Battery Life Formula
Runtime (hours) = capacity (mAh) × efficiency factor / load (mA). Efficiency factor accounts for Peukert losses, temperature derating, and end-of-discharge voltage. For required capacity: capacity = load × hours / efficiency. Always add at least 20% safety margin to required capacity.
Frequently Asked Questions
Battery life (hours) = battery capacity (mAh) / load current (mA). For example, a 3000mAh battery powering a 100mA device: 3000 / 100 = 30 hours. In practice, multiply by a Peukert efficiency factor (typically 0.7-0.9) to account for battery inefficiency at higher discharge rates.
mAh (milliamp-hours) is a unit of electric charge that indicates how much current a battery can deliver over time. A 2000mAh battery can supply 2000mA (2A) for 1 hour, or 200mA for 10 hours, or 100mA for 20 hours.
Peukert's law states that at higher discharge rates, batteries deliver less total energy. A battery rated at 100Ah at the 20-hour rate may only deliver 70-80Ah at the 1-hour rate. The Peukert exponent (k) ranges from 1.1-1.3 for lead-acid, and is closer to 1.0-1.05 for lithium batteries (meaning lithium batteries are less affected by discharge rate).
AA alkaline: ~2500-3000mAh. AAA alkaline: ~1000-1200mAh. CR2032 coin cell: ~220mAh. 18650 Li-ion: 2000-3600mAh. 9V block: ~500mAh. 12V 100Ah lead-acid: 100,000mAh (100Ah). 100Ah LiFePO4: similar capacity but 80% usable vs 50% for lead-acid.
Cold temperatures significantly reduce battery capacity. At 0°C (32°F), a lithium battery may deliver only 70-80% of its rated capacity. At -20°C (-4°F), capacity can drop to 50% or less. Alkaline batteries are even more affected. High temperatures increase self-discharge and accelerate aging.
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