How suitable is a Smartsolar MPPT 100/20 controller for handling a (spinbike) generator input?

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simplesimon asked Mar 20, '23

How suitable is a Smartsolar MPPT 100/20 controller for handling a (spinbike) generator input?

So the intended application is a spin bike generator - so a 3 phase brushless ac motor (from a skateboard) run off a flywheel of a spinbike with the intention to charge a 12V lead acid battery.

Input voltage is expected in the range of 20 - 40V dc (via a 3 phase bridge rectifier). Don't expect any overvoltage as the flywheel will keep the speed and hence voltage in check.
Will the unit handle the input? I calculate input is likely to be below 200W so shouldn't be close to the 290W max. with a 12V battery. I can't imagine any overspeed, hence overvoltage scenarios like might be the case with hydro generator or combustion engine generator

Battery charging: I'm assuming once the battery is full, will the controller autoamtically ramp up the impedance to reduce current drawn from the generator. (I assume this is how it reduces current drawn form a solar panel?) i.e. Pedalling will become easy
If the battery is very low charge, then I assume the controller will try to draw more current from the input - so pedalling will become heavy.
Does this sound reasonable or have I missed something?

Reason for using the MPPT controller is that I might use a solar panel later as an alternative input

MPPT SmartSolar Generator

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Answer 1 · 2023-03-21T02:15:17Z

snoobler answered · Mar 21, 2023 at 02:15 AM

As long as you don't exceed either of these limits, you should be fine:

The MPPT is a load to the source, so as the battery approaches full, it will simply reduce it's load making pedaling easier.

It will not draw more power than is available. If the pedaler is only putting out 100W, it will only draw 100W.

1679364812332.png (153.6 KiB)

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Answer 2 · 2023-03-21T06:25:15Z

Michelle Konzack answered · Mar 21, 2023 at 06:25 AM

But I would put after the 3-phase bridge rectifier an Electrolyte Capacitor with at least 100V and 4700uF because the Sinuswave could cause trouble with the MPPT

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matthias-nagel commented · Sep 17, 2023 at 05:48 AM

How did you figure out the size of 4700uF? What is the math behind it? I understand _why_ you need it and I have been dealing with a similar problem during the last weeks, but I was unable to find the proper size. Did you calculate the size or is it an educated guess?

Moreover, how do you propose to limit the inrush current? Put a coil in series? How big that one be?

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Michelle Konzack matthias-nagel commented · Sep 17, 2023 at 02:09 PM

Generally you should use 1000uF per Ampere.

But be careful with the Voc...

...because i had already 30 years ago a 68.000uF/100V Electrolyt Condensator exploding. Believe me, you do not want to repeat my experience!

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Answer 3 · 2023-09-16T14:41:58Z

simplesimon answered · Sep 16, 2023 at 02:41 PM

Thanks both for your comments. It's taken me forever to sort the mechanics but now it's generating! :-)

The current limiting has been useful to be able to control the resistance when pedalling and I added a cap. to the rectifier and see very steady voltage, which is reasonably inherent from the spin bike as you can't make sudden speed, hence voltage, changes with a flywheel.

I can get a solid 50W without too much sweat... but only sometimes. Basically it seems that when the voltage is too low (< 20V input to MPPT) then the power drifts lower, see graph attached - I assume a function of the MPPT. But it's not always like that, if I kick the voltage higher then I can end up with a steady 50W session at only 17V, see attached. Both sessions are with a fairly flat battery so the battery being full should not be a factor.

Anyway, I have a new generator on order which will produce higher volts, as originally intended, so I hopefully avoid the drift. Somebody made a calculation error mixing diameter and circumferance, doh!

power-constant-at-17v.png (163.2 KiB)

power-drifting-lower-from-21v-input.png (167.9 KiB)

50w-17v.png (134.3 KiB)

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matthias-nagel commented · Sep 17, 2023 at 05:46 AM

How did you size the capacitor? I have been trying to figure out a similar issue and was unsure how much ripple the MPPT can handle on the input side. (I have an alternator with a M2 rectifier.) Moreover, how did you limit the inrush current?

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Answer 4 · 2023-09-17T14:04:55Z

Michelle Konzack answered · Sep 17, 2023 at 02:04 PM

I have a 24V/240W DC Generator (Uoc~50V, Imax~ 8A) and used a 10000yF Elko plus a bridge rectifier at the PV in of my SmartSolar MPPT 75/15...

...and it works surprisingly good as a hydro power plant.

img-20230917-170008.jpg (621.2 KiB)

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