A parked car slowly drains its battery through clocks, alarms and computers, and AAA recommends a maintenance charger for cars that sit unused. A flat battery left flat is also harder to bring back.
Options
- Battery maintainer (smart charger). Monitors the battery and tops it up only when needed. The safest choice for long storage.
- Trickle charger. Supplies a small constant current. Fine for short periods, but a basic one left connected for months can overcharge.
- Disconnect the negative terminal. Stops drains, but you lose settings on some cars and it does not stop natural self-discharge.
Steps
- Park the car in a ventilated place and switch everything off.
- Connect the maintainer red to positive (+), black to negative (-), then plug it in.
- Check the voltage every month or so with the voltage checker. OPTIMA advises recharging if resting voltage falls below about 12.4V.
- If it goes flat, work out the recharge time with the charging time calculator.
Check your car’s manual first. Hybrids, EVs and start-stop cars can have specific requirements. If the battery will not hold charge after storage, see the replacement cost.
What drains a parked car
Even with the ignition off, modern cars draw small amounts for clocks, alarms, keyless entry and memory. Over weeks, that adds up. Batteries also self-discharge slowly, faster in heat. A flat battery left flat can sulphate, which reduces capacity permanently. Prevention is easier than recovery.
Choosing a maintainer
Look for a smart maintainer with a mode for your battery type (flooded, AGM, lithium if applicable), automatic shut-off or float mode, reverse-polarity protection and an appropriate amperage for your battery’s size. Read the manual, and avoid cheap chargers with no regulation, which can overcharge. Our charging time calculator shows how long a given charger takes to bring a battery up from a lower level.
Using it safely
- Charge in a ventilated space, away from sparks and flames.
- Connect the clamps before plugging in, and unplug before disconnecting.
- Keep the charger and leads off hot surfaces and away from water.
- Do not charge a frozen, cracked or leaking battery.
- Check the car’s manual for the right charging point, especially on cars with battery monitoring systems.
If you cannot use a maintainer
Start and run the car for a decent drive every couple of weeks. Short idling is not enough. Disconnecting the negative terminal reduces drains, but you may lose settings, and some cars need relearning. A portable jump starter in the boot is a good backup.
After storage
Measure resting voltage with the voltage checker. OPTIMA advises recharging a battery that rests below about 12.4V. If it will not hold charge after being charged, it may be time to replace; see signs your car battery is dying and the cost calculator.