I bought batteries locally, and they are crap. I have a lithium capacity tester and they tested at 300-400mah. With these in, the group dies at c.35%. I found out because at first i was using an external battery pack to charge the group, but it turns out it wasn't charging properly for several reasons. So now for simplicity, i charge using a wall charger. I am making a habit of charging very often to avoid being stuck in one gear, which really, really sucks.
Note 1: you can / may to some extent charge the group on the go with a battery pack or your phone, but that assumes you're staying on top of the battery in these things.
Note 2: the 2nd time my group died in a ride, when back home, and after charging, i had to press the RD button for several seconds to reset the cage (it was stuck). Then took me 5min to re-calibrate the gears.
Note 3: the group talks to my garmin edge: battery indicator as well as which gear i'm in. I bothered setting that up only 3 days ago. It's quite neat.
Bottom line: charge your group...