How they're built
A standard flooded battery has lead plates sitting in liquid sulfuric acid. It's a proven, inexpensive design and still the most common battery in older cars. An enhanced flooded battery (EFB) uses the same idea with stronger plates and additives so it can handle the repeated cycling of a basic start-stop system.
An AGM (absorbent glass mat) battery holds its acid in fiberglass mats pressed between the plates. It's sealed, spill-proof, more resistant to vibration, recharges faster and tolerates deep discharges better. That's why cars with start-stop, heavy electronics or a battery mounted in the cabin or trunk use AGM.
+ 01 Side by side
The practical differences
- Price: flooded is the least expensive, AGM the most, EFB in between
- Start-stop: AGM or EFB required; standard flooded wears out fast
- Deep discharges: AGM recovers best from being run down
- Heat: AGM generally holds up better because it's sealed and doesn't lose water
- Cold starts: AGM's low internal resistance helps deliver current in the cold
- Mounting: AGM can go in the trunk or cabin; flooded batteries vent and belong under the hood
+ 02 Switching
When you can change types
Going from flooded to AGM is usually fine if the size fits and the charging system suits it, and can be worth it in hot climates, for cars with big electrical loads, or cars that sit a lot. Going from AGM to flooded or EFB is the swap to avoid. The car's charging strategy expects AGM, start-stop will suffer, and on cars with the battery inside the cabin, a venting flooded battery is a real problem. See AGM battery replacement.
Questions about AGM vs flooded battery
Is AGM worth the extra money?
If your car requires it, it isn't optional. If it doesn't, AGM is worth considering in hot climates, for cars with heavy accessories, or for cars that sit for long periods.
How can I tell what battery my car has?
Look at the battery label for AGM, EFB or a flooded design with removable caps, check the owner's manual, or ask the tech, who checks it from your year, make and model.
Do AGM batteries need a special charger?
A charger or maintainer with an AGM setting is best, because AGM batteries prefer slightly different charging voltages.