What is second battery for?

Semikrazy · September 11, 2016

I just bought a 91 GMC 3500 7.4 that was a special order when new. Original owners bought it to tow a large horse trailer. It has a second battery that is isolated from the main battery but is charged by the alternator. Any idea what's it for? The guy I got it from owned it since 94 and he doesn't know either.

Bunks-tj · September 11, 2016

Usually for running electric. Maybe lights or a heater for the trailer.

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ChumminsDQ · September 11, 2016

Id assume running aux lights in the horse trailer But also they have lots of lights so without a second battery it can draw alot from a single battery.

Semikrazy · September 11, 2016

It came from factory or dealer installed like this. Does camper special models come with two batteries? I'm going to go get the truck tomorrow and maybe find out more about it. The second battery is the original battery so I'm sure it's bad now.

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Semikrazy · September 11, 2016

Thanks guys. Since it's already set up I'll just replace that battery.

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Chevor · September 11, 2016

Plow truck?

Semikrazy · September 12, 2016

The original window sticker shows the additional battery was an option. Kinda cool finding the original window sticker with a bunch of other papers on this truck. The original owner specked it out nice.

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barry G · September 12, 2016

before the obd2 control, the more amps drawn by battery triggered a tickle to the alternator to do more.

I would have let it go.. up to you.

electric trailer brake used to be quite a monster.

frame muncher.

Today you could go from 600cca to 1000cca..and not even notice. the alternator just feeds battery with a brain now.

feeds the truck demand another way.

2 batteries worthless.

maybe diesel gurus play with them.

up until 96, the batteries claimed to be just for starting, acc key on.

that was so sick..decades of looping on batteries doing it all.Precision claim was terrible.

Today, that claim sticks. Battery is for starting.. no looping a bomb anymore.

big alternator will feed your truck. Little battery to start.

Edited September 12, 2016 by barry G

Z45 · September 12, 2016

Common option for vehicles that towed a camper or trailer of some kind with need for electrical power to the trailer. I ordered it on a 1996 1500 4x4 just because I could, for no special reason. In the snow belt it was a option for snow plow equipped trucks. Dealers often installed them as accessories.

Edited September 12, 2016 by elcamino

bronyaur · September 15, 2016

I have a second battery and use it to run a winch. A winch can drain your battery in nothing flat. I can combine the two batteries for extra power, or reverse their role. It's also nice to switch to the auxiliary battery when you get in your truck to go to work at o-dark-thirty and find that your main battery has given up the ghost because it's old, or your son left the map light on two days ago, or.... A battery separator/combiner from Hellroaring Technologies can be invaluable if you're running off-road or frequent the middle of BFE. And, it helps to upgrade to (at least) a 150A alternator.