The number we can actually cite
Battery University BU-806a states a guideline: each 8°C (15°F) rise in temperature cuts the life of a sealed lead-acid battery in half. A VRLA pack specified around 25°C (77°F) does not live that same life in a closed console that sits in Florida sun. Once heat damages the pack, capacity does not come back. That is the source we use for sealed lead-acid heat.
Where the box actually sits
A canvas cover on a lift-kept pontoon, a gasketed Whaler console, and a Yamaha jet compartment under a white deck are ovens. Flooded cells lose water faster in that heat. AGM cannot be rewatered; BU-201a also flags AGM as heat-sensitive. A charger that never rests adds electrical heat on top of air temperature.
Flooded can survive heat with air and water
The same BU-806a page notes flooded lead-acid as more at home in hot climates if it is watered and vented. That is why a trailer bass boat with open trays and an owner who checks caps can outlast a sealed pack cooked under a seat. Venting still has to follow the boat rule for hydrogen. Heat is not permission to seal a wet cell in a plastic bin.
Lithium has a different heat story
LiFePO4 does not use the 8°C sealed-lead rule. It still has a BMS temperature range. A module cooked in a furniture box can shut down on high-temp protect. We treat that as a location problem — move air, move the box — not as a reason to ignore lead-acid heat data on the other batteries in the boat.
What we change on a hot-boat visit
Open the box, check the charger float, and pick chemistry that can live there. Cape Coral covers and Sarasota lifts are this call every summer. We come to the boat. The shade of the dock is not the temperature inside the seat.
Covers, color, and a closed well
A dark cover over a white deck still traps air in the well. We cannot paint the sun. We can open airflow, stop a charger that never rests, and pick flooded with water and venting when a sealed pack keeps dying in that box. The 8°C guideline is why we talk about the well as a climate, not as a piece of gelcoat.
Sources
- Battery University BU-806a — How Heat and Loading Affect Battery Life
- Battery University BU-201a — Absorbent Glass Mat (AGM)
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.