Jump start or replace? How we decide at the car

How do you test if a car battery needs replacing?

Four measurements decide it, and they take about five minutes at the car. None of them involves guessing from the battery's age or brand.

The first three show the state of charge and the health of the car around the battery. The fourth shows the health of the battery itself, which is what settles the jump start or replace question.

• Resting voltage: measured with the engine off, it shows how charged the battery is.
• Cranking voltage: how far the voltage falls while the starter is turning the engine.
• Charging voltage: measured with the engine running, it shows if the alternator is doing its job.
• Measured CCA: the tester compares the cold cranking amps the battery can deliver now with the rating printed on its label. A battery well under its rated CCA is at the end of its life, even when its voltage looks normal.

What voltage should a car battery read?

A fully charged 12 V car battery reads about 12.6 V or slightly more at rest. The figures below are for a battery that has been sitting with the engine off for a while, because a battery reads falsely high for a short time after driving or charging.

AGM batteries rest a little higher than flooded ones, and cars with smart charging move their voltage up and down on purpose, so a single reading outside the range is a reason to look closer, not a verdict. A low resting voltage only says the battery is empty. It does not say why.

When is a jump start enough?

A jump start is enough when there is a clear reason the battery went flat and the battery itself is still healthy.

After the jump the battery still needs a real recharge. Half an hour or more of steady driving is the minimum, and a deeply drained battery is better on a proper charger overnight, because an alternator is built to top a battery up, not to refill an empty one. The usual cases where a jump start is all it takes are these.

• Lights, hazards or the ignition were left on.
• The car sat parked for weeks, for example over summer travel or in airport long-term parking.
• A door or the boot was not fully closed.
• The battery is under about two years old and passes the test once it has been charged.

When is a jump start not enough?

A jump start borrows power and repairs nothing, so if the battery is worn out the car will be stuck again at the next start. A flat battery also reads low on CCA, so when the tester asks for a charge and retest, we do that before condemning it. We recommend replacement when we see any of these.

• Measured CCA well under the rated figure on the label.
• Voltage dropping below roughly 9.6 V while cranking, even after charging.
• A second or third jump start within a few weeks.
• A battery that will not hold about 12.4 V overnight with nothing draining it.
• A swollen case, a leak or a sulphur smell.
• A battery that sat deeply discharged for weeks and will not accept a charge.

Is it safe to jump start a modern car?

Yes, when it is done with a protected jump pack connected to the correct points. A good jump pack has reverse-polarity and spark protection and delivers a controlled current, which is what a car full of control units needs.

The risk comes from the old method, cables from another running car. Clamps that touch, a reversed connection or a cable pulled off while the donor engine is revving can send a voltage spike through both cars. Batteries also give off hydrogen, so a spark at the terminal is a bad idea. Done properly, the routine is this.

• Use the jump posts under the bonnet if the car has them, even when the battery is in the boot.
• Connect positive first, then negative to the marked earth point or to bare metal on the engine or body.
• On cars with a battery sensor on the negative terminal, do not clamp onto the terminal itself, so the sensor can measure the current.
• Never jump start a battery that is swollen, cracked or leaking.
• Disconnect in reverse order once the engine is running.

Can you jump start a hybrid, or a car with a bad alternator?

A hybrid can be jump started through its 12 V battery or the 12 V jump terminal under the bonnet, and nowhere else. The high-voltage pack and its orange cables are never touched. The 12 V battery in a hybrid only has to wake the computers and close the relays, so it needs very little to get going.

Using a hybrid to jump start another car is a different matter, and many manufacturers advise against it. Check the handbook first.

A car with a failed alternator will start from a jump and then run only as long as the battery lasts. That is useful for moving it out of a live lane or into a parking bay, and nothing more. It needs the charging fault repaired, and a new battery will not change that.

Why does a car battery die again right after a jump start?

Because the jump start treated the symptom, not the cause. That is why we test before and after a jump start instead of leaving once the engine fires. If the battery fails, we can fit a new one on the spot with a warranty of upto 24 months, and if it passes you keep the one you have.

One of these is usually behind a battery that goes flat again, and the test at the car separates them.

• The battery is at the end of its life and can no longer store what the alternator gives it.
• The drive afterwards was too short to put back what the start took out.
• The alternator is not charging, so the battery was never refilled.
• Something is drawing from the battery while the car is parked.
• The terminals are loose or corroded, so charge cannot get in or out properly.