grounding battery

bobnoffs · #1 05-12-2018, 02:48 PM

originally i wired my rv12 with the batteries [earthx] behind the panel . after considering an airtight battery box i have decided to remount the batteries fwf. the engine is grounded to a firewall passthru bolt that holds the ground strip terminals for the panel and the engine compartment. the engine has a second ground from another point on the block to another bolted terminal on the firewall.
do i really need to run a cable from the neg. of the batteries to that terminal where the ground strip terminals mount or can i bolt the cable from the battery ground to the firewall by the battery and save the weight of at least 2 feet of cable?
thanks for any advice.

jask · #2 05-12-2018, 03:53 PM

I would definitely not use the aluminum firewall to conduct electricity. You should run the negative of the battery directly to the post where the engine ground connects. Otherwise full amps attempting to run the starter would be flowing through the firewall. It would hamper starter performance and might cause a fire.

maniago · #3 05-12-2018, 04:45 PM

Very simply, dont cheat on your grounds. Ever. The ground is more important that the hot side to get clean and strong. If you short change it in your build, electricity will find a way to complete the circuit, but it wont be through where you want it to go....and drive you crazy chasing electrical gremlins.

Run the extra cable, and save yourself years of headache, worry, and mistrust of your airplane.

gasman · #4 05-12-2018, 05:46 PM

Quote:

Originally Posted by bobnoffs

originally i wired my rv12 with the batteries [earthx] behind the panel . after considering an airtight battery box i have decided to remount the batteries fwf. the engine is grounded to a firewall passthru bolt that holds the ground strip terminals for the panel and the engine compartment. the engine has a second ground from another point on the block to another bolted terminal on the firewall.
do i really need to run a cable from the neg. of the batteries to that terminal where the ground strip terminals mount or can i bolt the cable from the battery ground to the firewall by the battery and save the weight of at least 2 feet of cable?
thanks for any advice.

Run your battery ground cable to this bolt.

Energy actually runs from the ground (-) of the battery, to the + terminal of the battery. So the ground is VERY important....

BTW.... the firewall is stainless.... NOT aluminum.

bobnoffs · #5 05-13-2018, 05:11 AM

thanks guys,
i guess i will replace that battery to terminal ground cable on the fwf side of the firewall. appreciate the input.

Tankerpilot75 · #6 05-13-2018, 12:28 PM

My RV7A is having ? xenon strobe light? issues and I?m wondering if I might have a ground problem. My strobes quit working and after talking to a Whelen representative I?ve accomplished the following checks.

1. I disconnected all three plugs to the power supply (left and right wing strobe and tail strobe) and then plugged them in one at a time to see if a strobe light short was the problem. Result: no difference. None of the strobe lights worked separately.

2. Pulled the power supply and took the tail light strobe, placed them on the bench and hooked up an external power supply. Result: tail light strobe fired normally indicating both the power supply and tail light strobe were okay.

3. Reinstalled power supply, hooked up only the tail light strobe that I?d previously checked on the bench and guess what? Nothing!

4. Checked power going to power supply. Voltage was 12.3 volts from the battery, strobe light switch worked and ground showed a good continuity.

I?m at a loss as to what?s causing my problem. However I do remember when I replaced my Odyssey battery last year that my ground cable was attached to the firewall. Also, the aircraft has never had a great tradition of cranking easily. Most of the time I have to engage the starter several times to get it to have a normal starting crank.

My question is: where should the battery ground be connected to the aircraft? If not to the firewall, then where?

i am not the builder but I am the second owner. It?s had several A&P/IAs work on it and a couple of EAA tech reps over the last three years of ownership assist me on maintenance issues and none of them have ever commented on the battery being grounded to the firewall.

Carl Froehlich · #7 05-13-2018, 12:48 PM

Battery is grounded to the same firewall ground lug that you ground the engine.

A couple of things to check:
- Do you ground the engine on the starter ground lug or under some greasy engine bolt? Try the starter ground lug if not there to see if engine start is better.
- Is the battery ground lead and engine ground lead the same gauge (#2 or #4)?
- Do you use airframe ground for your strobes? If so, run a separate ground wire to the firewall ground and see if that solves your problem.

Carl

jask · #8 05-13-2018, 09:13 PM

What you are describing is a poor connection in your electrical system. When you are checking the voltage at the strobes, you need to check it under load. I would guarantee you are getting a significant voltage drop. You are also no doubt getting a voltage drop to the starter solenoid. Check all the connections for the power source to those items.

lr172 · #9 05-13-2018, 11:11 PM

Quote:

Originally Posted by jask

I would definitely not use the aluminum firewall to conduct electricity. You should run the negative of the battery directly to the post where the engine ground connects. Otherwise full amps attempting to run the starter would be flowing through the firewall. It would hamper starter performance and might cause a fire.

Could you elaborate on how a frame ground can cause a fire? Millions of cars have been produced that use frame grounds. Many ground to the block, but a good percentage ground directly to the frame. There is a separate ground strap from the frame to the block. Also, Aluminum is a very good electricity conductor, no less efficient than steel. Assuming your firewall is aluminum. Mine and most other aircraft are stainless steel.

Also, did you realize that a primary ground path for your starter runs through your aluminum engine block?

Larry

lr172 · #10 05-13-2018, 11:20 PM

Quote:

Originally Posted by Tankerpilot75

My question is: where should the battery ground be connected to the aircraft? If not to the firewall, then where?

i am not the builder but I am the second owner. It’s had several A&P/IAs work on it and a couple of EAA tech reps over the last three years of ownership assist me on maintenance issues and none of them have ever commented on the battery being grounded to the firewall.

Like Carl and many others, I have a ground lug on the firewall. The Battery, Engine Ground strap and airframe gound terminal strip and firewall all are directly connected to this lug. I have a Whelan strobe setup using an airframe ground with no issues. I only used dedicated ground wires for avionics and intstruments or anything directly connected behind the panel, where I used shared grounds.

Larry