I am overall very pleased with Ni-MH's but a lot of folks just don't understand the differences. Basically you get higher capacity in the same size cell as Ni-Cd but at a price (just like everything else in life).

1. Ni-MH are less tolerant to abuse (excessive high rate discharge, excessive overcharge, vibration, cell reversal) than Ni-Cd. NEVER charge one above 1C (1 capacity)

2. Do not use a Ni-Cd peak charger on and Ni-MH battery. If ran in "Auto" mode they will overcharge and damage the battery. Make sure your charger is rated for Ni-MH.

3. Ni-MH left discharged for more than several months will most likely never hold their full capacity again or not hold a charge. Ni-MH has a higher self-discharge rate  (3-4%/day) than Ni-Cd (1%/day).

4. Ni-MH capacity falls off with cycling whereas Ni-Cd stays pretty constant. For this reason and since Ni-MH's are generally cheaper I choose to replace them EVERY season. it's a small price to pay to save a model.

Once again batteries are one of those things that modelers are quite opinionated about but all the types we use have their pro's and con's and should be selected by APPLICATION not emotion.  

Per Red Scholefield of "The Battery Clinic" and other battery Gurus and experts, memory in NiCds has pretty much been eliminated.  20-25 years ago memory was an issue in NiCds, but NiCd battery development in those same years has pretty much defeated this spector!

I have both NiMH and NiCds in use as airborne electrical power supplys in my planes.  I have had NiMH batteries go bad for little or no reason whereas my failure rate for NiCds is almost non-existant.  I just prefer NiCds.  

I started using Ni-MH about two years ago.  I felt like the only one left still using nicads at that point.  I just bought my first lipos last year..haha.  I am only going to add two things to what Mike said.

1)  Less margin for error with cheap Ni-MH versus Nicads.  I didn't seem to have as much trouble with generic nicads as I have had with generic Ni-MH.  A lot of the cheaper NI-MH use high impedence cells which are trouble waiting to happen.  Now I only buy better quality cells.  You get what you pay for..I know I know.  Check out No-BS batteries.

2)  Form charging Ni-MH is an absolute must or you will never get the pack to perform to full capacity and it won't last (Your charger will false peak and drive you nuts).

I bought two Ni-MH packs right before my relocation a year ago and they both ended up in the garbage.  As Mike said, a few months of no attention doomed them.  I use two Schultz chargers and I always do the full three form cycles on the walwart.  There is a new generation of NI-MH that that show promise and an improvement in self discharge rates.

ditto on No BS - LOTS of information there!

I replaced all of my airborne packs this year from No-BS batteries as well.  I elected to go with the NiMh packs to save some weight while still having good performance. I have been very happy with them but I do test all of my packs every 60 days with my West Mountain Radio CBA II, I discovered a NiCd pack this season that was rated for 600mah but even after 3 cycles only gave me 320mah.  Into the trash it went and a plane saved.  More than paid for the battery tester.

David

http://www.westmountainradio.com/CBA.htm

 Three of us switched to NiMH's from Dave Thacker three years ago. We all bought Sirius charge pros. They quick charge or should I say all we do is quick charge. I never charge any of my 1650s at home. Three years later all of us are running the same packs and they are routinly tested at 90% to 95%. Friends of mine in the midwest have been doing this with Sirius chargers for the last 5 years with the same results. This charger is unlike anything else you are using. They are not cheap but I wouldnt use anything else for charging my batteries. Check them out for yourself. They are nothing short of amazing.........I have also never read a bad review on any of these chargers. No I dont work for them or sell them, but I do use them with great results.

FAQ:

http://www.siriuselectronics.com/index.php?main_page=index&a mp;cPath=51&sessid=79150b58d4b845d0fa5e20c16e6d3cf5

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