Sunday, Apr 14, 2013 at 20:07
The reason it all changes is because the MPPT regulator can separate the panels from the battery, allowing each to function at its optimum. How the strings are connected depends on the panels and the regulator being used.
If the panels are wired in series then the voltages are added together but the current for the entire string is limited to that of the "smallest" (i.e. lowest maximum current) panel. So if all the panels are identical then it is fine to wire them in series as long as the MPPT regulator can handle the combined voltage.
Some MPPT charge regulators (and we're talking "real" ones, not just marketing hype) can handle several hundred volts, though those targetted at campers are more likely to be limited to perhaps 50V or less. Some (perhaps more hype than MPPT) are limited to only 24V or so. In either case the string of panels (series) should be designed to produce a voltage within the MPPT controllers specified range, and to avoid having smaller panels limit the current of the entire string.
If widely differing panels are used, it clearly makes no sense to limit the current of a larger panel by connecting it in series with a smaller one. So in that situation they might be connected in parallel. The downside here is that the MPPT controller now has to produce an "average" best performance which might also limit the larger panels output. This will, however, generally be far less limiiting to the overall output than the small panel limiting the current of a series string!
The specs for a solar panel will state Vmp, which is the "maximum power voltage". This is often misconstrued to be the voltage that is produced when lots of light hits the panel. In fact, because the voltage is controlled by the load, what it actually means is "the voltage at which the greatest amount of power can be produced" - not the same thing at all! MPPT works by altering the load presented to the panels to give the best power output. This is possible because under any given light intensity and temperature a panel will produce a varying amount of power as it's voltage and current change. The voltage is controlled by the load to which it is attached, and a "real" MPPT controller will actually vary the load (and therefore the voltage) and test to see what voltage/current combination gives the highest power output at that time. If different panels are connected in parallel, the "best" result for the combination might be less than the sum of the two panels when individually optimised - this in turn is why domestic MPPT
grid-tie controllers often have several strings each with their own MPPT controller.
Having stated what a "real" MPPT controller does, it should be noted that many cheaper devices sold as MPPT in fact do not actually test and track for maximum power, but simply provide a load that prevents the panel voltage falling below a preset limit. This is not MPPT (though it is sold as such) but does allow a panel to produce more power than would be possible if it's voltage were dragged down to that of a depleted battery.
Without specific data is is hard to say what is best in a particular situation, but assuming the MPPT controller is a typical "camping" unit (i.e. fixed minimum panel voltage rather than true tracking) with a maximum voltage of about 24V, you need to connect the panels in parallel, as if connected in series 18V x 2 =36V and the controller can only handle 24V. If a better MPPT controller is used with a maximum voltage of (say) 40V or so, then two panels of similar (18V) max power voltage and similar max current could be connected in series, but panels with different specs should be in parallel to avoid current limiting.
It is also wise to note that:
1/ panels with greatly varying voltages need extra thought, so stick with one nominal voltage.
2/ panels have a maximum string voltage, so don't make series strings that take the string voltage above that level (it's often 1000V or so, but on some cheaper 12V panels it's a lot less or unspecified)
3/ the output of strings in series can suffer badly if one panel is shaded or faulty.
4/ cheap 12V camping panels often don't have blocking diodes, so panels can suck power from other panels (parallel) or even from the battery if the charge controller doesn't isolate them. But there's no reason why you shouldn't add diodes, they're available on eBay.
[I shall be offline for a few days, but I'll try to answer any further questions when I return!]
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