How long will a car battery stay charged in storage?
Last updated October 2026
It depends on whether the battery is still connected. In a newer car, a connected battery can go flat in a few weeks. Disconnected and kept cool, it can last months.
Connected to the car
Cars draw a little power even when they’re off, for clocks, alarms, and computers. Optima, which makes car batteries, says about 25 milliamps is normal, and 85 milliamps is common on newer vehicles. At 85 milliamps, Optima says a 50 amp hour battery “will be completely discharged from a full state of charge in just over three weeks.”
You don’t need to wait for it to go flat to do damage. Optima says sulfation starts below 12.4 volts and shortens battery life.
Disconnected
A new, fully charged battery that’s disconnected and stored well could take “several months or more” to discharge completely, according to Optima.
Temperature matters. Discover, a battery maker, lists these monthly self-discharge rates for AGM and gel batteries:
| Storage temperature | Charge lost per month |
|---|---|
| 46°F | 1 to 2% |
| 68°F | 3 to 4% |
| 86°F | 5 to 6% |
| 104°F | 10 to 12% |
A drive-up unit in summer can sit at the hot end of that table. Regular flooded car batteries can behave differently, and we didn’t find a manufacturer figure for them.
If you’ll be gone more than a couple of weeks, either disconnect the battery or rent a space with an outlet and use a maintainer.