Friday, 20 October 2017

The Cheapie controller is MPPT - not PWM

Well I've had a better chance to fiddle with this now, and I'm quite sure that this little UEIUA CPS-2420 controller is properly an MPPT controller (unlike the views of others)

So here is my evidence for that. PWM will essentially pull the panel voltage down to very close to the battery charging voltage. This is because it can't do voltage to voltage conversion. Given that people normally run a "nominal" 12V panel, which is often open circuit at 21V this is less of an issue than it may seem. Panels usually  have its maximum rated power at a voltage something like 17V ... in the case of my black 100W panel is actually 17.8V (... which is of course at the maximum obtained measuring at Standard Conditions , namely full sunlight but with a panel temperature of 25°C). So the drop from there to 13 or so optimal for charging isn't a significant loss. Such would be a case of a panel at more or less optimal voltage match for the purpose, and PWM just has to groom it a little.

But if you run panels in series not in parallel then things change and PWM is at a disadvantage.

So in the above experiment we see that with two panels in series we are clearly getting MPPT because:

  • circuit voltage of the two series panels was brought to 38.6V
  • the panel was delivering to the MPPT controller 1.5Amps
  • the battery was being held at a charge voltage of 13.5V
  • with an observable 3.56 Amps going into it
were this system a PWM system this would simply not happen.

So the youtube reviews you may watch on this controller make what I believe to be the error of solely determining the capacity of the controller based on a panel which is already closely matched to the usage (battery charging) and measure what it puts out into the battery.

I've already shown (in previous posts) that the system is capable of delivering a few extra amps to the load without taking all that power from the charging. In a previous experiment (with one panel) I was able to deliver 3.5Amps into my load (from the load port) while still pumping 0.56Amps into the battery (3.5 + 0.56 = 4Amps) yet it failed to deliver 4 amps to the battery when simply charging the battery. This leads me to see that it can (and does) produce more amps, but only if the entire system requires it (in its view).

Clearly this new test demonstrates that the controller is able to deliver the full panel capacity to the "system"(given as also observed the panel temperature pulling down its maximum power delivery capacity). 

This word SYSTEM is the critical point. The controller is intended to be a part of a system, not just a stand alone battery charger. As such if we take that viewpoint and explore its capacity as a system we can see that it is delivering the capacity of the panels into the system, and not just pumping it into the battery.

I went and did a second test (because the sun came out as I was writing this) and found the following:
  • two panels in series was loaded by the controller to produce 6.48 Amps @ 20 Volts input into THE SYSTEM.
  • the controller then poked 6 Amps into the battery @ 13.2 Volts (you need to recall that battery at rest unloaded is a different voltage to the battery under charge and the controller has NO WAY to know what the resting state of the battery is until perhaps night)
  • the controller still fed the load with 3.5 Amps @ 12.8 Volts
so if this was PWM we would still only get a maximum Amps of 5.5 (the short circuit amp rating and ignoring panel temperature) yet it was able to feed 6 + 3.5 = 9.5 Amps into the system

Considering that the combination of the two cells was in theory only able to deliver 11amps,  that this controller pulled 9.5Amps to feed to the other two demands on the system is pretty darn good.

Clearly MPPT, not PWM.

If your goal is to shove the most power from your panel into a battery (from which you draw load in parallel to charging) then (as observed) the CPS-2420 MPPT controller is perhaps not your best bet, especially if you have only one panel producing (an open circuit) voltage of around 20V.

For that role I've found the little T20 (which I bought earlier because it was cheap) to be at an advantage there as it puts out a few more amps into the battery (with nothing on its load) than the CPS-2420 does.

Its a nicer looking unit in some ways but perhaps more confusing because it has more options, allows you to configure more and the menu is not straight forward to navigate ... this is them side by side,

With two solar panels in series the T20 it was still only delivering the same amount of amps as with one panel (and pulling the whole rig down to the lowest common denominator voltage) which is what PWM does. Here it is on the two panels (which in series produce the same amps, but double the volts).

as you can see its not able to take advantage of the potential 40V available to it. Because its PWM.

So where does this leave us?

It means to me that the choice of which controller needs to be a decision based on "what are you goals".

