A friend of mine has bought one of these to limit an over charging problem with a motorcycle battery
I wonder if anyone has any thoughts as to what is in the small black box?

 

The gold object is a high wattage low ohm resistor, used to dump excess voltage by passing current to ground. The black box is likely to be a zener (maybe a tl431 adjustable zener) and a power transistor to switch the resistor across the 6v rail.

 

hi thanks for your reply
yes thats exactly what I thought but I just cant draw a diagram with the 3 wires in/out of the black box, its the two black wires that are confusing me

 

The black box may be as simple as a zener diode that starts to conduct as the voltage approaches six volts. Also known as a "shunt voltage regulator". A lot like used to be used on some brand of motor cycles made in the UK. Cheap and easy but not very good. When it fails the voltage goes too high and the lights all burn out, often at high speed in the dark. The gold colored part is a series power resistor, the other part of a shunt voltage regulator circuit. Not even a slightly original idea, shunt regulators are less efficient.