I think remember Hugo saying the big cap (is it a super cap?) should last up to 2 weeks keeping the RTC going. This should charge almost immediately after turning the unit on so there's no need to "give it a good charge" as you would a car battery.

For the record, I recently went overseas for 15 days with my empeg powered off and the RTC kept time fine. And yes although I left it in the sled in my car, I had disconnected the -ve battery terminal (for other completely unrelated reasons)