Understanding the sizes of batteries

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 May 2017

Power is the rate of flow of energy into or out of a gadget or a substance. That is, the amount generated or consumed in a unit of time (per second, per minute, per hour etc). Clearly then, if power is multiplied by the time elapsed, the result is a measure of the energy transferred.

For example: A typical domestic light bulb rated at 60 watts (60W) consumes (and radiates) 60 units of energy in one unit of time. Now, scientists and engineers use the second as the standard unit for measuring time, however, for commercial trading, the electric companies use the hour.

Thus, if the bulb is left on for one hour, the electric company will bill you for 60 watt-hours (Wh) while scientists and engineers will talk about 216,000 watt-seconds(Ws). Watt-seconds are joules (J) – the international standard units of energy. The two amounts of energy (60Wh and 216,000J) are exactly the same; only expressed in different units.

The electric company uses watt-hours because watt-seconds are very small units; in fact, even the watt-hour is too small so the preferred unit is the kilowatt-hour. That is, thousands of watt-hours (kWh).

Battery manufacturers do things in a different way. They know that the nature of the chemical reactions going on inside batteries produce a fixed voltage. They also know that when volts are multiplied by amperes the result is power. So, instead of giving a power rating in watts, they state the electric current (amperes) that the battery produces.

Consequently, instead of stating the energy content in watt-hours, they show it in ampere-hours (Ah). Thus, the typical car battery may be rated at 40Ah. This means that it can produce one ampere continuously for 40 hours and then go flat. Alternatively, it will generate 40A for only one hour…or any other combination of amperes and hours whose multiplication product is 40Ah (e.g., 8A for 5h. etc)

Mobile phone batteries indicate their energy capacities in milliampere-hours (mAh). Just like the 1,000 millimetres in one metre and the 1,000 millilitres in one litre, there are 1,000mA in one ampere. So, when you see you phone battery saying 2,000mAh, this is equivalent to 2Ah.

However, the 2Ah in a cell-phone battery is not the same amount of energy as 2Ah in a car battery. The reason is that the former is supplied at 3.8 volts while the latter is at 12 volts. So, to make a proper comparison, the two must be converted into similar units of energy – say, watt-hours. This is obtained by multiplying the Ah by the voltage of the battery.

Thus, 2Ah at 3.8V is 7.6Wh while 2Ah at 12V is 48Wh. Clearly, the 2Ah-car battery has a lot more energy than a 2Ah phone battery – over six times the amount. So the natural question is: How many cell-phone batteries would be enough to run a car?

Well; that is a story for another day.