In fact I still like this little white controller (even though it lied about being MPPT and is really PWM) because it provides some additional features which I find useful. I mainly use it with a small panel (10W) to keep the battery conditioned on my ride on lawn mower.
I like that I can set it up for myself:
  • battery max voltage (so it won't over charge and will cope with a variety of battery types)
  • battery min voltage (where it will kill the load were I running one, and should it go below a voltage value I set)
  • a display of the current battery voltage (so I don't need to pull out my multimeter to test it)
  • I can leave it alone and know its doing its sole job
Keeping my mower battery trickle topped up (in a good manner and voltage sensitive) is an ideal usage. The mower sits idle in winter and can result in dead battery (from self discharge and sulfation). This controller and a 10W panel prevents that.

Even if you wanted to a run a short term load off your battery then this little controller does well as long as you manage that load yourself.
If you have timed operations in mind then this little controller does that too ... if you want more amps out of it then you'll have to run your panels in parallel (to minimise the losses of voltage, while doubling your amps). Just make sure you've got your diodes set up right with that ...

As I've read elsewhere PWM isn't simply worse than MPPT, it provides things differently and at a lower cost (although at this price point that's moot).

As it happens these two controllers are amazingly low cost and both do things differently, so picking one will be determined by your needs, your setup and your system.

Following are some sections from pages on the web which I have found salient (and their URLs for reference):
From this link

The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can be a better choice than MPPT and there are factors which will reduce or negate the advantages the MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed with respect to the following factors:
1. Low power (specifically low current) charging applications may have equal or better energy harvest with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency regardless of the size of the system (all things being equal, efficiency will be the same whether using a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency curves for MPPT controller are printed in their corresponding manuals and should be reviewed when making a regulator decision.
2. The greatest benefit of an MPPT regulator will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced. A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site should be a factor considered when making a regulator choice.
3. Systems in which array power output is significantly larger than the power draw of the system loads would indicate that the batteries will spend most of their time at full or near full charge. Such a system may not benefit from the increased harvesting capability of an MPPT regulator. When the system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may be unnecessary in this situation especially if autonomy is not a factor.

and this link suggests too:
The Solar input nominal voltage must match the battery bank nominal voltage if you’re going to use PWM
So if you do have a single "nominal 20V" panel (which probably puts out its max power point at 17, but amps at 14 will still be close to max), then you are matching the battery to the nominal voltage (especially when its a hot climate like Australia) and so you'll see less advantage to MPPT. Which I think is the other testers issue; I don't think he applied sufficient voltage advantage for MPPT to give its best run.

So know your needs and pick you animal :-)


I like to do things in a scientific manner, if you have any issues with my conclusions then proper scientific method would suggest that you examine my methods and identify what I did wrong. Try and replicate the results if you can before just slamming me.

Thursday, 19 October 2017

territory intruder

I've had problems with Wrens being ... well bird brained about things before (here). But now that I'm living more or less in Wren Country its becoming more common.

Sitting at the table this afternoon I was conscious that the Wren "territory calling" sound was becoming a bit more acute ... so I looked out the window and sure enough, this Wren had found another rambunctious intruder ...

It alternatively attacked the other bird and sat calling for the rest of the tribe (Wrens are group birds with one dominant male) to assist with driving this intruder off ...

The regular "shopping bag over the mirror trick" seemed to work ... and I have enough bird guano on the bike to start a company selling it..

Friday, 13 October 2017

Cheapie MPPT Controller further discussed

well we had sunlight worthy of mention today, so I thought I'd extend the testing I did on my cheapie controller and give it a quick whirl with one of my 100 W panels.

Now first up lets say that the panel is specified as:

  • Max Power = 100W
  • Open Circuit V = 21.6
  • Short Circuit A = 5.97
  • and "Rated V" = 17.8 (projecting the MPP?)
  • and "Rated A" = 5.62
which is interesting as it implies (to my limited understanding of Solar Panel Specs) that the maximum power of 100W is at 17.8 @ 5.62 

So I thought I'd wire it up to my system in quick way with the following diagnostics:
  • Volt Meter (Fluke 11)
  • Ammeter (Lexa cheapie in Amps)
  • 150A Watt / Power Analyzer (ebay jobbie)

Knowing that some items can suck more peak power than my MPPT controller may be able to stand and deliver, I thought that I'd wire my load directly to my 120AH battery. So here is that.

and for those who can't quite follow the mumble, the summary position is that less amps were going into the battery than was being sucked out of it. In particular the charger was putting about 1.6A into the battery while the little fridge was sucking out 2.89 A ... a short fall of power.

