Building a battery pack
After a few years, my Futaba T6EX Radio Control transmitter's battery pack became dead, it would only hold battery for around 15 minutes, making it very dangerous to try any model airplane model even for short time. The options were to buy a new one or to build one myself. Obviously I opted for the second option.
The original pack consisted of 8 AA sized NiCd rechargeable batteries in series, giving 800mAh at 9.6v (9.6v = 8 x 1.2v). Although I considered AA batteries at the end I chose AAA batteries because they are smaller. A home made pack of AA batteries would have resulted a bit bigger, making necessary to mod the transmitter's battery holder (that is, to cut the plastic). As modern AAA NiMH batteries hold approximately the same capacity as the original batteries, I made my life easier and went with these.
The first step was to open the original battery, cut the connector to reuse it later, and put the old batteries y the recycle bin. Then I soldered the batteries as seen in the next pictures, joining the + side with the - side of another. The soldering iron should be quite hot as the big size of the battery will dissipate the heat quite fast, making it difficult for the tin to keep the right temperature while in contact with the battery. I recommend to slightly scratch the surface of the battery connectors with something sharp to make it easier for the tin to attach. Try to avoid overheating the batteries to reduce the chances of damaging them.