BMS ebike battery

Hello All,
in relatively new in this forum. My name is Jiri, generaly I am good in electrochemistry, and li-ion technology.
I started to make battery replacement, for electric bikes batteries. I know about batteries and their behaviour, if somebody wnat to ask, just ask me, but I do know anything abotu BMS units. May you help me how to chekc if BMS unit of the battery pack works properly?
I can outline of the proces: Take battery pack, disassemble it, remove the li-ion cells itself and now, how to check the BMS?

Thank you a lot and I am sorry for not so clever question.

Yours,
Jiri

Hi Jiri,

No need to dissemble your battery. Simply meter the parallel groups to make sure they are all of equal voltage after an overnight charge with the bms connected properly.

Is your battery "unbalanced" after an overnight charge?

Why are you attempting to test a battery problem?

Thank to e-bike for your answer and suggestion, but generaly this way I can not recognize if the BMS or individula cell(s) fault.
Do you have any idea how to test the BMS faster? IS there anything like BMS tester device or somebody in forum mas suggest me?

Thank you a lot.

Jiri

It can be somewhat challenging to test a BMS with no pack. I built a test fixture that used a string of 1k resistors and a bench power supply to simulate pack voltages. This can test the main charge and discharge transistors and the control circuit.

The BMS is typically has several functions:
Any cell over voltage (usually 4.3v)
Any cell under voltage (around 2.5v)
Over current, usually programmed for the maximum allowable current for the cells
Short circuit, a higher current but has no delay in cutting off the load
Balancing, which it does by turning on a resistor when the cell reaches the balancing point (usually 4.2v).

To really check everything, each cell circuit needs to be individually tested for over and under voltage response and the balancing point measured. Just measuring the balance voltage is difficult. When the shunt triggers and turns on the resistor, the cell voltage (or simulated cell voltage) immediately drops and the shunt turns off. When you get it right at the threshold, it will pulse with very short on times that are difficult to measure. I use a LED clipped to the shunt resistor to detect the short pulses. An oscilloscope also works.

To fechter, thank you very much for your reply, it clarified me a lot!
Do you dealing with battery repasing? May you help me with construct BMS tester? May you outline the scheme of the bench supply, resistors with LEDs and connection to BMS? I am sorry for bothering you, but I can help you or andwer to the quedtin relating to the electrochemistry.

Thank you very much.

Yours,
Jiri

Here is a topic describing my bench test for generic BMS boards:

https://endless-sphere.com/forums/viewtopic.php?f=14&t=94617

fechter said:

Here is a topic describing my bench test for generic BMS boards:

https://endless-sphere.com/forums/viewtopic.php?f=14&t=94617

Click to expand...

Thank you very much Fetcher!
It is great!

BR,
Jiri