Can I charge a boat battery while it is still connected?

Yes, you can usually charge a boat battery while it remains connected, provided the charger is regulated and set correctly for the battery. A permanently installed marine charger is designed for this; with a portable charger, check its instructions and avoid repair or equalisation modes while boat equipment remains connected.

Check the battery and charger

Read the battery label before connecting anything. Check its voltage and whether it is flooded lead-acid, AGM, gel or lithium iron phosphate (LiFePO4). The wrong charging profile can damage the battery, and excessive voltage can damage connected electronics too.

For a typical 12V lead-acid battery at around 20 to 25°C, absorption voltage is commonly 14.2 to 14.8V and float voltage around 13.2 to 13.8V. These ranges are a reference, not a menu of settings. Gel batteries often require a lower maximum voltage than some AGM batteries, so follow the battery manufacturer's figures.

• Flooded, AGM or gel: select the specified profile and use battery temperature compensation where supported.
• LiFePO4: use the battery maker's approved settings, a functioning battery management system and suitable low-temperature charging protection; many batteries prohibit charging below 0°C.
• Older unregulated chargers: avoid leaving these attached to a connected boat system because their output may rise excessively as the battery reaches full charge.

A 10A smart charger suits many 100Ah lead-acid batteries, but the battery's permitted charging current takes precedence.

Connections and the battery isolator

For a portable charger on a conventional negative-earth installation, follow this sequence unless its manual specifies otherwise:

1. Switch off the charger and disconnect its mains supply. Turn off unnecessary boat loads.
2. Inspect the battery for cracks, leakage, swelling, loose terminals or damaged cables. Do not charge a visibly damaged battery.
3. Connect positive to the intended battery bank's positive terminal or designated charging point.
4. Connect negative to the designated negative charging point. If a battery monitor uses a shunt, connect on the system side of that shunt so it records charging current.
5. Select the correct profile, then connect and switch on the mains supply.
6. When finished, switch off and unplug the charger before removing negative, then positive.

Ventilate the battery compartment, keep sparks and flames away, and use a dry, RCD-protected shore-power supply.

The battery isolator can usually be off when the charger connects directly to the battery. This disconnects most domestic equipment while allowing charging. If the charging connection is downstream of the isolator, the switch must remain on for current to reach the battery. Automatic bilge pumps are often wired separately, so check that yours still operates before leaving the boat.