This is discussed over and over again here on RCU. New Nimh packs need to be Form charged at C/10 and discharged at C/5 for 2 to 3 times (while monitoring the pack intermittently for heat) before fast charging. Below is from Hangtime Hobbies website, his business is batteries.


Q: What's up with 'Formation Charging'? Can't I just charge and fly a new pack?



A: The term 'Formation Charging' describes the initial charge/discharge 'forming' process that fully activates the 'chemical engine' and balances the cells in a new pack. NiMH packs have a particular need for repetitive c/10 charge/cycle conditioning, a 'break-in' procedure needed to get them to full rated capacity. Both NiMH and Nicad packs benefit from the process and the intent is to make sure that any new pack has been verified to be fully operational and that a 'start-up' capacity number is established for the pack before it's put into service. To 'Form' a new NiMH pack do three 10% 16-24 hour charges followed by a 300 to 500 ma discharge routine between each charge. In other words: Do a slow charge at the slow charge rate (or as close to it as you can reasonably get with your equipment) as shown on the pack label. Charge till the pack is warm, followed by a controlled discharge with a cycler. Do it 3 times. The recommended Formation Charge/ Slow Charge rate for our packs is printed right on the label of the pack as well as on the data card the pack was shipped with. New Nicad packs should get at least one slow charge followed by a 300 to 500ma discharge. Record the capacity numbers reported by your equipment for comparison cycling as the pack ages. Never check your brain at the door! As one respected modeler put it, "There's nothing more suspect than a new battery pack." Before you fly anybody's pack be certain it's fully operational and safe to fly.