Should you disconnect your car battery for storage?

For a car going into storage for three months or more, keeping the battery connected to a smart battery conditioner is usually better than disconnecting it. Disconnecting stops the car's electronics draining the battery, but the battery keeps losing charge slowly on its own, and by the end of a winter it can be low enough to struggle or to have started deteriorating. A conditioner holds it at full charge the whole time, and the alarm, clock and settings carry on as normal.

Disconnecting still has its place. It suits older cars with little in the way of electronics, and owners who would rather take the battery home and look after it themselves. Those are the two routes our storage packages offer. On Storage Type A, at £30 a week, the battery is disconnected or removed by the owner. On Type B, at £35 a week, the car stays connected to the owner's own conditioner, and Type C, at £40 a week, adds a monthly engine run to temperature.

Why a stored car's battery goes flat

Two things drain the battery in a car that isn't being driven. The first is the car itself. Even locked and switched off, a modern car keeps powering its alarm, its immobiliser, its keyless entry and the modules that remember its settings, and a tracker adds another steady draw. The second is the battery. A lead-acid battery loses charge slowly even with nothing connected to it, which is why the RAC points out that a disconnected battery still discharges over time, and that one left unplugged and unused for a long time can be damaged beyond repair.

The damage comes from sitting part-charged. A battery left low for weeks at a time sulphates, as crystals build up on its plates and cut the amount of charge it can hold, and cold weather makes a weak battery weaker still. CTEK describes a battery maintainer as preventing exactly this, by supplying only as much charge as the battery needs, whenever it needs it.

How long can a battery be left?

For a week or two away, a healthy battery in good condition usually copes with a modern car's standby draw. Beyond that it depends on the age of the battery, the car's electronics and the temperature, and the longer the car stands, the stronger the case for a conditioner. A tracker or an alarm with interior sensors can draw more than a basic immobiliser, and an older battery has less in reserve to begin with. For three months or more, which is our minimum term, every package we offer includes one method or the other. Left connected with nothing to maintain it for that long, a battery can easily run flat.

When disconnecting makes sense

Disconnecting is a reasonable choice for an older car with no alarm, immobiliser or electronics worth keeping powered, and for an owner who is happy to take the battery out and keep it on a charger at home over the winter. With us it is also the least expensive option, at £5 a week less than Type B.

If you disconnect it yourself, the RAC's advice is to remove the negative terminal first and, when the time comes, to reconnect the positive terminal first. Keep the loose leads tucked away and insulated so they can't touch the battery posts by accident. The RAC also notes that some cars have electronic systems that take a short time to reset after the battery has been disconnected.

Expect to reset a few things when the battery goes back on. Depending on the car, that can mean the clock, the radio presets or an older radio's security code, and one-touch electric windows. Mazda's owner's handbook for the MX-5, for example, says the windows' automatic function has to be reset after the battery has been disconnected. Find the radio code and the handbook's reset instructions before you take the battery off.

If the battery has been off the car all winter, give it a full charge before it goes back in rather than relying on it to start the car straight away.

Why a conditioner is usually the better choice

The RAC's advice points the same way, recommending a battery maintainer because a disconnected battery still discharges slowly. On a modern car, a conditioner also keeps the alarm working and the car's settings intact.

A conditioner, also sold as a battery maintainer or smart charger, watches the battery's voltage and tops it up only when the charge drops. That is what makes it safe to leave connected for months. CTEK, which makes them, notes that a simple standard charger delivers a higher current to recharge a flat battery quickly and can do damage if it's left connected for too long. Names on the box aren't a reliable guide, because some smart maintainers are marketed as trickle chargers. What matters is that the unit is designed for long-term connection and suits your battery, which is worth checking if the car has an AGM or lithium battery.

On Type B and Type C the car stays connected to your own conditioner for the whole time it's with us, so bring it along when you check the car in.

Starting the engine isn't a substitute

Some owners try to keep a battery healthy by starting the car every week or two. A few minutes of running takes more out of the battery than it puts back, and it leaves moisture in the engine and exhaust. That's why the monthly engine runs on our Type C package go all the way up to temperature, and why how often you start a car in storage matters less than how long it runs each time.

The battery as part of a winter lay-up

The battery is one part of preparing a car for winter storage, alongside the tyres, the paperwork and a proper wash before the car goes in. Whichever package you choose, the car is stored indoors under a cotton cover, with someone on site seven days a week, and its tyres are rolled regularly. Every package has a minimum term of three months.

When you want the car back, give us 24 hours' notice through the check-out form, by phone or on WhatsApp. On a conditioner, the battery should be fully charged on the day you collect, so the car is ready to start without a jump or a morning on a charger.

Talk to us about your car

If you're not sure whether your car is better disconnected or kept on a conditioner, get in touch and tell us what you're storing and for how long. We store modern performance cars and classics alike at our facility just off the A21, 10 miles from Tunbridge Wells.