My 12v Lead Acid battery was not holding a charge. Rather than buying a new lead acid, I replaced it with a 4 cell A123 Systems Lithium Ion Battery. The A123 pack had lots of cranking power, but only 2300 mah, so it will start the bike easily, but it will not crank for a really long time. As long as you bike is tuned and running well, you are good to go!
The Li Ion Batteries can also be charged at a high rate, so that can take all of the amperage the generator provides, and will recharge quickly (from completely flat to fully charged in less than 15min.) So although they won't crank for a really really long time, they will be recharged and ready to start your bike after a short ride.
The size and weight advantage is substantial, and will be perfect after I remove that stock airbox to open up the frame Cafe Racer style. I will be able to hide the little battery anywhere.
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