GMC dual battery install

Project Bedrock


Upgrading to dual batteries gives working Chevy/GMC 1500s a big electrical buffer; Odyssey and Cole Hersee make it easy

By Bruce W. Smith

Turning the key to discover nothing but a dead battery isn’t fun. But it happens.

Inverters, coolers, radios, warning strobes, laptops, battery chargers, printers and other necessary accessories put a big drain on any pickup’s stock electrical system when the truck is on a jobsite with the engine off.

Even with the engine running the amp load needed to power such an array of accessories can put a strain on the stock electrical system of today’s half-ton pickups.

Add a plow , connect to a weak trailer battery, or flip on auxiliary driving, fog, or work lights and the battery is going to take a hard hit. Sometimes hard enough to leave you stranded.

In preparation for adding an inverter, warning lights, and a few other work truck “necessaries” on Project Bedrock — our 2011 GMC Sierra 1500 Crew Cab 4×4—we decided it was a good idea to double-up the batteries.

Stock Chevy/GMC 1500s come with a single Group 48 battery located in the passenger’s-side rear corner of the engine bay. There’s also an unused battery tray on the driver’s front corner; there’s no TP2 (dual-battery option) for the 1500s, so it’s bit of a mystery why it’s there.

Whatever the reason, it does make it easy to double-up the batteries.

Odyssey 78-PC1500 AGM battery, Cole Hersee Smart Isolator and SmartStart low-voltage disconnect are the heart of GM 1500 dual-battery upgrade.

All you need is a GM Battery Retainer (#14005061) that costs less than $5; a second battery; and a battery isolator designed to work with the new electronics.


The factory AC-Delco Group 48 battery is unique to GM trucks, and it’s rated at 615 CCA (Cold Cranking Amps) with 110 RC (Reserve Capacity).

It’s almost identical to a Group 34/78 or Group 78 battery. So we chose an Odyssey 78-PC1500-A AGM battery as the auxiliary, which delivers 880 CCA and 135 minutes RC.

The Odyssey is far more receptive to deep discharging and resistant to the hard pounding from off-pavement use than the OEM battery.

To connect the two batteries properly, we selected a Cole Hersee Smart Battery Isolator (#48530) designed to work with today’s electronic charging systems.

This 200-amp isolator senses the voltage of both main and auxiliary batteries and connects/separates them according to their charge, always protecting the main “cranking” battery.


For example, you pull the truck in to a jobsite, turn the engine off and leave all the warning lights on. When the isolator senses the batteries reach 12.7V it opens the solenoid relay to separate the two batteries and turns off the status light. This separation protects the starting battery while allowing the auxiliary battery to continue to power the auxiliary loads.

When you start the truck, the charging system brings the cranking battery back to 13.2 volts (full charge), then the isolator closes, the red status light turns on, and the alternator now recharges the auxiliary battery.

Wiring in the second battery is as simple as following the C/H wiring diagram.

Should need a “boost” at start, an optional “momentary switch” can be easily wired in to the Smart Isolator that connects both batteries for one minute once it’s activated

We also installed a Cole Hersee SureStart Compact Low Voltage Disconnect Switch (#48513-01) on the firewall to use as our “hot point” between the auxiliary battery and future strobe lights and accessories.

This unit sounds an alarm when battery voltage drops below 12.1 volts.

One minute later it automatically disconnects the battery it’s connected to (we will use it with the auxiliary) from whatever accessories are attached to it.

Automatically isolating the auxiliary battery in this way keeps it protected from total discharge, which could be the case when running lights or other accessories when one is away from the truck for a period of time.

This easy battery upgrade sets Project Bedrock up to handle any electrical need we might encounter in the field.

It should work nicely on your Chevy or GMC as well. – Pro


Odyssey Batteries/EnerSys Energy Products Inc.; 800-538-3627

Cole Hersee; Littlefuse Commercial Vehicle Products: 210-651-1845

Truck Supply & Outfitters; 205-553-4203