A 5,000 mAh Power Bank Does Not Hold 5,000 mAh for Your Phone

The load in the box is an average that may not exist

Very few devices draw a steady current. A sensor that sleeps at 50 microamps and wakes for two seconds at 80 milliamps every minute has an average nowhere near either figure, and getting it wrong in the optimistic direction is how a design that promised a year in the field returns after seven weeks. Work out the average from the duty cycle — how long at each level, times how often — rather than from the datasheet's active figure, and remember the cell's own self-discharge, which is the term that dominates once the average current gets small enough.

Cold and age both take capacity away

The capacity on the label is measured warm and new, and both conditions expire. A lithium cell in the cold delivers noticeably less than its rating and recovers when it warms, which is why a phone that dies at 30 % on a winter platform often restarts indoors with charge left. Ageing takes capacity away permanently, at a rate set mostly by how often the cell is cycled and how hot it gets while charging. Neither effect belongs in this calculation, which is why a design that has to work in year three at minus five degrees should be sized on a fraction of the rating rather than on the rating.

Frequently asked questions

What efficiency should I enter?

It depends on what sits between the cell and the load. A device running straight off the cell with a low-dropout regulator loses whatever the voltage difference costs and can be well under 85 %; a good switching converter can reach the low nineties; a USB power bank has a step-up and then the phone's own charger, two conversions in series, and the product of the two is what you get. When in doubt use 85 % for a single conversion and 80 % for a chain of two, then measure the real thing once and replace the guess.

Why is the phone battery quoted at 3.85 volts and the bank at 3.7?

Both figures are nominal voltages — a single number standing in for a voltage that actually falls from around 4.2 volts when full to around 3.0 when empty. Different cell chemistries and different charging strategies give different averages, and manufacturers pick the one that matches their cell. That is why watt-hours, which multiply the two together, are the only figure that can be compared between products, and why airlines print their cabin limits in watt-hours rather than in mAh.