How do I choose the right deep-cycle battery for my boat?

Choose a genuine deep-cycle battery that meets your daily electrical demand, leaves a sensible reserve and suits your boat's charging equipment. Check the shore charger, alternator and solar controller first, because a change of battery chemistry can require changes throughout the charging system.

Calculate your usable capacity

List each domestic load, its current draw and its daily running time. Multiply amps by hours to get amp-hours (Ah), then add the figures together. For a cycling fridge, count compressor running time rather than all 24 hours.

• Fridge: 4 A for eight accumulated hours = 32 Ah.
• LED lights: 1 A for five hours = 5 Ah.
• Instruments: 2 A for six hours = 12 Ah.
• Pumps and charging small devices: allow 6 Ah.

That adds up to 55 Ah a day. Add roughly 20% for uncertainty and small overlooked loads, giving a planning figure of about 66 Ah per day.

For ordinary marine lead-acid batteries, plan to use around 50% of nominal capacity between charges. Two days at 66 Ah calls for approximately 264 Ah nominal capacity, so a 270 to 300 Ah bank is a practical starting point. Some purpose-built traction batteries tolerate 80% discharge, but that allowance does not apply to every battery labelled "marine".

For lithium iron phosphate, usually called LiFePO4, planning around 80% usable capacity gives approximately 165 Ah for the same two days. Check the manufacturer's permitted discharge range. Your charging sources must also replace what you consume. A bigger bank only postpones a persistent energy deficit.

Choose your battery chemistry

• Flooded lead-acid: usually the lowest purchase cost and a reasonable choice for occasional use. Models with access to the cells need electrolyte checks and topping up with distilled water. They are heavy and must be installed upright.
• AGM: needs no routine watering and is useful where battery access is awkward. Choose a genuine deep-cycle model; AGM construction alone does not establish cycling performance. Repeated undercharging still shortens its life.
• Gel: suitable for cycling when correctly charged, but pay particular attention to charging voltage and current limits. An unsuitable charger can permanently damage it.
• LiFePO4: much lighter, with more usable energy and typically longer cycle life. It suits frequent anchoring and regular cycling, provided charging, protection and installation are compatible.

Compare cycle-life figures at the same depth of discharge and temperature. A headline figure measured at shallow discharge is not directly comparable with one measured at 80% discharge. Check warranty conditions and access to technical support too.