# Amp hour

Physical unit
Unit name Amp hour
Unit symbol ${\ displaystyle \ mathrm {Ah}}$
Physical quantity (s) Electric charge
Formula symbol ${\ displaystyle Q}$
dimension ${\ displaystyle {\ mathsf {I \; T}}}$
system Approved for use with the SI
In SI units ${\ displaystyle \ mathrm {1 \, Ah = 3600 \; As = 3600 \; C}}$
Derived from Amp , hour

The ampere-hour ( Ah ) is a unit of measurement for electrical charge . Although it does not belong to the International System of Units (SI), it is approved for use with the SI, making it a legal unit .

For batteries and accumulators in particular, it is common to specify the nominal capacity in milliampere hours (mAh); the prefix for units of measurement, milli, stands for one thousandth.

## definition

An ampere-hour is the amount of charge that flows through a conductor in an hour if the electrical current is a constant 1 A ( ampere ). It is often used to indicate the available charge of accumulators or batteries in ampere-hours (Ah).

The ampere-hour is a multiple of the SI unit coulomb (ampere-second; 1 C = 1 As):

${\ displaystyle \ mathrm {1 \, Ah = 3600 \; As = 3600 \; C}}$

## Use in accumulators

The charge storage capacity of accumulators in ampere hours (Ah) is often referred to as their capacity. However, it has nothing to do with electrical capacity ( Farad ) or power capacity ( Watt ). It only has something to do indirectly with the energy storage capacity ( kWh ) (see below). Therefore, one should rather speak of charge carrier capacity or nominal charge .

Only the product of the nominal charge with the terminal voltage (ampere-hour times volts ) gives the energy storage capacity of an accumulator in watt-hours (Wh). The nominal charge is therefore not a direct measure of the amount of energy such as joules (J), watt seconds (Ws) or kilowatt hours (kWh). With a given voltage, however, it can be used to estimate the running time of the electrical consumer and to estimate the energy capacity of the accumulator. In this case, the running time is proportional to the nominal charge: ${\ displaystyle t}$

${\ displaystyle t = {\ frac {Q} {I}}}$

With

${\ displaystyle t}$: Time (unit: hours h).
${\ displaystyle Q}$: Charge (unit: ampere-hours Ah).
${\ displaystyle I}$: Discharge current (unit: ampere A).

However, the actual maximum values ​​can deviate from the manufacturer's information due to other factors, such as age, condition of the battery or ambient temperature.

Examples: With a 12-volt car battery, one ampere hour means 12 watt-hours (Wh), i.e. 12 watts that can be accessed for one hour, while that is only 3.8 Wh with a 3.8-volt smartphone battery and with a 1.2 volt AA battery only 1.2 Wh.