Tip #93: Modern cars and EVs drain their 12-volt battery while parked
Modern cars keep computers, alarms and connected services running while parked, and EVs rely on a small 12-volt battery to wake up. Long sits, plugged-in accessories and short trips are the usual reasons it goes flat.
Tip #93 in one line
Modern cars keep computers, alarms and connected services running while parked, and EVs rely on a small 12-volt battery to wake up. Long sits, plugged-in accessories and short trips are the usual reasons it goes flat.
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Call nowWhy a parked car still uses power
Keyless entry listens for the fob, the alarm watches the doors, the telematics module stays connected, and modules wake up to check the car. Each draws a little current, and together they slowly drain a battery that is not being recharged. A healthy battery handles that for a good while. An older one, or one that only sees short trips, may not.
Add-ons make it worse: a hardwired dashcam in parking mode, a phone charger that stays powered, an interior light left on, or a door or trunk that is not fully latched.
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Call nowEVs have a 12-volt battery too
An electric car's big battery drives the motors, but a separate 12-volt battery boots the computers, unlocks the doors and releases the parking brake. When it dies, the car can look completely dead even with plenty of range. How an EV keeps its 12-volt charged differs by make and model; the owner's manual explains it, and it often recommends keeping the car plugged in or awake in certain ways during long sits.
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Call nowPrevention
If a car will sit for a week or more, a battery maintainer is a good idea where you can plug one in; check the manual for where to connect it. Unplug accessories, close every door and the trunk fully, and drive the car for a longer stretch now and then instead of only short hops. Have the battery tested once it is a few years old.
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Call nowWhen it is dead
Call (908) 444-0129 and tell the dispatcher the make and model, and whether it is a hybrid or EV. A jump start is one flat price, and the tech connects at the point the manufacturer specifies. If the car will not wake up or the main EV battery is empty, a flatbed takes it to a charger or a shop of your choice. The jump start guide covers hybrids, start-stop cars and EVs in more detail.
Software updates and remote features can draw power as well. On many connected cars, checking the car's status from an app wakes it up, and doing so often during a long sit adds to the drain. If the car will be parked for a while, check the owner's manual for any storage or low-power mode, and keep app checks to a minimum.
If a car keeps dying with a good battery, ask a shop for a parasitic draw test. It finds a module or accessory that stays awake when it should sleep.
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Call nowMore on batteries
Keep reading, or go back to every post on the blog.
- Dead car battery in a New Jersey winter: why it happens and what to do
- Tip #11: Jump-start in the right order, and finish on bare metal, not the battery
- Tip #12: Never jump a battery that is swollen, cracked, leaking or frozen
- Tip #18: Test an aging battery in the fall and replace it on your schedule, not the weather's
- Tip #19: If the car dies while driving, suspect the alternator before the battery
- Tip #20: A battery that dies every few days usually has something draining it
- Tip #85: Test your battery every fall once it is a few years old
- Tip #87: If your battery keeps dying, have the alternator load-tested at a shop
- All posts
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