Here's the explaination of why the Booster doesn't work on a Riva:
On any un-regulated battery (whether it's a Riva or a mod or whatever), as more current is drawn from the battery, the internal resistance of the battery causes the output voltage to drop. So, as the battery is loaded, it will go from 4.2V (or whatever it sits at unloaded when fully charged) to some lower voltage. As the voltage drops, the current going into the atty drops. Eventually, the voltage and current out of the battery drop to the point where the battery can handle the current, and the system reaches an equilibrium point. This is why, with an LR atty for instance, the loaded voltage on an unregulated battery will drop to something like 3.2 V, since the current being produced by the battery into that load is something the battery can handle.
In a system with a switching regulator, the regulator will try to maintain the output voltage NO MATTER WHAT. So, if the output of the regulator is set to, say 4.2 V, and the battery is outputting 4.2 V, as the current is drawn by the battery the output voltage of the battery will drop but the output of the regulator will stay the same. As the output of the battery drops, the switching regulator will actually draw MORE current from the battery to keep the output voltage constant. This will cause the voltage of the battery to drop further, which will cause the switcher to draw even more current, causing the voltage to drop, etc., until the battery bottoms out and stops working.
So really, you can look at it this way: with an unregulated output there is a kind of negative feedback that allows the system to reach an equilibrium. With a switcher in there, there is a kind of positive feedback that can make the system unstable.
This is why boosted systems need to use
batteries that can handle the current without dropping the voltage too much. In the case of the eGo, the PWM limits the current by reducing the duty cycle, which has the same effect as using an IMR battery.
Hope that is clear.