RV 8 AFT Battery Cable Routing

[kibrickj]

Perhaps a stupid/unnecessary question as I'm still figuring out some basics with QB FUSELAGE, but I was hoping to get pictures of the routing of the battery cables (both + and ground) for an AFT mounted battery on an RV8. Seems like its going to be a problem up through spar. Just trying to plan ahead.

Was thinking I should run ground cable up to firewall and both should by 02.

Any advice gladly taken...Newbie here.

Note: I will likely fly solo a lot, so prefer to have some weight aft.

[RYOUNG]

You should obtain Van’s OP10 and OP11. They provide suggested wire routing for the fuselage. This is how I grounded the battery to the airframe. I ran the no. 2 battery cable along the fuselage bottom on the left side to the firewall.
Although I show an Odyssey battery, I am going with a Concorde RG-25 for extra weight to help counter a IO-390.

[goatflieg]

I originally routed a #6 cable along the bottom of the fuselage; later I realized I needed a larger #2 cable to handle the load. By then my aft flooring was riveted in place. Keeping the panic at bay, I routed a separate #2 cable along the left upper longeron, under the cockpit sill, under the instrument panel, around/through the left gear tower and firewall to the starter solenoid. I used the already routed #6 cable to power the rest of the electrical system. The process was documented in the beginning of the May 6 2019 Blogspot blog entry here:
http://goatflieg.blogspot.com/2019/0...re-wiring.html

[kibrickj]

My understanding is that its best practice to have a single ground at the firewall which for me would dictate running a cable aft for both + and -. Not sure if its worth it for the battery cable ground considering the weight and other constraints. I'd like to prevent ground loops.

Martin, do you think there's enough room to run two 02 cable aft through the spar holes? Was there for you if you hadn't already sealed up floor with a single 6 cable?

All, although I'm an engineer by training, I only am smart enough to know what I don't know. Although I know its okay to deviate from a best practice in this case, is it the best thing to do to prevent this particular type of ground loop and run the ground cable up to the firewall where everything else is including the alternator and other noisy sources.

Thanks all for all past and future input.

[Mikeyb]

The grounding thing is almost like primer wars.

Like you I was interested in keeping the CG aft for solo flying.

I wired mine like RYOUNG described as per Vans DWG OP-10/11 using the AC as the ground path. There is a clear path for the battery + cable along the left side. Make sure you keep it clear of the passenger footwell boxes.

I measured the resistance in the ground path from the battery to the firewall ground connection. It was 3 milliohms which is comparable to a 10’ run of 2 ga copper wire with two bolted connections.

The ground loop issue is mitigated by grounding all the sensitive AC systems at the same point at the firewall. You have to connect the battery to the airframe at some point if you want the gas gauges to work.

It’s worth reviewing your battery location using the actual weight of the battery you will use. I ended up using a 4 pound EarthX battery. It it only makes a few tenths of an inch difference in CG between firewall and aft mounting. The 2ga wire run to the aft location is almost 4 pounds by itself.

[Ironflight]

Here’s another vote for a local (aft) ground to a longeron, then a single positive cable routed the way Van’s shows. I know many, many -8’s built that way with no issues, and mine has 2100 hours and 18 years of flying without issues.

Remember, you’re not going to the moon, you’;re just going to Wisconsin….

Paul

[fl-mike]

A local battery ground is fine. RYOUNG has a nice install.
The battery current is primarily during start. After that, the alternator is providing power and that firewall ground takes over, so to speak.

[kibrickj]

I LOVE this community. Thanks all for the pragmatic feedback. As I navigate between best practices and pragmatism, hearing from those that have traveled there before is priceless. I shall copy the grounding shown by young......Looked very well done to me too.

Maybe won't make it to the moon, but we'll see about Greenland at a later date.

THANKs all. I was inclined to go with the Odessey battery just to save a few bucks and likely add weight where I want it, but I'll cross that bridge later. I'm sure I end up with the lithium at some point. Again, thank you for sharing you great perspectives.

[TimR]

I'm going to jump in here. I'm at the same point in my -8 QB. Where did you all mount the master and starter relays? I need to get my hands on OP10 and 11, but looking to the brain trust right now. Right now, I have the master relay mounted to the battery tray, but I've been pondering placement of the starter relay. The master relay could be moved to the firewall and then place both relays under the hood. Thoughts?

[scard]

Originally Posted by TimR

I'm going to jump in here. I'm at the same point in my -8 QB. Where did you all mount the master and starter relays? I need to get my hands on OP10 and 11, but looking to the brain trust right now. Right now, I have the master relay mounted to the battery tray, but I've been pondering placement of the starter relay. The master relay could be moved to the firewall and then place both relays under the hood. Thoughts?

Master on the aft battery tray as you have it and the starter on the firewall.