Battery or alternator

What each one does

The battery’s main job is to start the engine. It holds a charge, hands a big burst of current to the starter motor, and then mostly sits back. Once the engine is running, the alternator takes over. It is driven by the engine’s belt, it powers everything electrical in the car, and it recharges the battery as you drive.

So the two depend on each other. A tired battery makes the alternator work harder. A weak alternator never refills the battery, so the battery goes flat and looks like the guilty party. That is why the same symptom, a car that won’t start, can come from either one.

Signs that point to the battery

• The engine cranks slowly after sitting overnight, then starts and runs fine all day.
• You hear a rapid clicking when you turn the key, and the dash lights flicker or dim.
• A jump start works, and the car then starts on its own for days afterward.
• The battery is several years old. Heat in summer and deep cold in winter both shorten its life, and Ottawa gives it both.
• There is white or blue crust on the terminals, or the case looks swollen.

A battery that is merely weak often gives you a warning on the first really cold morning of the year, when the oil is thick and the starter needs the most current. If it struggles at minus 15, it will probably leave you stranded at minus 25.

Signs that point to the alternator

• The battery warning light comes on while you are driving and stays on, rather than flashing for a second at startup.
• Headlights and dash lights dim at idle and brighten when you rev the engine.
• The car starts after a jump, runs for a while, and then dies on the road or won’t restart after a short stop.
• A battery that is only a few months old keeps going flat.
• There is a whine from the front of the engine that rises with engine speed, or a hot electrical smell.
• Electrical things act oddly together: the radio resets, power windows slow down, gauges twitch.

If the battery light comes on while you are driving, head for somewhere safe to stop. Once the alternator stops charging, the car is running on whatever is left in the battery, and that may not be much.

A dead car at the St. Laurent Centre

A customer once told us about a December evening at the St. Laurent Centre, bags full of gifts, and a car that wouldn’t start in the parking lot. He was sure it was the battery. After asking around for jumper cables without much luck, he walked over to the shop. One of the mechanics drove him back to the car, they got it started, and it came back to the shop for a proper look.

The battery wasn’t the problem. The alternator was. Within about an hour the car was fixed and he was back at the mall to collect his wife.

The useful part of that story is the wrong first guess. A jump got the car running, which made the battery look guilty, but a jump only proves the starter and the engine work. It says nothing about whether the car can keep itself charged. Had he bought a new battery and driven home, the alternator would have flattened that one too.

How we test both

Guessing is how people end up buying two batteries. At 2 and 1/2 Mechanics we test before we replace anything, and the checks go roughly like this.

For the battery:

• resting voltage with the car off, where a healthy, fully charged battery sits around 12.6 volts
• a load or conductance test, which shows how much cranking current the battery can still deliver compared with its rating
• the terminals, clamps and main cables, since a corroded clamp can make a good battery look bad

For the charging system:

• charging voltage at idle and at higher revs, with the headlights, blower and rear defroster switched on to load it
• a check for ripple, which shows a failing diode inside the alternator even when the voltage looks normal
• the drive belt and tensioner, because a slipping belt can’t turn the alternator properly
• the ground connections between engine, body and battery

If both the battery and the alternator test fine and the car still goes flat overnight, the next step is a parasitic draw test. Something is staying awake after the car is locked. A glovebox or trunk light that won’t switch off, an aftermarket remote starter, or a module that never goes to sleep are the usual suspects, and tracking it down is a matter of measuring the current while pulling fuses one at a time.

Before you buy a battery

Get the old one tested first, and get the charging system tested with it. A new battery on a failing alternator will be flat again in days, and a new alternator on a battery with a dead cell won’t fix a slow crank.

Make sure the replacement is the right one for the car. Many newer cars with engine stop and start use a special battery type, and some makes need the new battery registered to the car with a scan tool so the charging system treats it properly. Putting in the cheapest battery that fits the tray can cause problems that look unrelated.

Electrical testing at 2 and 1/2 Mechanics is billed at the inspection rate of $120 an hour, before tax. We call you with what we found and the price of the fix, and nothing gets replaced without your OK. More on this work is on our electrical, battery and starting page, and faults that bring up warning lights along the way fall under computer diagnostics.

If your car is cranking slowly now, don’t wait for the first deep freeze. Book online, drop it off by 9am, and if weekdays are full, the shop is open Saturday 8am to 3pm.