Batteries
There are two types of batteries, Alkaline and Lithium. You can measure the battery voltage on the touch port or directly on the battery. Make sure your voltmeter is calibrated correctly.
Alkaline Battery
This used to be 2 AA batteries soldered together. This is not a very good way to join batteries as the heat from the solder can damage the batteries. It can however still be used and has proven to work for many years if done correctly. The best replacement is the white battery pack available for the replacement. This pack also includes an extra fuse and has not been soldered so it will last for many years. This battery should last unit it reaches approximately 2.6v. But should be replaced if it is below 3v.
Lithium battery
This is a smaller blue battery with flexible wires from the ends. There have been some batches known to break at the point where the cable enters the battery. This plate in the end has also been found to break off on occasion. This battery should last unit it reaches approximately 2.8v. But should be replaced if it is below 3.4v.