Last Christmas my wife gave me an indoor helicopter, the type where the flight battery is charged directly from the transmitter (Tx). Whilst this method is very convenient it wasn't long before the Tx batteries needed replacing, especially as I was rapidly becoming addicted to this micro hovering marvel.
I tried swapping the four single-use AA batteries that power the 6 volt Tx with rechargeable batteries. However the Tx battery status indicator didn't register full charge so I aired on the side of caution and left the helicopter firmly grounded.

Single-use AA batteries are rated at 1.5 volts each, totalling 6 volts output in a four-cell Tx battery compartment. In comparison rechargeable AA batteries are rated at 1.2 volts each, totalling 4.8 volts output in the same Tx. The only way I could think of to bring the required operating voltage up to 6 volts was to somehow wire in an additional rechargeable battery (total five cells at 1.2 volts = 6 volts).

THE SOLUTION
As there was no room within the Tx to install an extra battery, an external battery holder was required and wired in series, (positive to negative) from the Tx internal battery compartment. To avoid having to solder wiring into the battery compartment and so probably invalidating the warranty on the Tx, I devised a simple non-invasive solution by wiring a two-cell external AA battery holder to an extension plug which just slots into one of the cell bays of the Tx battery compartment.

BATTERY EXTENSION PLUG
The extension plug is made from 13 mm diameter hardwood dowel, washers and round head screws which provide contacts in the Tx battery compartment.

1. Cut the dowel to size, allowing space for the screws and washers at each end, (use a battery as a guide for length). Ensure the screw heads are big enough so as not to tangle with the coiled spring contacts in the TX battery   compartment.

2. Cut a slot along the side of the dowel and trim battery holder wire leads to required length. Strip off about 10 mm of insulation from ends.

3. Screw in the screw contacts into pre drilled holes in each end of the dowel with the bare ends of the battery holder leads sandwiched between the washers. The neagative lead sits in the slot of the dowel and held in place with a dab of cyno glue.

4.  A two-cell battery holder is required as one cell space in the Tx battery compartment is taken up with the dowel extension plug. Attach the battery holder to the back of your Tx with double sided foam tape for easy removal.

5. File a small slot in the edge battery compartment cover of your Tx for the external battery holder leads to pass through. However, I suggest the cover is only taped in place as shown if you have any remaining warranty.  

CONCLUSION
I have also used this extension plug arrangement in a six bay 9 volt battery compartment Tx with a three-cell AA external battery holder. With this set up, eight 1.2 volt rechargeable batteries give a total of 9.6 volts. This slightly higher voltage did not cause any problems.

To revert back to using single use batteries it is a simple matter to remove the external battery holder and extension plug. I think a word of caution might be appropriate here. Whilst results have been successful with the battery conversion on Txs from three different manufacturers I cannot guarantee that the conversion will work on every Tx.

The big plus of course is that you will save money in the long term by not having to keep replacing single use batteries, and also you will be helping the environment.