Projects Trunk mounted battery

Owner Bill and I came up with this slick answer to a problem. How to mount your battery in the trunk and still look cool
Bill found this cool old Craftsman skill saw box, on eBay. With some slight mods, it was perfect.
Ran 2 gauge stranded wire, from starter, to a HD kill switch. Then from switch into box. Engine grounded to chassis in front and battery grounded to chassis in rear.
Naturally grommets were used, where all cables pass through metal.
Rebel wiring harness is next for this Hemi powered ‘32.
Previously had small gel battery under passenger floor. Not a fan

JT






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Looks good!

--louis

 

Looks good, but a couple of suggestions.
The switch really should be accessed from inside the car for the quickest battery dis-connect.
The battery box should have some ventilation, because the Optima can out-gas if over charged.

 

Louvers, good to go.

 

Could you louver the backside of the box that doesn't show?

My brother and I were just conversing about the same cable routing as you are using. Wonder if the frame steel will carry the electrons as well as a long clean cable would? You and he will soon find out.

 

might want to be vented outside the trunk, like, a hole through the floor?

 

Squirrel, do battery gases rise or fall? For some reason I thought they were lighter and the vent would have to be high in the box.

 

Thanks guys
Gave each and every one of these points thought along the way. Gotta start somewhere
Hate people that sit around and talk about what to do
And wind up never doing anything
Better to try and fail, then to never try at all.
All these Jalopies are works in progress


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kInd of depends where you want the gasses to go. If you want hydrogen and oxygen in the trunk, then vent the battery to the trunk. Or don't put a box on it, at all.

 

I agree with squirrel. I run a small hose from the bottom of the box through the floor an inch or so, with a couple holes near the top opposite side. Air flow under the car will create a small vacuum pulling trunk air through the box.

Frame steel is not as good of a conductor as a copper cable. Always better to run the battery ground directly to the engine block.

 

I like that setup.
Venting is a good idea, but should be 'discreet': Sin to louver a classic Craftsman box!
If you must cut something up, make it a Snap-On!
Agree with @V8 Bob, switch would be more sensibly located within reach of the driver:

1. Shut car off, reach for the switch before getting out. (thieves are watching cool rides!)
2. In case of a 'T-Bone' or 'rear ender', fire is possible...Kill switch? Can you reach it?

Very sanitary install: Both my wife's BMWs have batteries in that location.

 

Nah, your not following me. I agree to vent the box, the question I had was if you drill a hole in the bottom of the box as a vent and the gases rise to the top it isn't venting the gases properly. If the vent tube is at the top of the box and vents to the outside it would be safer. I'm asking because I'm adding a vent currently to a box and want to do it correctly.

 

The cross sectional area of a conductor has a lot to do with it's resistance. The resistance of a frame rail with 1 square inch of cross sectional area, might be lower than that of copper with 1/8 square inch of cross sectional area?

 

Oh, I was following you....

If you can find a way to vent it out of the car, above the battery box, that would be best. Next best might be to vent it below the car. Least best might be to vent it into the trunk.

 

I believe hydrogen is lighter than air, they use to use it in air ships, like the Hindenberg. hmmm.....

 

You're right that the area can play a role in conductivity, but running a properly sized copper cable will provide a very good ground, eliminating any issues that might be present with an old/rusty/poorly welded/abused frame. Copper is just a much better electrical conductor.

 

Looks awesome. Makes me want to do something similar.

 

Someone should tell all the truck and heavy equipment mfgr's that they're doing it wrong.

 

Like Squirrel says, it's all about cross-section. Even a 2x3x.125" square tube will have almost six times the cross-sectional area of a 4/0 cable, over twelve times that of a 2/0 cable. And steel isn't six times poorer at conducting electricity, so at the distances we're talking here, it'll be a better conductor than whatever cable you'll use. A frame ground will be more than adequate.

 

The point about having a single cable, with only one connection at each end, vs a cable connected to the frame at each end, is a good one...not because it starts out as a better conductor, but because it's easier to maintain. Those ground cable to frame connections are usually neglected.

That's one reason I don't like putting the battery anywhere besides the engine compartment...with relatively short cables to the engine.

 

Never said wrong, just not the best.

 

According to readily available info, steel can be well over nine times poorer than copper, or 3-9% vs the standard 100% for copper.

 

...so having at least ten times as much cross sectional area of steel vs copper, gives you the same effective resistance.

Is the extra weight and cost of the wire worth it? vs the extra failure points and cost of terminating the copper wires the frame at each end? that's up to you.

I'd probably go with the frame for runs more than about 4 or 5 feet.

 

In viewing the OPs pictures and him stating he is using #2 copper wire, I assure you his frame will conduct as much as his wire. Being a hot rodder at heart I always adhere to “ mores better and a whole bunch is just right” lol. I outfitted trucks for years with high amp draw beds and accessories. I keep one size cable and one size terminals to reduce my inventory, everything got 00, with designated grounds. I not aware of the OPs amp draw, but #2 wire at that distance seems a little light to me.


Bones

 

Shoulda looked that up before I ran my keyboard... But even so; I'm seeing numbers of between 3 and 15 percent for steel. Split the difference, that puts you at 9 percent. So with 2/0 cable, you still have more than adequate cross-section with a single 2x3x.125" tube. But framerails come in pairs, and both will be part of the conducting path. Granted, a riveted frame could have losses at the joints, but if they're big enough to cause electrical issues I'd be far more concerned about the structural integrity of said frame...

As was pointed out, the OEMs (including automotive) use a frame ground almost exclusively.

 

From Optima website:

https://www.optimabatteries.com/en-...-build-custom-battery-box-your-optima-battery

Trunk/Interior Mounting- If your battery is mounted inside the passenger compartment, the battery must be vented to the outside of the vehicle. OPTIMA group DS46B24R, 27, 51, 78, 34C and 31 batteries all have ports for connecting a vent hose.

Need to find the vent port, then use tube through floor, is what it sounds like.

Phil

 

View attachment 4161439 I mounted battery in vintage tool box on side of truck and ran new wire to ground and power. I never considered using frame for battery ground.

 

When using the frame for a ground source, remember to add a wire the same size as the positive wire for a ground to the engine. That is, put a designated ground from your engine to frame. To complete the starter circuit. Also ad a ground to the body.
And don’t use star washers between your terminals and the frame.


Bones

 

Hey Fellas, Thanks for all the good info.
Guess I will run a vent tube.
At the city garage Caprice Cop Cars have twin batteries in trunk. They both are attached to small vent tube.
They also use the chassis as the ground path.
This frame is an aftermarket job, so in good condition.
Wire is 2/0 stranded copper. Battery terminals are these screw style and eyelets were crimped using a new tool Bill found.
354 Hemi cranked over with mini starter. Still need to shrink tube that end and tidy up the cable, along frame rail.
Engine grounded to frame at motor mount










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Cool, I've got one of those with the saw sitting under my workbench I've been wondering what to do with. Great idea.