Jump to content

Solar panel strings


Recommended Posts

So I am not the only confused one. It is unclear.


Eugene, yes, it is a PWM. Max volts 125v.


After overworking a MPPT controller that failed 1 week after warranty expired, I thought I will try a high quality PWM controller by adding one more panel to compensate for it being less efficient. 


Panels are 310w, with a Voc of 45.35v, Vmp of 36.3v.


Large wattage panels have the benefit of less mounting but it gets tricky ito volts on PWM controllers.


I have 3 x 310 connected in parallel.

So using logic, controller can handle 1550w, I recon I can add 2 more panels parallel, reaching the 1550w limit, staying well away from 124v.

Link to comment
Share on other sites

I think with PWM your string configuration is dictated by your battery voltage, not by the Voc of the panels. Of course the Voc of the panels still needs to be taken into account.


Solar panels are constant current devices (mostly), so lets say you have a 310W panel with a Vmp of 36.3V, that is, it makes 8.54 amps. If you connect that panel to 3 x 12V batteries, it can't reach absorption voltage without going way above Vmp, and if you connect it to a 24V bank, then you pull it down to between 24V and 29V (still at 8.54 amps) so you reduce the panel power to between 205w and 250W (and it works best when the batteries are full!). So I know this is old news to people like us..., but... as I recall you have a 24V bank?

2 modules in series is Vmp of 72.6V still at 8.54 amps. On a 24V bank that means 610W on the roof, max 250W at the battery.


So all your panels in parallel is absolutely the only way this is going to work (for 24V).

Link to comment
Share on other sites

On a related note though, I hear what you are saying about frying MPPT controllers and what they cost, and how adding 20% more panels and living with it might just be a good trade-off. That just doesn't sit well with the engineer in me. The problems with MPPT however is that 1) Inductors that can do that sort of power is expensive, 2) high voltage MOSFETs that can also handle some current is expensive, 3) The Schottky diode that catches the reverse EMF has to be a Schottky for heat/voltage drop reasons, but once again a diode with a high enough reverse voltage breakdown is expensive.


So for those reasons it is almost always better to have one large MPPT controller, but then that configuration is precisely the one that leaves you with a single and expensive point of failure.


Picture this: Two wires coming in, add modules to the rated power... one fails... power is reduced. Just replace the one failed module. In my mind, this is doable: One MPPT controller, several buck-converter modules controlled by it. So you can use high-voltage lower current mosfets, same for the schottky, and a much smaller inductor per module.


Such modularity is probably going to cost more money though :-( Man... I suppose we keep at it until at some point it becomes affordable (usually because everything else became more expensive :-P )

Link to comment
Share on other sites

Plonkster, yeah, I reluctantly, and much slower than you :D  , came to the same conclusion. As much as I liked Eugene's Idea, on a 24v bank, I am stuck on panels in parallel.


My inverter is max 1600va, using maybe 700va max currently, hoping to do on average 1kw into the future (not to run it at max power all the time), I accidentally figured out my inverter can run my 9000btu aircon, on a certain setting!!!


Need +-600w more, so that is why I need at least 2 more panels. :-)


Now, based on your last post, looking again at a MPPT controller considering the Zumafication of the rand, I wondered what you will you think of this idea:

To reduce the costly one point of failure, what if I was to connect each 310w panel to its own BlueSolar MPPT 75/15 controller?

Then I set all 3 (or 5) of them at exactly same setting to ensure no debates or arguing between them?

And later, if there are more batteries, 1 panel, one controller so one can expand at ones leisure.


When one fails, it is cheap to replace.

Ja, maybe a little bit more wire, but I have a LOT of spare at present and even if I did not, it still makes no real difference.

Link to comment
Share on other sites

BlueSolar MPPT 75/15 controller are going for R 1 503.26 VAT incl = R 7 516.29 for 5 of them.


Ag dammit!!! 


I cannot charge a 225ah bank on 5 separate controllers. Amps per controller is too low.

Nor can I add 2 panels on one controller either, in series amps are too high, in parallel the watts are too high for the controller.



Link to comment
Share on other sites

I have almost exactly the same inverter as you, except I have the multiplus. If yours can start a 9kbtu ac, chances are mine can too, so I'm curious what setting it is you speak of (I assume it's a setting on the AC and not on the inverter :-) ). I've been told these inverters can peak at around 3kva for about a second before shutting down, but because my AC has a startup spike of around 5kw (estimated) I've never been tempted to try.


Multiple BlueSolar 75/15's is more or less what I had in mind, or perhaps a little larger. Perhaps bundle some panels.


I see why you can't use series. Two panels in series puts Voc above 90V. You'd need a 100/15 for that to work.


Some ideas. None are really much cheaper.


First idea, get 3 x 100/15 controllers. That should set you back around 6k total. With a 100V peak, you can put two panels in series and do Vmp=72V at 8.54 amps. This allows scaling to 6 panels while running each controller just over half-capacity.


Second idea, get 2 x 100/30 controllers. Will probably set you back 7.5k or slightly more. With this you can put 2 strings of 2 panels per controller and still run below half capacity, so you can scale to 8 panels.


Third idea, chat to the WRND MPPT guy, he's also in Brackenfell/Belville area somewhere. His MPPT is a little cheaper and you can probably swing 3 of them for the same price.


Or just wire them up in parallel and be done with it. You can always reconfigure later when the exchange rate is better or something :-)


Edit: Source of prices: I used sustainable.co.za and subtracted about 27%, which is what their markup appears to be compared to some prices I do know.

Link to comment
Share on other sites

Yes, setting on the AC, the display has a smiley face in Orange, that switches off when temp is reached, and when on, it uses about 300-400w, set at 27deg temp.

The startup, I can hear the inverter groan, but no adverse effect as the loads are protected by a online APC.

Note: At MAX there will be 700va running off the inverter, I wait for it to be like 400va before I switch the aircon on ... thereafter it does not matter ... just me being cautious.


Plonster, I like your ideas!!!


Option 1 : BlueSolar MPPT 100/15 (12/24V-15A) = R 1 829.91 incl @ existing stock prices = R 5 489.74 incl total plus one ekstra panel. Which is not going to be same make and model ...


Option 2: BlueSolar MPPT 150/35 (12/24V-30A) = R 4 993.03 incl @ existing stock prices = then the existing three panels in series are sorted ... then leave these three panels as one unit with controller. Then I can start the next batch of panels, in the 250w ranges, for they seem to be favoured by most for off-grid.


Using the same Victron controllers, set on identical settings, whilst using different panels per controller, should not be a problem I believe.


Like to stick with Victron but I am going to investigate WRND.


To continue in parallel needs two more panels of different brand. The Virgo in me gets upset, but, I have taught him to shut up now and then.  :D


Let me sleep this one through.

Link to comment
Share on other sites

The canbus models can be synchronised (the 150/70 and  150/85), so they go through the bulk/absorb/float cycle in tandem. With separate controllers, some of them are probably going to go to float a little before the others do, but in the end it should all even out I believe.


What I want to do with my setup, once there's a bit more money, is add some north-west facing panels. These long summer afternoons have a lot of west-facing sunshine that's going to waste.

Link to comment
Share on other sites

Speaking of canbus, I got this guy's canbus library to compile for the STM32F4.



My canbus transceiver chips landed today, at work (I'm booked off at home with a cold... in this heat), so from next week on... time permitting... I'm going to start hacking into the mppt can bus :-)

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...