Problems · won't start

It clicks. What the clicking tells you.

Turn the key, get clicking, engine does not turn. Almost every answer online is written by somebody who would like to book you in, and almost all of them stop at the same rule of thumb. The rule is sound. What none of them give you is the one measurement that settles it, or the reading that looks healthy and is not.

First, listen to the clicks

Two national chains describe the same distinction independently, and it is worth having before you touch anything:

It is a good first cut and it is not a diagnosis, because the second case has two quite different causes. A meter separates them.

The number that settles it

A 12 V lead-acid battery's resting voltage maps to its state of charge, and this table is not one publication's opinion — four independent sources give the same figures to the tenth of a volt. Rested means several hours after the engine last ran; measure it right after a drive and the surface charge flatters it.

The reading that looks fine and is not

Here is the part the chains leave out. A battery can rest at a perfectly healthy 12.6 V and still collapse the moment the starter draws. Resting voltage measures how much charge is in it. It does not measure whether the battery can still deliver that charge fast, which is the thing that has failed on most batteries that die.

The load figure is the one to take: with a meter on the terminals and somebody cranking, a healthy battery does not fall below 9.6 V. All four sources give the same threshold.

Rested above 12.4 but falling under 9.6 while cranking is a battery that is charged and finished. Rested at 12.0 and recovering after a charge is a battery that was flattened by something — a door left ajar, a short trip, a failing alternator — and the cause matters more than the battery does. That is what the third row is for: if the engine runs but the charging voltage never reaches about 13.7, the alternator is not putting it back and a new battery will die too.

A jump pack is also a diagnostic

If you have no meter, a jump pack answers the question by itself:

1. It starts and keeps running — the battery was flat. Find out why before buying a new one.
2. It starts, then dies as soon as you disconnect — the alternator is not charging.
3. It still clicks exactly the same way — the battery was not the problem. That is the starter or a cable, and no jump pack of any size will fix it.

That third case is worth knowing before you spend anything: people buy a bigger pack to solve a starter fault, and a bigger pack cannot.

The failure that catches people out

When a battery is flat enough to give a single click, some lithium jump packs refuse to run at all — they read the near-zero voltage as a reverse-polarity fault and will not engage. In the test we sourced for our jump starter page, packs without a manual override button could not be made to work; one had to be dropped from the test and another needed a fresh battery fitted first before it would agree to help.

So the feature to check on a jump pack is not the amperage on the front. It is whether there is a manual override, and whether it publishes a cold cranking figure at all — two of the seven units in that test advertise the largest peak numbers and publish no sustained figure whatsoever.

When it is not the battery

1. Corroded or loose terminals produce exactly the rapid-click symptom on a good battery. Look before you buy: white or blue crust, or a terminal you can twist by hand.
2. Age. Three to five years is the ordinary lifespan. A battery that starts the car all summer and fails on the first cold morning is not unlucky — cold raises what the starter needs at the moment the battery can give least.
3. No click, no lights at all is a different fault: a completely dead battery, a disconnected terminal, or a main fuse.
4. It cranks normally and simply will not fire — then the electrical side is fine and this page is not your answer.