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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).

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7  
I'd focus more on uprating the alternator. – Chenmunka Mar 10 at 11:31
    
One of the issues you'll run into with running a larger AH battery is, the battery is going to be physically bigger in most cases. You only have so much room on a MC ... If a larger battery is the only way to go, you may consider going to a capacitor setup. Also, not sure what you mean by "fluctuate"? Do you mean it gets brighter with engine speed? Or does it constantly get vary bright to dim and back again? – Pᴀᴜʟsᴛᴇʀ2 Mar 10 at 11:43
    
If you do change your headlight, make sure the wiring harness can handle it. Putting a 55W headlight in will increase your current draw from 3 to 5 amps. Also make sure you have enough room in the headlight shell for your new wiring, it's usually pretty tight in there! – TMN Mar 10 at 12:53

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.

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Great advice +1 – DucatiKiller Mar 10 at 16:39

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.

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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.

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