Car starter battery and AGM auxiliary battery in parallel

Matty · January 2021

I would like to setup a dual battery system in my car to power 12V appliances while driving and also while stopped. My car has a temperature compensating alternator and an ignition relay which disconnects the starter battery when the ignition is switched off. An auto electrician advised me to simply connect my car starter battery and 120Ah AGM auxiliary battery in parallel and power the appliances from the auxiliary battery. The idea is that the auxiliary battery will maintain a high state of charge by receiving charge from the starter battery while the ignition is on and isolate the auxiliary from the starter battery when the ignition is switched off. I asked whether this type of alternator could cause the voltage to drop too low to keep the auxiliary battery at 100% state of charge and his opinion was that yes this is true but it would likely still reach 95% SOC which is acceptable for me. My question is related to whether this is good practice. There are differing opinions. Some say that it is fine to place a car starter battery and an AGM battery in parallel and others say batteries should only be connected in parallel if they are of the same type, age and capacity. I am wondering whether this setup is a good one or whether I should add a DC-DC charger between the car starter battery and auxiliary AGM battery.

mcgivor · January 2021

This is a complicated subject which really needs thought, flooded starting batteries and AGM deep cycle batteries have different requirements, particularly when it comes to charging voltage. AGM require a slightly lower charging voltage to prevent venting, whereas flooded starting batteries accept a higher voltage for a brief period after doing their primary task, supplying a brief high amperage load.

When connected in parallel each battery will experience both loads and charging they weren't specifically designed for, which is a recipe for disaster, in my opinion. Keeping them seperate doing their intended purpose is the best advice I can offer.

Matty · January 2021

mcgivor said:

This is a complicated subject which really needs thought, flooded starting batteries and AGM deep cycle batteries have different requirements, particularly when it comes to charging voltage. AGM require a slightly lower charging voltage to prevent venting, whereas flooded starting batteries accept a higher voltage for a brief period after doing their primary task, supplying a brief high amperage load.

When connected in parallel each battery will experience both loads and charging they weren't specifically designed for, which is a recipe for disaster, in my opinion. Keeping them seperate doing their intended purpose is the best advice I can offer.

Is adding a DC-DC charger a potential solution to the problems that may be caused by having the car starter battery and AGM auxiliary battery connected directly in parallel?

mike95490 · January 2021

A DC-DC charger must be configurable for your auxiliary battery style ( AGM )
it will also have some losses, figure about 15% loss, so if you are charging 15A, you are consuming 17.25A from the vehicle

Wheelman55 · January 2021

Matty. It’s a simple process if you use a battery isolator like this: https://hellroaring.com/

I’ve put these in both my pickup trucks, including a 2018 F150 that is loaded with electronics.

The isolators are solid state and made in the USA by Hellroaring. They will field phone calls to walk you through the process.

It will take you less than a day to install. The hardest part is deciding where to put the sux battery.

Matty · January 2021

Wheelman55 said:

Matty. It’s a simple process if you use a battery isolator like this: https://hellroaring.com/

I’ve put these in both my pickup trucks, including a 2018 F150 that is loaded with electronics.

The isolators are solid state and made in the USA by Hellroaring. They will field phone calls to walk you through the process.

It will take you less than a day to install. The hardest part is deciding where to put the sux battery.

I keep my auxiliary battery in the rear of my vehicle. When my ignition is switched off, the starter battery disconnects so it appears I already have an isolator installed.

Car starter battery and AGM auxiliary battery in parallel

Comments

Categories