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Old 05-12-2010, 07:02   #61
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Originally Posted by goboatingnow View Post
However if you have loads that work the panel to the max ( either battery charging or just DC loads) then by definition MPPT provides more power from a given panel

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Dave
Yes in theory assuming a loss less conversion. In practice you only benefit from MPPT when the gain from the MPP out ways the losses that occur in voltage conversion etc.
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Old 05-12-2010, 09:22   #62
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I'm a bit confused by the concept of trying to control the battery charging voltage. When I used to design battery chargers (not lead-acid), I would monitor the battery voltage, over time, and use this to determine the proper charging current. As I increase the current the voltage rises, but voltage is what I measure and current is what I provide.
Two different school of thought there. Some believe that increasing the pressure will increase the flow.

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You can call it voltage or current regulation.
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Old 05-12-2010, 09:28   #63
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I don't understand regulators on both sides.

In that Cypress example it is the presence of voltage divider resistors R400 and R401 giving a voltage measure of the panel at Vin_sense and similarly current sense resistors R402//R406 allowing the voltage drop across them (aqnd hence current through them) to be measured at Iin_CSP and Iin_CSN. There is a corresponding volatge and current measurement on the battery side.

There is only one microcontoller and one switched charging path, but by sensing the current/voltage on both sides and thereby controlling them by switching Q400 and Q401 it is accurate to say that the panel and the battery are being controlled/regulated, hence it can be described in a general sense as having a regulator (or pehaps "regulation" reads better) on both sides.

Power supply design is a specialist area and its a fair amount of work for an EE to analyse the feedback path and closed loop response of an analogue power supply and derive the differential equations and/or model the power supply with a circuit simulator to predict performance. A microcontroler (digital feedback path) version even more so. In fact the only real way to know exactly how it is operating is by sitting on a bench under various loads and probe it with an oscilloscope.
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Old 05-12-2010, 10:58   #64
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Whatever magic MPPT you buy, be sure that it was built for marine use. I had the same experience as Paul with my Blue Sky controller--one drop of saltwater and it was smoking history. I went back to the simpler, cheaper, and potted marine controller.
Blue Sky does have a utility enclosure you can buy (it looks pretty standard) that will keep the occasional drips off. Unfortunately in my case it was slightly too large to fit where I wanted to put it, so, running out of time, I decided to omit the enclosure. Not one of my better decisions, as it turns out! If you can keep the Blue Sky unit dry it actually seems to work well.
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Old 05-12-2010, 11:37   #65
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...it is accurate to say that the panel and the battery are being controlled/regulated, hence it can be described in a general sense as having a regulator (or perhaps "regulation" reads better) on both sides
Yes, your words are better. The reason the MPPT controllers use PWM is so that they can make use of the special properties of AC circuits. Specifically using relatively lossless impedance rather than DC resistance to manipulate the voltages and currents to use the power efficiently.
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Old 05-12-2010, 11:43   #66
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(my confusion about voltage-controlled charging)
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Two different school of thought there. Some believe that increasing the pressure will increase the flow.
I should have better phrased my original posting. I won't bore you with my credentials, but I am extremely comfortable with Ohm's Law. Thanks to everyone for the replies.

I suppose the thing that bothers me is that some folks talk about setting the battery charge voltage to some specific point. Sure, you can do this if you have a strong enough charging source, but with the solar panels most of us have we are only going to be able to push enough power into the battery to slightly elevate the battery voltage. As the battery charges the voltage will rise until it hits our desired setpoint, at which time we may change the charging parameters.

Changing the subject back to basic MPPT theory, if you are still puzzled, here's a simple case. Start with a perfect battery (ideal voltage source). Add a resistor (source resistance) between the battery and the load. At what load resistance will you get the maximum power in the load?

It turns out that you get the maximum power when load resistance = source resistance. Half the power is lost in the source resistance and half is dissipated by the load. This is the most power you can get from that source. At this point the voltage across the load is one-half of the open-circuit source voltage, and the current through the load (through everything in fact) is half of the short-circuit current.

This illustrates maximum power transfer, and MPPT, but of course solar panels don't behave like classical resistive power sources. A solar panel is closer to a constant-current source, but not quite. There is however a load point where the panel will deliver maximum power. It couldn't be any other way.
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Old 05-12-2010, 11:53   #67
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I note that the Cuthbert paper is dated June 2009, so it has been out about 18 months and possibly reflects only the newest technology.

But from what it is saying, they are still NOT CONTROLLING anything in the solar panels. Yes, they discuss the changing voltage and amperage of the solar panels as they are exposed to different loads, but they are not actually monitoring the panels and they are not actively responding with a modified load.

What they don't say, but plainly infer, is that since the solar panels are charging a capacitor bank in the buck converter. A buck converter typically being a dc-to-dc UP converter, for laymen's terms, ost commonly a converter that takes a low battery voltage like 11.6 volts and UPconverts that to 14.4 volts, an alternator/car/boat running voltage.

