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hamish
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# Posted: 15 Sep 2016 09:02pm
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Quick sitrep,
400W poly panels in parallel
2xL16 370AH Batt
PWM Controller
Provides all the capacity I need April-Oct (But would like to add a small bar fridge to use the excess solar during the summer).
Thinking of taking the jump to MPPT to take advantage of the higher potential voltages to get to charging sooner during the day vs the pwm. Trying to determine the advantages of going purely to a series MPPT connection vs a series/parallel MPPT connection? Not to mention hopefully extending my solar only season from Mar-Nov type thing.
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MtnDon
Member
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# Posted: 15 Sep 2016 09:29pm
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The greater the difference in the PV voltage and the battery bank voltage, the greater the transforming loss. So unless the wire run from PV to controller is exceptionally long it does not pay to put too many panels in series. When looking at series connecting you also need to consider what the cold weather first light spike will be. Figure the safety margin by rule of thumb by multiplying the total (series) Voc rating of the panel x 1.25. That # should be less than the voltage maximum rating of the CC.
Series will definitely help produce usable watts earlier and later in the day, and under cloudy conditions.
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hamish
Member
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# Posted: 15 Sep 2016 10:15pm
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So lets say I break up my 400W parallel system and make it into 2 series strings in parallel
Existing 22V @ 24A (100W panel VMPC 21.96V IMP 5.9?A)
To 44V@12A (series parallel)
OR88V@6A and let the MPPT do its thing?
My main objective it to take advantage of the off days (earlier, later and cloudy days). With the tree foliage shading will occur. Existing wire runs from the panel to the CC are under 25'.
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MtnDon
Member
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# Posted: 15 Sep 2016 11:00pm
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With that wire run and a 12 volt battery I would series / parallel the panels and use an MPPT. That would be to avoid the high ratio of 88 volts to 12 volt battery and the loses that will incur. Nothing but a chainsaw will help with the tree shading 
BUT here's something to add confusion to everything. If the panels are all in parallel and one panel has full or partial shading, the only reduction in power from the PV will be the power that the shaded panel does not make. However, if two or more panels are in series and one of them has some shade that will drag down the total output from that series pair. BUT, there is another BUT...
Modern panels are equipped with bypass diodes that will eliminate much of that problem. The diodes are supposed to allow the power from the brightly lit solar cells to pass around the shaded cells. So in theory a few shaded cells on one panel should not adversely affect the output of that series string.
In the end with a good MPPT controller and modern panels you should have a net gain in energy going into the batteries.
Another reason for changing from 4 parallel panels to a 2S/2P or all series arrangement is that there is a chance you do not have fuses or breakers in each of the 4 existing parallel connections. That is a detail skipped by many DIY. That can cause a fire hazard... IF one of three or more parallel panels develop a short, the short would then shunt all the amps from all the other parallel panels through the short. There's a good chance that would overheat a wire or even cells within the shorted panel, with the potential of starting a fire. With two parallel panels the wiring is usually capable of handling the short circuit. The short problem would still exist but the fire hazard is less.
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