There is an ongoing debate on whether it is a good idea to use the chassis for the return current path on 12 volt circuits on van conversions. Normally, you would run a plus 12 volt wire from the house battery to the 12 volt load, and then a same size return wire from the 12 volt load back to the negative terminal of the house battery. An alternative is to eliminate the return wire and use the vehicle chassis for the return current instead – so, the negative return wire just becomes a short wire from the negative terminal of the load to a nearby point on the van chassis, and the chassis carries the return current. This saves a wire for each 12 volt load it is used on.
I’m not a fan of this procedure because over time, the connections to the chassis tend to corrode and then the connection becomes unreliable and often fails in unpredictable (hard to trace down) ways. I think that anyone who has worked on car electrical systems over the years has probably had to deal with problems caused by bad grounds. For me, its faster and easier and better just to run actual negative return wires even if it costs a few bucks more in wire.
One question that comes up when using the chassis as the negative return path is how good a connection does it make? How does the voltage drop on the chassis return compare to the voltage drop on a properly sized negative return wire? The test on this page sets up a circuit with a chassis return, then measures the voltage drop along the supply wire and along the chassis return and compares the results to see how they compare. The results may surprise you (or not 🙂 ).
I wanted to pull at least 50 amps in order to get voltage drops that were measurable.
I had an extra GoWise inverter, and decided to put it part way back in the van and run a space heater off it in order to get a 50+ amp load on the 12 volt wires coming into the inverter.
I fed the plus side of the inverter with a #8 wire 10 ft long from the plus terminal of the van battery to the plus terminal of the inverter. I figured that this would give an about 5% voltage drop on this wire, which would be large enough to measure accurately on my meter.
For the negative return from the inverter to the van battery, I used the van chassis. I ran a #8 wire from the neg inverter terminal over to the van van fender – I used the crease along the top of the rear fender well and drilled a 5/16th hole in it, scraped off the paint to bare metal. I put a 5/16 bolt through the hole in the fender crease and then put the lug on the end of the #8 wire over the bolt and then a copper washer and a nut. Tightened the nut well – this was difficult due to the position and my cabinets, but I got a good bit of torque on it with a ratchet and socket.
So, now the inverter is supplied by a 10 ft #8 wire and the return is the van chassis sheet metal, and by extending one of the leads on my volt meter, I can measure the voltage drop for both the supply and return lines with the space heater loading the inverter.
Here are the measurements:
The inverter was supplied off the van battery and the engine was running all the time to charge the van battery.
Current to the Inverter 63 amps.
Voltage drop along #8 supply wire: 0.416 volts
Voltage drop for the sheet metal return line: 0.111 volts
I ran this for half an hour and the numbers were consistent over that time.
The #8 wire got slightly warm to the touch.
None of the terminals on inverter, sheet metal or van battery got even warm.
Voltage at the van battery was 14.19 volts, and voltage at the inverter input terminals was 13.65 volts. – this lines up pretty well with the measured voltage drops along the supply and return lines.
Trying to find out how much of the sheet metal path loss was in the metal itself, I measured the voltage drop along this path. At the Inverter neg terminal, the drop was 110 mv, at the bolt I put through the fender, the remaining voltage drop was 8 mv. So, even on the sheet metal return path, most of the voltage drop was along the wire between the inverter and the sheet metal (about 2.5 ft) and not along the metal itself.
This makes the effective resistance of the whole return path 0.111 volt / 63 amp = 0.00018 ohms!
So, I have to say I was surprised that the chassis return worked so well and actually had a lower voltage drop than the #8 supply wire side. The voltage drop on the sheet metal side was about 1%. To get the same voltage drop on the supply side would take AWG 2 wire.
While this surprises me how well the chassis return worked, it does not change my policy on using chassis returns – I don’t. I’ve just had too many ground failures over the years – the latest one was on our promaster – this was a ground designed by a RAM or Fiat engineer and installed at the factory, and it still failed in 8 years – and it was definitely not easy at all to track down as the source of the problem.