The capacity of a battery means how much current (see below) can be delivered over a period, and is usually expressed in terms of current over time, e.g. Ah (ampere per hour) or more commonly mAh (milli-ampere per hour).

It is important to note that batteries are usually designed for a particular application. For instance, a car battery delivers a large current over a short time when starting up the car, while a watch battery delivers a small current over a long time.

Therefore, the capacity is not an absolute value. For instance, a 1 Ah battery should not be expected to deliver 1 ampere for one hour, but rather a lower current over several hours. The standard conditions used in labelling rechargeable batteries is 20 hours at +20°C. Thus, if a battery is labelled 1000 mAh, the manufacturer usually means that it has the capacity to deliver 50 mA for 20 hours at room temperature.

Flow of electricity. Measured in ampere (A). Note: current means the flow of electric energy at any moment — not over a certain period of time.

The life span of a rechargeable battery depends largely on two factors: the number of recharges, and the way it is recharged. Under normal conditions and proper recharging cycles, a rechargeable battery could be expected to last up to 3 years or up to 500 recharging cycles.

Battery life may be shortened for a number of reasons, including:

• Battery left on charger too long
• Battery not being fully discharged prior to recharging
• Too long time between recharging cycles

The gradually decreasing ability of a battery to reach its full capacity, often due to being repeatedly recharged without first having been discharged.

Note: depending on the type, some batteries can be 'rejuvenated' by a series of hard discharges and full recharges (if permitted according to the manufacturer's specifications). However, a battery cannot be reconditioned to a state better than its age or the number of recharge cycles — in other words, a battery cannot be restored to 'as new'.

Defines the relationship between voltage, current and resistance:

V = I x R

where: V=voltage in volt, I=current in ampere, R=resistance in ohm

Power calculation in a pure resistive circuit:

P = V x I

where: P=power in watt, V=voltage in volt, I=current in ampere

The total voltage of a battery with multiple cells serially connected:

Tv = C x V

where: Tv=total battery voltage, C=number of cells, V=voltage of each cell

The total current of a battery with multiple cells connected in parallel:

Ti = C x I

where: Ti=total battery current, C=number of cells, I=current of each cell