How Long Will Your Battery Last?

Battery life depends on two things: how much energy the battery stores (capacity in mAh or Ah) and how fast that energy is consumed (current draw in mA or A). This calculator gives you an accurate runtime estimate.

The Formula

Runtime (hours) = Battery Capacity (mAh) / Load Current (mA)

In practice, multiply by an efficiency factor (typically 0.7–0.9) to account for battery chemistry losses, temperature, and discharge rate effects. A 1C discharge (full capacity in 1 hour) is relatively efficient; higher rates reduce effective capacity.

How to Use This Calculator

Enter the battery capacity (found on the battery label, in mAh) and the average current your device draws. Optionally adjust the efficiency factor for more realistic results.

Practical Examples

Example 1: 10,000 mAh power bank powering a phone drawing 500 mA. Runtime = 10,000 / 500 × 0.85 = 17 hours.

Example 2: IoT sensor with 2,000 mAh AA cells, drawing 5 mA average. Runtime = 2,000 / 5 = 400 hours = ~16.7 days.

Example 3: 100Ah car battery running a 10A load. Runtime = 100,000 / 10,000 = 10 hours (before full discharge, which would damage lead-acid batteries — 50% DoD is recommended).

Tips for Longer Battery Life

• Use sleep modes to reduce average current in microcontroller-based designs.
• Match battery chemistry to the discharge profile (Li-ion for high drain; primary lithium for long-term low drain).
• Temperature affects capacity significantly: cold reduces lithium battery output.