Those of you considering using Lipos for Tx battery should investigate very carefully. I have not researched it myself but, when the DX7 was very new, there was someone who was trying to blame it for his airplane crashing on his second flight. As it turns out, he had installed a 3S lipo in his DX7. The pack's full-charge voltage exceeded the max input voltage for the board in his DX7 (12V, I believe) and smoked the Tx.