There are three things you should consider when buying batteries. The first and simplest is the size, but, given that most appliances are designed and built with a determined battery size the choice of battery size has already been made for you by the manufacturer.
Common Battery Sizes
AAA Size - This battery is usually found in low power drain appliances such as MP3 players, remote controls and digital cameras.
AA Size - Because of their larger size, A batteries have about three times the capacity of AAA batteries.
C Size - The C battery type is also known as 14A, LR14 and R14. Although the voltage of a C battery is the same as an AA, C batteries provide power for a longer period than AA batteries because of their larger size. C batteries are typically used in medium drain applications like toys and musical instruments.
D Size - This battery is the largest of the D, C, AA, and AAA family. D batteries are usually used in high current drain applications, such as powerful torches, 2 way radio systems, portable entertainment systems, or other equipment requiring extended run time. The D cell is also known as 13A, LR20 and 13D.
9V Size - Sometimes referred to as a PP3 battery. It is commonly used in appliances such as domestic smoke detectors, carbon monoxide alarms and digital clocks. PP3 actually refers to the type of connection or snap that is on top of the battery which consists of two connectors: one small and circular and another larger one usually either hexagonal or octagonal.
Coin or Button batteries - These are the small coin sized and shaped batteries commonly found in watches, digital cameras and other small appliances.
Materials Used in the Battery
Alkaline cells provide more energy for a given size of battery, and they have a longer shelf life. They’re well suited to devices with low to moderately-high power drain, particularly if they’re in more frequent use. One drawback with alkaline cells is that they’re not well suited to use in devices with very high power drain, such as digital cameras.
Single use Lithium
Lithium batteries are significantly more expensive than alkaline ones, However, they’re more suited to use in high drain devices- such as 2 way radios and digital cameras to run out much quicker than they would normally.
Rechargeable Nickel Cadmium (“NiCd” or “NiCad”)
These are an older technology and are being replaced by the more powerful NiMH batteries, not least because the cadmium in the battery is highly toxic and should always be disposed of properly and not just thrown away.
Nickel Metal Hydride (NiMH)
NiMH batteries generally have a higher capacity than NiCds, and have overtaken NiCad batteries for this reason. Please be aware though that in periods of prolonged non use, MiMH batteries will run ‘flat’. NiMH rechargeable batteries are less harmful to the environment than NiCd ones.
Single Use or Re-chargeable
Single-use batteries are produced on a greater scale than rechargeables, making them initially cheaper to purchase and widely available and despite their poor image from a ‘green’ recycling perspective re-cycling opportunities for them are now readily available.
These allow the battery to be recharged and reused multiple times and are thought to save the user money over single-use batteries in the long run but the initial cost is much more than single use batteries, and you need to buy a charger. Re-chargeable batteries are considered to be less environmentally friendly by some because of the heavy metals they contain.