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Flooded vs AGM vs Gel Batteries

Flooded, AGM and gel are three lead-acid constructions that differ in how the electrolyte is held. They need different charging settings, and using the wrong charger profile shortens battery life.

Ref FA-0032

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

All three are lead-acid batteries. The difference is the electrolyte:

Flooded (wet cell): liquid sulfuric acid covers the plates. Some have removable caps for adding water.
AGM (absorbed glass mat): the acid is held in glass fiber mats pressed between the plates. The battery is sealed and valve regulated.
Gel: the acid is mixed with silica to form a gel. It is also sealed and valve regulated.

AGM and gel are both "VRLA" (valve-regulated lead-acid) batteries. They recombine gas inside and do not need water added.

How to identify

Read the label. Flooded batteries often have vent caps or a removable cover and may say "add water". AGM and gel batteries are sealed with no caps and are labeled AGM, gel or VRLA. A clear hydrometer "eye" does not tell you the construction. Some AGM batteries look like flooded ones, so always read the label.

Constructions compared

Feature	Flooded	AGM	Gel
Electrolyte	Free liquid	Held in glass mat	Gelled with silica
Maintenance	Check water level if serviceable	None	None
Mounting	Upright only	Many positions, check maker	Many positions, check maker
Vibration resistance	Lowest	High	High
High-current starting	Good	Very good	Fair
Deep cycling	Fair to good (deep cycle types)	Good	Very good
Sensitivity to overcharge	Tolerant	Sensitive	Most sensitive
Charger profile	Flooded	AGM	Gel

Charging differences

Each type needs its own charge voltage and float settings. Gel is the most easily damaged by too much voltage. AGM needs tighter voltage control than flooded. Many chargers have a selector for flooded, AGM and gel. Use the battery maker's charge voltages, not a generic chart. Some vehicles also need the battery type set in the charging system or battery monitor after replacement.

Key points

Battery ratings (CCA, CA, RC) and BCI group sizes are explained in the battery ratings sheet and the BCI group size chart.
AGM is common in start-stop vehicles and in heavy trucks with large accessory loads.
Never add water to a sealed AGM or gel battery.
Flooded batteries vent hydrogen while charging. Charge in a ventilated area away from sparks.
Store all types fully charged. A discharged battery can freeze and sulfate.

Common mistakes

Charging gel or AGM on a flooded-only charger setting.
Replacing an AGM with a flooded battery in a vehicle designed for AGM.
Assuming "maintenance free" means AGM.
Mounting a flooded battery on its side.

Governing standards: BCI technical manual and SAE J537 (battery testing); battery maker's charging specifications.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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