Dead battery
You turn the key and get a click — or nothing at all. This is the guide in the order you need it: check it really is the battery, get started, charge it properly, and then find out why it happened.
1. Jump leads or a booster. Order matters: red to the dead battery's plus, red to the donor's plus, black to the donor's minus, black to bare metal on the dead car (not the minus post — a spark at the battery is an avoidable risk). Let the donor run 2–3 minutes, then start. Disconnect in reverse order.
2. A charger. If you don't have to go anywhere, this is the best route: an ordinary 5 A charger brings a 60 Ah battery from deep discharge to full in 8–12 hours. Overnight, not "for a bit". For AGM and GEL pick the right mode on the charger.
3. Replacement. If the battery is older than 5 years, or it goes flat again within two days of a full charge, charging won't save it — the capacity is gone. A load test at a garage confirms it in a minute.
The most common mistake: jump-start, drive 10 minutes, assume it's fixed. A short drive returns a fraction of what the start took. Either an hour of continuous driving, or a charger. And the next morning, measure the resting voltage:
| Morning, car parked | What it means |
|---|---|
| 12.6 V or more | Charged. If it happens again, the cause is in the car, not the battery. |
| around 12.4 V | Only partly charged — charging wasn't enough, or the battery is losing capacity. |
| below 12.2 V | The battery doesn't hold charge. Load test; probably replacement. |
Thresholds are a guide for ordinary lead-acid batteries (flooded, EFB, AGM and GEL have slightly different healthy bands; LiFePO4 behaves differently).
The car sat. A parked car never fully sleeps: alarm, keyless entry, control units. A healthy car with a healthy battery sits for weeks — we measure a drop of roughly 0.02–0.05 V per night. If yours drops tenths of a volt a day, something isn't sleeping.
Short trips. A start takes more than a 10-minute drive returns. Weeks of short trips drain a battery slowly.
An old or sulphated battery. Every deep discharge takes capacity. A battery that has once sat flat goes flat faster next time — and faster still after that.
A charger that wasn't charging. The car sat "on the maintainer" — a breaker tripped, a cable came out, it failed. The owner was at ease. We see this case again and again.
Does it keep going flat? The five causes and how to find yours →
Not the price of a new one. A deep discharge shortens the life of every lead-acid battery — and the longer it sits flat, the more. A battery that would have lasted six years lasts three after two or three deep discharges. The cheapest battery is the one you never let go flat.
A battery almost never dies overnight. It fades over days or weeks — and the whole time it shows in the voltage, only nobody is looking.
QZ is a monitor connected straight to the battery. It measures continuously, talks over its own mobile connection (no Bluetooth — it works while you're on holiday and the car is in the garage at home) and alerts you while there is still time to charge, send someone with a charger or call a garage. You'll also hear from your phone when a maintainer has stopped charging.
Honestly: QZ does not measure cranking current (CCA) or capacity — a garage tester does that. QZ watches how the battery behaves in real life and warns you early that it is getting worse.
Car didn't start after sitting for a while? The full checklist →