Bicycle generator battery charging physics questions (from a begginner)

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Martoia

Joined Sep 5, 2021
5
Hi all,

I’m building a simple bicycle generator to teach myself some fundamentals of electronics. I’d like to understand the properties of this machine, particularly the relationship between current draw, current supply, and load/resistance.

The machine looks like this: DC Generator (geared for 17v @ pedel speed of 60rpm) -> Diode -> 12v Buck Converter -> DC-DC Charger (selectable 10/20 amp charge current, tolerates 8-16v) -> 100 AH battery.

Question 1: I’ve read that a cyclist can comfortably sustain 75 watts of electrical power. 75w / 12v = 6.25a. However the charger ‘wants’ (excuse the simplification) to draw 20 amps at the high point of the charging cycle, or 240 watts. How will this feel to the cyclist trying to maintain 12v at a lower current of 6.25 amps, and why? My guess is that it will simply be impossible to turn the peddles whilst the charger is drawing 20 amps.

Question 2: A current-limiting resistor has been suggested to me as a solution to the problem of devices requiring too much current to operate comfortably via peddle power. I understand this insofar as I can see that limiting current will essentially reduce the operational wattage of the battery charger. But this is where I get confused. Resistance seems to be both the problem (by requiring too much current to be set in motion by the cyclist) and the solution (by limiting the amount of current in the circuit to a level the cyclist can maintain). How can this be?

Clearly I’m missing something in my mental model of the relationship between amps and ohms. Can anyone point me in the right direction?

Many thanks
 

Ian0

Joined Aug 7, 2020
5,147
Easier to understand if you remove the regulator from the equation.
Just have a generator (17V @ 60rpm) connected straight to the battery, and remember that
1) both the battery and the generator have internal resistance.
2) Generator voltage is proportional to rpm.
The pedals will turn freely until a speed is reached where the generator voltage exceeds the battery voltage.
Above that, the current will equal the generator voltage minus the battery voltage divided by the sum of their internal resistances, so the power required to turn the pedals will get large very quickly.
There will be a generator speed at which the output is 75W.

If you include the regulator, it will spend most of its time being unable to regulate, and therefore will be rather pointless.
 
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