What is a battery?
A battery is simply a‘chemical machine’designed to store electrical energy. The term battery is generally taken to mean two or more individual electric cells. There are two basic types of cell; primary and secondary. Primary cells, such as ordinary ‘dry’ cells, are not rechargeable and must be discarded when exhausted. Secondary cells, such as the Lithium-Polymer type are rechargeable.
Voltage (Unit of measurement: Volts; V)
Voltage is equivalent to ‘electrical pressure’.
Current (Unit of measurement: Amps; A)
Electric current is measured in Amperes, abbreviated to Amps or even just A for short. Values of currents smaller than 1 Amp are usually referred to in terms of milliamps (mA). One milliamp is 1/1,000 Amp, so 500 mA is 500/1000 of an Amp, or ½ Amp.
Electrical resistance (Unit of measurement: Ohm; Ω) All electrically conductive materials have some electrical resistance. This includes the materials from which batteries, motors and speed controllers (ESCs) are made.
The electrical resistance of a battery has a special name – the resistance of the 14.8v lipo is termed its internal resistance. The internal resistance of a battery is a major influence in determining how suitable a battery is for high currents.
Cell capacity (Ah)
Suppose we have a one gallon water tank. This would be able to provide a flow of one gallon per hour for exactly one hour before it was empty. We could say the capacity of the tank was 1 gallon-hour. Similarly, a battery that could provide a current of 1 Amp for 1 hour is said to have a capacity of one Amp-hour, also written ‘1Ah’ or 1,000mAh.
This is a term much used when referring to lipo batteries for use in RC models. The term indicates the amount of current provided by a battery in relation to its capacity, or C. For example, the 1C current for a Gens ace 5000mah is 5000mah or 5.0 Amps. The table below illustrates this concept:
From the table, we can see that 1,200mA is the 1C current for a 1,200mAh battery. Notice that 1,200mA is also the 2C current for the smaller 600mAh battery. To recap, when using the idea of C ratings, we are specifying a current in relation to the capacity of the battery in question, rather than mentioning a specific current.
The C rate is therefore a useful indication of how hard a battery is working. For example, a fully charged battery pack would become discharged in about one hour at 1C, or only half this time, 30 minutes if the pack was made to work twice as hard with a discharge rate of 2C.
Note that C rates apply to both charge and discharge currents. It follows therefore that C rates are also a convenient way to describe the‘speed’ at which a battery is charged. For example, an empty battery charged at a constant 1C would theoretically be charged in an hour. (In practice, the charge current will need to be varied when charging lipo batteries, as we shall see later on.)