Truck Bed Battery Box Brain Storming

**BigBrother** · 07-15-2022, 08:10 AM

So, I've been wanting a power source in the bed of my truck. Was sourcing some wires and 12 volt outlets and such and had an idea.

Why not just put a battery box and trace some outboard battery cables or jumper cable with permanent lugs to it from my current battery letting the alternator charge it, and having a double source of power. If I added a cut off switch I could isolate it if I decided to camp in the truck, and not run my primary battery down if I was using it to charge phones, lights, fans, etc....maybe put in a 120 converter so I charge my hand tools and such while they ride around with me.

I have an 8' bed with camper shell and bed slide. I would put it up in the front of the bed, in between the wheel well and behind the cab on the drivers side and beside the bed slide. It would never get in the way.

Anyone see any potential problems with my plan? Any ideas I havent thought of? Not going to burn up my alternator charging two batteries am I? Battery goes in the back because there's not enough room under the hood.

Thanks...

**JimmyD714** · 07-15-2022, 08:33 AM

You'll need an isolator. Some are relays some are diode based. This keeps the batteries from draining each other. A diode lets current flow one way only, the relay version mechanically opens the circuit so the coil of the relay should be energized by a wire that only has voltage when the vehicle is running. This would effectively disconnect the 2 batteries from each other when the vehicle is not running.

Fuse everything properly so you don't start a fire. I've seen way too many people skip this step. Trust me, you need it. The output of the alternator into the isolator needs to be fused as well as both outputs of the isolator going to your 2 batteries.

**BigBrother** · 07-15-2022, 08:39 AM

I hear you on the fuses.

diodes and isolators are above my paygrade in the 12 volt world. Unless I can go to amazon or ebay and get something like that in a "kit" I'm not savvy enough to do it. I know and am comfortable with my limitations.

I would want my batteries to be linked at all times unless I am plugging stuff in while the engine is off. Dont want to drain my starting battery. A manual cut off would protect against that.

**ScoobieMcDoobie** · 07-15-2022, 08:52 AM

I wonder if you would experience any significant voltage drop running a cable that long from the alternator? May be worth testing.
I would imagine you would need at least a 2-gauge wire to run that far.

I agree with Jimmy, using an isolator would keep you from havin to flip a switch each time you crank the truck up.
I think something like this could get you started down the right path. https://a.co/d/aNcqd0g

*Disclaimer I am no electrician.

**BigBrother** · 07-15-2022, 08:59 AM

I was searching amazon for "truck bed power source kits" and didnt look for "isolator"s. That's a quality link there scoob. Thanks.

Isolator will need to go in between the two batteries? With the isolator, I wont need a diode will it?

**Duck cutter** · 07-15-2022, 09:01 AM

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**JimmyD714** · 07-15-2022, 09:06 AM

The isolator is pretty straight forward to connect from what I've seen. You have an input from your alternator and 2 outputs to your 2 batteries. That's for the diode based type. For the relay type, same connections except adding connections to energize the relay.

If you do a web search you'll find wiring diagrams and some articles on this. It can definitely be done but the isolator is very important.

You don't want your batteries linked at all times, only when they're being charged. Otherwise they'll drain each other. That's what the isolator does for you. I'm not certain but my guess would be that if you're using two similar batteries, you could use a relay style isolator but if you're using a 2nd battery that is much different in size and style from the main battery that you'd want the diode style. Reason being the relay style literally connects the two batteries in parallel. The diode style isolates them electrically because the diode only allows current flow in 1 direction - from alternator to battery.

You can charge both batteries but electrically the batteries should be disconnected from each other when the alternator is off regardless of what types you have. Your main battery is only connected to your car's electrical system, your 2nd battery is only connected to whatever you're powering from that battery.

**JimmyD714** · 07-15-2022, 09:12 AM

Originally Posted by Duck cutter

7D78180E-7149-45E2-B429-93154F692741.png

I'm gonna have to disagree with that chart. Per the NEC, a 14 gauge wire cannot handle a 40 A load even in free air conditions, regardless of the length of the wire. I believe a free air installation allows for a maximum of 20 A and 15 A if inside any conduit.

We don't want this guy's truck catching on fire!

**Duck cutter** · 07-15-2022, 09:18 AM

Originally Posted by JimmyD714

I'm gonna have to disagree with that chart. Per the NEC, a 14 gauge wire cannot handle a 40 A load even in free air conditions, regardless of the length of the wire. I believe a free air installation allows for a maximum of 20 A and 15 A if inside any conduit.

We don't want this guy's truck catching on fire!

I’ve had that chart saved for a long time so it may not be current anymore

**BigBrother** · 07-15-2022, 09:23 AM

I plan on running 4 gauge copper from battery to battery. Probably 10 gauge tinned from auxiliary power source to each outlet.

**murraywader** · 07-15-2022, 09:26 AM

Just get an off road dual battery kit.

**JimmyD714** · 07-15-2022, 09:26 AM

Originally Posted by Duck cutter

I’ve had that chart saved for a long time so it may not be current anymore

The be on the safer side, Google the NEC ampacity chart and you'll find something better. Some of those numbers scare me on that chart you have. Doesn't matter if it's 2 feet or 2 inches, I'd never connect some of the loads shown in the shorter length columns to those sizes of wire.

**Glenn** · 07-15-2022, 09:38 AM

What are you wanting to power and for how long?

**BigBrother** · 07-15-2022, 09:42 AM

I want to be able to charge my phone if I am camping. Interior lights. Maybe get real fancy and add a radiator fan. Also add an inverter to charge power tool batteries...

**Glenn** · 07-15-2022, 09:50 AM

If it were me I'd get a deep cycle battery and keep it on a maintainer till you needed it then toss it in the truck and hook it up. Get a marine switch panel with a USB port and wire in whatever else you want like it's a boat. Simple and cheap.