Why Battery Runtime Matters
A car battery can support more than starting power. It may run lights, audio, coolers, inverters, alarms, dash cameras, and test equipment. Runtime changes because each load draws current from a limited charge store. A simple amp hour division is useful, but it is not enough for heavy loads.
Key Factors
This calculator estimates engine off runtime with practical electrical losses. It accepts battery voltage, amp hour capacity, reserve capacity, state of charge, cutoff limit, accessory watts, direct amp draw, parasitic draw, inverter efficiency, age derating, temperature derating, Peukert exponent, voltage sag, and safety margin. These inputs help model a real vehicle better than a flat rating.
Cold weather reduces available capacity. Old batteries also hold less charge. Large current draw can shorten lead acid runtime because chemical reactions cannot fully keep up. This is why Peukert adjustment is included. A lithium battery can use an exponent near one. A flooded starting battery often needs a higher value.
Useful Planning
Use the result for camping lights, mobile radio, sound systems, emergency accessories, and parked vehicle electronics. The safe runtime result is more useful than the absolute runtime. It leaves reserve energy for starting and avoids deep discharge. Deep discharge can damage starting batteries and may shorten their service life.
The reserve capacity field is helpful when the amp hour rating is missing. Many automotive batteries show reserve minutes instead. The calculator converts reserve minutes at a selected test current into estimated amp hours. This is still an estimate, but it gives a practical starting point.
Interpreting Results
Look at total current first. Then review usable amp hours, usable watt hours, Peukert runtime, and parasitic drain days. If runtime is too low, reduce load watts, improve inverter efficiency, raise battery capacity, or increase the minimum state of charge reserve. Use measured current when possible. Clamp meters and power meters improve accuracy. Record each scenario in the table, then compare results before leaving a vehicle unattended for long periods or remote trips safely.
This tool supports planning only. Battery chemistry, temperature history, alternator behavior, cable losses, sulfation, and actual battery health can change results. For critical systems, test the battery under load and keep a jump starter nearby.