Leaving Your Car Parked While You Travel: Battery Care

Every September the same thing happens across Al Nahda. Families come back from six weeks abroad, press the start button in the basement and get a click. The battery was fine when they left.

A parked car is not switched off in the way a lamp is. It keeps using a little power the whole time, and summer heat does the rest.

Why a parked car still drains its battery

The alarm, the immobiliser, the keyless entry receiver, the clock and any tracker all stay awake when the car is locked. Each draws very little, but together they run around the clock with nothing recharging the battery.

On top of that, a lead-acid battery slowly loses charge on its own, and it does so faster the hotter it is. A battery left partly discharged for weeks also starts to harden internally, which is damage that a later recharge does not fully undo.

Before you leave

A few minutes on the day before you fly make the biggest difference:

• Have the battery tested if it is more than eighteen months old. A weak one will not survive the wait
• Give the car a proper drive of half an hour so the battery is full when you park it
• Park in a basement or covered bay if you can. It is far cooler than a podium deck
• Unplug chargers and dash cameras, and check that no interior light is on and the boot is fully shut
• Keep the key fob well away from the car, as some keyless systems stay active while the key is nearby

For a month or more

The simplest answer is a person. Ask a friend or neighbour to drive the car for half an hour every week or two. It has to be a drive: a few minutes of idling takes more out of the battery than it puts back.

The other answer is a maintenance charger, sometimes called a trickle or smart charger, which keeps the battery topped up from a wall socket. It only works if your bay has power and the building allows it. Disconnecting the battery is a last resort on a modern car, for the reasons given in the questions below.

When you get back and it will not start

Do not keep cranking. Two or three tries is enough. A jump start will usually get the engine running, and the car should then be driven for a good half hour.

Have the battery tested soon afterwards. One that has sat deeply flat for weeks in the heat has often lost capacity for good, and it tends to let you down again within the month. The test costs nothing and tells you whether it has recovered.