Boy you guys are nice!!! I bought a 2700maH NimH JR battery and hooked it up to my Futaba receiver and of course it was hooked up backwards. The pack got immediately warm (it was still in my hand), so I pulled it and switched it around. The way I can tell the correct polarity, is that the plug on either Futaba, JR, or Hitec all have bevels on the plug. Match the plug as per the wiring on what you're used to (IE match the bevel in the correct way it's plugged in).

I posted my backwards battery question on the battery forum and got grilled for not checking polarity with a volt meter!

MPB