How
batteries are rated
It is important when choosing a battery to choose the right one in order for you to ensure you get the performance you expect from it. There are two main values which you will see stated on batteries; Voltage (v) and Ampere-hour (Ah) however, these values are only a guide, there are a number of other things you need to consider when choosing your battery:
Battery voltage is the amount of power a battery can deliver. Just like you need to apply a certain amount of force to an object in order to make it move, a battery will need to provide a certain amount of power to an electrical device in order to make it work. If you cannot produce enough force to push the object, it won’t move; in comparison, if the battery doesn’t provide enough power then the device won’t work. Because of this, it is important to match the voltage of your battery with the voltage quoted on your device.
All batteries are rated in volts DC (Direct Current, an electrical current which flows in one direction only. Alternating current is electrical current which periodically reverses its direction).
NOTE: during a batteries life cycle, its voltage is not constant. The type of battery being used and the conditions of its use will determine the characteristics of this voltage decrease (see Real Battery Characteristics below for more details).
Relates to the capacity of the battery; it is the amount of current that can be withdrawn continuously from a battery over a period of time at its rated voltage.
It is a unit of Electrical Current * time.
Electrical current (Ampere): Is the flow of electric charge through a conductor.
Time: Is measured in hours.
The value actually stated on the battery is the best possible capacity at a specified current under a defined temperature and featuring a specified cut-off voltage.
Cut-off voltage: is the minimum voltage level when a battery is considered no longer useable in a given application.
The Voltage of a battery is not constant, it will reduce over its life with the rate of this decrease depending on the type of battery e.g. Alkaline, Lithium etc, and the rate of discharge from the battery, i.e. the amount of current taken from the battery by the device. Batteries will still operate when their voltage decreases, but the power output from the battery will obviously decrease. Power output (watts) is the product of the discharging voltage (volts) and discharging current (amperes); your device will require a certain level of electrical power to operate. Therefore if the voltage drops, this will reduce the power delivered by the battery, and if it reduces too far then the device will shut down.
A common mistake
A common mistake is to estimate the operational life of a battery by using explicitly the ratings quoted on the battery. This may seem logical, but the ratings are only meant as a guide, it is impossible for companies to list all of the batteries characteristics on the battery so the information has to be limited to these ‘ideal’ often ‘maximum’ values.
Example, say a battery is rated at 20 Ampere-hour (Ah), you application drains 20 Amperes from the battery; you assume this to work for an hour. This is unlikely to be the case because running the battery at its quoted rating will cause the voltage of the battery to drop off, possibly quite rapidly. Once the batteries voltage drops below the applications cut-off voltage then the device will shut down, even though the battery is still capable of producing electrical power.
In order
to be sure that the battery you choose for your application will perform
to your requirements, check with the battery manufacturer and if possible
look at the datasheets for the batteries. Some examples
of such datasheets are available in out Technical
Information section