Without a reliable power source even the most high-tech piece of machinery is little more then a paper weight. Since batteries are relatively small and self-contained, they are a great source of power for many types of electronic applications.
The first thing to consider in choosing a battery is the specifications of the device it will be powering. First and foremost, it's crucial to consider the input voltage
of the device. You can find the input voltage on the datasheet or instruction manual of the device. What you want to make sure is that the input voltage of the device (which is often within a range of numbers), falls within the voltage provided by the battery. As you will find out, using a battery that has a higher voltage rating than the device excepts is a good way to fry the device.
After making sure the battery is within the input range of the device, there are at least two further things to look at, the specific voltage of the battery itself and the mAh, or milliamp hour, of the battery. Voltage
as opposed to Current
, is an all or nothing deal. Another words, if you are using a 9V battery to power your device, the battery will be "pushing"
all 9V into the circuit (as long as it is powered). The more voltage that you provide, the more "energy" you will be putting into the system. Further, without a way to regulate the voltage with various components (resistors, volt regulators, etc), it is still possible to cause ill effects to the system in the long term by stressing it too much, even if it is within the voltage input range of the device.
Consider, a small electric motor and an R/C battery pack that provides 11.1 volts. At full charge, (and in good working order), the battery will provide somewhere around 11.1 volts to the motor, which will vary based on the chemical makeup of the battery and the device itself. Some small electric motors often are rated at 3-6 volts, so providing 11.1 volts will significantly stress the system. Even though the motor will still probably turn, the voltage used will likely shorten the lifespan of the motor by a good bit given such a difference. On the other hand, using a 1.5V AA battery to power the same motor will result in the motor either not turning or turning weakly, often not having the effect desired in applications. However, a lower voltage than specified will usually not hurt a circuit.
When dealing with current, measured in amps, the device you are dealing with is "pulling"
the current from the battery. Every battery has an mAh rating which states how much milliamps per hour a battery provides. A device like an LED draws a fairly consistent current. A 40 miliamp LED will draw about 40 miliamps from the battery per hour. Motors, on the other hand, draw a variable amount of current depending on how much work the motor is doing. When the motor is turning a heavy wheel or carrying a large load it will draw significantly more current than when it is free running.
As an example, consider an R/C battery pack rated at 9.6V and 1600 mAh.