Boat Battery Replacement

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.

Call · Get a quote

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.

Like-for-like or a different type?

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.

Sizing the replacement bank

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.

Fitting batteries safely

Batteries are heavy and store a lot of energy, so installation details matter:

Setting up charging for the new bank

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 safety

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.

Engine start batteries

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.

Why a boat battery is not charging

Before replacing a battery, it is worth knowing it is really the battery. A bank that will not hold a charge may be worn out, but the same symptom comes from a charger set to the wrong profile, a slipping alternator belt, a failed split-charge relay, corroded terminals or a small drain left on while the boat is unattended. If the battery is not charging while the engine is running, the alternator and its charging link are checked first. We test the battery and its charging together, so you only buy new batteries when they are needed.

Boat battery not charging · Boat battery charging

Chargers for the new bank

A new bank deserves the right charging. A marine battery charger on shore power, a battery to battery charger from the alternator and a solar controller should all be set to the new battery type. Where the boat moves to lithium, we supply and fit Victron charging equipment and Fogstar LiFePO4 batteries. For a dedicated bow thruster or windlass battery, the type is chosen for high short-term current and charged from the main system.

Lithium battery upgrades

Frequently asked questions

How do I know my boat batteries need replacing?

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.

Can I mix old and new batteries in one bank?

It is best avoided. Batteries in a bank should be the same type, capacity and age, or the weaker ones drag the others down.

Do you supply the batteries?

Yes. We supply and fit batteries, including Fogstar LiFePO4, and set the charging system up to match.

Do I need to change my charger when I replace batteries?

Not always, but its settings must suit the new battery type. Switching to lithium often means changes to alternator charging too.

How do you replace a boat battery?

Isolate the system, disconnect the negative first, remove and replace the battery, secure it, reconnect positive then negative, and check the charger settings suit the new battery. Larger banks need cable and fuse checks too.

Why is my marine battery not holding a charge?

It may be at the end of its life, have been left discharged, or be drained by a small load. Testing the battery and checking for a drain tells you which.

Why is my boat battery not charging while running?

Common causes are the alternator, its belt or wiring, or a failed split-charge relay or DC to DC charger between the alternator and the domestic bank.

Batteries not lasting?

Tell us what is fitted and how you use the boat, and we will recommend the right replacement.

Plan the electrical work aboard