How battery runtime is estimated
A battery's rated capacity is measured at a specific discharge time, often 20 hours for lead-acid and 1 hour for lithium. Draw current faster than that and lead-acid delivers noticeably less. Peukert's law captures this effect with an exponent k: about 1.1–1.3 for lead-acid and close to 1.0 for lithium.
- Load current:
I = P / (V·η) - Runtime:
t = H·(C/(I·H))^k × depth of discharge
Worked example: camper battery
A 100 Ah, 12.8 V LiFePO₄ battery powering a 150 W load through a 90% efficient inverter supplies about 13 A. With k = 1.03 and 90% usable depth of discharge it runs for about 7.3 hours.
Good to know
- Discharging lead-acid below 50% sharply shortens its cycle life.
- Cold reduces capacity, especially below about 10 °C. This estimate assumes room temperature.
- Pro sizes the bank: capacity needed, batteries in series and parallel strings for your required runtime.