How often should you start a car in storage?

If a stored car's battery is kept on a conditioner, it doesn't need starting at all for the battery's sake. If you do start it, once a month is enough, provided the engine runs until it is fully up to temperature. The routine to avoid is a quick start every week, with the engine switched off again after a few minutes. That puts moisture into the engine and exhaust without driving it back out, and it takes more charge from the battery than a few minutes of running puts back.

A monthly run to temperature is how our Storage Type C package works. The car stays connected to its owner's battery conditioner, the engine is started once a month and run until it is properly warm, and because there is a private road on site, engines can be run without a car going out onto a public road. The reasoning behind that routine applies just as much to a car kept in a garage at home.

Why a quick start does more harm than good

Burning petrol or diesel produces water vapour as well as heat. In a cold engine, some of that vapour condenses on the cold metal inside the engine and along the exhaust, and it is only driven off once everything has been hot for a while. Start a cold car, let it idle for three or four minutes and switch it off, and the water stays where it formed. Some ends up in the oil, and some sits in the exhaust, which can then start to rust from the inside. The creamy residue owners sometimes find under the oil filler cap of a car that only does short trips is often that same moisture mixed with oil.

The battery doesn't gain from a short start either. Turning an engine over takes a heavy burst of current, and the alternator needs time to replace it. The RAC has warned that five to ten-minute journeys are too short to recharge a battery properly, and that starting a car occasionally and leaving it running for a few minutes may end up draining a weak one.

Once a month, all the way up to temperature

In our experience, a monthly start that runs the engine all the way up to temperature works better than a quick start every week, because a short start leaves its moisture behind and takes charge out of the battery, while a full warm-up burns the moisture off before the engine is switched off.

Up to temperature means more than the needle lifting off the bottom of the gauge. The coolant warms first and the oil takes longer, so let the engine carry on running for a while after the temperature gauge has settled at its normal position. Let it idle gently while it warms rather than revving it, and keep an eye on the gauges and warning lights while it runs. On a cold winter morning an idling engine can take a good while to warm right through.

The AA's laying-up advice for a laid-up car is to start it once a week and let it run for about 15 minutes to keep the battery charged, or to connect the battery to a mains-powered battery maintainer instead. Both of those deal with the battery. Once a conditioner is doing that job, starting the car is about the engine, and a longer, less frequent run suits the engine better than a short, frequent one.

What a monthly run tells you

Running an engine once a month also brings problems to light while there's time to deal with them. A warning light, a weep of coolant or a rough idle found in December is far easier to sort out than the same fault found on the morning you want to take the car out.

Our three packages differ on exactly this. On Storage Type A, at £30 a week, the battery is disconnected or removed by the owner and the engine isn't run. Type B, at £35 a week, keeps the car on the owner's conditioner. Type C, at £40 a week, adds the monthly run to temperature. Over a 26-week winter from October to early April, the monthly runs on Type C cost £130 more than Type B. Every package has a minimum term of three months, and the full comparison shows exactly what each one includes.

If the car has a SORN

A car with a SORN can't be used on a public road until it has been taxed again, other than to go to or from a pre-booked MOT or other testing appointment, as GOV.UK explains. Running the engine is a different matter. A SORN is about public roads, and a car kept on private land can still be started and run there, which is one reason a private road on site is useful for cars laid up with a SORN.

Deciding between a SORN and staying taxed is one of the first jobs when preparing a car for winter storage, because it decides whether the car can go out on the road during the winter at all.

Starting a car stored at home

If the car is in your own garage, a few rules keep a monthly start safe and worthwhile. Never run an engine in a closed garage. Exhaust gas contains carbon monoxide, which you can't see or smell, so open the garage doors fully before starting, or run the car outside on your drive. Check the oil and coolant levels before the first start after a long spell, which the AA also recommends before a laid-up car is used again.

If you can't let the engine run long enough to get it properly warm, leave it off and keep the battery on a conditioner instead. A car that isn't started at all over a winter is usually better off than one started for five minutes every weekend, because it avoids the moisture and the drain on the battery.

Electric cars

An electric car has no engine to warm up, so the question becomes its 12-volt battery, which runs the locks, the alarm and the car's computers and can go flat like any other. The AA suggests pressing the start button of an electric car for 10 minutes once a week to keep that 12-volt battery topped up. A maintainer on the 12-volt battery does the same job without anyone needing to visit the car. Check the handbook as well for the manufacturer's advice on how much charge to leave in the main battery before a long lay-up.

Booking a space with monthly engine runs

We look after modern and classic cars of every kind in a dry, alarmed building that is manned seven days a week, and every car goes under a cotton cover with its tyres rolled regularly. If you'd like your car kept on its conditioner and run up to temperature every month through the winter, get in touch to book a space and choose Storage Type C. We need 24 hours' notice whenever you want the car back, and a WhatsApp message is the quickest way to ask.