JS Burly 123

I have received the JS Burly 123 rechargeable batteries and charger ( $ 30.00 ). Off the charger they read 3.9-4.0 vlts.. These were said to work well in an ARC-LS. I have an ARC-LS Standard light. After several charge cycles I am only averaging around 20-25 minutes of continuous ON time per charge. Then the light goes out all at once. They are then down to almost 0 vlts.. I don't know the mah rating of the cells but thought the run time would have been around 45 minutes. Does this run time sound normal ?. From what I've read I figured on 40 % run time with the rechargable vs 2 hours with the disposable cells. But I'm only getting a 21 % runtime. Just wanted to see what others are getting on their ARC-LS with the rechargeables. Thanks for any replies.