Two months later that cell exploded. More information and pictures here.
I managed to kill a trustfire battery, I left it charging in the 9x on 12.5V for a few days. I think that 12V is just too much heat for the protection circuitry to dissipate. Good thing is the batteries worked as promised and did not catch fire, although they are not completely idiot proof. I will look into it some more but for now only charge at 9v.
Why? Bruce explains it pretty well. To summarise.
I used 2400MaH Trustfires that I bought on E-bay, search for 'trustfire 18650'.
Here they are compared to a typical AA battery, big fellas.
Close up, you might just be able to see the tab that needs pushing down to release the wire.
The goal is to make this into a standard battery pack so it can be used with any charger or for something else. Here I have a three lead jst-XH plug and an XT60 plug.
I only had a spare 3 cell balance lead so I cut the plug of the original pack.
And our battery pack is done.
First tape them together with opposing polarity.
Now for some soldering, I use a 140W gun iron for the heavy wire and battery work, and this 24v goot for all my fine work.
Plenty of temp for a short time along with lots of flux is the trick to get the solder to stick the the battery terminals.
This short piece of 16AWG wire will connect the + and - of this end. The smaller grey wire will be the 3.7V line.
<<insert image of back end>>
(till then the grey can connect to either end of the short red wire).
Not too much heat or you will damage the protection circuitry that is in the end of the battery. If you are a gumby practice on some old D cells first.
Heat helps to hold the two wires together while soldering or you can wrap the end with a strand of bare wire.
Next we need to mod our radio other wise it will blow up with this battery.
This time we are only going to work with the board on the back of the unit.
Remove the screws so we can access the other side of the board. The 12V input plug can then be pulled so we can lift the board up to get to the other side. The pots and switches will still be attached, be careful.
Here is out battery socket, Bend it up gently so we can get to the tracks underneath.
Left pin will stay as ground so you can leave it as is, we want to isolate the middle and right pins.
There are three short tracks connecting the pad to ground, cut em.
Using the soldering iron on the other side, remove the diode and external power wires. We are going to change it so we can charge the batteries directly and have reverse polarity protection for both the batteries and external power.
I bent it back down no problems however it will not be as straight as it was. Back to the other side.
I first use a very sharp knife to cut the tracks and and then something blunter to clear the copper away. Ground is on the left, and 7.4V on the right. The center pin will not be connected but will be 3.7V.
Again a sharp knife but this time just scratch the green away to reveal the copper underneath. I made a mistake here, we only need to make a pad below the center pin.
Track modifications for this side finished. Center pin isolated and a pad made below it. The right pin is also isolated. Left pin you can leave as is, here I have fixed my mistaken hacking.
Solder the diode from the other side in place as shown. The very thin wire is our fuze, I used the core from some kynar wire, but any strand of copper will do. If we connect a battery or power supply to this the wrong way the small wire should burn out.
Re-locate our leads for the power supply jack and we are done.
We can now connect any 2 cell battery via its balance lead to run the radio. With the Trustfires inside and their clever protection circuits we can stick 9 or 12v into the side of the unit and safely charge without any special charging equipment. We can even charge the unit while using it. They are a perfect fit also.
If you cut a notch in the battery cover the The XT60 connector could be used to power other equipment such as Fat Sharks.
Damn its beeping at me. Don't worry, last step is to go to the open9x setup and calibrate the battery sensing circuit. Measure the voltage on the battery with a multimeter and adjust the voltage on the 9x match. After looking at the tests I set the low voltage alarm at 6.8V, although 7V might be a safer bet.