Don't have a lipo unless you have a regulator. Spektrums take 6V and don't like 7.4 unless its the D3 etc models
Get a LiFe!!
they are 6.6V and are small enough. I have done the mod to a couple of Dx3s/Dx3R/Dx3c works great and they are cheap and chargings easy.
You need to open the TX and solder on a female end of a servo extender.... Presto done
Get the smaller LiFe around 1500 mah lasts for AGES!!
Something like this
ONe
Two unsure if this fits as I have one from ebay like the first one but measure the hole
** I did a dx3R today Ill try and do a guide + pics next time