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?

Well as I identified in my first post (with a small panel into a small battery):

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 it) to 0.55Amps at full short circuit load. This is about a 27% increase in power over PWM.

you only get that sort of step up in amps with Voltage to Voltage conversion ... or MPPT. I believe that the evidence supports that it is (indeed, I demonstrate that it is in subsequent posts (eg this one), some of my other readings made it unambiguously clear that it must be MPPT as not only was it putting more amps into the battery than the panel was producing it was keeping the pair of  panels at 38V ).

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 ...