The boat is never fully off
Stereos remember settings. Float switches wait. Alarms and stereo amps wake. On a slip or a lift, those loads nibble the house bank every hour the charger is dark. Owners describe a battery that “dies at the dock.” The cells may be fine; the week of silent draw is not. Battery University’s sulfation article is what happens if that pack sits low.
Chargers fail as often as batteries
A GFCI on the pedestal trips. A cord comes loose. A charger on the wrong chemistry never finishes. The owner sees a flat bank and buys a battery. We test the charger and the cords with the batteries. Port Charlotte and Cape Coral lifts are full of this pattern in summer thunderstorms.
Find the draw before you replace everything
A clamp meter on the negative with the battery switch “off” tells you whether off is actually off. If current still flows, find the load. Memory draws are small; a livewell pump stuck on is not.
House versus start
A parasitic on the starting battery is how a cruiser will not crank on Saturday after a week of cabin lights on the wrong switch position. Isolators exist to stop a dead house pack from flattening the starter. Wikipedia’s battery-isolator article is that device. The visit is to see whether the boat has one and whether it still works.
What we do
Measure rest voltage after a sit, inspect the charger, and look for loads that never sleep. Replacement happens when the pack is sulfated or aged. Replacement without finding the draw is a repeat call in three weeks.
Common loads that never sleep
Stereo amps, pump float switches that stick, and a battery monitor that is itself a draw show up on clamp meters. We isolate by pulling fuses when we can. The point is to find the circuit, then decide if the pack is already sulfated from the week it sat low. A new battery on the same stuck pump is a two-week repeat.
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.