I've been struggling with iffy gumsticks too...mostly Sonys, one Sharp, a couple of new "powerstream" batteries. They're all weak, even the new ones. I tried most of the tricks mentioned here, except freezing them. Of the old batteries, it seems the Sharp is the best...with wall charging, my NH900 seems to do the best. Though I've noticed with my equipment, I appear to get the best charge by connecting my RH10 via USB to a Mac. Obviously no data transfer is possible there, but it charges and charges and charges. Sure wish there was a reliable source of new gumsticks and or the Sony lith batteries.
Oddly, even though the RH10 charges from a USB on a Mac, it will not charge when plugged into my portable USB battery. Even though the battery was low, the display only showed a full battery icon. I disconnected immediately...