ADVICE - Why is my battery only charging to 36v ?

[bilabonic]

Hi All

Have had a 36v/20Ah battery on my self build for around 20months and it initially charged to 42v.

It is now only acheiving 36v after a charge and its dropping to around 34/35 whilst in use.

Is the battery on its way out ?

What will it cut out at ?

Thanks

 

[Deleted member 4366]

After years of measuring stuff in industry, I learn't one important step in problem solving. When an instrument gives an unusual or unexpected result, first you should question the instrument, before trying to solve a non-existent problem.

After confirming the 36v by using another voltmeter or checking the accuracy of your present one, the first thing to check is the charger. If the charger output gives a measured 42v and charging switches off at 36v, it's time to open the battery.

You need to peel everything off until you get to the BMS, where you measure individual cell voltages on the multipin connector. You put your black probe on the position where the thick black from the cell-pack joins the PCB, and then probe each pin on the multipin connector, plus a final reading with your red probe on the thick red wire from the cell-pack. Those results will tell you a lot about the health of the battery.

 

[bilabonic]

Thanks Dave, i checked with a Fluke and it was 35.9 v (used a bit since last charge).
Plugged charger in and monitored, it started at 37.7v and ramped up to 42.1v.
AS SOON as i connected the supply to bike/controller it dropped 2v....

Is it worth opening it as i suspect its just old/requires replacing so each cell would be roughly the same position.

 

[Deleted member 4366]

Dropping the voltage to 2v under load means that the BMS has detected an error, so switched off. Typical errors are: Too much current = short-circuit somewhere; over temperature - unlikely; cell reached LVC - normally most likely, but not consistent with battery charging to 42.1v; faulty BMS - could be.

 

[KirstinS]

Just a thought but what type of cells are used in your pack ?

phylion type seem to still charge up to 4.2v each when knackered

But the voltage collapses as soon as a load is placed which means the BMS low
Voltage cutoff can kick in

Could explain the readings , maybe ?