Buck Gizmo -Volts vs Amps ? UPDATED

[BFL13]

Update 5 Nov. (It didn't work out as hoped)

I got my little gizmo, see close-up photos here

http://www.ebay.com/itm/DC-DC-15A-Converter-Buck-Adjustable-4-32V-12V-to-1-2-32V-3-3V-5V-24V-Step-Do...

It is tiny! 2" by 2 3/8" Solidly built though. It has a few two-hole connection points on the board as seen in the photo, one is called "Key" What is that for? Thanks.

When I get to testing this I have been thinking I can use my two trickle float chargers as input to simulate a solar panel with the voltage inside the range of the gizmo.

Each charger is 2 amps and makes 14.2-13.4 volts , floating at 13.4 steady. So I think I could put them in series and get about 28 volts and 2 amps to feed the gizmo, then it should have about 4 amps at 14v output?

I will also try it on the 37 volt solar panel once I get its voltage down a bit (shading a cell or two ought to work) so it won't fry the gizmo.

(This was all just to explore the part the buck converter plays in an MPPT controller. I haven't thought of anything this gizmo could do that I can't do now with other equipment, but maybe something will occur)

[MEXICOWANDERER]

BFL, the Bucker is PRICELESS for use as a device battery charger. Just find the right tip to fit the accessory and you have a 12-volt powered battery charger that can recharge NiCad, NiMH, Alkaline, or Lithium rechargeable batteries. IMHO you haven't wasted a penny of your money.

[BFL13]

It was Mex's fault for posting about gizmos. At least it only cost $13 🙂

[westend]

Hey BFL,
If it's any consolation, Edison, Tesla, and all the other great inventors didn't get it right on the first attempt, either. This is part of the practical process of science. My hat's off to you for trying new things. 'Attaboy!

[BFL13]

BFL13 wrote:
My contention is that the standard ... solar controller is a straight buck converter....

The market will sell you a controller that is MPPT with a high intake buck converter you can set to 12v battery level for output. Nobody sells one of those without the MPPT feature.

IMO you can get one of these gizmos that will be a PWM 24v panel controller but for much less money.

Wrong! It turns out the controllers have more circuitry in them that is required to work with battery charging as the load.

So my gizmo cannot be used as a charge controller for 24v panels (as proven during testing)

My evil plan has been foiled! 😞

[BFL13]

BFL13 wrote:

westend wrote:
What's messing you up is that you're trying to fit your buck converter into the same relationship as the total function of an MPPT controller and that isn't the case. Salvo defined it above. The MPPT controller is able to control output to the battery by boosting more amps at the sake of voltage. Your buck gismo is more of a voltage source and can't vary voltage to increase amps to the battery.

That is even more mixed up than I am! 🙂 Oh well, it is late.

The buck converter has an adjustable voltage output (CV) which you can set for a lower voltage than its input voltage, so it makes more amps out from the same wattage.

OTOH, westend may be right! 😞 I will post my gizmo test in a new thread.

[jrnymn7]

I said,

"??? What I don't understand is why Isc does not apply when an mppt is employed?"

O.K., I think I understand better now.

The Isc exists. It's there, whether the controller "cares" about it, or not. And, in fact, the MPPT does not concern itself with Isc while in operation, because it is designed to calculate for the most efficient use of ALL the available power (W's) at that moment; which is a mathematical relationship between USABLE ("max") I and V, and the load being called for.

The solar panel is a Source for Current (Amp Hours). But the MPPT is a Voltage Regulator that ONLY operates in CV mode; NEVER in cc mode. The pwm, otoh, is also a voltage regulator, but it operates in BOTH cc mode and cv mode; controlling voltage indirectly thru current regulation, and controlling current indirectly thru voltage regulation.

So, essentially, MPPT's do not ever do BOOST mode, because they never do CC mode. Boost mode = CC mode. So to say the MPPT does such and such while in boost, is really a misnomer. MPPT's simply do not operate in boost mode. They may do such and such, at a time when a PWM would oherwise be doing boost/cc, but that's still two different controllers doing two different things during the same "stage" of battery soc.

There, indeed, appears to be no advantage to using an MPPT, over a PWM (or a basic little buck converter gizmo)... once the batteries have reached a state of charge, such that, they can no longer accept ALL the available current from the panel/controller. At this point, both types of controller are now operating in ABS/CV mode, and amps are tapering.

