Battery Runtime

Formula

Description

Battery runtime is estimated by dividing the battery capacity by the average load current, giving the theoretical number of hours until the battery is depleted. A 3000mAh battery supplying 500mA gives a theoretical runtime of 6 hours. However, real-world runtime is typically 70-85% of this theoretical value due to several factors: the Peukert effect (higher discharge rates reduce effective capacity), voltage regulator efficiency losses, and the minimum voltage cutoff of the circuit which leaves some energy unused in the battery.

Variables

• t — Runtime in hours (h)
• Capacity — Battery capacity in milliamp-hours (mAh)
• Current — Average load current in milliamps (mA)

Practical Notes

Battery capacity is rated at a specific discharge rate (typically C/20 for lead-acid, C/5 for lithium). Drawing current faster than the rated discharge rate reduces the usable capacity. For pulsed loads, use the average current including sleep modes. Temperature also significantly affects capacity: lithium batteries lose 20-30% capacity at 0°C compared to 25°C.