How long can a car sit before the battery dies?

A parked car is never fully off. Alarms, keyless entry, clocks, telematics and module memory keep drawing a small current, and the battery also slowly discharges on its own. How long a car can sit is simply how long that small drain takes to empty a useful part of the battery. So the honest answer is a calculation, not one magic number.

How do you work out how long a car can sit?

Divide the charge the battery can spare by the parked draw. In our arithmetic below, a 60 Ah battery drawing a typical 30 mA takes about 6 weeks to fall from full to half. Carmakers call the current a sleeping car draws "dark current", "standby current" or "parasitic draw". Subaru gives a clear reference point. Its dealer bulletin 15-308-23R sets an acceptable limit of 70 mA or less and says a typical Subaru draws 30–35 mA once asleep. Other makers publish their own limits in their service manuals.

Here is what those draws mean for an example 60 Ah battery that starts fully charged. We use "half charge" as the line because by then, around 12.2 V, battery makers already say to recharge (see the voltage chart).

This is our own arithmetic (draw in amps × 24 hours), not a manufacturer figure. Real life is usually worse, for three reasons:

• The battery is rarely full when you park. Short trips don't give the alternator time to recharge it. Varta says that with infrequent use or short distances "the alternator cannot fully charge the battery" (Varta).
• The battery may not be healthy. A worn battery reaches half charge sooner than the table suggests. Subaru's bulletin notes that "weak batteries mimic parasitic draw."
• Temperature. Varta says high temperatures speed up self-discharge. Cold cuts the starting power left in the battery, so a half-charged battery that would start the car in September may not in January. See why car batteries die in cold weather.

What do carmakers and battery makers advise?

In effect, check and recharge about once a month, in our reading of the dealer-lot rules below. Carmakers deal with this on every dealer lot, and their instructions are a good guide:

• MINI checks the battery voltage of cars parked on its dealer lots on a four-week cycle and recharges any that read below 12.5 V (about 71% charge). With the battery disconnected, it stretches the cycle to 12 weeks. It also warns that letting the voltage drop below 12.0 V "permanently reduces the battery life expectancy" (MINI SI M61 01 02).
• Lexus, in a bulletin for its 2015–2016 hybrids, tells dealers to disconnect the negative terminal of the 12 V battery for storage of 30 days or more, "to prevent the SOC [state of charge] of the auxiliary battery from decreasing during storage due to parasitic current." Hybrids should also be switched on for 30 minutes every 2 months (Lexus L-SB-0011-14).
• Varta (Clarios) says to charge a battery to 12.7–12.8 V before storage, check it every two months, and recharge if it has fallen to 12.5 V or less (Varta).

Read together (our reading, from dealer-lot rules for new cars): about one month is when carmakers expect to intervene on a car left connected, and even with the battery disconnected MINI still recharges every 12 weeks.

What makes a battery die sooner than the math says?

• A module that won't sleep. Subaru notes that when a car is parked where cell signal is weak, such as an underground garage, the telematics module can stay awake "drawing up to 140 mA for 14 days" before it hibernates (Subaru 15-308-23R).
• The key fob too close. A fob near the car can keep keyless-entry modules polling. Subaru's test procedure tells technicians to keep it at least 4.6 m (15 ft) away.
• Aftermarket devices. Subaru's dealer bulletin 07-236-25 lists alarms, remote starters, dash cams and GPS trackers as devices that "draw power continuously." A plug-in OBD dongle left in the port is the same kind of load (our note; see can you leave an OBD2 scanner plugged in).
• A real fault. A few hundred milliamps from a stuck relay, a faulty module or a shorted wire can flatten a battery in days (see the 300 mA row above). If yours dies after sitting only a few days, read how to find a parasitic battery drain.

Subaru also cautions that "a one-time dead battery does not indicate a parasitic draw." Rule out the simple things first: a door not fully latched, an interior light left on, or a battery that was already weak.

How do you leave a car parked for longer?

1. Start with a full battery. Drive it well, or charge it, before you leave it.
2. Use a battery maintainer if you have power where you park. Varta recommends a charger with a maintenance mode, because other chargers can overcharge.
3. Or disconnect the negative terminal, only if your owner's manual allows it. Toyota Canada advises against disconnecting the battery "under any circumstances" on models with a battery sensor on the negative terminal (Toyota Canada), and some cars lose stored settings when the battery is disconnected.
4. Check the voltage every few weeks if you can't do either, and charge it at 12.4 V or lower.
5. Hybrids and EVs have a 12 V battery too. Follow the manufacturer's storage routine for the main battery as well.

What should you do if you come back to a flat battery?

• Wear eye protection and keep sparks away. Batteries give off explosive hydrogen gas and contain sulfuric acid (East Penn).
• Never jump-start or charge a frozen battery. East Penn warns that doing so risks an explosion. A discharged battery freezes at a much higher temperature than a charged one; see cold weather.
• Jump-start in the right order. East Penn's sequence: positive (+) clamp on the dead battery's positive terminal, then the other positive clamp on the donor's positive, then negative (−) on the donor's negative. The final negative clamp goes on the dead car's engine block or frame, or on its remote negative jump terminal if it has one (check the owner's manual). Start the donor, let it run at least 30 seconds, then start the dead car, and remove the cables in reverse order.
• Then recharge it properly. A jump start plus a short drive is unlikely to fully recharge a deeply discharged battery. A full charge, then a test, tells you whether it survived.