Help with stray voltage

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chronosgarage

10 W
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Oct 23, 2020
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96
My build is 2000w hub motor with 20S12P and 20S9P battery connected in parallel in each cell parallel group. The two battery is connected to one bms that connected to the contactor. I have not connect the controller and the hub motor at all.

The problem is I have a voltage reading when I connect my volt meter from battery negative or battery positive to the bike frame. I’ve tried to find the source of the voltage without any success.

When I connected 20watts 47ohm resistor to the bike frame and battery negative (ground) the voltage drop to about 0.01 to 0.02 volt.

Is this condition normal, or there is something that need to be fixed?
 
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My setup is 2000w hub motor with 20S12P and 20S9P battery connected parallel in each of the battery parallel connection to a single bms, the bms is connected to a contactor. I have not connect the controller and the hub motor.

The last time my controller blew because of a pinch from the main positive wire to the frame. So before I installed the new controller, i want to make sure that everything is correct.

I’m having a voltage reading from the battery ground (-) to the bike frame. The voltage is about 1 to 2 volt before the contactor is engaged and about 7 to 8 volt when it is engaged. When I connected the battery ground to resistor and to the frame, the voltage drop to about 0.01 to 0.02volt.

I’ve tried to find if there is any short somewhere within the bike, but I can’t find any.

Is this something normal or something that need to be fixed?

Any help is appreciated...

Thank you...
 
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If nothing is electrically connected to the frame, then there should be no current flow. Put the meter on mA setting and measure current flow between frame to batt negative, and frame and batt positive. Both should be 0mA.
 
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The bms is not mounted on the frame and it is well insulated. The contactor is mounted to the frame, but when I dismount the contactor, the voltage remained the same.

I don’t use ground on the frame.

Sorry for the double post.

Thank you for your reply.
 
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Try measuring resistance between the frame and battery negative. If you see a low reading, like ohms, you have a short. If the meter shows high, more than megohms, then it's good. Your meter is very sensitive and you can get stray readings like that just from moisture or stray resistance in the controller.

You want the frame to be isolated from the battery circuit.
 
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fechter said:
Try measuring resistance between the frame and battery negative. If you see a low reading, like ohms, you have a short. If the meter shows high, more than megohms, then it's good. Your meter is very sensitive and you can get stray readings like that just from moisture or stray resistance in the controller.

You want the frame to be isolated from the battery circuit.
Click to expand...
Thank you for your reply.

I’ve tested the resistance between the frame and the battery, the resistance is 0.5 to 0.6 mega ohm.

I have not connect the controller or the hub motor.

Is there anything that should be fixed?
 
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chronosgarage said:
fechter said:
Try measuring resistance between the frame and battery negative. If you see a low reading, like ohms, you have a short. If the meter shows high, more than megohms, then it's good. Your meter is very sensitive and you can get stray readings like that just from moisture or stray resistance in the controller.

You want the frame to be isolated from the battery circuit.
Click to expand...
Thank you for your reply.

I’ve tested the resistance between the frame and the battery, the resistance is 0.5 to 0.6 mega ohm.

I have not connect the controller or the hub motor.

Is there anything that should be fixed?
Click to expand...
What brand and model is your multimeter?
 
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E-HP said:
chronosgarage said:
fechter said:
Try measuring resistance between the frame and battery negative. If you see a low reading, like ohms, you have a short. If the meter shows high, more than megohms, then it's good. Your meter is very sensitive and you can get stray readings like that just from moisture or stray resistance in the controller.

You want the frame to be isolated from the battery circuit.
Click to expand...
Thank you for your reply.

I’ve tested the resistance between the frame and the battery, the resistance is 0.5 to 0.6 mega ohm.

I have not connect the controller or the hub motor.

Is there anything that should be fixed?
Click to expand...
What brand and model is your multimeter?
Click to expand...

My multimeter is Klop DT 9205A. It Chinese brand.
 
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chronosgarage said:
Thank you for your reply.

I’ve tested the resistance between the frame and the battery, the resistance is 0.5 to 0.6 mega ohm.

I have not connect the controller or the hub motor.

Is there anything that should be fixed?
Click to expand...

Not really. That sounds pretty normal. You could try hooking up the controller and motor and repeat the test. If it stays in the meg ohm range, I wouldn't worry about it.
 
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Voltron said:
Maybe it's a stupid test, but if you take the batteries out of the frame and put them on something next to the bike, is the voltage still there?
Click to expand...
Thank you for your reply.

My battery is placed within the frame and also have 3 different place that is connected at every parallel cell group, so it becomes one big battery pack. There is a lot of cables and it’s hard to take the battery out without have to redo the cabling.
 
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fechter said:
chronosgarage said:
Thank you for your reply.

I’ve tested the resistance between the frame and the battery, the resistance is 0.5 to 0.6 mega ohm.

I have not connect the controller or the hub motor.

Is there anything that should be fixed?
Click to expand...

Not really. That sounds pretty normal. You could try hooking up the controller and motor and repeat the test. If it stays in the meg ohm range, I wouldn't worry about it.
Click to expand...
Thank you so much for your solution...
 
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