Re: [electricboats] One cell out of whack. Replace it or stay with it.

Beautiful accurate write up. BMS of quality protect cells first. Them implement user settable functions

 

Best regards

Lee Eldridge

0427874796

On 2 Jul 2025, at 02:50, Anton via groups.io <iamyouranton=gmail.com@groups.io> wrote:



I have used a JK BMS, and several other brands of BMS, and their main function is to disconnect the battery before damage happens. Your BMS is not doing that. It should do that regardless of any interaction between the BMS and charge sources  or discharge loads, any CANbus or other signal level stuff that is intended to control anything else should be irrelevant to the BMS simply disconnecting the battery before anything bad happens. Cell balance is only there to balance the cells when things are working properly, if one cell goes high, the BMS should simply disconnect the battery from everything. 

To reiterate, you definitely want your charge sources to be set up correctly, your discharge cutoffs in your inverter or other loads to be set up properly, it serves both the keep weird stuff from happening when the battery disconnects but also as a double safety, the BMS *must* disconnect when any cell goes low or high regardless of any other set points or parameters. 

So, your BMS is not functioning correctly, and you need to figure out why. 
As suggested, definitely double check the cell voltages against what the BMS is telling you with a quality DVOM, but also do a thorough review of all your connections, I have a bell going on in the back of my head that maybe your charger is hooked up to the batteries directly in such a way that the BMS disconnect is not disconnecting it. 
If that is not the case, maybe the BMS settings are set to a too high cell voltage. In any of the BMS I have used, cell voltage overrides any other settings, if one cell goes high or low, it doesn't matter what the overall voltage settings, delta, etc are set for, if one cell goes high or low, it disconnects that battery, that is the whole purpose of any BMS.

There was some noise on the forums in the past about JK BMS firmware being wonky and settings changing to lithium ion settings when it was disconnected from the battery, but I haven't experienced that. Make sure your BMS is running the latest firmware. 

Also worth noting, while testing, do not take your eyes off the meter while charging! It can go from resting voltage to high enough to damage the battery in minutes, from 3.4 to high enough to damage the battery in seconds. Do not leave it unattended until you have figured out what is not right. 
Having driven that one cell up to 4.25v, it kay be damaged, but for the purpose of trouble shooting the BMS, use that cell, it can be the canary in the coal mine and be sacrificial while testing. Once you have things working right, you can capacity test the whole battery assembly and find out if that cell is dragging the whole pack down enough to warrant replacing it.