I'm installing aft-mounted batteries. There seem to be alternative schools of thought as to whether to ground the battery negative to the fuselage nearby or to run a heavy ground wire forward to a firewall ground. My fuselage components are fully epoxy primed, so I wondered if there would be a high resistance problem using an aft battery ground.
I made up a low resistance test circuit, based on a Bob Nuckolls constant current design, but using Kelvin clips, and tested the fuselage ground return resistance using a Fluke 97 multimeter. It has a 40mV range, with resolution of 0.01mV, and with the Nuckolls 104mA constant current it resolves to 0.096mΩ.
With one clip on the firewall, and the other at various locations on the longerons near the battery box, it consistently measured about 1.7mΩ.
This compares well with M22759/16 2AWG wire, which has a nominal resistance of about 2.1mΩ over the same distance. So, looks like battery grounding at the lower aft longeron and a main ground on the firewall is the correct solution for me.
I made up a low resistance test circuit, based on a Bob Nuckolls constant current design, but using Kelvin clips, and tested the fuselage ground return resistance using a Fluke 97 multimeter. It has a 40mV range, with resolution of 0.01mV, and with the Nuckolls 104mA constant current it resolves to 0.096mΩ.
With one clip on the firewall, and the other at various locations on the longerons near the battery box, it consistently measured about 1.7mΩ.
This compares well with M22759/16 2AWG wire, which has a nominal resistance of about 2.1mΩ over the same distance. So, looks like battery grounding at the lower aft longeron and a main ground on the firewall is the correct solution for me.
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