How to safely test a lithium battery management system board

Question

I'd like to design a battery management system board (based on TI bq40z50-R2) for my own small (2-4s) lithium battery pack, but I am still unsure how can I test it safely.

I am worried that an error in my board design or in the configuration of the bq40z50 might cause a short circuit or some other unsafe condition to the batteries.

The tests would also include batteries short circuit, undervoltage, overvoltage, overcurrent.

I was thinking of buying for the first tests a 2s or 3s commercial lithium battery pack with protection (short circuit, overvoltage, undervoltage per cell,…) already integrated (unspecified IC).

I am unsure whether that protection circuit might interfere with the normal operation (measurement accuracy,…) of the bq40z50, invalidating the test.

I could also use stricter protection thresholds for the bq40z50: for example I'd set 4.23V for overvoltage on the bq40z50, whereas the protection IC in the commercial battery pack is set for 4.35V. This way I'd see if the bq40z50 sets the appropriate flag during an overvoltage test.

During such a test, can I also use a multimeter to monitor the battery' voltage or would this also interfere with the protection ICs?

Once these first tests are done, I'd proceed using the designated batteries for my battery pack (branded 18650 Li-Ion or RC LiPo without protection IC already in them).

Could this work or do you suggest an other procedure/hardware?

Is there another way to simulate a Lithium battery without actually using one?

Answer 1

If you wish to test just the BMS, there are battery simulators out there. The generic tool for such a job is a 4 quadrant power supply, it can both source and sink current in both polarities. This will allow you to test all possible output states with one device.

Answer 2

To mimic a li ion cell is going to be difficult due to there discharge characteristics. Every chart will show you what i mean, but speaking of lifepo4, they have a very stable charge/discharge curve. For a simple bms for this kind of battery i would suggest some cut of relay set at 3.0 V, and 4.0 V to stop charging. It depends of course on your total cells what you connected in series. A resistor board for balancing above 4.0 V per cell. With all this added you can safely test your bms board. But keep in mind, that my numbers are pretty standard, are very safe and for a lot of cycles, 4x more than factory standard cycles.

If you keep all the margins 0.1 V less then the margins of the TI bq40z50-R2, you can test whatever you want.

The C.I.D. chip in a protected cell, does nothing more to protect the cell from being overcharged and short circuit, thus prevent a thermal runaway. When it is popped, your cell is useless.

Answer 3

I've tested a lot of BMS's and batteries, and all were double-protected, that is there's a thermal fuse right next to the cell.

You can buy those resettable fuses pretty cheap (ca. 0.20 USD), so don't save on them. Some cells already have them, especially the flat ones, but I'm not sure if 18650 have resettable protection built in.

PS after a few burned fingers and a burned table I assure you that protection is a must with Li-ion cells.