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

mobydick 11

Active member
It sounds like you are looking for something like a "mil spec" battery terminal available on Amazon. Allowing you to bolt multiple wires to the same battery post. I've used the Audew and CrimpSupply brands, prefer the latter but they are mostly the same. I've also bought them in better quality from electrical retailers specializing in marine and heavy equipment. You'll need terminals, wrap and crimper for the wire gauge you are working with. TemCo is a great place to purchase this stuff, they have an Amazon storefront. Their copper wire and tinned terminals are American made for the most part and more reasonable than the Chinese sheeite everybody else sells.
Buddy cold also try the Installgear also . you just cut the end off a store bought cable and strip back and plug in then tighten the Allen bolt . I have started buying my fuse blocks in this style ,saves some work . I think I will melt some solder on the end before I tighten it in though


New member
That sounds about right..... As I recall the way it worked for me.....when I tried to buy something in a quantity of "2", they patted me on the head, pinched my cheek, tied my loose shoelace, and gave me a pass to the short bus. The "warehouse" was where I was dropped off.


New member
Phew! Started reading this thread in 2019 and have finished it today!

Great info everyone, you have all saved me a ton of cash on my trailer 12v system design. Was going to go down the DC to DC route but with my Wrangler I have enough voltage to charge.


Your link is to the manufacturing and wholesale website. For low volume, e.g. individual purchases, the website is different.

I absolutely agree, great resource.
Too bad their site is badly coded, I tried visiting it and there seems to be something wrong with their location based redirect, it either thinks I am in South Africa (I'm in Utah, USA) or if I try from my cell on TMobile it just says I'm in "-" and still redirects me.

Sent them an email about it, but it'll probably be a bit before they fix it.

Sent from my SM-N975U using Tapatalk


New member
Wondering if anyone can help me troubleshoot/pinpoint an issue I’m having with my system. I built it off of the basic plan from post #1. Only difference being the house batteries are two 35ah 12v agm batteries wired in parallel.

I’ve had this system since 2014 and it’s worked well enough for my needs. But, I had the hood open and I noticed that the solenoid has some rust around the small + terminal that goes to the keyed source. Not sure if that should be concerning or not. You’ll also notice the 4ga wire in this photo, it’s actually bulging just past the heat shrink, where it’s very firm rather than soft, like it was when new.

In the second photo, it looks like a chunk was taken out rather than melted but I don’t know what could have done that.

I’ve had this exact system in 3 separate vehicles so something may have happened from the latest re-install into my 4Runner. The system appears to still be functioning. I put my multi-meter on before starting the vehicle, house batteries were at 12.65v. After starting and running a few minutes they got up to 12.7 before I shut it down. I’m disconnecting for the time being so no further damage can occur until I fix it. Any ideas what would be causing the 4ga wire to firm up like that?



New member
Water intrusion and you now have green copper sulfate inside the jacket. Also rodents have chewed the heat shrink and jacket on the other cable.
Ok should the entire length of cable be replaced or will it be safe to cut it just a bit past where the corrosion stops?

Didn’t know rodents eat cables... that’s a new one for me. I’ll replace that then, it’s a short run so I have enough extra.

Hard to say what the bulge is.
Probably corrosion. Notice grayish colour where its heatshrink is slightly chewed away. Maybe distortion of plastic due to heat or chemical exposure. Since I duuno what type insulation is other than it has a weird translucent appearence wont speculate further on quality product.
Second picture is chewing of a rodent.
Now maybe inspect other wiring & hoses also...
Going to dig into there a bit more this week, check for other chewed wires/hoses. Not sure of the quality of the cable either. It’s been too long to find a receipt.

Isn’t the solenoid installed upside down also? Usually the instructions say dimple must be down.
Solenoid did not actually come with instructions. Have had it installed this orientation for 3 rigs/6 years/45k miles collectively, no issues whatsoever until now. Could be incorrect but checking through the questions on the amazon listing just now didn’t yield much more than “how dumb are you? If you need instructions you shouldn’t be installing” not helpful.


There is a whole niche industry in products designed to prevent rodents wrecking car wiring.

In Germany you see ultrasonic annoyance sound generators

apparently also used more recently to repel young people from street corners
  • Like
Reactions: WVI


Perform the following tests to to determine present wiring health:

Drain the house batteries down to about 12vdc, so that they need charging.

Connect your voltmeter test leads to the two large posts on the solenoid.

Start the engine. The voltmeter should read almost zero volts dc. Any voltage read here subtracts from the voltage seen by the house batteries. The contact's resistance causes that small voltage, and reduces current flow to the house batteries.

Place one dc voltmeter test lead on the starting battery's positive post. Place the other test lead on either house battery's positive post. You may need to lengthen your voltmeter lead/s.

There is basically a single wire conductor, separated by the solenoid's contacts/large terminals from one battery to the other. With the solenoid energized, contacts closed, there should be no more than a few tenths of a volts dc measured between batteries. The test leads connected from starting to house battery positive terminals shows total voltage drop/loss across all connections.

A clamp-on DC ammeter to read house battery charging current is very handy to actually know amp flow.