Most solar chargers are not smart chargers. To get a long lifespan from batteries you want to be careful to charge at a good rate, don't over charge, and don't under charge, the majority of solar chargers do not have any mechanism to do any of those.

I saw that voltaic backback mentioned above but it has a lithium ion battery so remember to fator in replacement battery cost every 3-4 years as lithium ion won't last 10 years like an nimh.