Replacing boat batteries is the moment to check that the bank is the right type and size for how you use the boat, that it is securely mounted, isolated and fused, and that every charger on board is set for it. We supply and fit replacement batteries, from like-for-like AGM swaps to Fogstar LiFePO4 upgrades, and set up charging to match.
Batteries rarely fail overnight. The usual signs are a bank that no longer lasts the evening, voltage that drops sharply when the fridge or anchor windlass starts, a slow crank on the engine, or a battery monitor whose readings no longer make sense. Our guide on when to replace boat batteries goes through these signs in detail.
Before you order new batteries, it is worth asking why the old ones gave up early. Chronic undercharging, deep discharges and incorrect charge settings shorten battery life, and a new bank will go the same way unless those causes are fixed.
The simplest option is to replace flooded, AGM or gel batteries with the same type and capacity. That keeps existing charge settings valid and is sensible if the boat's use has not changed and the old bank lasted a reasonable time.
If you want more usable capacity, faster charging or less weight, lithium iron phosphate is worth considering. It is a bigger change, though: alternator protection, charger profiles and battery management all need attention. Our lithium upgrades page and the LiFePO4 versus AGM guide explain the differences.
Rather than simply matching what was there, we can size the bank from an energy budget: what you run, for how long, and how many days between charges. Lead-acid capacity should be counted at roughly half its rating for usable energy, while lithium can deliver most of its rated capacity. The result may be a smaller lithium bank or a larger lead-acid one than you expected.
Batteries are heavy and store a lot of energy, so installation details matter:
Every charging source, including the shore charger, alternator regulator or DC-DC charger, solar controller and any inverter-charger, needs settings that match the new batteries. We check and set each one, and configure battery monitoring so the state of charge you see is accurate. We supply and install Victron chargers and monitors where an upgrade makes sense.
Battery banks can deliver enormous short-circuit currents. A dropped spanner across the terminals can weld itself in place and start a fire. Always isolate and disconnect before working on battery connections, remove metal jewellery, and cover terminals. Lead-acid batteries can release explosive hydrogen while charging, so keep sparks and flames away from battery compartments.
The engine start battery has a different job from the domestic bank: it delivers a short, heavy current to crank the engine and is then recharged straight away. It should be a battery designed for starting, kept separate from the domestic bank so that a night of lights and fridge cannot leave you unable to start. When we replace a domestic bank we also check the start battery and the arrangement that charges both, whether a split-charge relay, a DC-DC charger or a dedicated output on the charger.
Signs include short run times, sharp voltage drops under load and slow engine starts. Testing under load confirms it; our guide covers the signs in detail.
It is best avoided. Batteries in a bank should be the same type, capacity and age, or the weaker ones drag the others down.
Yes. We supply and fit batteries, including Fogstar LiFePO4, and set the charging system up to match.
Not always, but its settings must suit the new battery type. Switching to lithium often means changes to alternator charging too.
Tell us what is fitted and how you use the boat, and we will recommend the right replacement.