G'Day Smoo,
I have a DX7, & I'm using it in a Yak 54, with the 1100Mah battery that came with the radio, works fine, never a problem.
You MUST do a couple of things with the DX7, first charge your TX battery for about 24 to 30 hours using only the wall charger that came with it.
Next make sure your RX battery is fully charged using the wall charger, for about the same time as the TX battery.
Next locate the RX's, as per intructions, & then set all controls up, & rebind.
Before you fly, do a complete range check, with the engine running, & If all is well, you should be good to go.