What Is an Auxiliary Battery, and Should You Replace Both?

Published Monday, October 26, 2026 · Updated October 26, 2026, 6 AM ET · 5 min read · By Hassan, Owner of Roadmate Services

An auxiliary battery is a second, smaller 12-volt battery that keeps your car’s electronics running while start-stop shuts the engine off. It wears out like any battery. When it fails, start-stop usually quits first, and a weak one can drag the main battery down with it. If both test bad, replace both.

We saw exactly that on Sunday, October 25, in a driveway in Columbus. A Jeep Compass had been tested at a shop the week before, and the main battery was bad. When we got there, the auxiliary battery was failing too, with corrosion coming out of the case. So we replaced both.

Auxiliary battery at a glance

What an auxiliary battery does

Cars with start-stop turn the engine off at red lights and restart it when you lift off the brake. While the engine is off, something still has to run the radio, the screens and the fan. As AutoZone explains, that is the auxiliary battery’s job, so the main battery can save its strength for the restart.

Many newer SUVs and trucks have one, Jeeps included, and Batteries Plus notes they are often hidden under a seat, in the trunk or behind the glove box. Carmakers do love a game of hide-and-seek, and the battery always wins.

If you are not sure which kind of battery your car takes, our AGM vs flooded battery guide explains why start-stop cars need AGM.

Signs your auxiliary battery is failing

• Start-stop stops working, and the engine idles at every red light
• A message such as “Service Start-Stop System” on Jeeps and other Stellantis models, as Desert Battery Rescue lists, or an auxiliary battery warning
• The touchscreen resets, the windows feel slow, or several unrelated warning lights come on
• Corrosion or swelling on the battery case, like the one we pulled from the Compass
• The main battery keeps running down for no clear reason

A lot of these look like a weak main battery too, which is why both should be tested. Our guide to the signs of a dying car battery covers the main one. When a car starts sending you three warnings at once, it is not being dramatic. It is asking for a battery test.

Two batteries, one Sunday: the Jeep Compass

The driver works every day until after 5, and nobody was open when he was free. A shop had tested the car the week before and found the main battery bad, but he had not had a chance to get it done. On Sunday he called us, we booked him for the next hour and arrived on time.

With the hood open, the auxiliary battery showed its age: corrosion was coming out of the case. Putting a new main battery next to that would have been like pairing new shoes with the old socks that have holes. So we replaced both, a new DieHard AGM main battery and a DieHard Platinum AGM auxiliary battery, in about 30 minutes in his driveway.

He was very happy that someone came on a Sunday, so his car could keep working through the week. That is a good reason to have a battery done at home: you do not have to give up a workday to sit in a waiting room.

Should you replace both batteries together?

Test both first. If both are worn, replace them together. The two share one charging system, and Batteries Plus warns that a new battery paired with a worn one can drain faster and fail early. That matches what I tell drivers: replace both together so the old one does not drain the new one.

If one tests healthy, it can stay. Desert Battery Rescue makes the same point: test both, then decide.

1. Have both batteries load-tested, not just checked for voltage. A healthy battery rests around 12.6 volts (AutoZone), and our battery testing guide shows how to read it.
2. Look at both cases. Corrosion, leaks or swelling mean replace.
3. Match the type. Start-stop cars usually need AGM, and the replacements on our Compass job were both AGM.
4. Ask about a reset. Some cars need a battery management reset or registration after a new battery (Batteries Plus).
5. Clean the terminals while you are there. Our battery corrosion guide shows how to do it safely.

Most auxiliary batteries last about 3 to 5 years, the same as a main battery, according to AutoZone. If your main battery is that old, the small one probably is too. For the main battery’s life, see how long a car battery lasts.

Auxiliary battery replacement in Columbus

We test and replace main and auxiliary batteries at your home, work or parking lot, including batteries under a seat or in the trunk. See our battery replacement in Columbus page.

Battery replacement is quoted by phone, because the price depends on the battery your car needs. We work 6 AM to 8 PM daily, Eastern Time, weekends included, and arrive in about 32 minutes on average, depending on traffic, weather and location.

Frequently asked questions

The bottom line

If start-stop has quit or a start-stop warning is on, have both batteries tested. When both are worn, replace them together, so the new one is not working for two.

Here is to both batteries pulling their weight, and start-stop doing its quiet little job again.