Rechargeable batteries are good for the environment because they can be used hundreds and hundreds of times. In theory this prevents hundreds and hundreds of alkaline batteries from ending up in landfills. But all good things must come to an end. You'll know your rechargeable battery is on its way out, when it provides less and less run-time on your device. When it drops below 1/2 of the original run-time, it's safe to say it's dead.
But -- and this is an important but -- batteries may act like their dead if they've been sitting unused for a long period (3-6 months or longer). If yours has been sitting unused, it may be worth trying the following procedure to 'revive' it:
1. Run the battery all the way down. On a product like the Down Low Glow, this means run the DLG until the light output is very low.
2. Charge the battery all the way up.
3. Run it all the way down again. But this time, measure the run time with a clock.
4. Once again, charge it all the way up.
5. Once again, run it all the way down. Again, measure the run time with a clock. If the run in step 5 is equal to the run time in step 3, then it's safe to say, you're holding a dead battery. However, if the run time in step 5 is noticeably higher than in step 3, it's worth using the battery a few more times, to see if you can get it back towards a normal run time.