How to winterise your boat battery (and jet ski battery) on the Costa del Sol

In Málaga and on the Costa del Sol the winter is mild: a boat battery here doesn’t die of cold. It dies of boredom. Three, four or five months tied up in port, in a dry stack or in the garage at home, with the battery discharging by itself, some permanent load pulling on it and the salt air working on the terminals. This guide explains how to winterise your boat battery and your jet ski battery so they start first time in spring.

What happens to a battery that sits unused

A battery doesn’t need to be used to wear out. Left standing, three things happen to it at once:

• Self-discharge. Every battery loses charge by itself even with nothing connected. Flooded lead-acid batteries lose the most, AGM and gel considerably less, and lithium very little. Heat speeds up self-discharge, so our mild winter doesn’t slow it down the way a cold climate would.
• Sulphation. When a lead-acid battery spends weeks half charged or flat, sulphate crystals form on the plates and harden. That capacity never fully comes back. It is the most common reason for the battery that “was fine in October” and won’t start in April.
• Standing loads. The automatic bilge pump, the alarm, the radio memory, a tracker or a gas detector draw very little, but they do it around the clock for months. Added together, they drain a battery long before self-discharge does.

Add to that the salt air. Salty moisture settles on the lid and the terminals, corrodes them and can create small current leaks between the posts. On a boat that sleeps by the sea, cleaning and drying is part of the maintenance.

How to winterise a boat battery step by step on the Costa del Sol

The procedure is the same for a starter battery, a service battery or a dual-purpose one. Do it in this order:

1. Clean the terminals and dry the battery. Remove sulphate and corrosion with a brush, wipe the lid and leave everything dry.
2. Charge it to 100% before laying up. The last trip under engine is not enough: use a charger and let it finish the full cycle. A battery stored full lasts for months; stored half charged, it sulphates.
3. Disconnect the negative or use the isolator switch. That cuts the standing loads. If you remove the cable, always undo the negative first and insulate the terminal so it can’t touch anything.
4. Decide whether it stays on board or goes home. It depends on whether the boat is afloat, whether you have shore power and how heavy the battery is. More on this below.
5. Fit a maintenance charger suited to its technology. A smart charger with the right mode (flooded, AGM, gel or lithium) keeps it full without overcharging. An old unregulated charger left connected for months will ruin it.
6. Check the voltage once a month. With a multimeter, after several hours with no charge and no load. If you have no maintenance charger, recharge as soon as it drops below about 12.4 V.

On board or at home?

If the boat stays in the water, the automatic bilge pump must remain powered: never leave it without supply. In that case the battery stays on board, with every other load switched off and, if the berth has shore power, on a maintenance charger. If the boat is in a dry stack or on a trailer with no socket nearby, the practical option is usually to take the battery home: store it somewhere cool, dry and ventilated, away from sparks and flames, and charge it there.

Resting voltage: what the battery is telling you

Resting voltage is the simplest way to know the state of a 12 V lead-acid battery. Measure it with the battery disconnected and rested:

These are guide values for 12 V lead-acid: they shift slightly with temperature and battery type (a full AGM usually reads a little higher). With lithium the reading is different: the voltage stays almost flat, above 13 V, for much of the discharge, so a multimeter is no use for estimating charge. There you rely on the battery monitor or the manufacturer’s app.

Differences by battery technology

Flooded lead-acid (with caps)

This is the type that needs most care. Check the water level: the plates must be covered. If it is low, top up with distilled water only and, ideally, after charging, without going over the mark. Always charge in a ventilated place, because it gives off gas while charging.

AGM and gel

These are sealed: they are never topped up or opened. They lose less charge when idle and cope better with months of inactivity, but they sulphate just the same if stored half charged. The charger is what matters: it must have an AGM or gel mode, because too high a charging voltage damages them for good.

Lithium (LiFePO4)

Here the rule changes. A lithium battery is not stored at 100%: the usual practice is to leave it partly charged, at the percentage the manufacturer gives in the manual. Disconnect every load, because its internal electronics also draw a little, and never leave a lead-acid charger connected. And one general rule: lithium must not be charged below freezing. On the Málaga coast those temperatures are rare, but bear it in mind if you store the boat inland. There is more detail in lithium on board: when it pays off.

Jet ski: the same, with less margin

The procedure is identical, with one difference: a jet ski battery is small, just a few amp-hours, and it runs down much sooner. The upside is that it is light: take it out, bring it home and leave it on a maintenance charger for small batteries (a high-current car charger is not the right tool). To find out which type yours takes, see the guide to jet ski and personal watercraft batteries.

Before you go back on the water

A couple of weeks before the first outing, give the battery a full charge, let it rest and measure. Clean and tighten the terminals, connect the positive first and then the negative, and start the engine at the berth, not once you are outside the harbour mouth. If in doubt, bring the battery to our shop at C/ La Orotava, 100, in Málaga: the test is free, with no appointment, first come first served, and we will tell you whether it has another season in it.

If it doesn’t recover, it needs replacing. In the boat battery catalogue you will find Varta and Optima models such as the Varta LA70 12V 70Ah 760A (dual-purpose AGM), the OPTIMA BTDC-5.0 12V 66Ah 765A or, in lithium, the Varta LL100 12.8V 100Ah. If you are not sure what your engine needs, start with which battery an outboard engine takes or the boat and marine battery guide. And if you can’t come in, we offer battery delivery to marinas on the Costa del Sol: delivery only, we do not fit on board. Ask for a quote by WhatsApp or by phone on +34 952 31 01 18 and check the delivery time. You can leave the old battery with us: we recycle it at no charge.