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.
![logistics_2024-08-17_02.png logistics_2024-08-17_02.png](https://vansairforce.net/data/attachments/47/47740-8358b234b75a0980694e46b43d2c26dd.jpg)
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.
![fuselage_2024-08-18_01.png fuselage_2024-08-18_01.png](https://vansairforce.net/data/attachments/47/47741-46c363a2b50b8ffcd9859a44e76c969a.jpg)
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.
![logistics_2024-08-17_02.png logistics_2024-08-17_02.png](https://vansairforce.net/data/attachments/47/47740-8358b234b75a0980694e46b43d2c26dd.jpg)
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.
![fuselage_2024-08-18_01.png fuselage_2024-08-18_01.png](https://vansairforce.net/data/attachments/47/47741-46c363a2b50b8ffcd9859a44e76c969a.jpg)
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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