Because of the way the C3100 charges, it is usually worthwhile to use an oversize AC adapter. The early versions actually pulse charged at 10amps, which the original 3 amp supply really didn’t deal with very well. I ended up buying a ‘brick’ on ebay rated for 7 amps, and have used that ever since without problem. Given that you can buy these things for very little (You can get a 12V/7amp unit on ebay for about $15), there just isn’t any good reason to skimp. The reality is that any 12 volt adapter good for at least 5 amps will work just fine, and will allow the charger to deliver rated charge rates. Some may remember the gross inaccuracy of the charging information from early versions. This was because the calculation was based on the high current pulse, but the AC adapter couldn’t actually provide that much current even for brief periods, so it tended to report significantly higher capacity than was really present.