How To

What’s the best indicator of a car battery’s condition?

John Paul, AAA Northeast's Car Doctor, hears from a reader with a question about measuring a car battery’s condition.

The Car Doctor hears from a reader with a question about measuring a car battery’s condition. AP Photo/David Zalubowski, File

By John Paul

April 12, 2026

4 minutes to read

Q. My old Toyota Corolla, which I want to go on forever, has a voltage reading of 12.4 volts at the battery when the engine is off. Is this okay? At what number should I be worried?

A. Some people are under the false impression that anything above 12 volts is okay, but not really. Typically an automotive battery at 12.6 volts is at 100 percent state of charge. A 12.4 volt reading is still acceptable, and it is typical of a car that does not get driven often. Typically a charged battery should be 12.4 to 12.6 volts at rest. This is a sign of a healthy, charged battery. To properly evaluate a battery you should look at both voltage and cold cranking amperage. These combined numbers tell the battery’s true condition.

Q. My car has a sealed AGM battery in it. I know with old batteries with removable caps, you would check the water level and just charge the battery. My question is, can you jumpstart or charge an AGM battery just like a conventional battery?

A. Lead acid batteries are usually called flooded lead acid (FLA) batteries. In the FLA batteries the electrolyte (battery acid) is free flowing liquid. In AGM (absorbed glass mat) batteries the electrolyte is absorbed into special glass mats placed between lead plates. Since both are lead acid batteries (just different construction) you can jump start the car like you would with any style battery. Just like FLA batteries, AGM batteries are best charged at low amperage. Some battery manufacturers do recommend specific AGM battery chargers, which charge at a higher voltage, for optimum performance.

Q. We purchased a 2026 GMC Terrain AT4. We are having trouble figuring out the car wash mode. The dealership it was purchased from knew nothing about it. A local GMC dealership has never heard of it. The car wash we have used for 14 years has heard of it but could not figure it out. He stays in the car to wash it to prevent the car from automatically going into park. Any information would be appreciated.

A. I had some homework to do on this one but did find some information in the online GMC owner’s manual. Here is what is stated. If you get out of the vehicle during the car wash: Drive to the entrance of the car wash. With the vehicle still on, press the brake pedal and open the door. While your door is open and the brake pedal is pressed, shift to NEUTRAL by pressing the “N” button. Note: It is important to open the door before shifting to NEUTRAL. If you open the door to get out of the vehicle in a car wash after engaging NEUTRAL, PARK may automatically engage. The light on the “N” button should be lit. If it is not, press “N” again. Now you can unbuckle your seat belt, turn the vehicle off if desired, get out of the vehicle and close the door. The vehicle may automatically shift to PARK upon re-entry. Technology can make even the simplest tasks complicated.

Q. My wife loves her 2025 Toyota Corolla, and I enjoy driving it too. It has enough power for us and is easy to handle. However, the horn is really wimpy. Would that be a design item or a malfunction? Is there an easy way to increase the volume? I read your column every week.

A. That is a pretty common complaint about an otherwise particularly good car. There is no way to turn the volume up. Some owners have converted the one horn setup to two horns. Typically they order a kit (typically eBay) for the Japanese market cars that includes a high and low tone horn which are also louder. The kits from Toyota use a simple wiring harness that plugs into the single wire and then goes to the two-horn set up. Hella makes universal horns, but you would need to fabricate a wiring set up and mounting bracket.

Q. I had the brakes replaced on my 2015 Ram truck. This is a new shop for me. How do I know I got a good brake job?

A. You may notice improved brake performance and a lack of vibration. There should also be no noise or squealing when slowing down. The brake pedal should be firm and responsive, never spongy. Also, a good repair shop will provide you with documentation of their work. This should include what was replaced, the type/brand of brake parts and the warranty. The shop may even show you the old parts to make you feel comfortable.

Q. As a regular reader of your advice column, I would love your advice on gas treatments and cleaners. I buy most of my gas at BJ’s Warehouse club, as it is cheaper. Online information seems to run from gas treatment being totally useless to wonderful. What do you think? Use or not, how often and which one if needed?

A. When possible, I try to buy TopTier gas since it contains additives that help minimize and remove engine deposits. Although many fuel additives may recommend adding a bottle during every fill up, I find two or three times per year to be sufficient. Regarding brands, Lucus and Techron make quality products. Techron is typically my first choice. This was the recommended product years ago when I attended the GM training center to eliminate combustion chamber carbon build up. Techron is also relabeled by several vehicle manufacturers to keep engine combustion chambers clean.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your Car Doctor question to [email protected]. Listen to the Car Doctor Podcast on Apple Podcasts or other popular podcast sites.