# How to winterise your boat battery (and jet ski battery) on the Costa del Sol > Blog article — Málaga Baterías · Updated 2026-10-12 _How to winterise a boat or jet ski battery in Málaga and on the Costa del Sol: charging, disconnecting, maintenance chargers and resting voltage._ 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. Quick answer: clean and dry the terminals, charge the battery to 100% before laying up, disconnect the negative (or use the isolator switch), decide whether it stays on board or goes home with you, keep it on a maintenance charger suited to its technology and check the voltage once a month. On a 12 V lead-acid battery, if it drops below about 12.4 V at rest, it is time to recharge. 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: - Clean the terminals and dry the battery. Remove sulphate and corrosion with a brush, wipe the lid and leave everything dry. - 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. - 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. - 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. - 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. - 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: Resting voltage Approximate state What to do ≈ 12.7 V or more Full Nothing. Measure again next month. ≈ 12.4 V Half charged Recharge now, before it starts to sulphate. Below ≈ 12.0 V Flat and sulphating Full charge as soon as possible and a test: it may have lost capacity. 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. Battery didn’t make it through the winter? Bring it in for a free test No appointment needed, at C/ La Orotava, 100 (Málaga). If it needs replacing, ask for a quote: we also deliver to marinas on the Costa del Sol. Call 952 31 01 18 WhatsApp 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. … ## Buy or change this battery in Málaga Málaga Baterías — official Varta distributor. No appointment needed. Free quote and recycling of your old battery. - **Phone:** [952 31 01 18](tel:+34952310118) - **WhatsApp:** [+34 952 31 01 18](https://wa.me/34952310118) - **Address:** C/ La Orotava, 100, 29006 Málaga - **Directions:** https://maps.google.com/?q=Málaga+Baterías+C/+La+Orotava+100+Málaga ### Areas we cover Málaga city: Centro, Teatinos, El Palo, Pedregalejo, Ciudad Jardín, Cruz de Humilladero, Carretera de Cádiz, Churriana, Campanillas, Puerto de la Torre, Bailén-Miraflores, Palma-Palmilla. Costa del Sol: Torremolinos, Benalmádena, Fuengirola, Mijas, Marbella, Rincón de la Victoria, Vélez-Málaga, Torrox, Nerja, Alhaurín. --- [See the full product page](https://malagabaterias.com/en/blog/winterising-boat-jet-ski-battery-costa-del-sol)