Four chemistries, four chargers
Flooded wet cells, absorbed glass mat, gel, and lithium iron phosphate can all sit in a marine tray. They do not share a charge program. Battery University’s lead-acid charging article describes constant-current / constant-voltage stages and different voltage windows for flooded, gel, and AGM. Its lithium-iron-phosphate article is just as clear: four 3.2-volt cells, about 14.6 volts at the top of charge, and no conventional float. Putting the wrong program on a new pack is how a “fresh battery” dies in a season.
Flooded still needs air and water
A flooded marine battery vents hydrogen while it charges. Cornell’s boat-battery rule requires a way for that gas to leave the boat, and Wikipedia’s hydrogen page reminds you the gas has no odor. Caps, upright mounting, and a box that is not sealed against the hull are part of the install. Florida heat drives water loss. We check electrolyte where it is safe; we do not treat wet cells as sealed.
AGM and gel are sealed, not identical
AGM holds the electrolyte in glass mats. Battery University notes spill-proof mounting, better vibration resistance, and deeper cycling than flooded, with a warning that AGM is heat-sensitive. Gel uses a silica-stiffened electrolyte and wants a gel voltage, which is lower than many AGM programs. Swapping gel for AGM because both cases say “sealed” is a charger problem waiting to happen.
Lithium is a system
LiFePO4 modules that fit a group-31 footprint still need a battery management system, a charger that can be turned off at full charge, and an honest look at the alternator. Float from a lead-acid slip charger shortens LFP life. If the boat cannot change that charger, lithium is not a drop-in of size alone.
Read the label, then the charger
On a lift-kept pontoon in Sarasota or a canal boat in Cape Coral, the failure is often the program on the pedestal, not the logo on the case. Bring photos of every battery label and the charger’s chemistry settings. We match those before we talk about a replacement chemistry.
One boat, more than one chemistry
A cruiser with a flooded starter, gel house, and a lithium trolling experiment is three charger problems. We sort the labels first. We would rather standardize a bank than keep a museum. If you want to keep mixed chemistries, you need mixed programs. That is the whole chemistry article in one engine room.
Sources
- Battery University BU-403 — Charging Lead Acid
- Battery University BU-409b — Charging Lithium Iron Phosphate
- Battery University BU-201a — Absorbent Glass Mat (AGM)
- 33 CFR § 183.420 — Batteries (Cornell LII)
- Wikipedia — Hydrogen (odorless, flammable)
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.