On a boat, running out of battery isn't waiting for a tow: it's being left without radio, depth sounder, navigation lights or bilge pump. That's why marine electrics separate batteries by function — and choosing the wrong function is costly.

⚡ The essentials in 20 seconds

  • Two different jobs: starting the engine and house (electronics).
  • For the house circuit you need deep-cycle: AGM, gel or lithium.
  • The marine environment rules: vibration, heeling and salt.
  • Size the bank by real consumption × hours, then double (the 50% rule).

Starting vs house: two incompatible jobs

  • Starting battery: thin plates, lots of surface, brutal current for two seconds to start the engine. It hates deep discharges.
  • House battery (fridge, sounder, plotter, radio, lights, windlass): thick plates, slow, deep discharge over hours. A starting battery used this way dies in a season — you need a deep-cycle.
  • With a single engine and minimal equipment, "dual purpose" batteries exist, a reasonable compromise only for small craft.
⚠️

Using a single battery for starting and for the house is the recipe for being left without a start after an afternoon at anchor with the fridge and music. Separating the circuits (with a splitter or a battery panel) is the first thing any technician recommends.

The marine environment changes the rules

  • Vibration and slamming: rule out fragile-grid batteries; AGM and gel are fixed and sealed, and stand the pounding far better.
  • Heeling: an open battery lists and can spill acid; AGM and gel work at any reasonable angle.
  • Salt humidity: terminals protected with dielectric grease and watertight boxes aren't optional; salt corrodes connections in one season.
  • Winter lay-up: six months parked without a maintainer is half a life less. Disconnect, charge to 100% and check every two months.

Sizing the house bank

Add up the real consumers on board (sounder + plotter ~2 A, fridge 3-4 A, navigation lights, VHF radio…), multiply by the hours of use between charges and double the result, because a lead battery only comfortably delivers 50% of its capacity. A typical weekend of 90 Ah consumption calls for a 180-200 Ah bank in AGM/gel — or less in lithium, if weight and space matter.

Lithium on board?

LiFePO4 wins on weight, space and cycles, and delivers almost all its capacity; that's why it triumphs on cruising yachts and boats where every kilo counts. In return, it requires a compatible charger and alternator, and protection for low temperatures. For a recreational boat used in summer, a good AGM or gel remains the simplest and most economical option.

How to separate the circuits on board

The key to never losing your start is for the house and starting batteries to work separately. It's achieved with a charge splitter (lets the alternator charge both but stops them discharging into each other) or a battery panel with a selector. That way, even if you spend the afternoon at anchor draining the house battery with the fridge and music, the starting one stays intact to get back to port.

Lithium on board: requirements

LiFePO4 is tempting for weight and cycles, but it has homework: a compatible charger and alternator (or a DC-DC regulator), a BMS that protects against overcharge and against charging below 0 °C, and a bigger upfront budget. On cruising yachts and boats where every kilo counts, it pays off; on a summer-use motorboat, a good AGM or gel is simpler and cheaper.

Common mistakes in marine use

  • A single battery for starting and house: you're left unable to return to port.
  • Open batteries on a boat that heels: risk of acid spill.
  • Ignoring the salt: corroded terminals in one season.
  • Sizing the house bank tight, with no margin for a long night.

Frequently asked questions

Can I use one battery for both starting and the house?

It's not advisable except on very small craft with minimal equipment. The safe approach is to separate the circuits, so the house load doesn't leave you without a start to get back to port.

Which battery best withstands vibration and salt?

AGM and gel: they're sealed, fixed and work at any reasonable angle, unlike open batteries, which can spill acid and suffer more from pounding.

Is lithium worth it on a recreational boat?

If you want minimal weight and many cycles (cruising yachts, intensive use), yes; for a summer-use motorboat, a good AGM or gel is simpler and cheaper.

How do I care for the boat battery in winter?

Charge it to 100%, disconnect it and, if you can, leave a maintainer connected or check the charge every two months. Six months of neglect is half a life less, especially on lead batteries, which self-discharge and sulphate if left half-charged.

Need help choosing for your boat?

Tell us the length, the equipment on board and the use, and our technical team replies with the recommendation and lead time in under 4 working hours.

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