I wondered why the controller was not putting enough into the batteries to balance the load.

So here are the few more measurements I mumbled about at the end...

So the summary position is that the contoller has no way of knowing what is being drawn from the battery (what load its under) and (I assume) it assumes no load. There are controllers which incorporate this sort of externality with a thing (commonly? occasionally??) called a shunt sensor. This doesn't have one.

So my findings are that with my single 100W panel will give through more power, but interestingly it seems that it (the controller) won't push in (to the battery) much more than 3Amps (not shown in the video, I turned off the fridge giving more available power).

Which could of course be because controller reckons that's as much power as the battery can handle. After all it has no way of knowing what the battery capacity is ... is it 50Ah, or is it 100Ah, or is it ...

None the less with a load (on the load so the controller knows about it) it does take more from the panel. My load (the small fridge) is about 3.5 amps meanwhile the battery is getting 0.57A (in this specific example), which steps up to over 2 amps if I disconnect the load . So,  3.5 + 0.57 = 4.07 amps

Does this mean that the controller isn't MPPT?  Where did that power go?

I believe that the evidence supports that it is (indeed, I demonstrate that it is in subsequent posts, some of my other readings made it clear that it was putting more amps into the battery than the panel was producing). Also as I mention in the video the panel got to 69°C which means that we need to re-work the figures (because these things conform to known physics). This is coefficient is about 0.4 per degree variant from standard (of 25C) that the panel temp gets to. In my case its; about 44 degrees above 25 giving about 18% loss or in other words dropping my max A figure to about 4.15A ... or pretty close to what we got.

Where does this leave me?

Well it means that (depending on my fridge) I'm going to need more power input than I currently have or the system will basically just wear the battery down in the evenings which it won't be able to recover in the day time.

So, lets have a look next at what a second panel does for this ...

Wednesday, 4 October 2017

Cheapie MPPT controller (updated)

Well, normally I have the view that you get what you pay for. However often what things cost is a reflection of a complex mix of R&D costs, what the market will bear, profit motive ... A few other things.

Well China is now going very seriously into Solar Energy and it's not surprising that they are making efficient electronic devices to do small scale solar.

So with this in mind I bought a cheapie eBay controller and thought I'd see what it did.

This is it, and unlike ones I've seen reviewed on YouTube, this has two inductors.

It's the model CPS-2420 and it comes adorned with precious little documentation.

So, today I got it, and put it to the test. I attached it to a 12V flooded lead acid car battery and with a DMM (Digital Multi Meter) on it and found that it put my panel to about 17V and pushed 13V into my battery. Rough figures because it was constantly adapting output as the battery took charge and as the light conditions changed.

The little embedded system did a great job of ramping up load and determining the system capacity autonomously and heuristically. Best indicator of its effectiveness was that it put 0.67Amps into my battery when the panel is rated (and I've measured itto 0.55Amps at full short circuit load. This is about a 27% increase in power over PWM.

If you don't know the difference between PWM & MPPT then I suggest you do some reading, alternatively this YouTube link has a great and detailed overivew:

But in a nutshell its a way of getting the most power out of your solar photovoltaic panels and into your storage (battery).

This is the review of another similar model (the CPY version) by Adam Welch, but mine is the CPS version (and 20Amps) and as you can see looks a bit different to his.

So in some ways I prefer mine, and either way its Fukken Amazing for $30


I've  just done a quick test with two multimeters (one for amps, one for volts) and an inline wattage meter (that does both amps and volts) inline with another (bigger) panel...

So, on an overcast time I was getting

0.32Amps @ 12.98V into my battery while the panel was 0.41A 13.88V

now this doesn't take into account the losses in the two meters, which while small won't be nothing. I know for instance that my DMM showing Amps has an resistance (including leads) of about 0.5 Ohm which of course is significant.

Basically its not particularly efficient at very low light conditions.