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Battery Runtime Calculator

DC battery runtime from capacity (Ah) and load. Constant load, no Peukert.

Runtime

8.00 h

Usable capacity

80.0 Ah

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These calculators are engineering design aids intended for estimation and educational purposes. They do not replace professional engineering judgment or compliance with applicable standards (ABYC, NEC, IEC, SAE, or local regulations). Always verify final designs against relevant codes and manufacturer specifications.

What this tool calculates

Estimated runtime based on battery capacity (Ah), load current (A), and depth of discharge. Basic formula: Runtime (hours) = Capacity (Ah) / Load (A).

For wiring the battery to loads, use voltage drop and wire sizing.

Real-world considerations

Actual runtime depends on battery chemistry (lead-acid vs lithium), Peukert effect (for lead-acid at high discharge rates), temperature, and age. For lead-acid systems, 50% depth of discharge is commonly recommended to preserve battery life. Lithium systems often support 80–100% usable capacity.

Marine example

100Ah AGM battery, load 10A, usable capacity at 50% DoD = 50Ah. Estimated runtime: 50Ah / 10A = 5 hours.

Compliance & engineering note

This tool assumes steady current draw and does not model inverter inefficiency, surge loads, battery aging, or temperature derating. Always verify critical system endurance with conservative assumptions.

FAQ

How long will a 100Ah battery last at 10A?
At 100% discharge, 100Ah ÷ 10A = 10 hours. For lead-acid, limit discharge to 50–80%. At 80% usable: 80Ah ÷ 10A = 8 hours. Lithium allows deeper discharge.
What is depth of discharge (DoD)?
DoD is how much of the battery capacity you use before recharging. Lead-acid: 50% DoD extends life. Lithium: 80–90% is common. This calculator uses your DoD setting.
Does the Peukert effect apply?
Yes. Lead-acid batteries deliver less capacity at high discharge rates. This calculator assumes a linear relationship; Peukert effects are not included. For high-peak loads, runtime may be shorter than shown.
How do I calculate battery runtime for a fridge?
Fridges cycle on/off. Use average current over 24h or use the compressor run current. For typical marine 12V fridge: 2–5A average. Enter that as load.
What happens if I discharge too deep?
Lead-acid: repeated deep discharge shortens life. Lithium: BMS usually cuts off before damage. Always size for your intended DoD and recharge promptly.

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