Batteries on board: why a bank dies in a single season

A boat battery lives, on average, half as long as it could. The cause is almost always the same one, and it isn't battery quality.

Let's go through what kills it, how much capacity you really need, and how to tell a tired bank from a dead one.

What actually kills a battery

Deep discharge

A lead battery doesn't like going below half. Discharge it to thirty percent once, and you've lost part of its capacity for good. Do that twenty times in a season, and the bank is finished.

This leads to a simple consequence that surprises a lot of people: out of a hundred-amp-hour battery, you actually have fifty available. Anything below that you're borrowing from next season.

Chronic undercharging

The second cause is less obvious. The engine runs at anchor for an hour, the alternator delivers most of the charge quickly, and the last few percent trickle in slowly. As a result, the battery spends months living at eighty percent charge.

For lead, this is a slow death: sulfate builds up on the plates and stops dissolving. The battery still seems to work, but its capacity keeps shrinking.

Heat

Every ten degrees above twenty-five cuts service life in half. A battery in an unventilated engine compartment ages faster than an identical one in a cool locker.

Wintering in a discharged state

Discharged electrolyte is almost water, and it freezes at minus ten. Charged electrolyte withstands minus forty. More on this in the article on winterising.

Lead or lithium: what it means in practice

Arguing this in the abstract is pointless, so here's what actually changes on board, point by point.

Available capacity. For lead, it's half the rated figure. For lithium, about ninety percent. That means a hundred-amp-hour lithium bank replaces a two-hundred-amp-hour lead one.

Weight. Lithium is roughly three times lighter for the same usable capacity. On a small boat, that's noticeable.

Charge speed. Lead takes the last twenty percent slowly and reluctantly. Lithium accepts charge at full power almost to the end. In practice, this means an hour of engine running is actually enough, not just "sort of topped up."

Service life. Lead is three to five years with careful handling. Lithium is ten years or more, provided it isn't frozen or overheated.

Price. Lithium costs two to three times more upfront. Over cost per year of service, the gap is far less dramatic.

What lithium requires. Its own battery management system, and usually a different charger and a different alternator charging profile. You generally can't drop lithium in place of lead without touching anything else. This is the main source of hidden cost when switching.

How much capacity you actually need

The calculation is simple, no textbook formulas needed.

1. List everything that draws current and how many hours a day it runs: fridge, lighting, autopilot, instruments, phone charging, the bilge pump.
2. Multiply consumption by hours and add it up. That gives you daily draw in amp-hours.
3. Multiply by the number of days between charges.
4. For lead, multiply by two again. For lithium, by one and one tenth.

A common mistake is forgetting the fridge. It runs around the clock and in hot weather accounts for more than half of total draw.

The second mistake is sizing by an appliance's rated power rather than its actual work. An autopilot in calm water and the same autopilot in a fresh breeze draw very different amounts.

How to tell a bank is already dead

Voltage under load. A fully charged lead battery at rest reads around 12.7 V. If it shows 12.2 or lower after a night, either the charge is gone or the capacity no longer exists.

Quick to rise, quick to fall. A battery that "fully charges" in ten minutes and drains in an hour is the classic sign of a dead bank. It's only accepting a surface charge.

Mismatch between banks in a bank set. If one battery in a pair runs hotter or shows a different voltage, it's dragging the other one down with it. Replace both.

Swollen case, smell, leaks. Conversation over — replace it immediately.

Rules that double service life

• Keep the starting and house batteries separate. The starting battery delivers a large current briefly; the house battery delivers a small current for a long time. One battery covering both roles does neither job well.
• Charge to full, not to "good enough." Once every couple of weeks, let the battery reach a hundred percent from shore power or the generator.
• Don't mix different batteries in one bank. Different age, different type, different capacity — the weakest one drags the rest down.
• Keep the terminals clean. Corrosion on a terminal creates resistance that keeps a battery from ever fully charging.
• Fit a battery monitor. A voltmeter tells you little; an amp-hour counter tells you the truth. It's the one instrument that pays for itself right away.

What to take with you when it's time to replace

Batteries have strict limits on dimensions and terminal placement — the box on board rarely has spare room.

• The full battery nameplate — type, capacity, cold cranking amps.
• The battery in its box, so you can see which side the terminals are on and how much clearance there is around it.
• Measurements of the box — length, width, height, allowing for terminals and the lid.
• The charger and its settings, if a switch to lithium is planned: it usually needs replacing too.

With this information, a compatible replacement can be found even when the original model is discontinued — batteries come in a long range of sizes, and a substitute is almost always available.

In short

What kills a battery is deep discharge and chronic undercharging, not age. Count available capacity honestly: for lead, that's half of what's printed on the case. Separate the starting and house batteries, fit an amp-hour counter, and charge to full every two weeks. That's enough for a bank to serve out its full life instead of half of it.