AGM vs. flooded car batteries: what’s different and which one your car needs

Start with what's in the car now. If it came from the factory with an AGM battery, usually because it has start-stop or a lot of electronics, replace it with AGM. Going back to a cheaper flooded battery shortens its life and can cause electrical problems. If the car came with a standard flooded battery, a flooded one is fine, and AGM is an optional upgrade.

Two ways to build the same thing

Both types are lead-acid. The difference is how the acid is held.

Flooded (also called wet cell or standard) batteries have plates sitting in liquid electrolyte. They're the traditional design, the cheapest, and they work well in cars without heavy electrical demands. Some have removable caps. Most sold today are "maintenance free," meaning sealed enough that you never add water.

AGM stands for absorbent glass mat. The electrolyte is soaked into fiberglass mats packed tight between the plates. Nothing sloshes. The battery is sealed and can recombine most of the gas it makes. That build lets it deliver and accept current faster and handle deeper discharges.

EFB, enhanced flooded battery, sits in between. It's a beefed-up flooded battery made for entry-level start-stop systems. If your car came with EFB, you can use EFB or move up to AGM. Don't go down to standard flooded.

Which cars need AGM

• Most cars with automatic start-stop, especially European brands and newer trucks and SUVs.
• Cars that came with AGM from the factory. There's often a label on the battery or the info is in the owner's manual.
• Many luxury cars with a lot of modules, cameras and seat motors.
• Some cars with the battery in the trunk or under a seat, because AGM doesn't vent like a flooded battery does.

If you're not sure, the original battery label is the fastest answer. Look for "AGM" printed on it.

Why a downgrade causes trouble

A start-stop car shuts the engine off at every light and restarts it maybe dozens of times a trip. While it's off, the battery runs the A/C fan, lights and screens. That's a lot of shallow cycling. A standard flooded battery isn't built for it and wears out fast, sometimes within months.

The car's charging system also expects a particular battery chemistry. Many cars charge AGM a little differently. Put the wrong type in and the car may overcharge or undercharge it. On some cars the battery also has to be registered, so the computer knows a new one went in. Battery registration and reset.

Is AGM worth it in a car that doesn't need it?

Sometimes. AGM handles heat and deep discharge better, resists vibration, and doesn't leak if the case cracks. It can make sense if:

• The car sits a lot and the battery goes flat now and then.
• You run accessories like a dash cam that records while parked, a big stereo or a winch.
• You've been replacing flooded batteries every couple of years and want to try something tougher.

It's less likely to be worth it on an older commuter car that's driven daily and has no extra load. A good flooded battery will do fine there.

Price

AGM costs more. On our prices page you'll see the range for standard batteries and for AGM and start-stop batteries. We tell you which one your car needs before we start, and the total before we install.

Heat

AGM generally holds up better in heat than standard flooded, but it isn't immune. Heat still corrodes the grids. An AGM in Tampa still won't last as long as it would in Michigan. Florida heat and car batteries.

Charging AGM at home

If you use a battery maintainer or charger, use one with an AGM setting. Older chargers that push high voltage can damage an AGM. Most modern smart chargers handle both.

Can you mix them?

In a truck with two batteries, both should be the same type, size and age. Mixing types makes them charge unevenly and the weaker one drags the other down. We replace them as a pair. See truck and SUV batteries.

The short version

Match what the car came with, or move up. Never move down on a start-stop car. If you're unsure what you have, tell us the year, make and model when you call and we'll check the type and size before we head out.