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Old 07-02-2012, 09:02 AM
PeterH PeterH is offline
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Default Traction Pack Voltage and Vehicle Ground

Hi there,

How would you correctly test an EV build to ensure the traction pack is correctly isolated from vehicle ground? This is AFTER everything but the controller is installed and functioning. UPS says my controller (Soliton Jr.) should be here today. :-)

Thought I'd ask that basic question before I started to panic and make all sorts of wrong assumptions.

Yesterday I was installing an Airpax circuit breaker (160 VDC 250 Amp) between the end of my battery pack and my distribution panel. I was very shocked (literally!) to discover that while holding the end of my wrench (taped with electrical tape along the handle but not at both ends) and brushing the side of my aluminum battery box with my hand, I felt a jolt of electricity.

Naturally, this was a very alarming discovery and is the cause of my opening question.

Thanks,
Pete
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  #2  
Old 07-02-2012, 10:05 AM
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Ziggythewiz Ziggythewiz is offline
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Default Re: Traction Pack Voltage and Vehicle Ground

Stick a meter on the chassis and the pack. You shouldn't get a voltage. When you do get a voltage you can make a connection with a light bulb to see how much of a connection there is.

Non-isolated gauges are a common way to get pack leakage to the chassis.
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Old 07-04-2012, 05:26 PM
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Exclamation Re: Traction Pack Voltage and Vehicle Ground

It might be a good idea to put a 10k ohm 5 watt resistor in series with an LED on both ends of the battery pack (144V for exampl) to vehicle ground. If either LED lights brightly there is significant leakage. Otherwise this should give you a voltage of about +/- 72 volts from either end to chassis. The 10k resistor will limit the current to about 14 mA maximum if you should touch any part of the battery pack while you are grounded elsewhere. This is enough to give you a bit of a shock but not enough to cause serious harm (unless you jump and fall or hit something).

There is probably enough leakage and capacitance in a complete system including controller and motor to give you a shock. And insulation breakdown of wiring or conductive dust from the brushes and commutator may provide an even stronger current. So you could even replace the LEDs with SSRs and trigger an alarm or trip the safety disconnect if either one sees enough current to turn on.
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Old 07-04-2012, 06:38 PM
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Default Re: Traction Pack Voltage and Vehicle Ground

Hook a 100k ohm resistor across a standard digital volt meter before using it to check for voltage between the frame and the pack. These meters have a very high impedance input and will often show voltage present when there is virtually no current to back it up. 100,000 ohms means only 3 milliamps could flow with 300 volts present. If this bleeds the voltage down to a low value then any leak you have is one with an even higher resistance than that. Those kind of tiny leaks are virtually impossible to completely eliminate and lack the current potential to hurt someone (but often can be felt as a little jolt.)
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