A device battery health log: what phones, Windows laptops and the Linux power-supply interface report

What it is

Phones and laptops carry a gauge that estimates how much charge the battery can still hold, and their operating systems show some of that estimate. Apple's support page states that the Battery Health screen's maximum battery capacity measures the device battery capacity relative to when it was new, that the figure might show as slightly less than 100 percent depending on the time between manufacture and activation, and that batteries of iPhone 14 models and earlier are designed to retain 80 percent of their original capacity at 500 complete charge cycles under ideal conditions (1000 cycles for iPhone 15 models). Microsoft's documentation describes powercfg /batteryreport as generating a report of battery usage characteristics over the lifetime of the system as an HTML file in the current path. The Linux power-supply class documents CHARGE_FULL_DESIGN as the design value and CHARGE_FULL as the last remembered value of charge when the battery became full, and warns not to mix charge (µAh) and energy (µWh) attributes. A battery health log records these device-reported figures on a fixed day each month.

Why it matters

A single reading is an estimate of unknown accuracy; a series shows whether it drifts smoothly, drops in steps, or jumps back after a software update or a full discharge. Only the series lets a household say which of those happened to a given device.

How to apply

• Fix one day per month and record, per device: date, operating system version, the reported capacity figure exactly as shown with its unit, the cycle count if shown, and on Linux both charge_full and charge_full_design (or the energy_* pair, never mixed).
• Save the Windows report file with the date in its name rather than copying numbers out of it.
• Keep an events column: full discharge, replacement, long storage, unusually hot days, software update.
• Compute the ratio of full to design capacity as a derived column at reporting time; keep the raw attributes.
• Record a device that reports nothing as a row saying so, with the reason.

Pitfalls

Vendors define "full" and "new" differently, so figures are not comparable across devices, only within one device over time. Re-calibration after a full charge and discharge is commonly reported to move the figure without any change in the cell. The log documents what the device reports; it gives no charging routine and does not predict when a battery will need replacement.