What is the total voltage of a fully charged battery?

This depends on the actual battery and what it is made of.

A 1.2 volt rechargable NiMh battery typically gives off a voltage of 1.3 to start with, then it rapidly goes down to 1.2 volt where it stays stable until almost empty, then it quickly drops below and down to .5 volt or even further down..

The same applies to Lead Acid or NickelCadmium batteries.

Different types of batteries have different properties.

NickelCadmium normally are able to give a lot of current until empty.

where as the current you get from a NiMh battery is rather limited.

Lead Acid are capable of a very high current but still dependant on the internal design of the battery.

Unfortunately one can not give a correct (for all) answer to a general question.

Regards.