What battery type and capacity should I choose for an electric boat motor?

For most new installations, choose a lithium iron phosphate battery (LiFePO4) approved for your motor, or the manufacturer's matched lithium pack if the system requires one. Calculate capacity in kilowatt-hours by multiplying electrical consumption by running time, then account for the battery's usable capacity and an arrival reserve.

Check motor compatibility

Start with the motor's installation manual. Check battery voltage, permitted voltage range, maximum electrical input and battery requirements. A battery may hold enough energy yet be unsuitable because its voltage is wrong or its battery management system (BMS) cannot supply the required current.

• Voltage: match the complete operating range, including fully charged battery voltage. A nominal 24 V or 48 V label is not enough.
• Current: check continuous discharge capacity and any specified short-duration demand. The BMS must supply the motor without tripping.
• Communication: some motors require an approved battery and a data connection to operate or provide accurate range information.
• Charging: confirm that the mains charger, solar controller and any alternator charging arrangement suit the battery chemistry.

A 3 kW electrical load draws approximately 62.5 A at 48 V. Current rises as voltage falls. Select the battery, cables and protection to meet the manufacturer's maximum-current specification, rather than relying on this nominal calculation.

Choose the battery chemistry

LiFePO4 is usually the practical choice where separate propulsion batteries are permitted. It weighs substantially less than lead-acid and provides more usable energy. Choose a pack with supplier support, a BMS, documented discharge ratings and protection suited to its installation location. Charging below freezing generally requires low-temperature protection or an approved heating arrangement.

Manufacturer-supplied lithium packs may use another lithium chemistry. Use the specified system where required. An integrated dinghy outboard battery cannot automatically be replaced with a generic LiFePO4 pack.

AGM, gel and flooded traction batteries remain workable where the motor permits them, particularly if you already own a sound bank and make only short trips. Their weight and lower usable capacity usually make them less attractive for a new installation used regularly. Never choose ordinary engine-starting batteries for repeated propulsion discharge.

For conventional marine lead-acid batteries, plan around approximately 50% discharge for reasonable battery life. Specialist traction batteries may permit deeper discharge, sometimes up to 80%; follow their specifications. Heavy current also reduces the energy lead-acid batteries deliver compared with their advertised capacity.