Adding an Aux battery the best way

My truck build will include a removable winch, a 3000 watt inverter and an onboard air system. But all of those depend on a reliable aux battery, de-coupled from the factory battery.

The aux battery has no continual electrical connection to the main battery. But an automatic charging relay closes as soon as the alternator starts putting out 14 volts. That way the aux battery can be freely used and/or drained without any impact on starting and running the truck.

I've seen the kit that contains a compact battery, but the battery is smaller than I want, and the cost is too high.

What I really want is a group 29 or 31 deep cycle battery, which I found at walmart for an affordable price.

https://www.walmart.com/ip/EverStart-Maxx-Marine-Battery-Group-Size-29DC-12-Volt-845-CCA/20531539
And a battery box to fit it:
https://www.walmart.com/ip/MSD-HM318BK-Battery-Box/16781380

But I don't want to take up valuable cargo space in either the bed or the cab. And the work required to fit it under the hood is prohibitive.

But I found another spot for it, that almost looks like it was MADE for it:




Thats the void behind the rear passenger wheelwell, and the battery box will fit up in there very nicely:



But we need to install a mount that's both capable of handling the weight and inertia of the battery, and easy to install and remove, since we'll be laying on the ground to do the actual install.

First we need a bracket that's mounted to the truck's structure, so we're not placing undue strain on the thin sheetmetal of the fender.

I fabricated an angle iron bracket that creates an outside ledge/rail just above the fender's lower lip.



That horizontal rail is suspended from 3 points,
From the wheel well, near the mud flap


From the bed structure at the front



And from the bed structure at the rear



I also painted the area while I was at it to minimize corrosion.

with that installed, the battery box still easily fits up into place



Next I took some 1/16" thick plate and bent it into a battery tray assembly such that it forms the floor and the inboard side of the new compartment. The open side of the tray will be bolted solidly to the outside bracket rail, while the inboard side will simply be suspended from the frame directly above it.

Here's the steel bracket that's bolted to the frame and will swing down and bolt to the tray assembly to support it.



And here it is installed.



The three 1/4" bolts that attach the tray assembly to the outside rail are welded to the tray, so you only have to wrestle it into place long enough for the bolts to drop into their holes, instantly securing it in place. But then I went ahead and put a nut and lockwasher on the center bolt, just to deal with bounces and jolts.



Speaking of bounces and jolts, the whole box is held down with a stainless hose clamp just snug enough to ensure that it stays put.



Obviously you can't reach the battery terminals once this is installed.

So I pre-installed cables that run to a 175 Amp Anderson connector. So installing the battery, and connecting to the battery are two different things.

So for now the battery is installed, but not yet hooked up.




Once I've made more progress on the wiring, I'll update this thread.

 

I'm guessing you have dual exhaust. If not, why did you install this on the curbside where you would get all that heat on your battery? I don't know about your Tundra but mine has the stock battery installed on the driver's side of the engine compartment, which also would have made a shorter wire run to the rear of the truck. On an older RV that I had, I installed a "Blue Sea Model 7622" that would allow your alternator to charge the house battery to a full charge before it would allow you to parallel the starting battery with the auxiliary one. There were a number of different configurations you could choose for assuring you never drained the starting battery below a certain level. This item can be found on Amazon or eBay or many others. I wish I had removed it from the RV when I sold it as the buyer wasn't interested. Cost today new is about $210. Info can be found at
ML-ACR Automatic Charging Relay with Manual Control - 12V DC 500A - Blue Sea Systems

 

No, I think just single exhaust. But I have a bed step installed on the driver's side, and that made access to that region a little more difficult. Not that it would have been impossible, but trickier. I've added an extra.. 30 inches to the cable run from under the hood to the rear battery. But the only location that the distance of cable run really matters is the run to behind the CrewMax rear seat where the inverter and compressor will be located. For that run I'll offset them toward the passenger side, to keep that run as short as possible, and use 4/0 cable for it, because for a 3000+ watt inverter, the voltage drop can matter a lot.

