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:
Rapid clicking, several a second battery side
There is enough current to pull the starter solenoid in, but not enough to hold it while the motor draws. It snaps in, the voltage collapses, it drops out, and it repeats. Battery, terminals or cables.
One loud click, then silence starter side
The solenoid closed and stayed closed, and the motor did not turn. That points at the starter motor — or at a battery so flat it managed exactly one engagement and nothing after.
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.
| Rested reading | Charge | What it means |
|---|---|---|
| 12.6 V or above | 100% | Healthy and full |
| 12.4 V | 75% | Usable, worth charging |
| 12.2 V | 50% | Low. Charge it |
| 12.0 V | 25% | Deeply discharged |
| Below ~11.8 V | 0% | Effectively flat |
The span from full to flat is under a volt. That is why “it reads twelve-something” tells you almost nothing and the first decimal place tells you almost everything.
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.
Scale runs to 14.7 V, the highest measured value here.
use it Convergent, four independent sources, 12 V battery state-of-charge and cranking voltage, read 31 July 2026 (archived copy; the original page has since been removed). The open-circuit voltage table for a 12 V lead-acid battery is not one publication's claim: four independent sources we read give the same figures to the tenth of a volt — 12.6 V rested is full, 12.4 V is about 75 per cent, 12.2 V about 50, 12.0 V about 25, and below roughly 11.8 V effectively flat. All four also give the same load threshold: a healthy battery does not fall below 9.6 V while the engine is cranking. Measure rested, meaning several hours after the engine last ran.
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:
- It starts and keeps running — the battery was flat. Find out why before buying a new one. A dash cam wired for parking mode draws on the battery while the car sits, and the cut-off voltage its kit is set to is one place to look.
- It starts, then dies as soon as you disconnect — the alternator is not charging.
- 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, many jump packs will not run until you override them — they read the near-zero voltage as a fault and refuse to engage, and a pack with no manual override cannot be talked out of it. In Car and Driver's 2026 test of eleven units on dead batteries, five of the seven that started needed their override or boost mode first, and one never recognised the battery at all. In CNN Underscored's load test of nine, one delivered nothing because its boost mode would not engage. The detail is on our jump starter page.
So the feature to check on a jump pack is whether it has a manual override, not the peak figure printed largest on the box, which is only the opening jolt. In that load test the pick held about 360 to 480 A for several seconds at 7.5–9.5 V, and even its 800 A cranking rating, the like-for-like figure, the reviewer calls still exaggerated, though closer to what was measured.
When it is not the battery
- 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.
- 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.
- No click, no lights at all is a different fault: a completely dead battery, a disconnected terminal, or a main fuse.
- It cranks normally and simply will not fire — then the electrical side is fine and this page is not your answer.
use it Convergent, four independent sources, 12 V battery state-of-charge and cranking voltage, read 31 July 2026 (archived copy; the original page has since been removed). The open-circuit voltage table for a 12 V lead-acid battery is not one publication's claim: four independent sources we read give the same figures to the tenth of a volt — 12.6 V rested is full, 12.4 V is about 75 per cent, 12.2 V about 50, 12.0 V about 25, and below roughly 11.8 V effectively flat. All four also give the same load threshold: a healthy battery does not fall below 9.6 V while the engine is cranking. Measure rested, meaning several hours after the engine last ran.
marked AutoZone and Firestone, Car won't start and is making clicking noises, read 31 July 2026. Two national service chains, independently, describe the same distinction: rapid clicking several times a second points to a battery that can trigger the solenoid but cannot hold the current to spin the starter, while a single loud click followed by silence points to the starter motor or to a battery flat enough to close the solenoid once and no more. Marked rather than measured because both sell the repair, and because neither publishes a measurement behind the distinction.
use it Car and Driver, Best Portable Jump-Starters for 2026, Tested, May 2026. Each unit jump-started beater cars from the magazine's own lot, chosen for completely dead batteries that ran flat again within minutes, so every unit met the same flat start. For most units the write-up says whether it started the car and whether it needed its override or boost mode first; for some it gives no result. Weight and lead length recorded, lead length measured from the base of the lead to the end of the clamp. Eleven units, eight with a spec sheet. The page was first published in February 2023 and updated with new products in May 2026.
use it CNN Underscored, I tested 9 jump starters that promise to revive your car, but only these 2 deliver, 30 April 2026. Nine units on a carbon pile load tester instead of a car: each was hooked to the tester, which draws current to simulate starting a vehicle, and the amps and volts it could sustain for multiple seconds were recorded; the load was repeated to see how many each could take before it could not deliver. One reviewer, one load bank, no vehicle. Amp-and-volt figures are printed for two units only, the Gooloo GT4000S (roughly 360 A at 9.5 V, 480 A at 7.5 V, then 420 A at 9.5 V) and the Noco GB40 (for example 160 A at 7 V, or 100 A at nearly 10 V); the other seven are described relative to those two.
not measured Nobody publishes how far below 9.6 V a battery has to fall before a given starter refuses, or how much of the drop is the battery and how much is the cables. The threshold is a pass or fail, and where it fails it does not tell you which part failed.