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Old 03-05-2012, 09:02 AM   #8 (permalink)
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honda cb125 - '74 Honda CB 125 S1
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The alternator has a minimum speed it needs to spin to charge the battery, the stock pulleys tend to give you that alternator speed at idle so under-drive pulleys will spin the alternator slower and not produce the voltage needed while at idle, it's also going to produce fewer watts over all, so if you do a lot of highway driving in the day time you might be ok, but if you do any cold weather driving, night time driving or short trips, then your battery is going to wear out faster, a few people on here have installed on board battery chargers so they can pull off the alternator belt all together and just charge the battery at home, that works for short trips or if you install a larger deep cycle battery it can work for longer trips, but it's also an added cost.
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