Battery Calculator
Estimate car battery health from rest voltage, calculate runtime from capacity and consumption, and find a compatible replacement battery for your car.
✔ 100% free
Mobile friendly
⚡ No account
Loading tool…
Advertisement
Voltage tells the story
A healthy lead-acid battery at rest (engine off, 30 minutes after the last charge) reads between 12.4 and 12.7 V. Below 12.2 V it is only about half charged, and below 12.0 V it is deeply discharged.
- 12.6 V + : fully charged
- 12.4 V : about 75% charged
- 12.2 V : about 50%
- 12.0 V : about 25%
- Below 11.8 V : deeply discharged or faulty
Runtime from capacity and draw
Runtime in hours = battery capacity (Ah) × efficiency ÷ average draw (A). A 70 Ah battery powering a device that draws 10 A lasts about 5–6 hours in practice.
Choosing a replacement
Match the original battery type, capacity (Ah) and cold cranking amps (CCA) to the vehicle documentation. Bigger is not automatically better — the charging system must be able to recharge it.
FAQ
Cold Cranking Amps: the maximum current the battery can deliver for 30 seconds at -18 °C while keeping voltage above 7.2 V. Cold climates need higher CCA.
Typically every 4 to 6 years, depending on climate and use. Frequent short trips, extreme heat and excessive electrical load shorten battery life.
A parasitic drain is draining it. Common culprits: interior lights, an alternator that keeps charging (bad diode), alarms, or a module that does not sleep. Test the draw with an ammeter.
Advertisement