28-10: Second Battery Box

When ordering my Firewall Forward parts, i ordered a second FF-01403 Battery Box. I need the second battery as part of the redundant electrical system design, and i decided that having two identical battery boxes would give me the flexibiltiy of using either the EarthX or Odyssey batteries (and also allowing the batteries to be interchangeable). 

The only real location i could find for the battery box was on the upper firewall on the left hand side, in the rough location where the plans have you mount the VA-168 Sensor Probe Mount. 

There are a couple of issues with this location - firstly, is that the existing firewall passthrough below the battery box only gives about 1.3" from the firewall flanges. Is this enough? 

The second issue is that the top of the firewall leans forward - this battery box is too large to place solely on the upper portion, meaning it has to straddle the 'angle change' of the firewall - you can see the gap below. This means that it won't be possible to rivet the battery box to the firewall, as was done with the original box. 

Here you can see how the battery box needs to straddle the angle change on the firewall, and the gap created behind it. 

After seeking some advice on the forums, i think the best option is to mount the battery box using some 3/4" 0.062" angle on each side. This can be flush riveted to the sides of the battery box, then the angle can be drilled for some AN3 bolts to mount to some nutplates and doublers on the aft side of the firewall.

Biting the Bullet

I find there is a lot of thinking and hang wringing in this build process when you go 'off reservation' and do something unique. I tend to overthink things for ages, and stagnate toward actually getting the job done. In the end, i just need to drill a bloody hole and get it done. 

To that end, i started by deburring the battery box, and countersinking the lower holes for the rivets that hold the box together. I did not countersink the holes in the back wall of the box - as these wont be used. 

The ol' suicide countersinking method

The tops of the battery boxes need to be flush with the firewall, so i initially drilled one #40 hole in the top of the side brackets, and match drilled this to the battery box when it was laid out flat on my backrivet plate. 

I then placed the battery box on the firewall, and rotated the brackets around the top hole so they were flush to the firewall. These were clamped, then removed and match drilled to the battery box. 

This shows how much gap there is between the battery box and the firewall. 

The rivets will be AD4 flush rivets whish need 1/4" of edge distance. One hole was a little close to a ligntning hole, so i used an AD3 rivet here to ensure edge distance. 

This hole was going to be too close to the lightning hole for an AD4 rivet.

The rivet holes were then countersunk using the suicide method once again.

The bolt holes were laid out #40 on the other sides of the angle, then match drilled to the firewall.

My old homemade nutplate jig for AN3 bolts was starting to wear out from all the match drilling through the nutplate rivet holes, so i made a new one:

I matched drilled some 0.062 doublers to the holes in the side brackets and added a few additional rivet holes for strength. I then drilled the bolt holes to size, and drilled for the nutplates using the new home-made jig. The doubler was then bolted to the firewall, and the rivet holes into the firewall were mtach drilled. 

The firewall was then dimpled, and the doublers countersunk to accept the dimples.

Everything was cleaned and etched, then I shot some primer inbetween the bits of the battery box which needed to be riveted together, including under the attach angles. Then riveted it all toghether. I was able to reach everything with the squeezer. 

Everything was scotchbrited, blown off and cleaned with iso ready for painting.

The battery box (and battery box angles) were then primed using high-temp VHT primer, and coated in VHT Gloss Black engine enamel. 

The doublers and nutplates were back-riveted to the firewall. I used bolts and temporaty nuts to hold them in place. 

The two bolt holes which were supposed to hold the VA-168 sensor manifold were open sources of exhaust into the cabin, so i screwed a couple of stainless 10-32 cap head screws into the back of the nutlpates to close these holes off. 

Finally, i was able to bolt the new battery box to the firewall. 

Battery box seals were attached

I ended up with approx 1" of clearance to the firewall flanges.

This was as close to the firewall passthrough as i dared to go!