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HiTech
Explorer
Feb 23, 2013

Initial test manual home brew $8 MPPT controller

The $8 (plus shipping) laptop DC to DC Power Supply (i.e. the guts of a laptop power supply) quick initial results:

Early morning sun it puts out a few hundredths of an amp into the battery but it pulls the Unisolars down to below .5v so of no use there.

Into a pretty full group 27 starting battery in morning sun it ran quickly up from 12.7v to 15+. The watts in to the battery started out in the 50's. by the time it ran up to the 70's of watts the amps actually went from the 4's to over 5 as the voltage climbed (different from my PWM controller), then dropped back into the 4's as volts climbed more. Watts continued up as the solar panel voltage approached closer to Vmppt.

With the default hook up I get roughly .5v higher voltage at the solar cells than I do at the battery, so a gain, but a small one.

Now to find ways to let the input run at a higher gap from the output, or get it to stick around 16.5 v.

The only specs I have are at the link above:

Module Properties: non-isolated step-up module (BOOST)
Input voltage :10-32V
Output voltage: 12-35V (adjustable)
Output Current: 10A (MAX)
Input Current: 16A (MAX) (Please enhance heat dissipation if more than 10A)
Output power: natural cooling 100W (MAX), enhance heat dissipation 150W (MAX)
Easy to drive 65W 90W dual-core notebook.
Use a 12V battery drive 19V 3.42A notebook, the module temperature about 45°c
Conversion efficiency: 94% (measured at Input 16V, output 19V 2.5A)
Output Ripple: 2% (MAX) 20M-bandwidth
Operating Temperature: Industrial (-40°c to +85°c) (ambient temperature exceeds 40°c, lower power use, or to enhance heat dissipation)
Full load temperature rise: 45°c
No-load current: 25mA typical
Voltage regulation: ± 0.5%
Dynamic response speed: 5% 200uS
Short circuit protection: None (Please Install fuses and protection circuits at Input port.)
Input Reverse Polarity Protection: None (Please Series diode at the input port.)

Pictures of the controller chips on the back are at the link.

Jim
  • Testing with a 75 watt lightbulb directly hooked to the buck converter output, no dice. It works as expected. The load cannot pull the solar output down more than a few tenths away from Voc.

    Trying with a 1500 watt resistance heater it is better, but still cannot get down to Vmpp, much less below to be able to scan back and forth for the Vmppt.

    I'm back to the 700w inverter and an old battery. I have about 400w of load. It is very jumpy, going from 18v out of the panel to 14v very quickly. In addition the setting on the pot jumps around with any minor change in light, especially even thin clouds. It appears the panel max output is closer to the theoretical Vmpp of 16.5v +/- .5v, while the Max output of amps & watts from the buck converter into the battery is closer to 14.5-16.0 v at the panel, as observed yesterday.

    Collecting more data.

    Ok it has to be the varying efficiency of the buck converter. At the moment max panel output is at 16.2v +/- .1, but max amps into battery are at panel voltage of 14.8 +/- .2v. Panel temp averages 126F.

    There is some loss of low light performance with this set up. In 100% building shade where all the lighting is indirect I get .42A panel direct, or 10% of panel direct full sun output. With the buck booster in the circuit and adjusted to the MPPT (14.9v) it is only getting .38A. So a 10 percent loss of available shade power, but only 1% of normal output. It would not surprise me to see a similar loss through the PWM controller though at those power levels.

    Jim
  • Jim,

    congratulations getting the solar panels to operate closer to their Vmp so they harvest more power when the sun is bright.

    Now to add cheap battery protection, could you just put a cheap PWM controller between your new contraption and your battery?

    (Sal had actually suggested this months ago on another tangentially related thread about protecting a battery from overcharging). The idea is the PWM controller will get higher current from your homebrew MPPT rig and pass it all through when the battery needs it. But as the battery becomes fully charged, the PWM charger prevents overcharging.

    Ed
  • Potentially yes but I think I would need a little different power supply. It would have to be adjustable constant voltage, but still be able to pull down the PVs on the input as you vary the voltage like this one. The voltage jumps all around based on changes in condition and I have to rescan with the pot to find the power point, even if it is at the same voltage, with a new pot position.

    With 3 out of the 11 Unisolar segments covered in building shade, the Vmpp for panel output has dropped to under 13v, to just a couple of tenths above the battery voltage. I think the fast MPPT scan times for optimizing power on partly cloudy days has some merit. I had wondered if it was just marketing hype but I can see it mattering under some conditions.

    If you get the set voltage even a little too high, output plummets as the IV curve suggests. If you get the voltage a little low, you pay hardly any penalty at all.

    Jim

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