Compare current full-charge capacity with original design capacity to calculate capacity-based battery health, and separate that long-term measure from today's charge level.
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The calculation
Capacity-based state of health (%) = current full-charge capacity ÷ original design capacity × 100
Worked example
A battery designed for 3000 mAh currently holds 2400 mAh at full charge. 2400 ÷ 3000 × 100 = 80% capacity-based health; the difference is 600 mAh, or 20% of design capacity. At 50% state of charge, the estimated stored amount is half of today's 2400 mAh, about 1200 mAh, while health remains 80%.
Full-charge capacity is an estimate that depends on measurement conditions. An 80% capacity result describes this battery's measured capacity relative to its design value; it is not a universal replacement rule, and charging does not restore lost capacity.
Video chapters
- 0:00 — Two meanings of battery percent
- 0:13 — Compare two capacities
- 0:32 — Divide current by original
- 0:45 — Check the missing capacity
- 1:01 — Health is not charge level
- 1:18 — Rearrange to find a capacity
- 1:33 — What this percentage cannot prove
- 1:52 — The capacity-health calculation
Read the full transcript
A battery can show fifty percent charge and eighty percent health at the same time. Charge says how full it is right now. Health compares how much it can hold when full today with how much it was designed to hold when new.
Use two capacities in the same unit. Suppose the original design capacity is three thousand milliamp-hours, and the current estimated full-charge capacity is two thousand four hundred milliamp-hours. The current reservoir is smaller even when both batteries are shown as fully charged.
Divide the current full-charge capacity by the original: two thousand four hundred divided by three thousand equals zero point eight. Multiplying by one hundred turns that fraction into eighty percent capacity-based battery health.
The difference is six hundred milliamp-hours. Divide six hundred by three thousand and you get twenty percent of the original capacity no longer represented in this estimate. Eighty percent retained plus twenty percent difference returns the whole original reference.
Now discharge this same battery to fifty percent of its present maximum. That is roughly twelve hundred milliamp-hours stored, while its capacity-based health still reads eighty percent. Recharging raises the fill level, but does not turn the current maximum back into the original three thousand.
The same ratio can solve backward. If design capacity is three thousand milliamp-hours and measured health is eighty percent, multiply three thousand by zero point eight to recover a current full-charge estimate of two thousand four hundred milliamp-hours.
This is a capacity-based estimate, not a complete diagnosis of power delivery, internal resistance, or safety. Reported maximum capacity can be recalibrated as measurement improves. Service decisions depend on the device and its maker, so there is no single universal replacement percentage.
Compare present full-charge capacity with the original design capacity in matching units, then multiply the ratio by one hundred. Here, twenty-four hundred divided by three thousand is eighty percent health, regardless of whether the battery happens to be full or half charged today.
Sources
- Calculator Academy — Battery-health calculator
- Apple Support — Battery capacity and health
- Apple Support — Battery-health recalibration
- US Department of Energy — Battery evaluation methods (PDF)