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View Full Version : Measured 12volts temporarily between frame and pack ?!!?



Hugues
09 March 2013, 0800
I was working on the bike today and felt something like a little static schock on my fingers. although i was wearing gloves. The second time i felt it and i decided to check with the voltmeter if there was any voltage between my frame and the 0 volts lug of my battery pack,

surprinsgly enough i measured something like 12 volts initially, then it slowly went down to zero in 5 or 10 seconds max. I measured the resistance between the same 2 points and it reads infinite.

Was I dreaming ? I did not really ride the bike today, just a few minutes a very low speed due to a technical problem. The 96 volts input wire from battery pack into my DC-DC converter was disconnected since last night.

Not sure what i really felt under my glove, but for sure the 12 volts reading was there for a few seconds.

Could it be static building up ? at 12 volts ?!?, or my DC-DC caps that were slowly discharging through the frame ? orthe BMS caps, if any ?

Anything to worry about ? Any other test i can run to exclude any short between battery pack and frame ? Visually, all look in order.

Skahle
10 March 2013, 1742
Interesting topic. It makes sense that a capacitive charge can build up between the frame and a fully isolated system that moves massive power close to it. The fact that it dissipated quickly when you connected a fairly high-impedance load (voltmeter) shows the amount of stored energy was small. If your system is not isolated then power supply caps sounds reasonable. BTW, you can't feel 12VDC unless you lick it...might have crept up a little higher?

The real question is fully isolated or not? In the 80's I designed a system that was used by the DOE for EV testing. It would periodically test for high or low side pack shorts to chassis for isolated systems like these. Our system would connect a very high resistance between one side and chassis and measure the voltage across it, then wait a bit (in microprocessor terms) and do the other side. It is easy to think it is isolated, let a wrench hit the chassis and find the pack has shorted and we were designing for experimental cars which had packs ranging up over 300VDC.

I think you are okay, but double check clearance on all pack components to chassis and keep measuring it. Also measure the motor case to pack and chassis.

BaldBruce
10 March 2013, 2020
Pretty normal reading. My frame usually sits at roughly half the 72V pack volatge. Since the pack is floating in relation to the frame, it can be at any potential it wants to be. As long as the voltage you read is not constant, your pack is indeed isolated and you do not have a frame leak. I have no idea what you felt through your glove, but the fact that you measured a changing voltage between your pack and the bike frame means your main pack is in fact isolated and you are in no danger from it. In general, it is always a good practice to check this voltage before working on anything electrical on your bike. If it is bleeding down to zero volts, then you are safe. If you read a steady voltage. STOP and troubleshoot the source of the unwanted connection. (Motor brush dust and bad DC to DC converters being the first two to check in my experience.)

jonescg
10 March 2013, 2233
My first Voltron pack used to read 50-60 V between any terminal and the frame. In fact the battery packs which were built into aluminium enclosures used to show the same voltage before they were even installed into the bike. I figured it was capacitative charge accumulation between the aluminium case and the terminals, as it steadily dropped with the multimeter across it. The problem largely went away after I wrapped the packs with PVC tape, but if you went looking for it you could get a tickle.

That's why the new pack is going to be completely isolated and enclosed in polycarbonate.

Hugues
10 March 2013, 2332
thanks for your input guys, interesting topic.

my urge to always try to understand things makes me ask a few more questions, if you allow me:

- so we agree there is a certain quantity of energy then that is stored in the frame, albeit small (voltmeter reading brings it down in a few seconds)
- but it builds back, while the bike is: not moving, all controller, DC-DC , BMS disconnected, even their ground wires. By which process does it build up then ?
- a small thing that i noticed and that may serve as a hint: after i unplugged most of my gear, the voltmeter reading was bringing it down to zero much faster than before when it was plugged.
- the charge seems to reach a plateau of around 12 volts after few hours. Could it be the caps of the controller or the DC-DC ? Just a wild guess here.

i could remove the controller and dc-dc from the bike to check, but the bike is out for body work now....