Trouble balancing cell voltage with 13s BMS

Hi all!

I just put together a 13s8p battery using new vruzend caps that is being used with a generic "1000 watt" DD kit. I used recycled Panasonic NCR18650a cells from user tumich on this forum. The cells arrived with voltages between 3.4 and 4.1, so when I assembled I made sure the difference in voltages was no greater than .1 within each of the parallel groups. After wiring the BMS and charger both bought from vruzend.com, I plugged it in to charge. After charging, the cells remained out of balance with the lowest group being at 3.6 volts and the highest at 4.15 volts. I tried draining the battery some by riding the bike for 20 minutes then plugging it back in to see if this may help solve my issue but I haven't had much success. I guess I am just checking to see if the BMS is not working correctly or if I made a mistake by not balancing the cells beforehand or a different error.

Please let me know if you have any thoughts and thanks for reading!

The BMS can only balance very slowly. If the cells are significantly out of balance, you should manually charge the individual low cells to try getting them all to the same level, then the BMS can probably maintain the balance.

It will take time to balance. The BMS balances by loading the high parallel group with a small load and bringing it down level to the lower groups. The load it applies is usually very small, often well under 500mA.

I believe most BMS will only balance while its on charge. The BMS will apply charge voltage to the pack until one group reaches 4.2v. It will then cut charge and apply load to that group until it reaches a lower voltage and then it will apply charge voltage accross the whole pack again. It will cycle like this until it gets the pack to its full voltage.

If there is one low group this can lead to a lot of stress on the BMS as it will have to load up all other groups to try and balance them down.

Its best to balance the cells prior to building the pack so you don't work the BMS so hard. The BMS is really there to maintain balance in a pack rather than balance groups that are 0.5v apart.

I hope you don't fry the load resistors in your BMS...

Thanks for the help. Do you think it will be evident if the load resistors are busted?

nifty150 said:

Thanks for the help. Do you think it will be evident if the load resistors are busted?

Click to expand...

Not always. Sometime they will look overheated, but sometimes not. If the cells are attached and the balance shunt is blown (stuck on), the resistor for that channel will be warm and you could feel it if you touch them. You can also test by measuring the voltage across each one. If the cells are below the balance voltage, the resistor should have zero volts across it. If you see cell voltage, then it's probably bad.

Yeah, knowing for sure a BMS is fully operational is not that straight forward which is one reason some people avoid them.

Is there a way you can charge each parallel group individually to 4.2v? Do you have access to the cells in the pack or is it wrapped?

lionman said:

Yeah, knowing for sure a BMS is fully operational is not that straight forward which is one reason some people avoid them.

Is there a way you can charge each parallel group individually to 4.2v? Do you have access to the cells in the pack or is it wrapped?

Click to expand...

I do not have access to a individual cell charger so I will discharge them individually with a resistor and some alligator clips to 3.6 then try charging them all together.

Thanks!

lionman said:

Yeah, knowing for sure a BMS is fully operational is not that straight forward which is one reason some people avoid them.

Is there a way you can charge each parallel group individually to 4.2v? Do you have access to the cells in the pack or is it wrapped?

Click to expand...

I do not have access to a individual cell charger so I will discharge them individually with a resistor and some alligator clips to 3.6 then try charging them all together.

Thanks!

Get some tp4056 charger boards and charge each bank on its own. They can be found on eBay for 1-2€ pee piece