Don't forget to check my Stampede webpage and my 12V power supply made from an ATX one.
My home-made solution for TX battery
What to get?
I bought a New Bright 9.6V 1250mAh NiMH pack and charger for toy RC cars for $3 or $4 at a Wal-Mart clearance sale. I know it could only contains 8 AA cells and I had the idea to use them for my Airtronics MX-3 transmitter. Here's a picture of my pack and charger:
(forget about the opened battery pack, I took the picture after opening it)
First I was surprised that it is a peak charger, there is a standard AC/DC adapter with a tamiya plug, and a base to plug the adapter and the battery. The LED on the base is red when charging and become green when it's finished. It's not like the old 700mAh nicad pack you had to let for 8 hours or so and check yourself, with this one it fully charges in about 4 hours. A battery pack and a peak charger for this price? pretty good deal!
Battery pack
I pry-opened the pack to discover 8 AA unmarked cells wrapped in green and glued together, they were side by side, I had to separate the pack in the middle and bend it in two to have the good shape for an MX-3 pack. There was already some protection on the tabs but I added some electrical tape to insulate cells and also to make the pack stronger, especially at the middle.
The plug needed for my MX-3 battery pack is a 3 prongs zplug, it looks like a standard servo plug or the connector you find in PC to plug your HD LED or Power LED, so I took one of an old PC case and trim a little bit two of the corners like on a zplug to make it fit, then solder the 2 wires.
The white electronic component you see seems like a thermal cutoff (labelled 115°C 2A 250V) meaning that when the battery temperature reachs 115°C (about 240°F) the component acts as a thermal fuse and break the connection. I let it there but I doubt the pack will overheat in a transmitter.
Testing
Double check polarity, plug the pack that fits nicely, and power on!
It works!
Charging
Now I needed a way to recharge the pack directly in the transmitter. The MX-3 has a standard female coax power plug (I think it is a 2.5mm inner diameter / 5.5mm outer diameter), I bought a random AC/DC adapter in a flea market for 99¢ only to take the wire and the right male plug. I then opened the base and discovered a double sided PCB with IC and surface mount components, which seems of good quality, it is the logic for the peak charging. I drilled a little hole at the top of the base to put the wire, made a knot in it in case of, and solder the positive and negative wires on the right place on the PCB.
I then plugged the AC/DC adapter, the tamiya plug on the base, and the coax power plug in the transmitter, and voilà: