First off, be careful with those battery packs. I used old 18-volt cordless battery packs from the tools you get at Harbor Freight, and if you’re not careful you can zap yourself pretty good if they are still holding a charge. If you have access to new Ni-Cad batteries (look on eBay) I would go that route, although $ is always a limiting factor.
Solar Laptop/Device Charger
Use reclaimed batteries, a solar panel, and a charge controller to make a charging station.