But there is definitely an advantage to the MPPT while the batteries are capable of accepting more than the available current. For even though the PWM makes use of the higher current rating of the two, Isc vs. Imp, its operating in cc mode, and according to Kirchoff's Law, results in less over all power going to the batteries.

BFL said: "increasing the load does make the voltage drop"

I don't see that happening. When the batteries have a high soc, when either controller would in abs/cv mode, any additional load will simply be supplied by the surplus current available. It's no different than putting a 110v 5a load on a 110v circuit capable of 15a, and then adding another 110v 3a load to the same circuit. Voltage is constant, at 110v, and the current is variable; but limited at 15a... just like a dc circuit in CV mode. It should be no surprise then, when V remains the same, as I increases, while the batteries are being floated IN CV MODE, not cc mode (like a smart charger).

------------

In order for the OP to make the most of every watt produced by the panel, on a day when sunshine varies, he would have to continually monitor the power available, just like the MPPT does. This could get rather tedious and time consuming. And seeing as the bulk of the Ah's replaced are replaced during the early stages of charging, when the batteries have such a high acceptance rate, and see that at certain times, charge times are limited by hours and quality of day light, there is a definite advantage to making the most of every W available. And this is where the MPPT proves its usefulness.

And yes, if solar is being used to supplement early morning generator charging, then there is no advantage to using an mppt, but if solar is being used for the early stages of charging, as well, then it can be very useful.

[BFL13]

westend wrote:
What's messing you up is that you're trying to fit your buck converter into the same relationship as the total function of an MPPT controller and that isn't the case. Salvo defined it above. The MPPT controller is able to control output to the battery by boosting more amps at the sake of voltage. Your buck gismo is more of a voltage source and can't vary voltage to increase amps to the battery.

That is even more mixed up than I am! 🙂 Oh well, it is late.

The buck converter has an adjustable voltage output (CV) which you can set for a lower voltage than its input voltage, so it makes more amps out from the same wattage.

[jrnymn7]

It must have something to do with the controller being a current regulator during cc, and a voltage regulator during cv... or not! lol. I'm tired.

I'll try again tomorrow. I need to get a better grasp on the difference between Isc and Imp. Hopefully a light will go on tomorrow.

And I'm thinking it's just like a charger putting out 10a, and 7a are going to the battery, because that's what it will accept, and the other 3a are going to the fridge or whatever, but the voltage remains constant, at its max set point (voltage regulated). Or not!

[westend]

What's messing you up is that you're trying to fit your buck converter into the same relationship as the total function of an MPPT controller and that isn't the case. Salvo defined it above. The MPPT controller is able to control output to the battery by boosting more amps at the sake of voltage. Your buck gismo is more of a voltage source and can't vary voltage to increase amps to the battery.

[BFL13]

You got yer IV curve for the panel. On it you find a point near the "knee" that when at that voltage ( Vmp) the panel is at max watts. That point is known as the Max Power Point. Whatever the amps turn out to be at that watts and that voltage is a by product sort of, called Imp.

The buck converter doesn't care about input amps. It only knows the input watts and voltage. The output voltage being lower with the same watts makes for higher amps in the output. The amps you get is sort of accidental by-product. So my 230w panel rated at 8.3a Isc (less for Imp) will do 15 amps as output from the controller's buck converter with the battery in the high 12s. If battery voltage is higher amps are lower.

Meanwhile with a 12v panel and a PWM controller, you get Isc as its output passed through the controller which does not start controlling until Vabs. (the CV set on the buck, down from the panel's voltage of something higher.)

In this case Vmp and Imp are totally meaningless. Only apply to MPPT.

You go out and disconnect the 12v panel and measure Isc. Say it is 7 amps at the moment (it changes all the time) now connect up and you should get 7 amps to the thirsty battery.

With a 24v panel you go out there and measure Isc (panel disconnected) and you get say 7 amps or whatever. But now connect up and with the big intake buck the controller has, you get say 14 amps to the thirsty battery out of the same watts with the lower voltage.

The charging amps you get is entirely accidental by-product of what watts divided by voltage happens to be at the output of the buck converter. The watts is a bit less than the watts at the intake.

This is where I get lost. The watts intake at the buck converter is a bit less than what the panel COULD do but is actually only what is "demanded" which could be a lot less than what the panel could do.

