True enough, the ability to make use of the stuff you have on hand adds another layer of preparation. As long as you know its limits ...

It's not feasible for these solar lights to recharge 2000 mAh AA Eneloops or similar. They don't produce enough current. They may produce enough to charge a AAA NiMH (these run between 600-1000 mAh) using a wad of aluminum foil as a spacer.

The batteries in the first solar lights were NiCd, and they were junk. They use low-capacity NiMHs now, about 600 mAh, in an AA format. The cells are the standard length but are often skinnier. So, if you take them out and plunk them in a low power LED flashlight, expect them to rattle around and have intermittent contact. Paper makes an effective spacer.

Solar light batteries suffer the same problem as other NiMHs -- partial charging and discharging reduces their capacity over time. I played with some old ones last winter and several cycles of full charge (with a smart charger) and full discharge brought them back into a usable condition. Surprising -- but you never know unless you try.