As Michael said if you have no voltage increase when the bike is running and revving above 1500 rpm, there is a charging fault and just running the bike will slowly drain it. Check the earth lead (negative battery lead) and check the connections to the rectifier and voltage regulator.
You can also check voltages at the 3 yellow wires from the 3 phases of the alternator, and these are prone to corrosion in the multiplug connector.
Points only systems will keep running at even very low voltages, down to 6 volts but you will get misfires as the battery gets lower.
Battery chargers with low outputs will take a long time to recharge your battery, on a twin I believe it would be 14AH (14 amp hour) battery so if your charger only outputs 500 ma (1/2 an amp) it will take around 28 hours to charge your battery back up fully. Voltages are an easy way to measure batteries, but not the most reliable method (as a forklift truck engineer working on battery powered trucks this is something I have a fairly good knowledge of).
You need to do one thing at a time, as your battery seems to be dropping off, that needs to be corrected before worrying about electric starters which will drain a battery very quickly (they can draw 100’s of amps so as an example a 14 A/H battery being asked to supply 280 amps will be dead in 3 minutes). So before anything else you need a good battery and charge system, bearing in mind lights, indicators, stop lights, running the coil for ignition etc. all take power which the battery won’t keep up with unless you have a good charge system to keep the power flowing.
Another thing to bear in mind is if you use the bike for a short journey and park it up, and then another journey and another rest, the battery will pick up a little as it stands (to a certain extent anyway) and can make it seem as if the battery and charge circuits are OK for a while, so your 150 miles if done in one go would kill a battery if it’s not charging but a few trips with rests will allow it to recover slightly each time.
