what most pilots here do (i have read) is have a few packs, one guy has an ingenious idea, have 3 packs (he runs a Nextstar), he flies for a bit, but doesn't DRAIN THE PACK FULLY, so it charges faster, he brings it in for a landing, has a bud slap that pack in a cooler with that reusable ice pack stuff, and gets one fresh off the charger, so by the time he is done with the new pack, he slips the "cooler pack" into the charger..... and so on and so forth, as for you, since it uses 2 packs, I don't think that is the most efficent route, I would maybe have one or 2 extra "pairs" of packs, if you can talk the wife into one, hey, Christmas is around the corner....
, as for charging, I guess they use like a SLA (sealed lead acid battery), same thing your car runs, only it doesn't rip your arm out of it's socket as you drag it over, they work good, a simple search can turn up a few, also check with the local phone company, they may have some lying around that are perfectly fine, but they changed them out for safety's sake[8D]