I've often read that if the rpm drops when you disconnect battery the glow plug is either bad or to cold heat range.  I've tried Fox RC long, Fox Miracle, Thunderbolt RC long, and Sig RC long and they all have rpm drop when battery is disconnected.   All new plugs in a variety of engines.  So what's up with that ?

Could be a number of things, as already stated, it's most likely a little too rich, but could be: engine showing signs of wear, water in the fuel, you might be taking the igniter away too early before the engine has a chance to warm up.

Generally it's a sign the plug is a bit too cool for the particular combination of engine/fuel/prop load and phase of the moon . Plug heat range is one of the things that sets the ignition timing and the closer the timing is to ideal the more power and therefore the revs you get. With the engine running and power connected to the plug it's got both catalytic heat and electrical power to heat it, in effect overdriving the heat range. Disconnect the power, the plug cools down a little, the timing changes to less than optimum and the engine loses revs.

If you find a rev drop with the hottest plug available then adding nitro or raising the compression will advance the ignition timing. Personally though I don't worry if there's a minor rev drop although lately I've started using the Taipan 4 stroke plug which gives no discernible change when using zero nitro fuel in an Enya 60-IIIB and several others.