Saturday, February 29, 2020

I get over 4 amps using the DC-DC Buck Converter! Lighting up a 60 watt 12 volt car halogen headlight!!

I appreciate this discussion. I've been scrutinizing all the treadmill motor bicycle vids on youtube. Claims of 300 watts - for example by "pedalpower" - this is actually only after the 120 inverter. So the amps are still around 2 amps. Why is this? It's all because of ohms. So I just got a 12 volt 60 watt halogen car headlamp bulb - and first I wired it up to my generator 12 volt 3 amp output. It worked but not as bright as when I hooked it up to the treadmill motor. But I was using a DC-DC buck converter - to make sure I could increase the amps. So I have a digital amp reader - I was getting spikes of close to 5 amps and a steady current of over 4 amps. So the reason I did this experiment is because when I charge up the 12 volt Chafon generator - I can only get 2 amps charging!! Why? Someone else uses a DC-DC Buck converter and also got over 4 amps on a 55 watt 12 volt bulb. Well I discovered this quote on a treadmill motor forum:
"One thing that really bothers me is your use of the battery as a load. If you have a battery of 12V and a resistance of 0.1 ohms at a point where the circuit is at a potential of 14.4V, you only have a voltage of 2.4V across that resistor."

So based on that analysis - what I did was use the DC-DC buck converter to increase my output volts from 12.6 to 20. Sure enough the pedaling resistance increased - and by how much? I could barely pedal in highest gear - whereas at 12.6 volts I can pedal with a steady 2 amps - in 2nd to highest gear. So you can do the Ohms math on this. It's TWICE the resistance! The above quote is ohms based on 24 amps input. So in other words I was still able to get 2 amps but not steady because the resistance was now twice as much! And why just 2 amps still? Because the Chafon generator has a built in charge controller that pulls down the 20 amps to just about 12 volts again. So it's better to just use the DC-DC buck converter to maintain a very low ohms resistance and still get the same 2 amps.

So the thing to realize about treadmill motors is that the amps are based on the highest voltage rating - because it's a 120 volt powered motor so that moving at walking speeding then you get the highest torque as the difference between the 120 or 90 volts with the volts of walking rpm. So that means unless you know the RPM rating of the motor then it's hard to say - Treadmill motors are rated between 7200 rpm and down to say 4000 rpm. So the lower rpm means you get higher volts are a lower rpm. So that means a motor could have a high horsepower rating but also have a very high RPM rating - meaning that the torque at walking speed is actually LoWER despite having a higher horsepower rating.

So now just to corroborate my claim of the 2 amp limit on charging 12 volt batteries with a treadmill motor - watch this video: https://www.youtube.com/watch?v=GX1G6Wo7vVY
Now read the comments.
Someone comments:
"13v 3A, only producing 40W. 100 Ah 12V battery is 1200W. It will take you 15 hours of constant cycling to charge 50%."
Actually that's being generous - he didn't have a charge controller. So he was getting closer to 2 amps as his average current.
But then read his response!
"You missed some reading i guess. I produced around 5.50 Amps at 13V thereby makng it ~ 71 Watts"
Now read my response:
"Actually Mr. Electron is only getting a 2 amp draw average while charging the 12 volt battery. But he gets a 5 amp draw while charging the 60 watt lightbulb. Why? Because the internal resistance of a 12 volt battery is much lower and so the amp draw is much lower - even though the total amphours of the 12 volt batteries are 48 Ah. The internal resistance of a 12 volt car battery is 0.02 ohms. The internal ohms of a 60 watt 12 volt light bulb is 12/5 so 2.4 ohms. So Mr. Electron - you have ANSWERED a question I posed to another youtuber - but my lithium inverter generator has a 120 volt input for charging. So I think if I invert my DC motor amps (stepped down in voltage via a DC-buck converter) - into AC then I should get a higher load draw from the 12 volt battery since it would be higher volts so higher ohm resistance. But then it still has to convert it back to 12 volts inside the Chafon generator."

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