Well since I carry my empeg in a pelican case I think my best bet for battery powered empeg is a sealed lead-acid battery. For 11.99 you can get a 12 volt 4Ah battery from batteries.com. Im pretty sure these dont need any special charging circuit I could be wrong though.
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Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam.