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Do alternators have a power band?


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I have an '05 R1200RT which is blessed with, I believe, a 720 watt alternator. At what engine speed is this alternator able to produce that power?


Lately I have been riding more while using the heated seat, heated handgrips, heated clothing, GPS, and also use the high beam whenever possible. I have assumed that the alternator has minimal output at idle and that as long as I am riding at highway speeds, then the alternator is producing near-peak power, but maybe I am wrong. Does the power output of the alternator linearly increase with engine speed? Is there an engine speed at which it is most efficient? Is it ever worthwhile to deliberately use lower gears to help run a heavy electrical load?


Any thoughts from the electrically-enlightened on this forum is welcome.



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Hi Jay,


No one answered this for you yet? OK...


An alternator's output is regulated and so does not increase linearly with speed (or at least not as far as the bike's electrical system is concerned) except at very low speeds as noted below. Alternator output in an automotive system will usually be set at 14.something volts and will provide as much current as necessary to maintain this voltage, up to the output capacity of the alternator. Exceed this and you will begin to draw from the battery and then your days (or hours) are numbered. An alternator may not be able to produce its full rated output at low speeds but usually things are designed so that an adequate amount of power is achieved quickly as you come off idle. I don't think that any of this is much a concern with our bikes though as the alternator is capable of full output at any normal cruise speed (and 720 watts is a lot of power for a motorcycle application.) From that point on up to redline the output will be regulated as necessary to stay within the appropriate range.


So, not much concern unless you are creating an unusual condition with a very heavy load (as with lots of heated gear plus auxiliary lighting and such... no need to be concerned with a GPS, MP3 player, and other electronic gadgets as they draw very little power) and are operating a significant amount of time at very low speeds. In this one case it might be worthwhile to run in a lower gear than you might normally. If this is the case for you it would be a good idea to install a voltmeter so you can see what is going on with the electrical system and respond accordingly.


Hope this helps,

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Your alternator should be putting up its maximum output by 4000 RPM. As Seth mentioned, it is really putting out whatever current it takes (up to its limit) to maintain a certain voltage.


You have to work pretty hard to get enough load to exceed its capacity but, you can do it. Nice long highway ride will perk the old battery back up.

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