Bucking and MPPT?

Efficiency for any controller is not constant and is a function of supply side voltage and demand side load.

I know you don't want to hear this, but its the energy transfer that's important, not the amps.

With that said, there are way to many variables that can effect day-to-day operations so there is no good way to measure performance without measuring in a lab under controlled conditions.

So IMHO, you will never really know how your system compares day-to-day.

The solar controller has a decent efficiency percentage per its specs, but the Eco-Worthy's funny little "demo" it does for PWM vs MPPT shows a percentage efficiency wrt that, and the percentages are all over the map depending on the situation for insolation etc not clear how they get that number.

So there is a sort of standing efficiency for the set-up and then there is another efficiency for how much the MPPT or maybe bucking plus MPPT is doing for you--or something like that!

I just wanted to know if that jump in amps was about right going from just bucking to bucking plus MPPT when it went into Bulk that time. I thought maybe catching it at that moment where battery voltage was still 13 but the amps went up by that amount would reveal something.

12thgenusa wrote:
I'd be really unhappy if the efficiency was only 85%. Mine is closer to 95%.

The Rogue display shows volts and amps into the controller as well as panel watts output. It only shows volt and amps on the output side if the controller. Efficiency is easily calculated.

LOL...

I said as an estimate for controller, all associated cabling, junction box connections, etc...

I have no idea how any particular system is wired/configured so I just threw out a low side number to take into account these variables.

A 95% system efficiency is very good considering most high controllers are somewhere near this value and lower end controllers are lower.

I'd be really unhappy if the efficiency was only 85%. Mine is closer to 95%.

The Rogue display shows volts and amps into the controller as well as panel watts output. It only shows volt and amps on the output side if the controller. Efficiency is easily calculated.

A DC meter measures the average value of voltage or current, which is fine for a constant value, or values that contain small changes in magnitude that vary with time.

A True RMS meter measures the current or voltage magnitude value that is needed to calculate power when the DC voltage varies a large amount in respect to time.

I don't really spend much time looking at my system parameters since its capacity is a little more than I actually use, so its pretty much topped off daily.

The panel's current can be considered constant for a given irradiance, so to change panel power, the panel voltage must be manipulated.

Power Out divided by Power In equals Efficiency so use your Power Out data and divide it by approximately .85 and this will approximate the power in - losses of controller and cabling.

Why an RMS meter? Using the ordinary multimeter on the panel terminals gets me the Voc (34 of rated 36.8 eg, and I got 28v when it was connected and operating. Is that 28 not valid because the meter is not RMS?

What panel voltages do you typically see when operating at fairly high amps and what would be the actual and rated Voc at the time?

How does panel voltage fit into the watts picture anyway? My controller's watts reading is made up of battery voltage and amps to the battery. No reading of the input side except for the "demo" it does comparing MPPT vs PWM--on that demo I see it uses actual panel Isc at the time not sure how that feature works.

I suppose the amps on the input side to the controller where it is still "24" volt would go with panel voltage to get the watts on that side of the controller? Then compare with watts on the output side?

BFL, you are welcome.

Any true RMS meter should be able to measure the panel voltage while its being loaded dynamically.

It is measured directly on the TriStar meter connected to my MPPT 45.

Thanks Ken. Is there a way to measure "panel voltage" while it is connected and operating? I can get the Voc when it is disconnected of course.

BFL13 wrote:
...I am trying to see on the IV curve where the power point was before tracking started and after but I can't make sense of it where in this case the voltage at the bottom is not batt voltage like with 12v panel IV curves.

BFL, the MPPT algorithm basically adjusts the solar panel's output voltage, for a given amount of irradiance, to transfer the desired power to the load.

If the Sun's irradiance is constant, the panel will create a nearly constant current for a very wide range of panel output voltage.

If maximum energy transfer to the load is not required, the panel voltage is adjusted to a lower voltage level which creates a lower energy level transfer.

So the tracking point is moving all the time in an effort to deliver the energy required by the load.

The Maximum part of the MPPT algorithm only occurs when the controller's output energy demand is greater than the energy available/provided by the panel - the controller delivers the maximum energy available.

KJINTF wrote:
OK guess I'm a bit dense
I now understand what it is you are going on about
It's the magic tracking, the "T" in MPPT we are discussing

The magic in a quality MPPT controller is hidden in the tracking function.
Finding a moving target as in Vmp can be difficult, array sweeps, output interruptions, etc all controlled by the tracking algorithm. Yes some older Blue Sky units were simply set it and forget it, IMO a joke. That might work somewhat OK for a fixed install but can not possibly do good for a RV's install where sun shade temp changes etc.. are part of the mix.

Tracking functions include finding and holding on to the Vmp of the array. It has several other tasks for example limiting output current by reducing the input power (the reason why many MPPT controllers can be connected to larger than "allowed" PV array), It moves away from the Vmp thus not harvesting all the available energy. Typically the PWM charging function Bulk ABS Float etc is in constant communication with the tracking function. A quality controller will always have tracking enabled 100% of the time (when actually passing current the loads), which is sometimes not true for the buck conversion stage that can be bypassed when the array voltage drops to low. The Buck conversion output is directly controlled by the tracking function. Tracking simply reduces the input power limiting the output current that can charge the battery with the PWM function. Most good PWM operations will go from 0% to a full 100% another place to limit/increase current to the loads.

Now all we need are measurements of Array Voltages/Currents as well as Battery voltages/currents.

That data will show you what is happening in each of the charging modes

I have a morningstar MPPT controller that I use with three "12V" panels in series. It's interesting to watch it track when first connected to the panels. Watching my Trimetric, in bright sun, it will output about 5A into a well discharged battery bank. After about 30 seconds, the current starts to rise quickly to near 12A as it begins to search for optimal Vmp. In full sun it will soon hit the 15A limit of the controller. In partial sun it will start converging with the the current going up and down as it searches, finally settling down. Then, if it is suddenly shaded for a few minutes the process will repeat Not sure what the algorithm is to determine when to search, but interesting to watch. If I pull off a panel and drop to two panels the process will repeat as well to find Vmp.

I suspect it starts assuming a Vmp in the range of 18V or so for a single panel then does a search to find the real Vmp.