How to test a Battery Management System.

[trike]

I have a Golden Motor 36v 16ah battery which has quickly lost run time. I think the cells are not too bad. The battery just dies when the orange battery indicator light goes out. I think the BMS cut off is 32v is that so. I think the BMS is faulty. Can any one tell me how to test the BMS. If anyone knows the right way to test the cells I would like to know that as well( that could come in handy some time in the future ).

[_GonZo_]

Ok here we go.

Well first you have to know a bit how and what a Battery Management System does.

Usual functions:
- Control max. Pack Voltage during charging cut of charge when pack reaches max voltage for your pack should be around 42V
- Control max. cells voltage during charge cut of charge charge when any cell reaches over max. voltage around 4,25V for your cells.
- Control min. pack voltage during discharge, cut of current when pack reaches min. voltage for your pack around 30V
- Control min. cells voltage during discharge, cut of current when any cell reaches min. voltage around 2,5V for your cells.
It can have more functions or less than this but this are the basic ones like:
- Temperature control, if the pack temperature passes 60ºC will stop charge or discharge.
- Balancing function keeps cells valanced in voltage.

Now how to check it:
Firs you need acces to the cells or at least to the balancing connector of the pack.
And you need a voltmeter.
The blanacing connector is something like in the picture with more pins in your case.



You can check directly on the cells or on that connector the voltage of the full pack and the voltage of each cell.

The best moment to check is when battery is almost empty, at that point problems are easier to find.
we are going to find out if the BMS is wrong or the cells.
First you have to check if all the cells have the same voltage, difference between the cells should not be over 0,1V, so for example the lowest can be 3,6V and the highest 3,7V if the BMS has balancing function the difference should not be higher than 0,02V

Then connect the pack to the charger for a while, and check the voltage increase on each cell if any of the cells increase voltage more quickly than the others that cells is dead or near to dead.

Now disconnect form the charger and put it on discharge with the motor or something else, and check again voltage of the cells and follow them till the pack empties.
When the pack empties all cells should be around 3V if one or more cells are way under then the BMS have cut because one or more cells are damaged and reach minimum level per cell, instead of cutting at full pack voltage end that is around 30V
As well you have to been able to see during this discharge if one or more cells droop voltage more that the others under the same load if there is one cells or more than one cell that droops more than the others that cell/s are dead or close to dead (provably are the same as during charge and provably the ones that are lower voltage when empty)

(The voltage, temperature and other values I posted here are the standard for Lipoly cell and packs they can be sightly different on each BMS model and differently adjusted to battery different models and uses)

Hope it helps and ask if something is not very clear, because my mother tong is not english.

[_GonZo_]

I just seen a picture of a GM BMS I know them and It has balancing function.
Do not know if have installed temperature control.

[trike]

Thank you for your help. The battery cuts off the power when the orange battery voltage light goes out. When I turn off the battery and turn it on again ,all the lights come on again. I can then drive away until I reach the next hill and the motor is under load ,then it cuts off again. I have a turnigy power analyzer in circuit. The power cuts off when the voltage goes under 36v which is when the orange light goes out. That voltage is too high for low voltage cut off. I think it should be 32v. It used to cut off when the red light went out. That is why I think the BMS is faulty.

[_GonZo_]

It is possible that the BMS is faulty but I bet more for a cell or cells bad.
When you put load on the pack (going up hill) the bad cell/s droop voltage under minimum cell voltage so the BMS cuts off.

What I mean is that the full voltage pack can be 36V but one or more cells are under the minimum voltage for example all the cells are around 3,9V except one that is at 3,3V so when you try to get current from the pack that cell goes down to 2,8V while the rest of the cells keeps around 3,7V so the BMS cuts off. (it is not exactly this way but I think is the way to make me understand)

[trike]

GonZo was right. I tested the cell groups(set of 4 cells). 1 set is 3.25v with the other sets being between 3.73v and 3.76v. 1 or more of the cells in the low voltage group is faulty. I think I will replace all 4. I will email Gary and hopefully he will be able to send me 4 new cells. Thank you to everyone for their help.

[_GonZo_]

Halt !!!

Those voltage readings were under load or no load?
If they were under load ok that cell is dead.

[Just]

"Those voltage readings were under load or no load? If they were under load ok that cell is dead" - and if the measurement was done on the battery disconnected from the load?

[Morgen 3Eman]

Hi _Gonzo_,

Hey, since  you've seen a GM BMS, did it have anything that looked like a high power FET that may function as a master on/off switch for the battery pack? 

TTFN,
Dennis

[Just]

As for the FET switches in BMS, I have 48V BMS with 10 FET switches on it? Why only 10 FETs for 16 cells? On another 48V BMS, there are  only 9 FETs (one is missing).

What purpose of the FETs? Might one missing FET cause any problems with BMS?

Thank you!