Why does my car battery keep dying?

One dead battery can be bad luck: a dome light left on, a door not quite shut, a week of not driving in the cold. Subaru's dealer guidance says a one-time dead battery is more likely a user error or a one-off event than a fault (Subaru 15-308-23R). A battery that dies again and again is different. Something is either using more charge than the car puts back, or the battery can no longer hold what it gets.

There are four usual causes. Work through them in this order, because the first two checks are quick and cheap and they change what the later ones mean.

The four usual causes at a glance

1. The battery is worn out

Every lead-acid battery wears out. AAA puts typical life at three to five years, ranging from 58 months or more in the northernmost US regions to under 41 months in the southernmost, according to Consumer Reports. Heat does more of the damage than cold; cold is just when a weak battery finally shows it. The Canadian Automobile Association warns that a battery "between three to five years old ... could fail."

A worn battery can read a healthy voltage and still fail, because voltage shows charge, not how much current the battery can deliver. That takes a load test or a conductance test. Consumer Reports suggests an annual load test once a battery is 2 years old in a warm climate, or 4 years old in a cold one. Charge the battery fully before it's tested; a test on a half-charged battery says little.

2. Short trips never recharge it

Starting the engine takes a burst of charge, and the alternator needs driving time to put it back. Varta: "If the car is used infrequently or only for short distances, the alternator cannot fully charge the battery" (Varta). Subaru tells its service staff to ask about exactly this, because a car driven only on short, infrequent trips "may not be driven long enough to fully recharge the battery after starting, leading to a gradual discharge" (Subaru 07-236-25). Winter makes it worse, with heated seats, rear defrost and headlights all drawing on the same alternator.

The tell is a battery that recovers after a long highway drive or a night on a charger, then slides again over a week of errands. It's also why a battery often dies again soon after a jump start: the drive afterwards is rarely long enough to put the charge back (our inference from Varta's point). A smart charger once a week or so, or a regular longer drive, fixes this kind. See how long to drive to charge a car battery for why there's no single right number of minutes.

3. The alternator isn't keeping up

If the alternator, its belt or the wiring between them is failing, the battery runs down a little more on every trip. With the engine running, Optima gives about 13.3–15.0 V as the normal charging range (Optima).

Don't condemn an alternator on one reading, though. Many newer cars manage charging to save fuel and deliberately hold a lower voltage at times, so a reading near 13 V isn't proof of a fault; our battery voltage chart has the details. A shop's charging-system test checks output under load and is the reliable answer.

4. Something drains it when the car is off

Every parked car draws some current to keep clocks, alarms, keyless entry and modules on standby. Subaru says its cars typically draw 30–35 mA once asleep and sets a limit of 70 mA (Subaru 15-308-23R). A fault drawing hundreds of milliamps can flatten a battery within days. In our arithmetic, a full, healthy 60 Ah battery can spare about 30 Ah before it needs recharging: 300 mA uses that in about four days, while using it in a single 12-hour night takes about 2.5 A, the kind of drain a light left on causes.

Common culprits:

• A module that won't go to sleep. Subaru's dealer guidance says a faulty module can "continuously send messages and prevent the entire network from entering sleep mode" (Subaru 07-236-25). A key fob left inside the car can keep it awake too.
• A light that stays on: glovebox, trunk, under-hood, or a door switch that thinks a door is open.
• Aftermarket devices. Subaru names alarms, remote start systems, dash cameras and GPS trackers, and warns that "even a small, acceptable draw from a device can add up to an excessive overall draw" when combined with the car's own. OBD2 adapters left in the port belong on the list too; see can you leave an OBD2 scanner plugged in.
• A drain that only happens in the cold. GM's PIT5745D describes dead batteries on many 2018–2022 models that happened only after nights below about 4 °C (40 °F), because a seat memory module woke the car's data network. It wouldn't show up in a warm service bay.

Measuring the draw takes a meter, patience (modules can take 20 minutes or more to sleep) and some care. Our guide to parasitic battery drain walks through it, including how to trace a drain to its fuse.

Loose or corroded battery terminals

A loose or corroded cable can look like a dying battery: it limits the current the starter gets and the charge the battery receives. East Penn lists loose cable connections, corrosion and damaged terminals among the things to inspect (East Penn Form 0004). Wear eye protection near a battery and keep sparks away: batteries give off explosive hydrogen gas.

How to tell if it's the battery, the alternator or a drain

1. Charge the battery fully, then let it rest for several hours and read its voltage. A flooded battery should read about 12.6 V.
2. Have it load tested. If it fails, replace it, and still do step 3.
3. Have the charging system tested with the engine running.
4. If both pass, measure the parked draw. A battery and alternator that test fine, plus a battery that keeps going flat, almost always means a drain.
5. Log the battery between drives. A battery that falls steadily overnight, or only on cold nights, tells you when the drain happens, which narrows down what to look for. A Bluetooth battery monitor can do this logging for you.