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I've made an electrical generator by spinning the shaft of a DC motor by connecting it to my bicycle.

I've tested it with a multimeter, and on 1 kOhm, 2kOhm, 3kOhm, and 4kOhm resistors I'm producing about 70 Volts. Current is varying accordingly with V=IR. Therefore power P=IV is increasing as resistance decreases.

My question is how do I find the optimum resistance to produce the most electric power? By this calculation, power would increase as resistance decreases. However there has to be some limit to this, as I was only putting in about 200 W into the bicycle. According to the equations a 10 ohm resistor would lead to 7 amps and 490 W, but obviously I'm not producing more power than I'm inputting.

Can someone please help me approach this problem? What is the optimum resistance for optimum power?