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Many, many people have managed to fry their 9X by hooking up a battery wrong one way or another.  If you've already done that, you can find one of several guides on how to repair it by Googling "9X reverse polarity fix".

This document is here to describe why it is such a problem and how to best avoid it.

A big part of the trouble comes from FlySky's initial decision to use a very common connector for its battery input.  It is a JST XH 3-pin connector.  So what?  That happens to be the very same connector used as the balance plug on the vast majority fo 2-cell Lipo batteries.  That, in itself, wouldn't be a problem... except they didn't wire the connections the same as a standard balance plug.  So people wanting to use a 2S Lipo (which is fine, especially with our 9Xtreme board) may see that the plug fits, so they plug it in without a 2nd thought... and poof.

Others are using a 3S Lipo.  OK, the balance plug is too big, so it won't fit.  But they notice that the spacing of the pins are the same as other plugs commonly used on batteries and even though it is a 3-pin connector, you can stuff a 2-pin connector in there just fine.  But... if you get it shited to the wrong spot... poof.

Then there are those that use the stock 8xAA battery holder that came with the 9X.  It has to be the lowest quality battery holder I've ever seen.  It is slightly too small for AA batteries.  You stuff 8 of them in there, and sometimes the metal springs start to move and cut through the plastic until they make contact with another nearby... and poof.

How to avoid all this?  Its easy... buy this battery from HobbyKing.

See that black connector on there?  That's a Futaba style servo connector and it happens to fit in the 9X battery connector slot.  It isn't the correct connector, but it fits.  And the nice thing about it is how it is wired.  The middle wire is positive, while one of the outside wires is negative (the other is blank).  This ensures that this plug will work fine with the 9X regardless of the orientation.

If you wish to take it a step further, you could splice a male Futuba extension connector to the original plug that came with your 9X's 8xAA holder and end up with something like so:

But I don't think that is necessary and it takes up precious room in the battery bay.

This battery is fantastic for this radio.  It is cheap, LiFE is an inherently safer chemistry than LiPO, it fits perfectly, it lasts a really long time, and it is a great voltage (9.9V) for use with or without the 9Xtreme.  Downsides?  You do need a charger capable of charging LiFE batteries and it is not suitable for a 9X with stock firmware since they are hardcoded to start complaining about a "low" battery at about 9V {which is silly since the radio operates just fine down to 7.2V stock or even much lower with the 9Xtreme}.  But if you're using a stock radio then you're most likely not on my pages.

If you wish to use a different battery, that's fine... just make sure it is wired correctly for the 9X and that you are using a connector such that it is impossible to plug it in wrong because... well... poof is not fun.

Middle pin is positive, outside pins are negative.