Charge first when the history is short
A battery that went flat on a long weekend and still takes current may recover with a correct chemistry charge (BU-403) and a rest. BU-903’s open-circuit table is the clue after that rest. If voltage and cranking come back, you bought a charge, not a funeral.
Replace when sulfate or age has won
BU-802: a pack that sat empty for a season usually will not come back. A starting battery that cranks slow after a full charge, or a trolling string that dies in twenty minutes after a night on the charger, is done. Age of the neighbors in a series string matters. One healthy cell does not save two dead ones.
Chargers confuse the picture
A battery that looks full on a pedestal and empty on the water may be the charger, the cords, or a parasitic — or a pack that can no longer hold. We test off the charger. Kissimmee owners often have a tender that has been “on” for months at the wrong voltage.
Lithium is a BMS event sometimes
A LiFePO4 module that shows zero may have disconnected on low voltage or cold charge inhibit. That can be a reset and a correct charger, not a new module. We read the sheet before we condemn it.
What we will say at the boat
Charge and retest, replace this one, or replace the string. We would rather be plain. A recharge that will fail next Saturday is not a favor.
Series strings change the replace decision
A single starting battery can earn a charge-and-retest. A 36-volt trolling string of mixed ages usually cannot: the new cell will be dragged. We would rather replace the string than recharge one and hope. If you already know two of the three are original to a boat you bought used, say that — it saves a test cycle that will not change the quote.
Sources
- Battery University BU-403 — Charging Lead Acid
- Battery University BU-802 — What Causes Capacity Loss
- Battery University BU-903 — How to Measure State of Charge
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.