Watt hours (Wh)
× 90 Wh
Electrical voltage (V) With a few exceptions, current vehicles are only equipped with 12V starter batteries. It was not always the case which explains why some vintage cars that are still running require special 6V batteries.
In this situation it is inadvisable to install a 12V battery, as the electrical components would be destroyed by the higher voltage. Large commercial vehicles usually operate a 24V system. To achieve this two heavy-duty 12V batteries are connected in series.
Ampere hour or Amp hour (Ah) The Amp hour rating is a measure of the electrical energy stored in a battery. It is defined by the amount of energy, a battery can deliver continuously without recharging for 20 hours at 25°C, without falling below 10.5 volts. For example a 60Ah battery will deliver a current of 3A for 20 hours (3 x 20 = 60).
For a small car with only a few electrical devices, a battery with 40-45Ah maybe sufficient. Premium vehicles and sports cars are generally equipped with batteries with capacities of up to 110Ah. Commercial vehicle batteries can be rated up to around 240Ah. The charge capacity of a battery reduces with increasing age and other factors such as ambient temperatures and humidity.
The Ah rating on the label is a legal requirement on all starter batteries sold in Europe, but this can differ in other parts of the world. North America for example uses Reserve Capacity (RC) which reflects the discharge time in minutes at a 25A discharge.
Cold Cranking Amps (CCA) CCA is another important rating to consider. It specifies the battery s ability to start an engine in cold temperatures. In general, it is much easier to crank (start) an engine in a warm environment than in a cold one. The rating refers to the current, a 12-volt battery can deliver at 0°F (-18°C) for 10 seconds whilst maintaining a voltage of at least 7.5 volts. The value on the label is determined by a precise test procedure, which in Europe is defined by EN50342-1 standard, and must comply with requirements of EU.Reg.1103/2010.
The higher the CCA rating, the greater the starting power of the battery. Replacement batteries should always be equal or exceed the OE battery in ratings. Fitting a new battery that has a lower CCA than the original equipment could result in poor performance.
Watt hour (Wh) Watt hours are a useful measurement for deep-cycle supply batteries, used to power lighting and appliances for marine/ leisure applications. Unlike the short burst of high energy required from a starter battery, the need here is for long, slow duration of electrical loads.
First calculate a load such as a 40 Watt television switched on for 3 hours (40W x 3 hours = 120Wh). By adding all the appliances (W) x required duration (h), we can work out a total electrical need between battery recharges. This might be met by a single battery or a multiple bank of batteries. This subject of supply batteries and power calculation is covered in more detail on page 40.
BATTERY CAPACITY THE NUMBERS EXPLAINED
Ampere hour/Amp hour (Ah)
× 60 Ah
Cold Cranking Amps (CCA)
25°C 10.5 V 0°F (-18°C) 7.5 V