My motorcycle (honda CBF150) came with a 12V 3AH battery. I want to change the stock 35w headlight to a higher power one and connect them directly to the battery (the stock ones are not, and they fluctuate). I also want to add fog lights and usb chargers in the future. To do that, I'm thinking maybe I need to increase the battery capacity.
Will replacing the current battery with a 7AH battery be enough or will I have to change the whole system (rectifier and all).
Sure you could put a bigger battery in .What may happen is that the total proposed draw is too much for the dynamo and it will go flat.Even if it does not go flat because your appliance usage patterns are intermittent you will get a lower battery life because of the constant cycling .If dealing with the charging system is too hard or too expensive then consider LED lamps which should draw only 20% of the current leaving more headroom for your proposed accessories.
As Autistic and Chenmunka say, if you want to increase the electrical power you're using (by having a higher-rated headlamp), you'll need to generate more power - by having a bigger alternator or dynamo/generator. Simply fitting a bigger battery won't help, as it'll still discharge all the time you're using more power than you're generating.
It sounds like you're already seeing the result of this even with your current lights, if they fluctuate with engine speed - you're only getting the full brightness when the alternator is turning faster and so generating more power.
As others have stated, the bigger battery won't cut it. But you may be able to play the game from the other end: use a headlight bulb with a lower power draw. Some LED bulbs draw less power than their incandescent counterparts for a given light output.
