Battery Life & Runtime Calculator (mAh, Load Current, Wh

Battery Electronics: Capacity, Discharge Efficiency & Peukert's Law

Battery capacity is rated in milliampere-hours (mAh) or ampere-hours (Ah), representing the total electrical charge a battery can deliver under nominal conditions.

Why Batteries Don't Deliver 100% Rated Capacity

• Internal Resistance & Peukert's Law: Drawing higher currents causes internal resistive voltage drop (I^2 R heating), reducing effective usable capacity.

• Low Voltage Cutoff: Electronics shut down when battery cell voltage drops below a cutoff threshold (e.g., ~3.0V for Li-ion) to prevent permanent chemical damage.

• Ambient Temperature: Cold temperatures slow electro-chemical reaction rates, reducing runtime by 20% to 50% in sub-freezing conditions.

How to Use This Calculator

Enter your Battery Capacity Rating (mAh) (e.g. 5000), Nominal Battery Voltage (V) (e.g. 3.7 (Li-ion) or 12.0 (Car/Solar)), Device Load Current Consumption (mA) (e.g. 250), Battery Chemistry & Discharge Profile — choose from Lithium-Ion / LiPo (3.7V, ~90% effective efficiency), Lithium Iron Phosphate / LiFePO4 (3.2V, ~95% efficiency), Lead-Acid / AGM (12V, 50% max recommended DoD, ~70% eff), and Discharge Cutoff Safety Reserve (%) (e.g. 10 (prevents deep drain damage)). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Battery runtime, energy storage, and C-rate electrical formulas: Runtime (Hours): t = (Capacity (mAh) × (1 - Reserve) × η) / (Load Current (mA)) Stored Energy (Watt-hours): Energy (Wh) = (Capacity (mAh) × Nominal Voltage (V)) / (1,000) Power Consumption (Watts): P = V × I = Voltage (V) × (Current (mA)) / (1,000) C-Rate: C = (Load Current (mA)) / (Nominal Capacity (mAh))

Worked Example

For example, running a 250 mA device on a 5,000 mAh 3.7V Li-ion battery with 10% safety reserve (90% efficiency): . Runtime is . The battery stores 18.50 Watt-hours (Wh) of energy, power consumption is 0.925 Watts, and the discharge rate is 0.050C.

Understanding Your Result

This calculation gives a precise result based on the inputs you provided. For practical decisions, consider whether the result changes meaningfully with slight variations in your inputs — if a 5–10% change in input causes a significant output change, gather more precise input data before deciding. Use this tool to quickly compare multiple scenarios side by side.