Diagnosing a Weak Car Battery

A car battery rarely dies without warning. It usually drops hints for weeks, and the hints are easy to explain away: it was a cold morning, it was only a short trip, the car is getting on a bit. The trouble is that the same symptoms can come from a tired battery, a failing alternator, a corroded cable or something quietly draining power overnight, and replacing the wrong part is an expensive way to find out.

Start with the symptom you have noticed, and work from there.

If the engine turns over slowly

A healthy starter spins the engine with a quick, even rhythm. A weak battery gives a slower, labouring whirr that takes a beat too long before the engine catches. It shows up most on the first start of a cold morning, because low temperatures cut a battery's output just when thick oil makes the engine hardest to turn. A slow crank that improves on warmer days, in a battery more than three or four years old, points strongly at the battery.

If it clicks instead of starting

Rapid clicking with the dashboard lit up usually means there is enough charge to work the starter's solenoid but not enough to turn the motor. That's a flat or failing battery, or a poor connection. A single heavy click with no cranking at all, while the lights stay bright, is different. It more often points to the starter motor or its wiring.

If nothing happens at all

No lights, no click, no chime when the door opens usually means a completely flat battery or a cable that has come loose. Check that both terminal clamps are tight and can't be twisted by hand before assuming the worst.

If the car starts with a jump, then dies

If the engine starts on jump leads and then stalls within minutes of disconnecting them, the alternator probably isn't charging, and the car is running on whatever the battery had left. A red battery-shaped warning light while driving points the same way. Despite the symbol, that light usually means the charging system has a fault, not the battery itself.

When you jump-start, connect the final black clamp to bare, unpainted metal on the engine or body of the car being started, away from the battery, rather than to its negative terminal. Batteries can give off hydrogen, and that last connection is the one that can spark.

If it runs fine but is flat again by morning

A car that drives normally all day but won't start after sitting overnight has either a battery that can no longer hold a charge or something draining it while parked. Common culprits are a glovebox or trunk light that stays on, a dash camera wired to run constantly, or an aftermarket alarm or stereo. Repeated short trips can also leave a battery permanently undercharged, because the alternator never gets long enough to replace what the starter used. A garage can measure the current the car draws while switched off and track down the cause.

If you see crust, swelling or smell rotten eggs

Pop the hood and look. White or blue-green crust around the terminals can block current. With the engine off, remove the negative cable first, then clean the terminals with a baking soda solution and a wire brush, and reconnect the negative cable last. A swollen or cracked case, liquid around the battery, or a smell like rotten eggs means the battery is damaged and may be venting gas. Don't charge it or jump-start it in that state. Have it replaced.

If you own a multimeter

A basic multimeter gives a quick reading. Set it to DC volts and touch the probes to the terminals with the engine off, ideally after the car has rested for a few hours.

• About 12.6 volts or a little more: fully charged.
• Around 12.4 volts: roughly three-quarters charged.
• Around 12.2 volts: about half charged, and worth charging properly.
• Below about 12.0 volts: discharged. If it won't climb back after a full charge, the battery is likely worn out.

With the engine running, most cars should read somewhere around 13.7 to 14.7 volts. A reading close to 12.6 with the engine on suggests the alternator isn't charging. Voltage only tells part of the story, though. A load test, which many parts stores and garages will run in a few minutes, shows whether the battery can still deliver current under strain.

If you are not sure how old the battery is

Three to five years is a typical lifespan, and heat wears batteries out faster than cold does, so drivers in hot climates tend to replace them sooner. Look for a date sticker on the case, or a stamped code. Many manufacturers use a letter for the month and a digit for the year, though the format varies by brand. A battery past four years that shows any of the signs above is living on borrowed time.

If it does need replacing

A standard battery for most cars costs roughly $100 to $250. Many parts stores will test your old battery and fit the new one at no extra charge, and most add a refundable core charge that you get back when you return the old battery for recycling. Cars with automatic start-stop usually need a tougher AGM or EFB battery, which costs more, and some models need the new battery registered with the car's computer so the charging system treats it correctly. Check the owner's manual, or ask before you buy, to avoid a second trip.