How to use the battery life calculator
The formula
Run time (hours) = capacity (mAh) × 0.85 ÷ load (mA)
The 0.85 factor is the honest-planning adjustment: nameplate capacity assumes a slow, gentle discharge down to empty, which real devices never quite get. Choose a lower factor in the dropdown for lead-acid batteries (which should only be half-discharged) or cold, ageing packs.
How to use it
The capacity is printed on the battery (mAh for small cells and power banks, Ah for big ones — multiply Ah by 1,000). The load current is on the device label, or measure it: mA = watts × 1,000 ÷ volts.
Example
A 2,000 mAh battery running a 100 mA load: ideal maths says 20 hours; with the 0.85 factor the honest estimate is 17 hours. Those are the values loaded when the page opens.
Frequently asked questions
How is battery life calculated?
Hours = capacity (mAh) × efficiency ÷ load (mA). The 0.85 default factor turns the optimistic nameplate capacity into a realistic planning number.
Why 0.85 and not the full capacity?
Battery capacity is measured under gentle laboratory conditions. Real loads draw harder, batteries dislike being emptied completely, cold weather cuts capacity, and devices switch off before the battery is truly flat. 0.85 covers typical cases; use the dropdown for harsher ones.
My battery is rated in watt-hours (Wh) instead. What then?
Convert: mAh = Wh × 1,000 ÷ battery voltage. A 50 Wh laptop battery at 11.5 V is about 4,350 mAh. If your load is given in watts, mA = W × 1,000 ÷ V.
Does a bigger load shorten life more than proportionally?
Often yes — especially for lead-acid batteries (Peukert's effect): doubling the current can more than halve the run time. This estimate is linear, so treat high-drain results as optimistic.
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