The short answer: replace boat batteries when a proper capacity check shows they no longer hold enough to cover your use, not simply because they are a certain age. Short run times, rapid voltage collapse under load and a bank that will not reach full charge are the usual signs. Test first, then decide between like-for-like and an upgrade.
Replacing a bank is also the moment to fix whatever shortened the old one's life. Otherwise the new batteries inherit the same fate.
The clearest sign is capacity: a bank that used to last a weekend now struggles through an evening. Voltage that looks fine at rest but drops sharply as soon as a pump or windlass runs is another. Lead acid batteries that swell, get hot on charge or need constant topping up are past their best.
Plenty of banks condemned by owners turn out to be fine batteries let down by poor charging or a hidden standing load. A resting voltage check after full charge, a load test and a look at the charging voltages will separate the two. Spending on new batteries before that is often money wasted.
Chronic undercharging is the big one. Lead acid batteries left partially charged for weeks sulphate and lose capacity permanently. Alternators with simple regulation, chargers set for the wrong chemistry and deep overnight discharge all contribute. A mixed bank of old and new batteries also drags the new ones down.
If the old bank died young, find out why before fitting the replacement. The answer is usually in the charging system.
Replacing lead acid with lead acid is cheapest upfront and needs no changes to charging. Moving to LiFePO4 costs more but gives far more usable capacity for the weight, tolerates deeper cycling and holds a steady voltage. It does, however, require charge sources and protection set up for the chemistry.
As Fogstar approved installers with a LiFePO4 specialism, we will talk through both honestly. For a boat used lightly on a mooring, a good lead acid bank can still be the sensible choice.
At minimum, charger and alternator settings should match the new batteries. With lithium, a battery management system, suitable alternator protection and correctly rated fusing become essential. It is also worth adding a battery monitor if you do not have one, so you can see how the new bank is actually used.
Battery banks store a great deal of energy. A dropped spanner across terminals can weld itself in place and start a fire. Terminals must be protected, cables correctly sized and fused close to the battery, and batteries secured against movement in a seaway.
Safety note: changing chemistry or rewiring a bank is work for a qualified marine electrician. A like-for-like swap is simpler, but the fusing and securing still need to meet marine standards.
A replacement is a chance to match the bank to how you now use the boat. Work out daily consumption from your actual loads rather than copying the size of the batteries being removed. If the old bank was always too small, fitting the same again repeats the problem; if it was oversized for a boat that rarely leaves the berth, a smaller bank may be perfectly adequate.
Consider the space and weight available, too. Lithium's lighter weight and greater usable capacity can allow a meaningful upgrade in the same footprint, whereas extra lead acid capacity often needs more room than the battery box offers.
The engine start battery has a different job from the house bank: short, heavy bursts of current rather than long steady discharge. It is usually kept on a traditional lead acid chemistry designed for cranking and charged alongside the house bank. When a start battery struggles, the cause is often a charging or connection problem rather than the battery itself.
Keeping start and house banks separate, with appropriate charging to each, means a flat house bank after a night at anchor never stops you starting the engine. If your boat lacks that separation, it is worth adding when the batteries are replaced.
It varies widely with chemistry, charging and use. Good charging extends life far more than the brand of battery does.
Generally no for lead acid. Mixing new and old batteries in one bank tends to shorten the life of the new one.
Not always, but it must have a suitable LiFePO4 profile. Alternator charging usually needs attention too.
We will test the bank and charging properly before recommending anything. Free assessment, written quote for any replacement.