Car Auxiliary Battery Malfunction Warning

Car auxiliary batteries are very similar to the main car batteries, except that they are smaller and feature smaller capacity and smaller maximum discharge current.

Car auxiliary batteries ensure constant voltage for various car electric systems, especially when the main battery is disconnected or used in the start/stop application.

Published: September 27, 2022.

What Is A Car Auxiliary Battery?

With the increase in the amount of electronics in modern cars, the energy demand has increased the load on the main car battery, which is generally designed as starting battery and for powering lights and other similar electric systems.

The Start and Stop systems' introduction has increased the main battery's load even further.

Note: Don't confuse auxiliary batteries with the second, deep-cycle battery often used as a car audio battery.

In order to ensure constant voltage for certain key systems (car's computer, for example), some brands have started adding a second battery, taking some load off the main battery.

For example, Mercedes-Benz started adding a second (auxiliary) battery in mid-2000. Today, auxiliary batteries range in capacity from ~1Ah to 12+Ah, while some brands offer auxiliary batteries that are not even batteries - they are capacitors.

Note: Auxiliary batteries and capacitors may be recharged using the car's braking system, but these cars are not "hybrid cars" or "electric cars" since this recovered energy is not used for propulsion again but for powering various electric and electronic systems.

Auxiliary batteries last like the main batteries, from 3-6 years, sometimes even more or less, depending on the local conditions.

Auxiliary batteries are often labeled as "AUX" batteries (AUX12, AUX14, etc.) which is very important - under certain conditions, these batteries are working heavily, and replacing them with a typical starting AGM or deep-cycle AGM batteries will create many other problems.

Auxiliary Battery Malfunction Warning

Cars with auxiliary batteries also feature extensive diagnostic systems and warn the users when the auxiliary battery needs to be replaced.

In most situations, it is not required to go to the dealership to replace the battery - replace it at home, if You like DIY activities, just be sure that the new battery is the same (or better) as the old one in terms of terminals, capacity, discharge current and other charge and discharge characteristics.

Note: auxiliary batteries are heavy workers, so be sure not to go cheap when looking for a new one.

However, going to the dealership ensures a more thorough car diagnostics and verifying if the auxiliary battery must be replaced or if something else may be of an issue.

Note: while the main batteries are often located near the motor, under the hood, the auxiliary batteries are often located in the rear of the car and/or near the fuse box.

Long Story Short: Having a malfunctioning auxiliary battery is not a pleasant thing to have, but it can be solved fairly quickly - either replace it on your own or contact the dealership and let them replace the battery and do a few more diagnostic tasks, better safe than sorry.