How to measure cell voltages directly from BMS?

[zzoing]

i need to measure all the cells from the signalab bms without unwrapping all the battery.

can someone please return to me this photo with pics of where the multimeter pins go to get the cell voltages?

here is a photo:

[dogman]

You can do it on the wires/pins coming from the plug. Right there where the plug is soldered to the board.

Or you can take the measurments from the plug itself, after unplugging the bms from the battery. Like this.

One cells voltage, sm.jpg (119.23 KiB) Viewed 299 times

Either way, you touch two wires, and march both probes down the line to get each cell without having to do subtractions, like you would if one probe stayed on the end wire.

[zzoing]

Thanks! i kindof should have know that! doh! i was so busy thinking about measuring from the board i forgot the wires LOL

ok

i left my board plugged 2 months while waiting for a new controller, it's gone down to 16 ah but it seems stable at 16. any suggestions appreciated. i have to measure again next time it's flat.

[dnmun]

it is best if you measure the cell voltages while the pack is charging. you can see which cell is highest and will cause the BMS to shut off charging.

don't disconnect the sense wire plug. measure the voltages right on the BMS pcb. this improves the chance you will see a bad connection in the sense wire plug.

the first cell is measured with the black probe on B- and the red probe on the first pin on the back side of that sense wire plug where the pins are exposed and you can stick the probe into the solder at the base.

DO NOT short out adjacent pins when you use the voltmeter. touch only one pin at a time with one probe, so have the BMS in a stable position and just be careful.

[dogman]

True enogh, in my picture you see just how close you are to disaster. Doing some math would be safer.

Best thing of all, splice in some 8s jst plugs on the wires, then easily plug in a Cellog 8 anytime. The smart ones have done this, But I keep procrastinating on it.

[ice_robin]

nice job

[dnmun]

the original poster asked how to measure the voltages on the BMS. he did not ask how to measure the voltages on the end of the sense wire cable.

the only way you can tell if the BMS is functioning to balance the cells in the pack is to measure the voltages of the cells while the pack is charging.

if you wanted to measure how rapidly the cells self discharge, then you would disconnect the sense wire cable and measure the voltages on the ends of the sense wire cable as the cell voltages dropped down from the balancing level of the BMS.

the BMS will consume current while the cells are draining down so it would affect the voltages and you would not have an accurate idea of how much each cell is draining itself down normally as you would when you disconnect the sense wires.