Buying a 12 v battery pack can be confusing if it's your first time replacing your battery. Determining your exact match for you power tool will depend on a some key factors: brand, voltage, capacity and chemistry.
The easy part first: find your brand of power tool and then you will know what battery brand you need. Next, look at the voltage on the current battery pack that you have. Voltage or volts is an electrical measure of energy potential. In this case, I am going to assume you need a battery pack with the energy potential of 12; in other words, the amount of "pressure" of electrons that pass from a negative connector to a positive connector. There is more to voltage than this, but I will cover that another time.
Now let's concentrate on the battery chemistry you can use for your power tool battery replacement. There are three chemistries mostly likely that would work for you: NiCd, NiMH and Li-ion. NiCd (nickel-cadmium) is the most economical at first glance, but not always the best for heavy jobs where you need your battery to last a long time. It does charge quickly though and performs well in low temperatures. NiMH (nickel metal-hydride) will hold a charge longer and is environmentally friendly. Li-ion (lithium ion) is the lightest weight which is great for professional uses, has a higher energy density, and a no memory which is helpful when recharging the battery.
Finally, you need to make sure that you buy a battery capacity that is suited for your needs. Capacity quantifies the total amount of energy stored within the battery and is labeled like this: Ah. Capacity or runtime is important when replacing your 12 v battery pack because you want your power tool to work for you as long as you can and higher capacity means less time between charges which will save you money in the long run.
At Batteryship.com you will find many choices for all your 12 v battery pack needs. Not only will you find the right price you will also be getting 30-70% more value with these aftermarket products than you will with the OEM (original equipment manufacturer) replacement parts.