How to make a cheap isolated dual-battery setup for $50

[dlichterman]

Excuse my electrical incompetence, but what determines the size of the solenoid? The original post showed an 80A and it was later changed to 200A for adding a winch. Is that to run a winch while the vehicle is off? I'm just looking to have a small 20Ah battery to power some lights and charge small electronics in my bed. Is 80A necessary or could I drop to 65A (to get the cheaper Blue Sea ACR). I'm running a '13 Tacoma with the 140A alternator, if that changes anything.

The size of the solenoid should be the max amount of current you expect to be actually handled. I picked a 200A/300A surge 10s solenoid so that I could self jump(which has come in handy).

 

[uhkhu]

With your stated use & 20Ah battery, you can easily go with a 65amp ACR.

Does the solenoid only see load from the connected equipment when the truck is running (when engaged)? If I'm running a small compressor (20A) from the second battery while the truck is off, does that current even reach the solenoid?

 

[uhkhu]

Is there any need for a charge controller on the second battery? I'm installing the original basic configuration with a 20Ah AGM secondary. Is there risk of overcharging this if it's installed as shown on the first page of this thread?

 

[randy h]

There is no risk of overcharging from your alternator. Remember your Starter battery does not overcharge, and you are merely sharing your alternator output between two batteries now.

 

[uhkhu]

Right now I'm putting in a small 20Ah AGM to power a 12v outlet and usb ports in the bed, but I'd like the ability to upsize in the future (probably not larger than 50ah ever). I followed the original post and went for 4ga wire and 80A fuses for each battery. Are these overkill? What dictates the sizing of wires and fuses for this set up? I'm not planning on self-jumping ever and the only thing I can see is running a compressor (~20A max) off the batteries, assuming they'd be connected while the truck is running.

 

[SEREvince]

Go cheap or go home... err..something like that anyway. No beers yet, I'm waiting for an (overdue) delivery of an Ikea bed for my son. I figured if I can assemble Ikea furniture I can build anything!

I was going to order the Genesis Offroad JK dual battery kit, and I checked in here to get reviews and low and behold I find this amazing thread. Of course I notice right away that Genesis makes no mention of drinking beer in their install directions so I knew I was on the wrong track.

I ordered and installed the MORE Offroad 12+ JKU Dual battery tray.
https://www.mountainoffroad.com/jkdbt12-dual-battery-tray-for-12-and-up-jeep-wrangler-jks.html
Helpful hint: Trim the TIPM mount a bit back further than they show. Other than that it's pretty easy and the top plate provides a nice mounting surface.

Two Deka AGM batteries from OReilly's.
34EXT 83996195332 Starting
34MAGM 83996016187 Dual purpose/ marine
Also picked up the add a circuit fuse here.


500 Amp Isolator
https://www.amazon.com/gp/product/B00EHV8Z3M/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

Ebay
2AWG wire (using for winch install also)
200amp inline circuit breakers 0/2awg x 2 (beware uneeksupply takes 2-4 weeks no matter what Amazon or Ebay tells you)
2awg/ 5/16 ring terminal ends
2x battery terminal ends
Adhesive heat shrink 1" dia x12"
Battery terminal covers

Momentary switch (can't recall source)

Already have the Temco terminal smasher from a previous project.

Still need to see If some spark plug wire boots will work on the isolator terminals.

I'm at about $550ish WITH 2 new AGM batteries. Just past what the Genesis kit alone would have been. Theirs is way sexier looking and you can add some supercomputer thing as well.

Help me if I forgot anything! I did print out a copy of the updated diagram and took some notes in crayon on it.

Questions:
On the isolator the second small connection goes to chassis ground? There are no markings so I assume there is no polarity?
I know my crimper isn't the best, but I've tried and failed to pull a terminal off so it works. I have very little solder experience. I'm thinking I might crimp and then fill with solder. I have on hand electrical rosin flux core. Can I just run that?
The momentary switch is kinda confusing me. Do I need two power sources? So one power source to the isolator and then a split pos wire that has power and goes back to the isolator?

Thanks for the awesome and informative thread!

