Lithium Battery voltage is acceptable down to about 2.8 Volts. We use a 3.6 Volt AA size battery manufactured by Tadiran or SAFT. 

You need to use care when removing and replacing this battery. It is soldered into a multi-layer circuit board. A common error is to damage the plate through holes. This sometimes leads to an internal short which destroys the board. Replacement boards are $1,850.00 each.

We suggest you measure the current drawn from the Lithium battery by the circuit board before you solder the battery in place. Current of more than 50 micro Amperes indicates other faulty components on the board that should be replaced. Otherwise the battery will discharge well before its expected life,

We sell replacement batteries, P/N 40-202-005, for $16.00 each