A Runtime Figure Is A Load Assumption In Disguise

Runtime is presented as a property of a device and is in fact a property of a scenario. The battery holds a fixed quantity of energy, and the hours quoted are that quantity divided by an assumed rate of consumption.

Energy is fixed, consumption is not

A pack stores a defined amount of energy, expressed as a capacity at a nominal voltage, and that figure is a genuine physical property that can be measured.

Runtime is derived by dividing it by an average power draw, and that draw is chosen by whoever wrote the specification rather than observed in the field.

Because the divisor is an assumption, doubling a runtime claim can be achieved without touching the battery, simply by testing a lighter task.

Test conditions are chosen to be favourable

Standby and low-activity periods are usually included in the assumed pattern, since idle time consumes very little and stretches the average considerably.

Screens are tested at moderate brightness, wireless functions at low activity, and motors under light load, all of which are legitimate settings and none of which are typical of hard use.

Ambient temperature is also part of the scenario, and cells deliver less usable energy when cold, which is why cold-weather runtime is consistently below the claim.

Consumption is rarely proportional to effort

Many devices draw a base current merely to remain on, so light use consumes far more than a proportional share of the pack's energy.

Others accelerate steeply, with a motor working against a heavy load drawing several times the current of light work rather than a modest increment.

This is why a single runtime figure cannot be scaled to a different task, and why quoting a range with the conditions attached is more informative than a single number.

Capacity itself is stated in different ways

Amp-hour figures depend on the voltage they are measured at, so the same pack can be described with different numbers by quoting cell voltage or pack voltage.

Nominal capacity is measured at a low discharge rate, and drawing current quickly reduces the energy actually available, which is a property of the cell chemistry.

Usable capacity is smaller again, because protection circuits reserve a margin at both ends of the charge range to preserve the cells' life.

The figure ages along with the pack

A published runtime describes a new battery, and capacity falls with both cycles and calendar time from the moment of manufacture.

A pack that has lost a meaningful share of its capacity delivers proportionally less runtime while behaving normally in every other respect, which is often read as a fault.

The specification that predicts long-term satisfaction is therefore not the runtime but whether the pack is replaceable and still sold when that decline becomes noticeable.