I'll respectfully disagree about putting loose Nimh's into a 4 cell battery holder. 99% of the time that works just fine, but you're relying on the spring tension of a part made in China by 8 year olds who work for $1 an hour for the safety of your airplane. If a cell vibrates out of place or one of the springs isn't as tight as it should be, you take your plane home in a garbage bag. A welded (not soldered!) 4 or 5 cell battery pack made with Sanyo cells is the most reliable one that you can possibly have, and the cost isn't much different. Have a look at www.hangtimes.com for what I consider the best value in batteries on the market today. They aren't the cheapest, but they are top quality and the cheapest manufacturer that I know that sells the top quality items.