As PsyBorg stated, its a charging port for the battery. Just pulled up the manual and found a section on it. Not sure how to use it, must have to buy some special charger from Frsky? I just use a 2s lipo in mine.
Good find! My manual doesn't have that, nor did the one I pulled up online. I bought the frysky rechargeable battery for mine and the charger. However, the charger doesn't have a cord for this port, I have to take the battery out of the radio to charge it.
It is a charging port that was added fairly recently. You can use the FrSky QS-7S charger (Q X7 FCX07 Li/NiMH Dual Mode Charge) with it but you need a special cable. I'm not clear if the charger comes with the cable. May depend if it is old stock since the older Q-X7S TXs did not have this port either.
If you are in the US, Aloft has a NiMH only version ...
Personally I'm not a fan of this port since it has exposed pins which in theory expose the battery terminals and are potentially a shorting risk. There may be a protection circuit that prevents that but I think FrSky should have provided a cover and I would tape over it when not using it.
my qx7 came with a cable for that port. i plugged in 2s lipo and tried to use cell checker with the provided cable only to receive an error message. multimeter shows that the pins are connected to eachother in the internal battery port and that one on the outside of the tx acting exactly like a extension cable for the balance plug. i don't know what gives but i wasn't able to recharge my battery with regular lipo charger
It is not a balance charge port. I don't have one but as far as I know the center pin is not connected. You have to use the FrSky charger. Possibly it would work to use a non balance mode on a smart charger but you will need to keep the current low.
Previously you had to unplug the battery to use the FCX07 charger so it could be a design change and a phase out of the FCX07. If you want to use a Lipo you are on your own...how it was with the X9D and how it should be imo.
After the really good decisions FrSky made with the orignal X9D battery and charger setup, they have made a number of missteps with that aspect of their TXs from non removeable to misusing USB B to using and unpopular size of LIon to making a charge port with exposed pins and no cover...so possibly they are still confused on this issue or have decided to return to the basics...just speculation on my part. If it is a return to basics that is a good thing imo. Trying to respond to customer demands for this aspect is a fools errand. Just enabling the TX to be powered from any resaonable battery by giving it a good voltage regulator and some reverse polarity protection and a large battery bay with an included NiMH battery and charger good for most people as was done on the X9D is a good solution and they should just have stuck with it.
That's good for older Q-X7's that don't have the new port. If you can find the connector then you can presumably to the same sort of thing with the new port. However the connector they use for this new port is a bit strange. See here... https://alofthobbies.com/nimh-charger-for-the-qx7s.html
You just need to make an adapter wire from the output to the charging jack. I have the B6 also, Just set it for NIMH (if you have a NIMH) and the mah of the battery and it will charge and shut off automatically. I used to use a LIPO but didn't like the TX sitting around with the battery installed.
here is a pic of what i did and it seams to work great. i just removed the middle wire, i figured i did not need it since im useing the nimh oem bat. pack. (the green one) also when i put a volt meater to the middle wire i got nothing idk why its even there?