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How to test motorcycle charging system wattage at different rpms?

Hi I have a motorcycle that is rated at 435 watts at 7000 rpms, but I do not ride at that rpm and ride mostly at 3000 to 4000 rpms and want to know how many watts are being produced at these rpms so I can figure out how much heated gear I can run with out discharging the battery. Thanks. Mike.

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2 Responses to “How to test motorcycle charging system wattage at different rpms?”

  1. bikinkawboy said :

    Yeah Mike, if you can find the regulator-rectifier, there should be three wires coming into it from the alternator and probably 3 wires coming out. One will be a ground wire and the other 2 hot wires. One wire, a fairly heavy one, should go to the positive terminal of the battery. It is probably connected somewhere to the heavy positive battery cable, either at the starter solenoid or lug at the battery terminal. This big wire should be the wire that carries the DC current to the battery and wiring harness. If you take an inductance ammeter and lay against the wire, it should show the current flowing through it. In lieu of an inductance ammeter, get a ammeter with screw lugs on the back and fabricate some wire connectors so that the current flows through the ammeter on its way to the battery. There will also probably be a wire that attaches to the same lug on the battery cable that will allow current from the regulator to bypass the battery and flow directly into the wiring harness to support other loads, so it’s necessary that you measure the amperage between the regulator-rectifier and the battery.

    To get an accurate measurement of the current, you’ll want to throw a heavy electrical load onto it, so that means headlights on bright, brake lights on, your electric vest hooked up and so on. Only by overloading the system will you be able to measure the true maximum output at a certain rpm. Any load less than that will be matched but not exceeded by the regulator. For example, the alternator may be capable of producing 20 amps at X rpm, but if the load is only 10A, that’s all the alternator will produce.

  2. orestes_otm said :

    Somethings not quite right with your numbers?
    watt = voltage x current, watt / voltage = current
    435w / 12v = 36.25 amp, no way your bike puts out 36 amps. at any rpm.
    I would suggest you simply check the rating on your main fuse. It should be around 20 amp. 12v x 20a = 240watts
    subtract out the headlight @ approx. 10a
    another 5a for horn, turn signals & stuff
    what ever is left you can use, but remember anything generating heat will use alotta power.


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