Solar Powered Outdoor Lights....battery question?????

I had one of those lights quit working and I had to break it to get inside and fix the broken wire. It is now my toy that lays on my desk. It has 2 ni batteries in it rated at 600. I replaced them with 2 rated at 2500. They charge great and work great. But the light don't seem any brighter. I did find out with the one I have that when the switch is turned off the batteries will not charge either.