Car parked for months: why the battery dies and how to prevent it

We see it every week: someone comes back to their holiday home in Torremolinos or Rincón de la Victoria after three months away, turns the key and hears nothing but a click. It isn't bad luck: it is what happens to a modern vehicle left standing for a long time, and the coastal heat makes it worse.

Why a car that isn't used goes flat

Two things happen at once. The first is natural self-discharge: a lead-acid battery loses charge on its own even with nothing connected to it. In a new battery the loss is slow; in one several years old, much faster. And it accelerates noticeably with temperature.

The second is parasitic drain. A modern car is never fully switched off: with the key out, the alarm, the key-fob receiver, the control units' memory, the clock, the infotainment in standby and, on recent models, a connectivity module that wakes up at intervals all stay alive. Each draws very little, but together they run 24 hours a day.

That is why a car from twenty years ago could sit without trouble while a well-equipped current one goes flat far sooner. If yours drains dramatically faster than normal, there may also be an abnormal draw worth measuring.

Málaga's heat works against you

Most people blame the cold, but cold only slows a battery down; heat destroys it. It accelerates self-discharge, evaporates electrolyte and speeds up plate corrosion. A car in the July sun, or in a poorly ventilated garage sitting above 30 °C for weeks, suffers far more than the same vehicle in a cool northern garage. That is why a realistic service life here is around 5 to 7 years (4–5 conventional, 5–6 EFB, 6–7 AGM).

How long can a car sit?

It depends on the vehicle's standby draw and the condition of the battery. As a cautious guide, with a healthy, charged battery:

• A few weeks: it should start with no problem.
• A month or a little more: risk zone, especially in summer and in heavily equipped cars.
• Two months or more: quite likely it won't start, and every week deeply discharged causes permanent damage.

A battery that is already four or five years old can run out of strength in half that time.

What to do before leaving it parked

Test the battery. Drop by for a free battery test: with the Midtronics we see in two minutes the resting voltage, the real CCA against the rated figure and the state of health. A healthy battery reads 12.6–12.8 V at rest; below 12.4 V it is already partly discharged. If it is marginal, leaving it three months guarantees trouble.

Leave it charged to 100%. A battery left at 60% sulphates far sooner. If the car has only done short trips, it isn't full.

Disconnect the negative terminal. If you are leaving it many weeks with no socket nearby, disconnecting the battery removes the parasitic drain. Always take off the negative (earth) first. The trade-off: the car may lose memories and settings (radio presets, clock, seats, gearbox adaptations, radio codes on older models). If you want the alarm to stay active, this option isn't for you.

A maintainer is not a charger

A conventional charger pushes current in until you unplug it; forgotten, it can overcharge the battery. A maintainer charges, detects when the battery is full, stops and only tops it up when the voltage drops. It can stay connected for months without harm, which makes it the best solution for a holiday home: plug in during October and in March it starts first time.

• Choose one compatible with your technology: AGM and EFB need specific profiles, and an old charger with no AGM mode can ruin an AGM.
• On a hybrid or EV, the one that dies is the 12 V auxiliary battery (usually AGM), not the traction pack. Check the manual: there is normally a designated connection point.

The myth of starting it five minutes a week

It is the most repeated and most counterproductive advice going. Starting up and idling does not recharge the battery: the start itself uses a fair amount of energy, and at idle the alternator delivers very little charging current, much of which goes into the car's own consumption. Recharging lead-acid from a low state needs real driving, with the alternator working between 13.8 and 14.7 V.

Each "weekly start" leaves the battery slightly worse. If you move it, really move it: twenty or thirty minutes at a stretch, ideally with some dual-carriageway. A loop on the Ronda Este does more than ten starts in the garage.

I'm back and it won't start

Don't keep cranking: every attempt drains the battery further and heats the starter motor. There is a full guide in what to do when your battery won't start the car.

And a warning we give constantly: jump-starting again and again fixes nothing. You get borrowed energy for that one start, but the battery is still discharged and, if it spent months empty, it is probably sulphated: lead sulphate crystals on the plates that reduce the active surface and don't go away. It starts today with help and is dead again tomorrow.

If the car sat only a short while and the battery is recent, a slow, complete charge usually recovers it. If it was discharged for months, if after charging it won't hold 12.6 V at rest, if the CCA is far below the rated figure or if it is already five years or older, it needs replacing: we fit it on the spot in about fifteen minutes and recycle the old one free of charge.

Come by before you leave or when you get back

We are at C/ La Orotava, 100 (29006 Málaga), San Luis industrial estate next to Carretera de Cádiz. No appointment, first come first served, and the test is free. We are an Official VARTA Distributor and also stock Exide, Tudor, Motobatt and Optima, with indicative prices by size and technology (conventional €85–140, EFB €130–230, AGM €180–280, premium AGM €280–380) and the legal warranty under RDL 7/2021. Hours: July and August 08:00–16:00; September to June 09:00–14:00 and 16:00–19:00, Monday to Friday. Confirm by phone on 952 31 01 18.