Boat Lithium Battery Not Charging: Troubleshooting Steps

Equipment and information required

Confirm the source is actually producing
Mains charger powered and not in a fault state, solar controller showing PV input, engine running for a DC-DC charger with its ignition trigger live. Start here, because it is most often the answer.
Check the charge profile and voltages
A charger still set to a lead-acid profile may reach a voltage the bank will not accept a meaningful current at. Absorption set too low is the classic cause of a bank that charges to 90% and stops.
Read the BMS for a cut-off reason
Low temperature, high cell voltage or a cell imbalance will all stop a charge. The pack usually reports which, and that is faster than any measurement you can take.
Measure voltage at the charger and at the bank
A large difference under charging current means voltage drop in the cable or a poor connection. The charger reaches its target voltage at its own terminals and stops, while the bank never gets there.
Check the fuses and the shunt connections
A blown fuse on the charging run, or a loose connection at the shunt, produces exactly this symptom. Measure across each in turn rather than inspecting visually.
Verify the monitor is not lying to you
A monitor that never synchronises can show a bank stuck below 100% that is actually full. Check the charged-voltage and tail-current settings before chasing a charging fault that does not exist.

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