dual battery conversion

Daveo91Burb said:

Yep, I have the solenoid you refer to. I don't remember exactly how it is wired but its harness is intact enough I think it will be obvious when I put it together. So what about the isolators that are available on the market? Would they help with the unmatched batteries?

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They will do nothing for unevenly matched batteries but are great for when you have a mystery draw that drains the batteries or for parking for long periods of time.

I have no need for them but my buddies 99 crew dually wont keep a battery with out them lol too many computers


What cicuitguy is referring to is wiring the system where the 2 batteries are separate from each other. which can get pretty complicated

I made a custom battery tray in my 89. Made a bracket for the 175 amp fuse, I am not running an isolator however. not sure if I plan to put one in or not. Will post pictures soon.

Lots of good info here..

We used at least 2 batteries on all our tow trucks. Granted they were being used different than most applications. All I can add is make sure all the grounds are intact and clean. I added several extra, to the body, engine and frame. Can't have enough.

I would put in a dual battery switch, to give you the option of using either battery or both. If they are both tied together without isolation you eliminate the back up battery option. If there is a problem both batteries discharge if tied together.

fast 99 said:

We used at least 2 batteries on all our tow trucks. Granted they were being used different than most applications. All I can add is make sure all the grounds are intact and clean. I added several extra, to the body, engine and frame. Can't have enough.

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Actually, you can have "too" much but more like improper placement. Is your trucks TBI , EFI, or carb? If you have no computer, youre probably safe. If you have electric controls for your tow stuff, id make sure your grounds dont intefere with ignition or computer systems. to avoid that just make sure you dont have, say, an ECM ground at the same point as a winch or power window or hydraulic pump motor ground. Or be in the same path. The best example of not the best grounding was GM TBI using the thermostat housing/intake for ECM ground. because the battery cable was on the bracket for the alt and that put the ECM ground in the path of high voltage ground returns through to the battery

If you dont have that issue or you got it without doing that, then awesome

AuroraGirl said:

Actually, you can have "too" much but more like improper placement. Is your trucks TBI , EFI, or carb? If you have no computer, youre probably safe. If you have electric controls for your tow stuff, id make sure your grounds dont intefere with ignition or computer systems. to avoid that just make sure you dont have, say, an ECM ground at the same point as a winch or power window or hydraulic pump motor ground. Or be in the same path. The best example of not the best grounding was GM TBI using the thermostat housing/intake for ECM ground. because the battery cable was on the bracket for the alt and that put the ECM ground in the path of high voltage ground returns through to the battery

If you dont have that issue or you got it without doing that, then awesome

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Kind of see what you're saying but in the real world I have never had or heard of a drivability problem caused by an extra ground. Not saying it can't happen, just never saw it. Have seen countless issues with a poor grounds and that housing ground was notorious. Point I was trying to get across is power has to return to the battery. Sometimes that is overlooked and yes my example was a high amp extreme. Probably wouldn't apply unless extra lighting, winch or plow is installed.

You need an battery isolator. Make sure whatever isolator you select has a capacity greater than the alternator output. The isolated will maintain charge on both batteries. You can use a ford starter relay to connect both of the batteries during cranking.

I have seen latching relays in conversion vans, I think the way the work is all the add on electrical accessories are powered by a second battery that is isolated from the rest of the vehicle. Once the engine is running, the latching relay closes and the alternator will recharge the battery.

I’m sure any RV/travel trailer service shop could help you.

There are battery isolator kits that do a good job, or you could try the very simple setup that was stock on my ‘75 GMC. It’s not fancy or elegant, it it works.
- 2nd battery is mounted in the stock tray on the drivers side.
- from the firewall power junction they ran an additional charge wire over to a relay on the driver’s side fender.
- The activation wire for the relay is an ignition-on brown wire that runs up to the fuse panel.
- the battery side of the relay has a wire to a 2nd power junction and then over to the battery

When the ignition is on, the relay activates and the battery positive is connected through the relay to the 2nd power junction on the firewall. This charges the battery.
When the ignition is off the relay disconnects the battery from the rest of the truck, and it only powers the 2nd junction on the firewall. That 2nd junction is where the camper power was connected. With the ignition off you could run the camper power as long as you want and it would only drain the 2nd battery.

The stock relay is not that big. However, since all it does is allow the battery to charge it only has to be big enough to handle alternator output when charging. All the high amp accessories would draw directly from the 2nd junction connected to battery positive.
The wiring diagram for this setup is shown in the ‘74 and ‘75 wiring diagrams on this site.

dual batts in my crewcab.
used to run an isolator- nothing but probs, went thru 3 of them.
i wired mine in parallel. simple, cheap, easy. if ya got a big block or diesel, it doubles your amps for a good kick at turning them over.
i run 2 1100cca rv batts with a $10 manual batt cut-off switch. when i have my truck camper on, i use the cut-off so the camper is running on one battery, and always have a fully charged one to start the truck.
been flawless and problem free for over 20 yrs now

fast 99 said:

Kind of see what you're saying but in the real world I have never had or heard of a drivability problem caused by an extra ground. Not saying it can't happen, just never saw it. Have seen countless issues with a poor grounds and that housing ground was notorious. Point I was trying to get across is power has to return to the battery. Sometimes that is overlooked and yes my example was a high amp extreme. Probably wouldn't apply unless extra lighting, winch or plow is installed.

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There is no issue, save for if one creates a ground loop.

fast 99 said:

Kind of see what you're saying but in the real world I have never had or heard of a drivability problem caused by an extra ground. Not saying it can't happen, just never saw it. Have seen countless issues with a poor grounds and that housing ground was notorious. Point I was trying to get across is power has to return to the battery. Sometimes that is overlooked and yes my example was a high amp extreme. Probably wouldn't apply unless extra lighting, winch or plow is installed.

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I know, thats why I said its probably fine in anything youre doing unless it was EFI, and even then still need some specific pathing in relation to the factory wiring to cause an issue.

Point is tho is that you can do them properly and have both worlds lol, grounds are good.

Daveo91Burb said:

So my '91 Burb does not have dual batteries but the '90 parts burb I had for awhile did. Both are 350 TBI. I took everything along those lines from the '90 before I got rid of it including all wiring, the tray, even the batt itself. Is it as simple as installing the tray on the drivers side and then wiring the positive to the terminal block on the firewall and then grounding the negative? I will want to use an isolater so the two batts don't have to be exactly equal and one will not drain the other. Where does that go? Any other suggestions?

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There are multiple ways to wire in an additional battery(s). Each has their purpose so to speak. Figure out what you want the battery for and what you will want to power with it and then you can decide what is the best method.
Your intended purpose of an isolator above is partly incorrect I believe, because while charging you will want/they will equalize.
Basic options:
1. direct wire in parallel. Use, more available capacity in all situations (mostly beneficial for starting or high draw like a winch).
2. Simple 1-2-combined switch, a "perko switch". Abiltiy to "store one battery for use if the other isn't cutting it for some reason.
3. Simple relay connecting the 2 when ignition is on, to allow charging but no battery combining when key is off. Intended use, for accessories only, with automatic charging and auto disconnect when key isn’t on.
4. Voltage sensing relay. Expensive version of #3 that does mostly the same thing but less wiring and messing around and only combines batteries when charging voltage is preset.