Hey Leo, OK then what has been done to my bike so that it runs with the battery disconected?
You are both correct in what you are seeing.
With these bikes, the voltage from the alternator is proportional to the engine rpm. As an example, if the rpm is a little low at say 1100 rpm, the alternator output drops off to maybe 11 or 12 volts, depending on what lights, etc. are on.
If the rpm is higher at say 1300, then alt output may be 13 or 13.5 volts.
If Leo's bike was running at 1100 rpm or less with lights on, the voltage may have decreased enough that the ignition HV became too low and the engine stopped.
If katmol's bike was running at say 1300 rpm, then the alt output was high enough to continue to give good ignition HV, and keep the engine running.
That's the reason, that the battery and alternator are connected in parallel. There is that area around 1200 rpm, where its a balancing act. When the alternator voltage falls below 1200, the battery takes over and supplies the current to the ignition and other loads. As rpm goes above 1200, the alternator takes control, supplying current to the ignition and other loads, and in addition starts feeding current back into the battery to charge it.