Why did you need a bigger battery?
Rectifier charges the battery and stock rectifier will work fine as long as the battery is of same power as it came stock. When you put in a bigger battery you are asking the rectifier to do more work than its capacity. It is like using a pickup to pull the load of a Shehzor truck, if you get my point. So the solution is using additional rectifier. But again since coils on the bike are stock so you will need coils than can aptly provide better current flow now that you have more traffic going through it. Long story short, better keep it stock.
I hope @imranbhatti comes across this thread and shares his view, an expert on the tpoic.