How start a 2-battery pickup using the second battery?

Pickup has 2 batteries. Passenger-side battery starts the truck. If it loses charge how do I use the driver-side battery to start the truck. I assume that you disconnect the driver-side battery from the truck cables and then connect both batteries with jumper cables. Does that sound right?

Are you sure only one of the batteries cranks it? I’ve not seen one like that. The only reason I can see to have two batteries is from cranking power. What kind of truck is it?

Absolutely. The 2 batteries should already be connected to each other by the battery cables. I have heard of some Ford diesel pickups that automatically disconnect one battery after engine starts. The only reason deisel pickups use 2 batteries is because you need beaucoup amperage to start a diesel engine. I?m assuming you have a diesel. No offense, but why can?t you tell us the application (make, model, year,

and engine size) so we?re not shooting in the dark here? A lot of us have access to Alldata and other repair manual programs and even battery charts to see what size and group # applies to your truck. These sources are useless without the application. Please post back with proper info.

The second battery is supposedly to provide extra power as needed for the camper. My mechanic has said that the left side battery is the one that starts the truck. Sorry for the lack of info to start with. The truck is a '95 Chevy Silverado heavy duty chassis 3/4 ton, 4WD, automatic trans. The previous owner had driven the truck back and forth to Alaska a couple of times. I consider it to be my “truck for life.”

Hope this isn’t a double reply–my laptop gets ahead of me some times. Please see the response to the first message, and I appreciate both of you answering. The truck is gas, not diesel, and I didn’t learn all the details about how it is hooked up from the original owner.

If the second battery was installed with an isolator, there should be some way to bypass that with a switch. Maybe not. Anyway, I see no harm in just using jumpers from one battery to the other. This will effectively bypass any isolation device, but should not hurt anything. Isolators are generally set up to keep the auxiliary battery from draining the “starter” battery if the aux. gets low from use in the camper.

I think that’s right. When I first read the OP I thought it had to be a diesel. But I do remember working on
a gasoline 3/4 ton van ambulance that had a switch in between the 2 front seats to connect or disconnect the auxiliary battery from
the main
battery. The switch resembled one of those old fashioned kitchen timers with a knob/handle except it was
black and had only 2 positions- connect and disconnected or something similar. Tomorrow I can get on alldata

for a wiring diagram cause I’m curious about this setup. Meanwhile, Adriver, if you want you could take the batteries
to a Pep boys/Autozone/Advanced Auto.
Click here: see post 2-2-08; 5:58;00 PM. You know, be tactful and maybe they’ll use the info in the bottom half of the post.

Why not simply swap the batteries over? If your going to have to disconnect them anyways. JMHO.

If there is not an isolator installed already, painless wiring makes a nice setup:

If it doesn’t have an isolator, then when the battery is dead, they’re both dead! So the issue of jumping would be moot. Assuming there’s an isolator in place, you don’t have to disconnect anything. Just jumper from the aux battery to the main battery to get it started. I ran an '89 2500 like this for years with a plow setup. The isolator just insures that the aux battery can’t backfeed the main battery but it can be charged from the alternator.

Since they share a common ground, you want to connect to the main battery POS(+) post first to avoid making sparks around the dead battery.

Thanks to all for info on this situation. At this point I would like to check to see if the isolator is doing its job–allowing the aux battery to provide camper power and allowing the alternator to recharge it. Any ideas on a good way to do this check?

Sorry, but I’d need to see a schematic of your particular set up to offer much help. My understanding of isolator circuits is they are meant to keep the aux. battery and its loads from draining the starter battery when you are watching the TV in the camper. I can think of a couple different ways to set that up, so I don’t want to guess what you have, but jumping between the two should not hurt anything in an emergency start situation.


Measure the aux battery terminal voltage with the engine off. Start the engine and check the terminal voltage again. It should be around 12.5VDC off and around 14VDC with it running.

You can check for battery voltage at the plug where the camper connects. It depends on which style plug you have as to which pins should have the power on them. Simple to look up on the web or post back with the connector style and I’m sure someone can help.