The capacity of a cell indicates the charge it can deliver, under specified discharge conditions, before its voltage drops to an unsatisfactory level (1 Ah = 1 A flowing for 3600 seconds, i.e. 1 Ah = 3600 coulombs). This level may be determined by the voltage requirement of the equipment or, in the case of rechargeable batteries, by a voltage below which the cell cycle life is degraded.
The Ah capacity of a cell is proportional to the weight of the balanced active ingredients, but the total weight includes the case, seals, electrode substrates, connectors, electrolyte, separators, etc. and generally far exceeds that of the active materials alone. Generally, the smaller the cell, the poorer its capacity/weight ratio. Small cells will also dissipate any heat produced during discharging faster, as well as reach ambient temperature faster. The latter is especially important in cold weather.
In most cases, cell voltages lie between 1 V and 4 V. The higher voltage cells require non-aqueous electrolytes, because the electrochemical window of water is only of the order of 2.5V. Cells may need to be connected in series to make a battery of higher voltage. Cells connected in parallel make a battery with more ampere hours.