I'm rebuilding a generator for an antique car - it's a 12V Lucas C40 dynamo. The old service manual says the max continuous output is 22A, and they recommend using a 500W variable resistor. They don't provide values for the resistor but say to calculate the value using a number from a chart (which is 22A) divided into 13.5. I get 0.613. But I don't know what that means. Is it 613K ohms? Is that the top end of the range needed for the resistor? and what should the low end be?

 

Fraction of a OHM. This is pretty close to the calculated amount fixed resistance.
https://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/

https://www.amazon.com/YDELEKS-Aluminum-Wirewound-Resistor-29X52X250MM/dp/B0CKQB361P

You will need to bolt it to a large heatsink with a cooling fan (or cooling water loop) for continuous use.

 

I would find a discharged car battery and two head lights and see if it can to that job.

 

I would find a discharged car battery and two head lights and see if it can to that job.

Funny but I would think they guy wants something repeatable for actual testing.

 

In Detroit I used banks of car headlights for loads. When you are in a building with truckloads of headlights and no power resistors, you use what you have.

They are not a constant resistance. At 0 current the resistance is low. A low car battery is much more like real world testing where a low battery pulls no current at low voltage, and pulls current only above 12V.

 

Car headlights are a good test load as they put startup stress on the supply. I tend to keep the lamps from my car when they fail as the other beam is ok. And they are designed to run hot with no fans needed.

 

IF you are measuring the current then it does not matter that the headlight resistance varies, just keep adding them until the current is what you want it to be. What I found made a convenient resistor was a section of dull bandsaw blade. Just mount it firmly because it gets hot and glows. Cheap fairly compact, and convenient if you have a shop scrap bin handy.