Slightly OT, but thought I would chime in as I've been testing this exact setup for the better part of a year.

My practical experiments indicate that the only way to make this work is to bypass all the internal circuitry in these lights and hard-wire them directly from the solar panel to the battery terminals. The cheapies I'm using (including some tossed in the dump, NiCd batts and all -- grr!) produce a little over 4 VDC directly, though at a such a miniscule current that you can't really overcharge.

I find that they will effectively charge low-capacity NiMH cells (in the 1200+ mAh range), but are barely enough to maintain high-capacity cells (2500s) at their existing voltage.

Bottom line: with 2-3 of these rewired units plus cheapo NiMHs ($1 ea.) and a decent 1-AA LED light, you have more-or-less perpetual light as long as you have sunshine. Naturally, YMMV.