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BFL13's avatar
BFL13
Explorer II
Nov 04, 2014

Buck Gizmo Test Results

This test was to see how much the amps went up at the output of the buck converter compared with the input amps. This is the gizmo again:

http://www.ebay.com/itm/DC-DC-15A-Converter-Buck-Adjustable-4-32V-12V-to-1-2-32V-3-3V-5V-24V-Step-Down/281206555382?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D25217%26meid%3D8539ba1eede94f48b9c4da74610ac108%26pid%3D100011%26prg%3D10629%26rk%3D5%26rkt%3D10%26sd%3D351129151726&tfrom=351129151726&tpos=unknow&ttype=price&talgo=origal

So first I attached four pigtail thin wires to the terminals set screw type and then attached fatter wires to those and got all set up.

I put my two, 2a float chargers in series and tested the output at the "working" clamps at 27.0v

No load bench test of adjustable voltage CV screw pot. With 27v in, and meter on output, set it to 14.8v The little screw turns many times and you can set voltage exactly all the way through its range. In this case I stopped at 26v output in case I would fry something hitting the input voltage, no idea, but I had not even started the test yet so that would be annoying to fry it now. ok

Next test is with no input, connect to battery bank. The input voltage was the same as the battery voltage on the output, 13.82v, then with converter off, voltage down, 12.81

So it just passed through the voltage going that way from out to in across the circuit board.

So now the biggie, with battery connected 12.8v, connected input 27v with ammeter in the pos input line showing amps bouncing around between 1.55ish to 1.75ish----------so what were the output amps????

The same!!! Rats, phooey. Ok , maybe the batts won't accept any more, try it with a load. On Trimetric without the buck converter on, got 12.3v and -20.3a. Turn on gizmo, 12.3v and -18.3 so 2a from gizmo. Tried a few more like that pretty much the same results

Maybe a slight hint of higher amps? Hard to tell with these small amounts and rough instruments but it sure didn't double the amps like I expected.

OK final desperate test using an actual 230w solar panel as input.

Dull day so panel de-rated already so ok for input to gizmo at
Voc 31.0 Isc 0.3a Batts now 12.8, no solar Tri shows -0.3 to -0.4 back and forth as normal with converter off. Add solar power to gizmo, amps on Tri now 000 to +0.1
So same as panel Isc for gizmo output or maybe a titch more , hard to say.

RATS!!!

So what went wrong? Should amps have been more at the output or not?
Thanks.
  • BFL13's avatar
    BFL13
    Explorer II
    Nov 06, 2014
    It seems to show that the $13 RJ Special solar controller with its fancier buck converter is quite a bargain when the straight $13 gizmo doesn't have those extra circuits.

    Same with the $102 Eco-W controller compared with that $105 gizmo, where the Eco-W is set and forget while you would have to twiddle CC and CV all day with the gizmo.
  • jrnymn7's avatar
    jrnymn7
    Explorer
    Nov 06, 2014
    Salvo wrote:
    This regulator is only capable of supplying 10A, but your battery wants more. The regulator goes out of regulation. That's why I sad to lower regulator voltage to just above Vbat. Now you should be able to increase Vreg and watch current increase. I don't think there's much voltage overhead.

    Icharge = Vreg - Vbat / charging cable resistance

    You need to keep Icharge under 10A.


    Does this mean a gizmo with CC would work?
  • BFL13's avatar
    BFL13
    Explorer II
    Nov 05, 2014
    Thanks Ken and Salvo. At least I got an education along with my $13 new paperweight :)
  • ken_white's avatar
    ken_white
    Explorer
    Nov 05, 2014
    BFL13 wrote:
    I get that the controller has an MPPT up front that fiddles its resistance to keep the panel at Vmp (sampling for wattage with feedback on that) so it will maintain best wattage for later use by the buck converter to have the best output wattage so it can make optimum amps at any battery voltage (I think :( )

    So now as well as that, it has another feedback thing happening at the CV end?

    I do see the MPPT controller still doing higher than panel amps when the controller is in PWM and there is enough "demand" for those amps, so it isn't MPPT doing that


    Most MPPT controllers have multiple feedback loops and use a microcontroller to control them.

    See -> uC

    And here -> Demo Board

    And here -> More Stuff
  • BFL13's avatar
    BFL13
    Explorer II
    Nov 05, 2014
    ken white wrote:
    BFL13 wrote:
    ...Ok so the CV in the controller's buck converter has some magic way to match the battery voltage (is that impedance matching?) that my gizmo is missing? What is that magic way?


    A controllers feedback loop is designed to optimize current, not voltage so the design is quite different.


    I get that the controller has an MPPT up front that fiddles its resistance to keep the panel at Vmp (sampling for wattage with feedback on that) so it will maintain best wattage for later use by the buck converter to have the best output wattage so it can make optimum amps at any battery voltage (I think :( )

    So now as well as that, it has another feedback thing happening at the CV end?

    I do see the MPPT controller still doing higher than panel amps when the controller is in PWM and there is enough "demand" for those amps, so it isn't MPPT doing that
  • ken_white's avatar
    ken_white
    Explorer
    Nov 05, 2014
    BFL13 wrote:
    ...Ok so the CV in the controller's buck converter has some magic way to match the battery voltage (is that impedance matching?) that my gizmo is missing? What is that magic way?


    A controllers feedback loop is designed to optimize current, not voltage so the design is quite different.
  • ken_white's avatar
    ken_white
    Explorer
    Nov 05, 2014
    BFL13 wrote:
    OK but the current source "24v" 230w panel has an Isc of only 8.2 amps but when the controller (a buck converter, yes?) is in "12v" mode on the battery it can crank out 15 amps.


    The solar panel controller is not really a buck converter in the classical sense since its designed to be a battery charger.

    Power supplies in general are CV devices and are designed to be operated differently than a battery charger even though the basic topology is similar.

    So, your results should be expected.
  • BFL13's avatar
    BFL13
    Explorer II
    Nov 05, 2014
    OK but the current source "24v" 230w panel has an Isc of only 8.2 amps but when the controller (a buck converter, yes?) is in "12v" mode on the battery it can crank out 15 amps.

    EDIT saw Ken's above Egads, Ken and Salvo agree on something! :)

    Ok so the CV in the controller's buck converter has some magic way to match the battery voltage (is that impedance matching?) that my gizmo is missing? What is that magic way?
  • ken_white's avatar
    ken_white
    Explorer
    Nov 05, 2014
    BFL13, the buck converter steps down the voltage by adjusting the duty cycle of the switch clock (150 kHz).

    The basic buck converter is a constant voltage power supply so if the output voltage is higher than the battery voltage, the duty cycle will increase in an effort to raise the battery voltage and it can go as high as 100% if there are no boundary control circuits.

    If the CV cannot match the battery voltage level, the solar panel will then be basically connected straight through to the battery bank since the transistor is 100% on, or saturated, which means its acting as a switch.

    The buck converter will only have a varying current output if its CV is slightly greater than the battery voltage and then the current will be a function of the battery bank open circuit voltage and the battery banks internal resistance.

    So, your results are valid.

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