I'm new to this forum but I've had a solution to the "low battery" problem for a while now...and it works very well for me. Like most people with this issue, I also went through the quick fixes of firmware update, dielectric grease and serial battery replacements...at best with very transient success.
I accepted that a dead supercap was the problem, but unlike those with access to the failed component (Pro2), those of us with Vantage Vue consoles have the main board encased in resin.
So the only solution was an external power supply. I set this up with a 5W solar panel feeding into a regulator that charges a 1.2Ah 12V battery (its very small). The battery output is fed into a dc-dc converter which reduces the output to 3V, which I feed directly to the battery terminals of the vantage vue.
All the components are housed in a 4x4" weatherproof box mounted on the same pole as the ISS (with the wiring
running through drilled holes directly into the pipe)
Costs: solar panel $9, regulator $7, DC-DC converter (buck converter) $2...all from ebay and for about $20 I have a much more robust, durable and modular solution.
Basically, the system has enough capacity to power the ISS for exponentially longer than the supercap ever could.
Its all mounted neatly and looks pretty high-tech (IMHO!) and has been working well for about 6 months now.
Hope it helps a few of you with the same issue.