Free battery life calculator: convert mAh and volts into watt-hours, then divide by your device's wattage to see how many hours one charge will last.
Battery Life Calculator turns a battery's rated capacity and your device's power draw into an expected runtime, so you can plan a shift, a trip or a sensor deployment instead of guessing.
Enter the capacity in milliamp-hours (mAh) and the pack's nominal voltage. Multiplying the two gives the stored energy in watt-hours: 10,000 mAh at 3.7 V holds 37 Wh. Divide that energy by the load in watts and the units cancel to hours, because one watt is one joule per second.
The efficiency input covers what the ideal equation ignores: converter and wiring losses, plus the share of capacity you never intend to discharge. A USB power bank feeding a laptop through a boost converter typically lands near 85%, while a direct DC load on a lithium pack can reach 95%.
Nominal voltage is the number on the label, not the fully charged value: 3.7 V for a lithium-ion cell, 3.2 V for LiFePO4, 12 V for a lead-acid block. If you only know amp-hours, multiply by 1000 so the capacity fits this field.
Treat the answer as an upper bound. Usable capacity shrinks at high discharge rates and in the cold, and keeps shrinking as the cells age, so leave headroom for anything critical.
At 3.7 V nominal the pack stores 10 Ah x 3.7 V = 37 Wh. Applying 85% efficiency leaves 31.45 Wh usable, and dividing by 5 W gives about 6.3 hours.
Rated capacity assumes a gentle discharge at room temperature. High current, cold cells and age all reduce usable capacity: a cell rated 3,200 mAh can deliver roughly 2,300 mAh at a 2C load. Lower the efficiency input if your load is heavy or the battery is old.
No. Use the nominal voltage printed on the label, such as 3.7 V for a lithium-ion cell or 12 V for a lead-acid battery. The peak charging voltage would overstate the stored energy.