When the charge in that capacitor bank varies, the solar panels will see a different effective load, and their ouput voltage and amperage will vary, yes. BUT THEY ARE DOING THAT BY THEMSELVES, IN RESPONSE TO THE LOAD, AND THE MPPT CONTROLLER IS STILL NOT DOING THIS AS A CONTROL ACTION.

If it is, they've failed to make clear what the "control" and "sensing" circuits actually are. The only conection to the solar panels is shown as the buck converter module, and again, that module is not controlling the panels--it is simply responding to them.

Or did I miss something in there? (Been known to happen.<G>)

If you stick a firehose (solar panel) in a funnel (MPPT or buck) , and the funnel backs up and overflows, you don't say the funnel is "controlling" the firehose. The firehose is putting out the most it can, all the time, and the funnel is simply reacting to it. Not controlling it. If the three little piggies shut their front door to keep the wolf out--they're not controlling the wolf. They're simply restricting the wolf's options.

Now, maybe this reactive situation is making (allowing? putting?) the solar panels run at their maximum power point, but that's just a matter of reaction, not control per se. What these claims remind me of, are the folks who are selling four and "five stage" charge controllers. You know what they call that fourth or fifth stage? "Off". It's the new math.
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Old 05-12-2010, 12:59   #68
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hellosailor,

Looking at the Cuthbert/Cypress Semi paper, the MPPT converter is monitoring both the panel output voltage and the panel output current (see the Iin_CSP and Iin_CSN connections to the current-sense resistor [I can't quite read the connection names, but they're something like that]). The input capacitors C405, 407, 407, are there to provide a low impedance for the switching inductor L401 to work with. During normal operation there shouldn't be much voltage variation across these capacitors, so the panel will see a fairly constant load. I suspect that the input inductor L400 is there for noise suppression and impedance control as much as anything else. The FET pair Q400, Q401 are driving L401, which feeds C400, 401, 402 -- this is the essential Buck circuit.

By the way, a Buck switcher is a step-down topology.

The panel voltage and current sensing is used by the controller to adjust the Buck PWM duty-cycle to get the maximum power out of the panels. I'm not sure I understand what you are saying, but the MPPT switcher adjusts it's operating point to draw more or less current from the panel, and the panel "responds" by decreasing or increasing it's output voltage. Call it "control", or call it "reaction", the panel is just a passive circuit and it behaves accordingly.

I suspect that we are discussing two different topics, thus the confusion. MPPT has to do with taking power from the solar panel. Charging the battery is what you do with that power. If the battery can't accept all the potential power, then the MPPT controller adjusts it's operating point and less than the maximum power is taken from the panel. At this point it's no longer MPPT.
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Old 05-12-2010, 13:04   #69
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But from what it is saying, they are still NOT CONTROLLING anything in the solar panels. Yes, they discuss the changing voltage and amperage of the solar panels as they are exposed to different loads, but they are not actually monitoring the panels and they are not actively responding with a modified load.
Or did I miss something in there? (Been known to happen.<G>)
Its a good observation, but terminology may be confusing the issue:-

"they are still NOT CONTROLLING anything in the solar panels"
True, but it is controlling the voltage across the panels measured on the two wires coming from it, and the current flowing along those two wires, which is in effect, considered to be controlling the solar panel.

"Yes, they discuss the changing voltage and amperage of the solar panels as they are exposed to different loads, but they are not actually monitoring the panels and they are not actively responding with a modified load."

Yes it is at the input to the controller. An MPPT controller is actually monitoring the panel and by feedback the voltage and current is held on the "knee" which by P=VxI gives the power in Watts and at the knee that VxI number is at the maximum. At the same time imagine "looking" into the controller on those two wires from the panel. In DC terms the equivilant resistance R of the controller input is that V across the panel divided by I flowing in the wire (R=V/I). Completely disregard the output side to the battery and the resistance of the battery, voltage levels, charging mode etc, but just in terms of the resistance looking down the wires into the controller from the panel, the R looking into the controller is being varied to control the V and I to keep the panel on the "knee" of the curve .

A normal non MPPT solar controller doesn't have this varying resistance but a fixed one which by selection may be right for a perfect fit on the max power part (the "knee") of the VxI curves.

The resistance seen is not a capacitor or inductor etc but that in the whole path, in the most part it is the voltage across the semiconductor device, that sets the resistance "seen" looking in and that is also how it is varied.
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Old 05-12-2010, 18:46   #70
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"By the way, a Buck switcher is a step-down topology."
Mea Culpa, I'm thinking of "buck-boost". [Slap!] Must. Find. Caffiene.

I suppose the best thing is that MPPT is now finding a broad enough market that custom chips are being designed and marketed for it, which really ought to help drop the pricing for them.

Like the aliens said at Roswell, "I have no idea how it works, I just fly it!" :-)
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