Cheap MPPT regulator from China - true and not fake?

Re: Cheap MPPT regulator from China - true and not fake?

Okay, lessens time.

Voc of 21 - 22 is normal for a "12 Volt" panel. You do not want lower Voc than this because that means you will also have lower Vmp, and that's the number that matters when charging batteries. If it is too low to begin with the further Voltage drop caused by the panels heating up and wiring resistance may result in the Voltage at the battery not being sufficient to fully charge. In other words you could start out with Vmp of 16, lose a couple of Volts to hot panels and wire resistance, and only have 14 at the batteries which want to Absorb above that level; the batteries do not recharge.

Second, there is no difference between the function/performance of two 12 Volt panels in series and one 24 Volt panel providing the Vmp & Imp are exactly the same in each configuration. But they are not always the same, starting with some people calling GT panels with a Vmp of 30 "24 Volt". Nor do all "12 Volt" panels have the same Vmp.

Then, depending on which type charge controller is used, you have the conversion differences to a 12 Volt system. A PWM controller passes only current, so a 'standard' panel with a higher Vmp and lower Imp may actually provide less Watts out than one with the V*I ratio reversed. For example some "100 Watt" panels have a Vmp of 18.9 and Imp of 5.3. On a PWM controller you get 5.3 Amps max, regardless of battery Voltage. Another "100 Watt" panels may have a Vmp of 17.5 and Imp of 5.7. With that you will get more current and thus more power if all other factors (such as V-drop) are equal despite the fact the panels are the same Wattage. Put them on an MPPT controller however and the output is the same for either panel.

Voc has nothing to do with the charging ability; it does not exist under operating conditions. It only has bearing on the maximum input Voltage of the charge controller. If it is too high, some components in the controller may burn up. It does not matter if it is a cheap controller or an expensive one: exceed the V max in and it will fail sooner rather than later.

Do not try to find panels to fit a particular charge controller. Get a decent controller to begin with. Unless you are just experimenting with solar power, buying cheap equipment is a complete waste of money.

Re: Cheap MPPT regulator from China - true and not fake?

jimindenver wrote: »

Well actually what I was referring to is the efficiency of converting from higher voltage to lower. Not only is there the example of a 300w system with two 12v in series producing less than a 275w grid panel, i also know of a 960w system using 160w 12v panels in pairs that I can easily out do with four 250w GT panels by 25%. I can easily max out a TS-MPPT-60 with four 250w polys. he is pulling 48a, not 60+. (I know his wiring, it's not there)

This started quite the debate on a RV forum until the manuals for the controllers came out and in them states that the efficiency drops as the difference between input and output increases. I believe Rogues manual was quoted as dropping 4% in efficiency and that wasn't even close to the loss when running really high voltage such as three or four GT panels in series and bucking it down to 12v. The numbers may be small for you but these are RV's and what we consider a good sized bank, you all laugh at. For us a little goes a long ways and lose 25% to a bad choice and you will be running a generator more than you like. Then again on a RV it starts with what fits on the roof, you don't always get to pick and chose.

Your comparison is inaccurate because the basis isn't equal. As I said a "12 Volt" or "24 Volt" panel doesn't mean anything, neither does the Watts. I suspect that the example sited involved two "12 Volt" panels in series operating at Vmp 35 vs. a 275 Watt GT panel whose Vmp is 30; the lower Voltage wins in efficiency on an MPPT controller because it has to do less work to down-convert to a 12 Volt system. The actual efficiency loss for a 5 Volt difference is inconsequential compared to other loss potential.

You are quite right that three GT panels in series bucked down to a 12 Volt system would be quite inefficient. Not just on an RV system either. You could be losing 7% (or even more) of the power potential in fact, and yes the particular controller involved will also affect the outcome.

Too often people serve up anecdotal evidence as 'proof' when they don't actually know what they're talking about. The only way to compare two systems for evaluating efficiency is to eliminate environmental and operational variables, and that can't be done outside the lab.

So many times I've said that the PV formula for an RV is often "as much as you can fit" owing to space and insolation limits. The positioning of the panels can make an amazing amount of difference in conjunction with how they are wired. Any RV application is always "less than perfect" exposure.