Short answer
Most leisure boats have two separate circuits: a starting battery that only starts the engine and a house battery for navigation lights, instruments, fridge and pump. Keep them separate with a main switch or charge splitter so you can always start the engine. Measure consumption in amp-hours (Ah) and size the battery bank accordingly.
Battery types
| Type | Pros | Cons |
|---|---|---|
| Flooded lead-acid | Cheap, robust | Shouldn't be deeply discharged, may need topping up with water |
| AGM/gel | Maintenance-free, doesn't leak | Sensitive to incorrect charging voltage |
| Lithium (LiFePO4) | Light, handles deep discharge | Expensive, requires a BMS and suitable charging |
If you mix types, the charger and alternator need to be set up for each bank.
Calculate what you need
- List all consumers and their current draw in amps.
- Multiply by the number of hours per day. A 4 A fridge running half the time uses 48 Ah per day.
- Add it up and allow a margin. Lead-acid batteries should not be discharged more than about halfway.
- A battery monitor with a shunt shows state of charge as a percentage. Voltage alone gives only a rough picture.
Troubleshooting when it won't start
- Start with the main switch. Are the terminals tight, and are they clean?
- Measure the voltage at the starter motor while cranking. If it drops sharply, the battery or cables are weak.
- Check the ground cable between engine and battery. It often corrodes.
See the troubleshooting guide for an engine that won't start and find the manual for your engine under Engines & manuals.
Cables, fuses and fire safety
12 volts sounds harmless, but high currents can start a fire. The starter motor draws hundreds of amps, and a short circuit straight across a battery can melt a spanner.
- Fuse the positive cable close to the battery, with a fuse rated for the cable's cross-section.
- Correct cable size: long runs or high current need thicker cable to keep voltage drop down. Follow the manufacturer's table for your equipment.
- Tinned cable and heat-shrink resist moisture and salt better than ordinary automotive cable.
- Secure the batteries in a box or with straps. A battery that breaks loose in a seaway is dangerous.
- Ventilation: lead-acid batteries give off hydrogen when charging. The space they sit in needs airflow.
Check every connection when you fit out in spring. Loose or corroded connections are behind most strange electrical faults on board.
Write the installation date on the battery lid. Then you know how old they are when they start to fade.
Winter and charging
Fully charge the batteries before winter and keep them on a maintenance charge, or disconnect them. A discharged lead-acid battery can freeze and crack. With shore power, a marine charger takes care of this; see shore power on your boat and winterising.
Frequently asked questions
Can I use one battery for both starting and house loads?
It works on small boats, but you risk draining it and being unable to start. Two separate batteries are safer.
Can the engine's alternator charge lithium batteries?
Not always. Lithium batteries can draw a lot of current and overheat the alternator. Use a DC-DC charger or follow the battery manufacturer's instructions.
How do I know how much charge is left in the battery?
Voltage only gives a rough indication. A battery monitor with a shunt measures current in and out and shows state of charge as a percentage.
Sources
- SmartShunt, Operation (laddstatus, spänning), Victron Energy victronenergy.com
- Elsäkerhet för privatpersoner, Elsäkerhetsverket elsakerhetsverket.se