rather than average. This depends on the ability of the BMS to monitor Vbat (min,avg,max) in real time and store these values. Otherwise use a Vdc, Vac method of your choice for each node and determine where the Vac rises the most. and try to keep these balanced by logging and sorting and use a spreadsheet for each cell serial# or label# for quality control and improvement tracking.

## Comments

I would expect LPF cells to be 3.2V nominal after charge and rated for 2k cycles before failure when used within specs.

## Recommendation

All core Emus BMS functions only use the minimum, maximum, or average values rather than individual cell data, therefore only these aggregated values are normally stored during the periodic cell monitoring process. However, the individual cell data may still be useful for diagnostic purposes, therefore it can be acquired by request over USB, RS232, or CAN interfaces.

If you are not already using this software, I would get it. Then use the above method to obtain individual cell voltages using a pulse current test method at appropriate intervals to collect the data for all cells after initialization.

https://www.elektromotus.lt/files/bms/docs/Emus_BMS_User_Manual_v.1.2.1.pdf

This is a impedance chart comparison between good and bad Li ion battery link.

As we can see, the two curves are very similar, so it very difficult to determine whether the battery is good or bad by measuring internal resistance. We can further see, that ESR at 0 Hz is the same, so any classic method as loading the baterry and measuring the voltage drop gives no plausible result.

The advanced method would be measuring impedance $Z(\omega)$ and making a reference (good) curve and then compare.