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Author Topic: Battery Management System -> how to test?  (Read 8180 times)

Offline Just

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Battery Management System -> how to test?
« on: September 14, 2012, 11:08:13 AM »
It seems I have some troubles with my 48V BMS...

Actually, when I connect a load to the battery with BMS, the output voltage of the whole pack drops to 2.8V. But, when I disconnect the load, the output voltage gets back to 52V.

When I connect the same load to the same battery cells without BMS, the output voltage doesn't drop.

Is the problem in BMS? Might it be fixed?

Thank you
« Last Edit: September 22, 2012, 05:29:57 PM by dmitryl »
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Offline Just

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Re: Battery Management System -> how to test?
« Reply #1 on: September 22, 2012, 05:29:27 PM »
Any comments? Help?

Does anybody know how to test BMS?

Case#1: Is it possible that BMS shorts some cells and discharge them to 0V? Then the battery pack with BMS is disconnected from the load, the cells keep their voltages.

Case#2 (another battery pack): When a load is connected to the battery with BMS, the battery's output voltage drops from 48V to 2.8V. Why? Is there a shortage inside of the battery and BMS cuts an output voltage?

Thank you!
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Offline Cornelius

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Re: Battery Management System -> how to test?
« Reply #2 on: September 22, 2012, 06:34:25 PM »
In case #2; if that battery pack have a key-switch, have you considered that the switch might be faulty?

It is not uncommon that the contact points in the key-switch get so burnt and dirty by switching on/off a heavy load, that it cannot carry any current anymore...
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Offline Just

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Re: Battery Management System -> how to test?
« Reply #3 on: September 22, 2012, 07:17:49 PM »
"if that battery pack have a key-switch, have you considered that the switch might be faulty?" -  no, the switch is okay. When I disconnect the BMS from the battery and connect it (battery) directly to the load, so deep voltage drop doesn't happen.

I wander how BMS is able to drop a battery output voltage from 48V to 2.8V! What's the magic number 2.8? Why is 2.8V? What voltage should be measured on the battery pack output leads when BMS disconnects it from the load due to a low voltage on one of the battery cells? Should it be 2.8V?

Thank you!
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Offline Cornelius

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Re: Battery Management System -> how to test?
« Reply #4 on: September 22, 2012, 07:31:46 PM »
Why 2.8V, I do not know... :(

But to your topic question; I think you already answered it yourself. ;D

"How to test BMS": Disconnect it. If your problem then goes away, the bms are faulty. :D

But what is wrong, I do not know.
It could be one of the thin wires going to each cell that have come loose? Or something with the bms itself.
The last one would not be easy to find out, and I think a new bms costs around $30?
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Offline Just

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Re: Battery Management System -> how to test?
« Reply #5 on: September 22, 2012, 08:25:24 PM »
"new bms costs around $30" - I need one, which is able to maintain 25A continuous and 80A peak currents and it costs a bit more expensive.

Anyway, I tend not to throw things away before being sure they don't work :)
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