More Solar Questions

Hi

Having a bit of trouble getting my head around the black art of solar power and have been getting differing opinions.

Can you mix 32 cell and 36 cell Monocrystalline panels?

Will a Monocrystalline panel and a Polycrystalline panel work ok together?

Looking at adding some panels.

Thanks guys. All a bit confusing for us plebs.

Regards
Rowdy


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Reply By: Warb - Friday, Apr 12, 2013 at 13:30

Friday, Apr 12, 2013 at 13:30
The panels don't know what other panels are being used, so poly/mono etc. doesn't matter in the broader scheme of things. The only important points are to match voltages and currents to suit the design of the system.

1st question is whether these will be used as a single string (connected in series), or as parallel connections to a single charge controller, or with multiple charge controllers.

If you opt for multiple charge controllers then there is no problem at all.

If you opt for parallel connection to a single charge controller then assuming the panels have blocking diodes (and so don't feed power back through each other) and are of similar voltages, then again there shouldn't be a problem, except that if you use an MPPT controller it will only be able to track the maximum power point of the combined panels which might mean neither panel is working at 100%. Be aware that 12V panels are often supplied without any blocking diodes.

However the potential exists for problems if you try using differing panels in a string (connected in series) because the maximum current for the string will be reduced to that of the "smallest" panel. So for example combining a 12V 10amp panel (120W) with a 12V 5 amp (60W) panel will result in (basically) 5 amps at 24V (120W) because the smaller panel limits the current.
AnswerID: 508850

Follow Up By: abqaiq - Friday, Apr 12, 2013 at 17:13

Friday, Apr 12, 2013 at 17:13
Remember that it is voltage that charges the battery it must be higher than the battery has or power won't go in. Many of the panels are rated to put out a specified power over a limited exposure range, say 10:00 to 14:00 at other times they may not be doing anything for battery charging as the output will not exceed the battery voltage. Hence the idea of series wiring [higher voltage] and the more sophisticated controllers to get the maximum power to the battery. By getting to the charging point [higher than battery voltage] earlier in the day.

Been a while since I looked at this issue, back in Saudi now.

ABQAIQ

Be careful out there.
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FollowupID: 786470

Follow Up By: Warb - Friday, Apr 12, 2013 at 19:17

Friday, Apr 12, 2013 at 19:17
The output of the panels will always be pulled down to that of the battery unless an MPPT charge regulator is used. A "12V" panel is in fact probably 18V maximum power voltage, but if the battery voltage is 11V then the panel will be dragged down to 11V. It really doesn't matter what voltage the panels are specified at, they will only ever produce "battery voltage".

A 200W 12V panel will produce (for example) 11amps at 18V given sufficient light and a suitable load. But if connected to a battery that is sitting at 11V, under the same light it will produce 11amps at 11V, or just 121W.

The only advantage to using more panels is that they will supply more amps, but the voltage will still only be that of the battery. In fact, because solar panels have a maximum current, wiring them is series is worthless - the voltage will only ever be "battery voltage" and the current will be the maximum current of the smallest panel in the string. So a pair of 200W 12V panels in parallel connected to a battery at 11V will produce 22amps at 11V (242W) whilst the same panels connected in series will produce just 11amps (the maximum panel current) at 11V = 121W..... Wiring an 80W (i.e. 4.4A) panel and a 200W panel in series to a battery at 11V will give 4.4A at maximum, which at 11V is just 48W!!

All this changes if an MPPT charge regulator is used, because such a device (a genuine one, not a cheap Chinese gimmick) will alter the load to the panels to get the best power output regardless of the battery voltage. The MPPT section of the unit ensures the panels are producing the most power, and then the charger section regulates that power to the battery. Unfortunately a good MPPT charge controller costs a fair whack!
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FollowupID: 786484

Follow Up By: Member - Rowdy6032 (WA) - Sunday, Apr 14, 2013 at 12:47

Sunday, Apr 14, 2013 at 12:47
Firstly, I would like to thank you for your input. Very informative. Slowly getting my head around it.

In your last para you state that "All this changes if an MPPT regulator is used".

When the MPPT charge regulator is used should the wiring be done in series or parallel?

Regards
Rowdy
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FollowupID: 786617

Follow Up By: Warb - Sunday, Apr 14, 2013 at 20:07

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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FollowupID: 786645

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