Car Battery Runtime & Charging Calculator

How long a battery runs a load, and how long it takes to recharge.

Runtime
Charging time

Battery health guide (12 V lead-acid, resting)

Resting voltageApproximate state of charge
12.7 V or more100%
12.4 Vabout 75%
12.2 Vabout 50%
12.0 Vabout 25%
below 11.9 Vdischarged

Measure after the car has rested for a few hours. Discharging a starter battery below about 50% shortens its life; deep-cycle and lithium batteries tolerate more. The final part of a charge (absorption) slows down, so real charging times are longer than the simple estimate.

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