After modifying your Turnigy 9X radio with ER9X or similar firmware you might need the battery mod. There is no point running a 3S LIPO in the 9X radio as the electronics runs perfectly fine on lower voltage and more voltage only generates heat. In the main menu of ER9X you can set the voltage alarm to suit 2S, adjust this accordingly and your radio will be running for a long time with less buildup of heat inside. I choose to buy TrustFire Lithium Battery with 2400mAh capacity.
Start by using sandpaper or something to scratch the surface of the connectors with, I
used a needle file and when done I applied some soldering Flux to ensure best result. Tin the
connectors but avoid heating the batteries longer times as they might
get damaged (you know - do not expose to heat)
When all 4 ends are prepared it is time to add a small piece of wire to connect 2 batteries into 1. Positive to negative, a serial connection (3.7V+3.7V) gives us the voltage we need.
To hold the batteries secured together I applied hot glue on both sides where they touch side by side but you can use heat-shrink tube as well if you prefer that.
To the other end we solder the original connector that was attached to the plastic battery holder. I extended the wires slightly. Make sure you solder positive and negative to the right end of the batteries or you kill your radio with magic smoke.
In order to make it easier to charge the battery-pack later on I added a JST connector as well.
Now the radio is humming along with a specially dedicated battery-pack that does not get damaged if you forgot to turn off your radio as the TrustFire's have built in over and under voltage protections.
Very convenient and they lasts a long time. Happy flying out there