I want to determine the available capacity of the alternator for adding additional loads.

It was suggested, "... that to figure out the actual capacity, get a Big Ass (tm) rheostat and hook it across the battery, set to max resistance, with the bike running at say, 4500 rpm. Put your voltmeter across the battery as well, and crank the resistance down until the voltage reads 13.2 volts. Measure the resistance of the BA rheostat, apply a bit of Ohm's law, and voila, your excess capacity."

Minimum system voltage is 13.5 volts. The alternator has a rated capacity of 400 watts.

Any suggestions what I can use for the BA rheostat?

  • The capacity is often limited by the losses. Minimum is 10% for DCR of coil meaning short circuit current is <10x rated current. So if you can measure output winding resistance internally before then 14.2V/DCR *10% is your max theoretical capacity. But a heater load test is best. Commented May 22, 2017 at 1:50
  • 1
    If you already know that the alternator is rated at 400 Watts (over it's full temp/speed range) ...then you know that it can provide about 28 Amps. The likelihood is that you never run it at full capacity (400 W). Just put an Ammeter in circuit and measure your running amps with all lights/accessories on, subtract from 28 A and that's the max you could expect to get to use. Commented May 22, 2017 at 5:09

2 Answers 2


One way is to use auto lamps as loads - adjust the quantity to provide the needed current draw.

A 55w headlamp bulb will take about 4.5A. Brake or indicator lamps take about 1.5Amps. Position lamps take about 1/2 amp.

  • Yes I suppose I could keep adding various loads but I was hoping to fashion some sort of variable resistor to do the trick.
    – Hattman
    Commented May 22, 2017 at 2:02
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    You could, but finding something that can dissipate ~400W is not easy, especially if you are only doing it once or a couple of times.
    – Kevin White
    Commented May 22, 2017 at 2:05
  • @KevinWhite Battery load tester comes to mind...almost any auto shop will have one handy & it's made for testing 12v power supplies.
    – Robherc KV5ROB
    Commented May 22, 2017 at 5:22

Easiest solution ever (migrated from EE site):

Take the bike to your friendly neighborhood auto shop & tell them you need to know the excess capacity from your alternator from a charging system test.

They'll hook the bike to a load tester & load test both your battery & charging system, then they can tell you how healthy your battery is (how close it can come to pushing labelled cranking amps), and how many "excess amps" your alternator can push out before your voltage sags below a set threshold.

If you're very friendly with the staff, you can often get the test done for free, or at least not have to shell out more than a couple coffee's worth of cash for it.

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