Auxiliary Battery Replacement

[cocot]

2017 Express van 3500 6.0L

Factory isolated auxiliary   battery located on drivers side frame rail.

I believe the 2nd battery is identical to the starting battery. (Group 78 ,700 ca 55 ah)

I would like to replace the 2nd battery with a deep cycle or AGM for longer run time for my inverter, 12volt fridge etc.

 

How will this affect the charging of the 2nd battery?

Are the many different charging modes applied to both batteries when the truck is running?

Does the factory isolated battery get charged independently of the starting battery when it is depleted and the starting battery is charged?

How does the computer handle the 2 batteries?

Basically, can I run a deep cycle battery or AGM as the house battery?

 

Someone please advise!  

[Jsdirt]

I've been playing around with AGMs in various equipment for several years now. My advice? Save your money.

 

The charging systems of 2007 - up vehicles uses a duty-cycled field, controlled by a computer. Since an AGM at rest is at a higher voltage than a regular lead acid battery (12.80v vs. 12.60v), the computer cuts the duty cycle prematurely. In other words, the AGM never gets a proper charge. I've had an Odyssey in my Silverado for 6 years now. I've had to charge the battery regularly with a 4-stage charger with an AGM specific setting, pretty much since the day I installed it. The OE charging system, plus the normal draw of all the electronics at rest, lowers the capacity of the AGM to a point where you're not realizing the benefits of the AGM. I also had one in my '05 Polaris Sportsman 800, and noted the exact same behavior. You'd be better served installing a higher amperage output alternator.

 

The only benefits of using an AGM, IMO,  is in a custom application, like a hot rod, or experimental aircraft - you can mount them in any orientation, even upside-down, plus, you can run a smaller battery than you need, saving space and weight.

 

Now, if you were to install that AGM into a dual battery system with one lead acid battery in the mix, the AGM will be constantly feeding the lead acid battery, causing depletion to happen much sooner. I would HIGHLY advise against doing that. AGM's aren't cheap ... and modern charging systems weren't designed to charge them. You'd actually be better off with an old school "dumb" charging system from the 60's through the '00's running an AGM, but even those aren't designed for it. They're just  one notch better in keeping the charge up, since there's no computer pulling back the duty cycle.

[cocot]

Thanks for the reply.

What you say makes sense.

I'm disappointed in the fact that the factory supplied a starting battery as the auxiliary battery.

Since the AGM is the only option for that battery group size with more capacity, that is why I was considering it.

A casual glance at the battery holder tells me that I probably can't use a bigger flooded battery without it hanging over the sides.

I will measure the tray and see what fits.

 

I happen to be a large buyer of wholesale batteries so I will have the rep figure out what fits!

 

 

[Jsdirt]

No problem.  I agree - a deep cycle would've been better suited. Sounds to me like yet ANOTHER instance of GM trying to save a buck at our expense. Par for the course today ...

 

Good deal! Let us know if you find one. That'll be valuable info going forward. 

[cocot]

As far as a Group 78 flooded deepcycle, I can't find one.

Only AGM. Gel would be better only because I know for a fact that batteries (not all) labeled gel are only in a gel state until they are charged and then they become sealed lead acid.

This is why I wanted a tech to explain how the alternator, computer and such determine the charging current and voltage. 

 

If anyone knows of a group 78  flooded deepcycle made by any manufacturer, I can try to cross reference it with Dekka or my other supplier.

[Jsdirt]

I just recently learned from a professional tuner that having an AGM in my truck could be the reason why the OBD-II monitors refuse to run, no matter how many miles I drive it. Every time I try to get a sticker, it comes up as "not ready" - nothing was cleared, and the battery hasn't been disconnected in 6 years. That guy told my buddy (who has a '10 Camaro with nearly 900 RWHP), who was looking to install one to save weight and space,  "If you want problems, install an AGM".  

 

Yet another reason to steer clear. If it were my truck, I'd just get the measurements of the battery tray and the space above it, and install the largest flooded deep cycle battery I could find, even if you have to go down in size from a Group 78. You might even find what you need at a heavy equipment dealer, used in skid steers, backhoes, etc..

[cocot]

11 x 8 x 8