As for charging, I have a 140 amp dual battery isolator from Amazon. They're $45 with cables, $16 without. Because it only has to carry the current from the alternator back to the rear, the wire that came with mine is plenty big.

For manual bypass, when the factory battery is dead, I'm installing a cheap "manual battery disconnect". Which means I'll have to pop the hood to use it. But when you close that disconnect, it will manually connect the rear battery's hot directly to the factory battery. In those cases I'll treat it like I'm jump starting over cheap cables: be patient, and don't try to start the truck until the rear battery has had a few minutes to bring the factory battery's voltage up a bit. That way I'm not asking that relatively small and long cable to carry the full current of the starter.

Bob

 

I now have phase one complete:
The isolator switch, fuse and manual connector panel is installed in the engine bay.

• There's a heavy gauge hot wire from the main battery to that panel
• and a heavy gauge hot wire from that panel back to the fuse bypass solenoid panel.
• There's also a 12ga. wire running from a toggle switch back to the activation terminal of the fuse bypass solenoid.
• (If the main battery is dead and you use the manual connector, you run the risk of blowing the two 300amp fuses at each end. So at the front the manual connector bypasses the front fuse, and when you flip the toggle switch it activates the "fuse bypass solenoid", which bypasses the rear fuse as well. It's only momentary, just long enough to get the truck started.)

The fuse and fuse bypass solenoid panel is installed up under the bed right behind the cab.

• There's a heavy gauge (4/0) link feeding hot from the aux battery QD into that fuse

The rear winch power QD is installed back next to the hitch receiver.

• There's a heavy gauge link from the fuse/bypass solenoid panel to the hot side of the rear winch QD

The QD to power the inverter is installed under the rear seat

• There's a heavy gauge link from the fuse/bypass solenoid panel to the hot side of the inverter QD

The inverter is perched on its two mount points in the center bay under the rear seat.

• I just screwed down a couple 6" long pieces of 2x4 at each end, the two ends of the inverter rest on them.
• This elevates the inverter to increase airflow, and allows to slip my rolled up jumper cables underneath it.
  
• I originally intended to mount the inverter to the rear cabin wall behind the passenger side rear seat.
  
• So I removed that seat the got it all in place, but I chose not to proceed, because under heavy use these inverters need generous airflow for cooling.
• And I just didn't see a way to get adequate airflow back to behind there.
• In the current location it can get plenty of airflow and is easily accessible/portable if I need to get to it.
• And I can see its voltage/amperage information screen if I care to.

And finally, there's a heavy (1/0) ground wire bonding the rear wall of the cab to the aux battery QD's negative side.

• Initially I didn't have this in place, and when I tried to run something like a 16 worm drive saw off of the inverter, the inverter would shut down due to low voltage.
• The voltage drop from the aux battery to the inverter can be pretty substantial, even though it's only 4 feet away.
• I had accounted for that on the hot side, but assumed the cab ground would be adequate.
• It wasn't. But after I ran the dedicated heavy ground wire, it worked just fine.

Phase two will be finishing up the 120V

• I'll install a weather-protected 120V AC outlet in the bed wall near the tailgate
• I'll run a cord from that outlet up to plug into the inverter
• That'll save having to have a drop cord coming out of one of the passenger doors.
• Flip the inverter on under the seat and the outlet in the bed comes live.
• I'm considering installing a switch in the center console to turn on the inverter, as well as a gauge to show its voltage and output amps.
• I'm not sure it's worth the effort.

and hooking up the onboard air

• I have a heavy duty 12v airpump (harbor freight) that I'll be converting into a tanked air compressor.
• The aluminum tank will mount under the truck near the driveshaft
• The pump/compressor itself will sit behind the passenger rear seat. It'll be removable, with both electrical and air pressure QD's, which I'll install at that time.

Phase three will be a front winch QD. But first I need to fabricate a frame-mounted front hitch receiver, which will be mostly concealed behind the front bumper. That QD can just be powered off of the front isolator switch panel.

Bob