You do not say what type of regulator you are using. If you are using a mechanical regulator be aware that Cyclon batteries are not suitable for use with these chargers. Cyclon batteries should be charged with an electronic regulator. How are you measuring your charge rate? I assume you are putting a multi meter across the battery terminals with the engine running. A reading of around 6 .3 volts for a fully charged 6v battery with no load on it is about right. You should see between 6.8 and 7v when the engine is running on fast tick over. A mechanical regulator cut out should operate between 6.3 and 6.7 volts according the BSA service sheet. Outside of those limit’s the cut out will need adjusting. The cut out operating voltage is determined by measuring the output voltage of the regulator at terminals E &D on the regulator. If your battery is fully charged then the ammeter will not indicate a charge until the battery voltage drops below the threshold of 6 volts. However, from what you describe it would seem that the charging circuit is doing its job. I would suspect that the battery is unserviceable. A reading indicating a nominal 6 volts can be had from a U/S battery, but the voltage will start to drop quite quickly when you apply a load. Also check all your lamps to see how many watts that they are collectively consuming. The dynamo is a willing animal and it will keep on trying to supply enough current to meet the load that is being demanded of it. The problem is, that if the battery is not 100% then it is a bit like trying to fill a bucket with water that has got a whacking great hole in it. Also check that you have a really good earth path for everything. Especially check the battery to frame earth, and try running a separate dedicated earth from the headlamp. I suspect though that the battery is the problem, and if you are using a mechanical regulator then you need to fit a lead acid battery. A mechanical regulator is not sensitive enough and it will cause premature battery failure.