How long does a boat battery last?
I've just bought a 2015 Beneteau Oceanis 38 and the house batteries look original, but they still seem to hold a charge. How long do marine batteries actually last, and what signs should I look for to know they are dying before everything goes dark at anchor?
Answer
A standard marine lead acid or AGM battery typically lasts between three and five years, while a modern lithium iron phosphate (LiFePO4) bank can easily exceed ten years. The exact lifespan depends entirely on how deeply you discharge them and whether they are left sitting flat over the winter.
Lifespans by battery chemistry
Your replacement schedule depends on what sits inside your battery box.
- Flooded lead acid (FLA): Expect 3 to 5 years. These are the traditional £100 to £150 110Ah leisure batteries from brands like Numax or Varta. They require regular topping up with distilled water and die quickly if deeply discharged.
- AGM (Absorbent Glass Mat): Expect 4 to 7 years. Costing around £200 to £300 for a 100Ah unit from Victron or Trojan, they handle vibration and deep cycling better than flooded batteries.
- Gel: Expect 5 to 8 years. They handle slow, deep house loads well but die quickly if your alternator pumps out more than 14.1V to 14.4V.
- Lithium (LiFePO4): Expect 10 to 15 years. They cost £500 to £1,000 for a drop-in 100Ah replacement from Relion or Battle Born. They offer 3,000 to 5,000 cycles compared to 300 to 500 cycles for lead acid.
What kills marine batteries early
Batteries rarely die a natural death. Boat owners usually kill them. The most common cause is sulphation. Leave a lead acid battery partially discharged for weeks on a swinging mooring, and lead sulphate crystals harden on the plates to permanently reduce capacity.
Deep discharging is another killer. Taking a standard flooded or AGM house bank below 50 percent state of charge (around 12.2V at rest) repeatedly shortens its life. Overcharging from a cheap shore power charger or a faulty alternator regulator boils the electrolyte away. Always ensure your battery compartment is well ventilated, as charging lead acid batteries produces explosive hydrogen gas.
Heat also destroys batteries. Mount your house bank inside a hot engine bay right next to a diesel engine running at 80°C, and expect its lifespan to halve.