Here is a summary of the thread above that was posted by WomanOfHeart...
All mini and super-mini batteries are 3.7v batteries, and they charge to around 4.2v initially, as noted by haft2doit.
However, the Joye 510 and Ego/Riva/Tornado batteries do not work the same way as all the rest.
They use Pulse Width Modulation (PWM) to output a steady 3.2v until the battery needs to be recharged.
All of the other batteries start out at 4.2v and drop quickly, continuing to decline until the need to be recharged.
However Riva has a new SE battery that does not have the PWM, and therefore works like all others, if I understand correctly.