Sunday, November 27, 2016

Karmann Ghia EV Conversion - Ground Isolation

Proper Isolation of High Voltage System is CRITICAL!

I don't think there is enough information on how to isolate the 12 volt vehicle ground from the high voltage battery pack. When I set up the high voltage system, it was completely isolated from the vehicle electronics by design. The DC/DC converter is one area where the two systems interact, the model I used is an isolated model. The error I made was with the Xantrex battery monitor. It does not isolate the 12 volt negative from the high voltage negative and neither does the voltage prescaler that is required to measure voltages greater than 36 volts.
I figured this out the hard way when I mildly electrocuted myself by touching the chrome bumper with my body while touching the motor which had a residual charge with my arm. At that point I realized that there was a problem. If the systems are isolated, there will be virtually no voltage between the the voltage positive and the body of the vehicle (ground). I traced the problem to the battery monitor. As it turns out, there is an inexpensive and simple solution. It is an isolated DC/DC converter power supply which outputs 12 volts and 1 watt. I bought it on ebay for $6 from China. Unfortunately I had another incident where a tool I was using to secure the high voltage battery shorted to one of the battery leads from the vehicle chassis. This sent current through the battery monitor frying it.

Isolated power supply: DC-DC Converter Isolated Power Module In 10-16V Out 12V DIY Favor

 This type of isolated power supply is a MUST for your EV conversion. It improves the safety of the system and prevents damage to your 12 volt system by a short circuit. You should test your system with a voltmeter to make sure it is truly isolated. There should be no significant voltage when you test the high voltage battery positive and the vehicle ground with a voltmeter. If it reads a high voltage value, you should find the area where the systems are in continuity and isolate them properly.
While we are on the subject of safety, be sure to add as many fuses as you need. Any area where the high voltage system is connecting to something should be fused and the fused should be properly rated for the expected load. For example, the load to the controller may be 400 or 600 amps. The load to the DC/DC converter may be 15 amps and the load to the voltmeter may be 1/2 amp or less. You may also want to place fuses between the batteries especially if they are in separate areas of the vehicle.

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