Battery, starting and electrical repair

What the symptom points to

• A slow crank, or just a click. The battery, a corroded cable or the starter. They look the same from the driver's seat and test differently.
• A battery light while driving. The alternator has stopped charging — or its belt has. The car is now running on the battery alone and will stop when it is empty.
• Flat every few mornings. Something is staying awake after you lock the car and draining it. We measure the draw and trace it, circuit by circuit.
• A dozen warnings at once after a cold night. Low voltage makes a modern car report faults in systems that are fine — ABS, steering, transmission. The battery is tested before anything else is believed.

The test

A starting and charging system test is a quick diagnosis at a set price: the battery measured against its rating, the current the starter draws, what the alternator puts out, and the cables and grounds between them. You get the numbers and an estimate for whichever part failed.

The battery itself is tested in every bumper-to-bumper inspection, free with any service — so a weak one is usually caught before the morning it lets you down.

Batteries on European cars

• The right type. Cars with start-stop need an AGM or EFB battery. A standard battery fitted in its place does not last.
• Registered to the car. BMW, MINI, Audi and Volkswagen need a new battery registered so the charging system knows it is new and charges it correctly. We do that with the makers' own software.
• The second battery. Many Mercedes-Benz and Volvo models carry an auxiliary battery for start-stop and the electronics. "Auxiliary battery malfunction" on the dash means that one, not the main battery.
• Age. Once a battery is five years old, we treat replacing it as maintenance rather than waiting for it to fail.

Alternators and starters

Alternators are a regular repair here, on Mercedes-Benz especially. When one is replaced we look at the drive belt and its tensioner at the same time — the belt is already off, and a worn one is often why the alternator was struggling. Starters are tested under load before they are condemned, because a starter that will not turn is often a cable that will not carry the current.

Wiring and electronics

Windows that will not go up, locks with a mind of their own, a sunroof that stops halfway, a blower that only works on high, a fault that comes and goes with the weather: these are wiring, switch and control-unit problems. Finding them takes wiring diagrams, an oscilloscope and the time to follow a circuit end to end. We repair chafed harnesses and corroded connectors, and replace a control unit only when testing proves it has failed.

Heat, cold and batteries

Heat is what wears a battery out; cold is when you find out. A summer under the hood cooks the plates, and the first cold morning asks the weakened battery for more current than it has left. That is why batteries seem to die in November. A test in the fall costs nothing with a service and saves the tow.

Electrical questions

How long should a car battery last?

Four to five years is typical. Short trips and heat shorten it. At five years we recommend replacing it as maintenance.

Does a new battery need to be programmed?

On BMW, MINI, Audi and Volkswagen, yes — the new battery is registered so the car charges it properly. Skipping that step shortens the new battery's life.

My Mercedes says "auxiliary battery malfunction". Is it urgent?

The car will keep running, but start-stop and some electronics depend on that second battery, and it will not fix itself. It is a small battery and a straightforward repair.

It started fine after a jump. Do I still need it tested?

Yes. A jump tells you the battery was empty, not why. If the alternator is not charging or something is draining the battery overnight, it will be flat again — and a deeply discharged battery is damaged each time.