Gel is not AGM with a different sticker
Battery University’s gel article describes sulfuric acid suspended in silica. The pack is sealed. You cannot add water. Charge voltage is lower than typical flooded absorption, which is why a “marine AGM/flooded” charger left on high will dry a gel house bank. BU-403’s lead-acid charging notes treat gel as its own program. If the charger has a gel position, use it. If it does not, the charger is the wrong tool.
Why gel showed up on cruisers
Gel house batteries were a way to get sealed cells and deep cycling into a cabin before AGM was everywhere. They still sit in older Sea Rays and sailboats. Replacing gel with AGM without touching the charger, or the reverse, is a season-long failure. Read every label in the box. Mixed banks of gel and AGM on one program are a compromise that usually hurts one chemistry.
Heat and gel
Sealed packs in Florida engine rooms follow the heat guideline for VRLA. A gel battery cooked on a high-voltage flooded program fails twice: voltage and temperature. We open the compartment and we look at the charger together.
Do not equalize gel like flooded
Flooded equalization is a high-voltage stir. Gel does not want that. A charger that auto-equalizes on a schedule will abuse a gel bank. Turn that feature off or change the unit.
On the visit
A photo of the gel label and the charger’s chemistry screen is enough to start. We match them. We do not treat gel as a generic sealed battery.
Reading a gel label next to an AGM neighbor
Older cruisers sometimes have gel house and AGM start on one charger. That charger has to pick a window; one chemistry loses. We would rather reprogram or split chargers than split the difference. If both cases still say gel, keep gel voltage. If you are replacing gel with AGM, the charger moves with the chemistry — not next season, on the same visit.
Sources
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.