Source for replacement yard solar lights?

[AllGold]

I have a set of 4 lights that uses a larger solar collector containing batteries with wires running to the lights. It's nice because you can put the lights in an area of northern exposure that wouldn't normally charge much and put the collector in a more sunny area.

Anyway, I found that a lot of higher capacity NiMH wouldn't run the lights, even if they were fully charged when installed. I don't know what the problem is but I'm guessing the setup is fussy about resistance in the batteries.