#171951  by Izzle
 Fri Apr 09, 2021 1:50 pm
As the theory was explained to me, when there's poor light (like early or late in the day, or overcast conditions etc) then the higher voltage produced by series connection allows recharging to occur more easily than the more normal parallel connection. I realise the current is much reduced but at least something is going into your battery. By using a DPDT switch, if the light conditions improve you can return to parallel set up, ie the panel connection is then optimised. I would accept on 'average' either type of connection works and probably produce a similar Ah's, but [u]would you get sufficient extra by going to the bother of fitting a DPDT switch?[/u]

As you suggested I did try to look up the performance details for my controller (Votronic MPP 350), but I was not able to determine outputs at the various panel voltages. All I found out was it would take the highest voltages and currents from my solar panels (currently old BP solar 2*80W - poly).

The campervan is really good for touring. It has an AA Solar 325Ah AGM battery so I can park up for 3 days with little solar recovery before moving on, then charging helped from my 140A alternator. The set up although small has worked well for 16 years.
 #172037  by Neddy
 Sun Apr 11, 2021 8:59 pm
Izzle, there is a commonly held misconception that at low light levels, higher voltage solar arrays provide higher charge rates than lower voltage arrays of the same wattage. This is simply not true - and in fact, the reverse applies! Take a look at the solar panel Current/Voltage graph below. (The red line marks PWM charge Voltages).

See how, as the light level falls, panel output Current also falls. But notice that even with very low light levels output Voltage is maintained. In other words, in poor light, you run short of Amps, NOT Volts. Serial panel connection (or the use of 24v panels) is counterproductive because (a) it increases array output voltage (of which there is no shortage) and (b) lowers array output current (which is in short supply).

Another factor that many people are unaware of is that at low light levels, panel Wattage falls to a level where MPPT functionality drops out even though the attached array may well still be producing power. To avoid wasting this energy, most MPPT controllers then connect their solar panels directly to the battery, thus running them at battery voltage - just as PWM controllers do all the time. In these conditions, panel current IS the charge current - that's why lower voltage/higher current arrays outperform higher voltage/lower current arrays across the board - but especially so at very low light levels.
Attachments
Panel IV Plus.JPG
 #172047  by scubadoo
 Sun Apr 11, 2021 10:04 pm
Izzle
Stick with a parallel setup. "Low light" output differences between series, parallel, 12V and 24V (or 2x 12V in series) panels are so insignificant that any gain achieved is unlikely to power your TV for an extra 5 or 10 minutes per day.

My Victron MPPT controller fed with 24V panels never enters PWM mode even at sunset in very "low light" while maintaining more than 96% conversion efficiency.

Solar input vs output into battery.
Sunset on a bad day.
Screenshot (1).jpg


About 40 minutes after sunset in all but no light and still in MPPT mode.
Screenshot_1 (1).jpg
Ten minutes later output was zero.
12 or 24V panels would make no difference.
 #172064  by Paul-Carter
 Mon Apr 12, 2021 9:49 am
Everybody is getting too bound up with tech. Pay some attention to the picks in Neville's post above.
It's really simple
1/ put as much solar on the roof as you can fit/afford and plan your layout for more if possible.
2/ a good quality controller.
3/ as much battery capacity as you can fit/afford ( my attitude is if your batteries are full before the sun goes down then there is a case for more)
4/ It is the low light situations that are important and the most common. The days with full sun are a bonus.
5/ don't take blind advice, take the time to educate yourself on the topic for all his faults Mr Google is a wonderful teacher.
When all this is in place you can tweek this as you like.
 #173221  by kiwipete
 Mon Jun 07, 2021 10:55 pm
I have 6 x 200 w panels on our caravan roof. There is no room left.
There are 3 x 40a Dualcom MPPT controllers.
Each controlker has a pair of panels.
Each pair of panels is series wired.
This charges 2 x 120a Full River batteries.
The issue is the batteries are failing over this winter with morning voltage down to 12.3v and SOC down to low 70's
Batteries were purchased Nov 2020
I am going to get them tested.
Also thinking of changing the panels to parallel wired?
Our power loads are around 7 to 10a in the evening with diesel heater, tv and 2 x LED lights.
Only thing running at bed time is the diesel heater, LPG alarm and fridge panel. Rverything else is switched off.
Daily charging of my laptop in mid afternoon via the 2000w inverter and ocasional printing etc.

Whats your thoughts?
 #173222  by ianganderton
 Tue Jun 08, 2021 1:49 am
kiwipete wrote:I have 6 x 200 w panels on our caravan roof. There is no room left.
There are 3 x 40a Dualcom MPPT controllers.
Each controlker has a pair of panels.
Each pair of panels is series wired.
This charges 2 x 120a Full River batteries.
The issue is the batteries are failing over this winter with morning voltage down to 12.3v and SOC down to low 70's
Batteries were purchased Nov 2020
I am going to get them tested.
Also thinking of changing the panels to parallel wired?
Our power loads are around 7 to 10a in the evening with diesel heater, tv and 2 x LED lights.
Only thing running at bed time is the diesel heater, LPG alarm and fridge panel. Rverything else is switched off.
Daily charging of my laptop in mid afternoon via the 2000w inverter and ocasional printing etc.

Whats your thoughts?
What state of charge are the batteries getting to by sunset?

Changing the solar panel configuration won’t change much unless there is a problem anyway (eg shading)


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 #173224  by Nut17
 Tue Jun 08, 2021 7:59 am
Unless you have a significant shading issue changing your wiring to a fully parallel connection may well make a small improvement in your solar harvest We also have six X 200 watt panels on our roof. A reasonably similar configuration but in our case two strings of three panels - each string wired in series to an SRNE 40 Amp MPPT controller. The one important consideration that I learned back in 2012 was the importance of having a shunt based battery monitor and not relying on voltage to keep an eye on the battery state of charge.
Many lead acid batteries will never reach 100% at this time of year if relying on solar alone due to the length of time that it takes to get the last 10 - 15 % of current into the battery. It is a common issue at this time of the year with solar harvest down to 30% or even less than that achieved in mid summer !! - So you may see 14.00 volts at 3.30 pm and falsely assume that your battery is full, when in fact that it needs to be held at 14.00 volts - or more for several more hours to get the battery fully topped up. This issue is then quickly exacerbated by sulphation degradation on the plates which occurs when the battery spends time partially discharged, even further reducing battery capacity. It is about now at I usually make my usual promotion of the benefit of converting to LiFePO4 ;) :-T
 #173229  by Paul-Carter
 Tue Jun 08, 2021 9:23 am
I have 10 120w semi flexibles in two banks of 5 in series paralleled together. It is good most of the time. I think the whole debate is a how long is your piece of string question because the situation is always changing. What is good this morning is not by lunchtime. Someone needs to invent something that can sense the output of each panel and switch the connections for best result but measuring individual power of each panel would be a challenge I suspect. The switching, not so much.
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