Vince

 

[dlichterman]

Questions:
On the isolator the second small connection goes to chassis ground? There are no markings so I assume there is no polarity?
I know my crimper isn't the best, but I've tried and failed to pull a terminal off so it works. I have very little solder experience. I'm thinking I might crimp and then fill with solder. I have on hand electrical rosin flux core. Can I just run that?
The momentary switch is kinda confusing me. Do I need two power sources? So one power source to the isolator and then a split pos wire that has power and goes back to the isolator?

TO answer a few of your questions - yes - the small connections on the isolator one needs to be +12V and the other to ground. Are you going to use the switch to control joining, or tie into an existing circuit(I see an add a circuit there)? I'm not sure how you plan to use the switch.

As far as crimping - especially if you're talking about the large gauge stuff - this is where buying the right tool is super helpful. I got the harbor freight hydraulic crimper to do all my cables and it is awesome for those huge lugs. Don't forget your 20% off coupon.

 

[SEREvince]

Are you going to use the switch to control joining, or tie into an existing circuit(I see an add a circuit there)? I'm not sure how you plan to use the switch.

Thank you.

I want to be able to self jump using the momentary switch from inside the vehicle. If I understand it correctly the add a circuit goes to the isolator to activate (combine) when the vehicle is running only, but not in the ACCESSORY position. It's the switch that I'm fuzzy on.

 

[dlichterman]

Thank you.

I want to be able to self jump using the momentary switch from inside the vehicle. If I understand it correctly the add a circuit goes to the isolator to activate (combine) when the vehicle is running only, but not in the ACCESSORY position. It's the switch that I'm fuzzy on.

Gotcha - I have a similar setup. So yes - you will need a second +12 running to the solenoid, but you can share the ground - so just +12V to the switch and out to the solenoid. If it has a light on the switch it might require slightly different wiring and you'll need to check the instructions.

 

[dwh]

Yea, default the solenoid is powered by the engine battery whenever the key is ign/run position (basically, self-jumping every time).

But if the engine battery is dead, turning the key on won't energize the solenoid. So you have to rig a switch that gets power from the aux battery and powers up the solenoid.

 

[SEREvince]

Yea, default the solenoid is powered by the engine battery whenever the key is ign/run position (basically, self-jumping every time).

But if the engine battery is dead, turning the key on won't energize the solenoid. So you have to rig a switch that gets power from the aux battery and powers up the solenoid.

That is the part I'm having trouble visualizing. Do I make a loop with no additional ground? House battery to the switch- switch to the solenoid. Solenoid is grounded. Push button = complete circuit. Car starts from combined batteries? Or angry pixies make Jeep inferno?

Edit: Do I fuse both POS legs?

Be aware, Simply energising the solenoid from an 'aux battery (self jump scenario). Power will backfeed into the circuit what normally energizes the solenoid. If all goes right, Backfeed duration should be very brief.
Potentially this circuit overloads if the big battery conductors are somehow faulty.

Circuit back to the fuse box is on a 5amp fuse. Suppose I should go less?

 

[dwh]

Or, use a SPDT switch. Connect center to solenoid. One input from the ignition circuit (normal automatic behavior)(no fuse needed, it's already fused). The other input from aux battery positive (self jump) (might as well use a fuse in that).

That way you can choose which battery powers the solenoid, and avoid any potential problem from a backeed.

 

[SEREvince]

Thanks gents. talking it out led to the aha(!) moment.

 

[SEREvince]

Thank you to all of the contributors to this thread.

May not be pretty, but it works perfectly. All that's left is the interior momentary switch and a dab of silicon where the wire exits the fuze box. I'm saving the switch until I have the dash apart for some other upgrades. Only thing I may change is to clean up the connections to the circuit breakers. You can see a bit of exposed wire. We'll see if I get any corrosion there. I may trim them back and then heat shrink over the connections. I'm guessing the clear covers will get brittle in short order.

 

[SEREvince]

That's good news. The circuit breakers made a clean layout difficult. I was blindly following the diagram out of ignorance and an abundance of caution. I think I can re-use the terminal ends so I'm only out a few $ and about 20 min of time.