I want to build a 24v 7S 3.7-4.2 18650 lion pack. My concern is the parameters my BMS should have to keep in balance all the 7 cells, not over charge them or over discharge them. cells from laptops: ICR18650-26c.
Please open /check this spreasheet with the values I think the BMS should have to be on the safe side. If values are incorrect , kindly provide feedback and correct values.
Thank you!
Best Answer
A battery protection module should not be used to prevent overcharge and discharge, but only for protection in case the normal charging and cutoff circuits fail. Your BMS specifies 4.28V and 2.8V as the upper lower limits. Those voltages are outside the range of healthy battery operation, but should be OK for protecting against catastrophic failure.
The balancer circuit should pass enough current to correct any imbalance condition. Theoretically that means it must match the charging current, because one cell could become fully charged before the others reach peak voltage. In practice a lower balance current is acceptable provided that the cells are not far out of balance.
Your cells have a capacity of 2.6Ah so they can be charged at 2.6A (1C rate). But the balance current is only 0.168A, so if you charge at 1C those cells better be in good balance! The answer to this issue is to measure the individual cell voltages and charge the battery slowly (ie. not much higher than 168mA) if they are significantly out of balance. Then you can safely use the BMS at full current for subsequent charges.