I cannot grasp how the increased load demand on the buck converter makes it do more amps with the same voltage so its watts must be more. It has to get those watts from the input, which must get them from the solar panel. That means you open up a greater potential difference. except that is volts talk , not watts talk? Increasing the load does make the voltage drop (loaded voltage) so that would widen the spread.

You are not setting the buck converter. Its output voltage is whatever the load's voltage is. Somehow that works back through the buck converter to the panel and makes its output go up if the load increases. There are wattage limits but everything seems to be a bunch of moving goal posts; drives me crazy 😞

[jrnymn7]

red, I think the only time bucking can stop is if the panel itself cannot provide enough power to maintain a V-input of at least one volt above the bucker's V-output setting. During cv/abs mode, amps are tapering, so any extra power available at the panel is simply not utilized.

The batteries are the load, and the panel is the power source. And just like plugging a lamp into an ac receptacle, the lamp does not receive the entire force of the entire ac power available, but only what it draws from it. So say, 115v at 2a, even though 15a are available from the ac panel breaker. (At least that's how I understand it).

[jrnymn7]

"The MPPT feature just fine-tunes the panel's output for the buck's input, but you still get some sort of panel input with no MPPT."

That's an interesting point.

Hmmm, I was just re-reading page 3 in my 'paralleling made easy' thread, where Salvo was explaining Kirchoff's voltage law. And I thought it applied here, and perhaps explained the advantage of the mppt feature.

Perhaps the mppt only shows its merit during boost, because it recognizes there is 'extra' power available that the pwm does not utilize. In Salvo's example above, the pwm is only extracting 120w at first, but the mppt is extracting 160w. Why is this? Well, it appears the pwm is operating according to Kirchoff's law, and is therefore only extracting just enough voltage to overcome battery Voltage and cable Resistance. So, if the batts are at 12.2v and cable resistance is .3v, the pwm will start at 12.5v in cc/boost mode, just like a typical power supply / 3 stage smart charger.

But the genius of the mppt is that it is not bound to Kirchoff's law, necessarily, because it does not start in cc mode, but rather in a sort of variable cv mode. And that "variable constant voltage" will be whatever is available at the time, from the panel. And when available power (Watts) exceeds that required by a pwm to operate in cc mode, the mppt makes use of this extra power, and thus produces more current.

So, while there is 160 watts available, and Isc is 10a:

... the pwm only uses 125 watts... (12.2v + .3v) x 10a = 125w

But while there is 160 watts available, and Isc is not limited to 10a:

... the mppt uses all 160 watts... (12.2v + .3v) x 12.8a... and thus operates at a slightly higher Charge Rate.

??? What I don't understand is why Isc does not apply when an mppt is employed?

[BFL13]

My contention is that the standard PWM solar controller is a straight buck converter with a lower intake rating (12v panels) than the buck converter in the standard MPPT solar controller designed to work with higher voltage panels (24v panels)

The standard MPPT controller can also do the lower intake 12v panels of course, like any buck converter can at less than its max rating.

The world is divided into "!2v" and "24v" panels where you must have an "MPPT" controller because it is the only kind that can buck the voltage from 24 to 12 to match the battery in an RV. People have taken this to mean you must have MPPT when in fact all you must have is the buck converter with enough intake rating.

I contend that the standard " MPPT" controller would do nearly as well without the MPPT feature, same as it does in Float when it is not in MPPT but it still bucks the voltage and gets more amps from the available input watts. You see this all the time when the battery only wants say 4 amps and you add a load of 8 amps, you can see 12 amps coming in from the solar controller that is NOT in MPPT at the time ( it is only in MPPT during Bulk)

The MPPT feature just fine-tunes the panel's output for the buck's input, but you still get some sort of panel input with no MPPT.

So I am calling the high intake rating buck converters a form of PWM solar controller that can do 24v panels and PWM is not just for 12v panels like the standard PWM controller is limited to.

The market will sell you a controller that is MPPT with a high intake buck converter you can set to 12v battery level for output. Nobody sells one of those without the MPPT feature.

IMO you can get one of these gizmos that will be a PWM 24v panel controller but for much less money.

[pianotuna]

Hi jrn,

Early MPPT units had a pot to turn to "tweak" them. This doesn't work out too well. Later units have a computer chip that "scans" the panel every so often to figure out the maximum amount of power that can be extracted.

Perhaps Salvo could tell us how an MPPT acts in absorption mode when the input voltage is 33 and the output is 15.3 volts on a Blue Sky 3024 MPPT controller?