Electrical charge is measured in coulombs and given the symbol Q. It is a product of current (amps) flowing in a given period of time (seconds). This doesn't have to be a stored charge; it could equally be the energy consumed by an electric heater over the same period of time. Shown is a simple circuit to charge a battery pack made up of both series and parallel combinations of cells. As it's an enclosed unit it can be considered as just one battery. Note we are not concerned with the battery voltage as this is determined by the internal combination of cells (around 1.2V each), nor the amount of current that can be drawn from a fully charged battery. Batteries are normally purchased with a designed capacity measured in Ah (Amperes-per-hour). A fully charged 200Ah battery can deliver 100 Amps for two hours or one amp for 200 hours. Subject to the limitations of the battery construction, batteries can be charged at very high currents for short periods of time, or for very long periods at tiny currents known as trickle charging. TEST YOUR UNDERSTANDING: Do you know what the cost of one-unit of electricity is for your home? When you have found out read your electricity meter and note the units used so far. After a suitable time, say one day read the meter again. From the information gained determine the cost of running your home for a day.
