Battery after sitting
The car sat in the garage for two weeks, today it was supposed to drive — and instead of a start, a click. This is not bad luck. It is physics you can measure, predict, and next time catch in advance.
A parked car never fully sleeps. The alarm, central locking, keyless entry and control modules draw current around the clock — on the order of tens of milliamps. It sounds small, but it runs 24 hours a day. Add the battery's own self-discharge, which grows with temperature.
A healthy car with a healthy battery sits for weeks without a problem. If your car fails to start after two weeks, the problem is not that it sat — it is that the battery was already weak, or something in the car is not sleeping properly.
These are measurements from cars we monitor in service, taken directly at the battery terminals every 10 minutes:
| Situation | Resting voltage | Decline |
|---|---|---|
| Sports coupé, one night in a garage | 12.63 → 12.58 V | ~0.02–0.05 V per night |
| Sports car, 7 days without driving | 12.77 → 12.67 V | ~0.017 V per day |
That is what a healthy car looks like: a few hundredths of a volt per day. At that rate a battery survives weeks of sitting. If your car drops tenths of a volt per day, something is wrong — and it can be found.
Measured on a car that has been sitting at least 4 hours (no driving, no charging), regular 12 V lead-acid battery:
Exact thresholds differ by battery type (flooded, EFB, AGM and GEL have different healthy bands; LiFePO4 behaves entirely differently). The table is a guide for common lead-acid batteries.
The battery is at fault when it is old or has been chronically undercharged. A battery that repeatedly sat discharged loses capacity (sulfation) — then it dies faster and faster even though the car is fine.
The car is at fault when a control module fails to go to sleep, or when added electronics raise the draw — a dashcam in parking mode can flatten a battery in days.
The charger is at fault when the car sits "on a maintenance charger" that is not actually charging — a tripped breaker, a pulled plug, a failed unit. The owner feels safe while the battery drains. We see exactly this case repeatedly in practice.
A battery almost never dies overnight. It weakens gradually — over days to weeks. All that time the decline is visible in the voltage; nobody is watching it.
QZ is a monitor connected directly to the battery terminals. It measures continuously, communicates over its own mobile network (no Bluetooth — it works while you are on holiday and the car is in the garage at home), and sends a warning while there is still time to charge, book a service or send someone with a charger. Even a maintenance charger that quietly stopped charging shows up on your phone — not three weeks later next to a car that won't start.
Honestly: QZ does not measure cranking amps (CCA) or capacity — a workshop tester does that. QZ watches how your battery behaves in real life and warns you early that it is getting worse.
Battery keeps dying repeatedly? Read the 5 most common causes →