Alkaline batteries are one-time use only and are designed to be thrown away and replaced with new units. Rechargeable batteries, however, can be used again and again and are typically sold in combination with a charging unit. If you can't remember whether a battery is dead or still has power, you can test it in one of a few ways.
M/F Examine the battery itself to see whether it has "testing" buttons. Some rechargeable batteries (for example, Duracel) have two buttons on the body of the battery itself. When you press down on the buttons, a small indicator on the battery appears showing whether the battery needs charging.
M/F Place your battery into the battery tester that came packaged with the battery itself. Often, the unit to test the battery is the same unit you use to charge the battery. Once inserted, the tester is able to tell you the exact charge remaining in the battery and how long it will be before you need to charge it again.
M/F Insert your battery into a device in your home, like a television remote control. If all else fails, a sure-fire way to test the charge of a battery is to use it like normal. If you insert your battery into your remote control and your remote doesn't work, this is a good indication that the battery needs to be recharged before you can use it.