sony: ICF-5500M / Captain 55: rechargable battery mods
The Sony Captain 55 is powered by only 3 baby cells. The battery current is 50 - 100 mA at normal listening volume. This is a rather large current. Above that, the minimum voltage that the radio needs, in order to function normally, is about 3,8 V. This means that the radio needs new batteries at a point where the batteries are not quite empty (considering that normal equipment can run on lower voltages).
Because of this power consumption I decided to use rechargable batteries.
At the time I bought this radio, the best choice was NiCads. But 3 NiCads deliver only 3 * 1,4 V = 4,2 V at max, right after charging. This voltage lowers rather quickly below 3,8 V. Not enough. So I put in a fourth NiCad in the radio itself, right below the timer. Some plastic had to be removed to make some space there. After that, the voltage was a nice 4,4 V (battery almost empty) to almost 6 V (battery full). The radio can use that extra voltage well and functions better. And, very important: the radio is not harmed by this 6 V.
A miniature electronic circuit was designed and installed in the radio (!) to watch the battery voltage. Below 4,4 V this electronic switch switched off the whole radio. Doing this, the NiCads were protected against too low battery voltage during use, and therefore they live much longer. I will put the schematic on this forum later.
To be able to charge the NiCad battery, the "power in" connector was modified also. This connector was connected directly to the battery.