Adding a second solar panel

[GeoffL]

Preamble: I have a single solar panel on the roof of the van, which I assumed was 100W (but now suspect may be more -- I don't know how to find out what type or model I currently have). It was on the van when I bought it a couple of years ago and came with an 8A PWM controller. Earlier this year, I upgraded the controller to a Victron SmartSolar 75/15 because we were off to Australia for a couple of months and i needed to leave the solar system connected to maintain the leisure battery (the 8A controller would have 'overcooked' the leisure bank). The best this system has managed so far is 360Wh on the summer solstice, which was in full sun for most of the day. Daily peak panel voltage is just over 20V under load and the panel has a 'matrix' of 9 cells by 4 cells. As we use 20 to 30Ah most days, I suspect that the current system won't be able to recharge the leisure bank within a few days of a night's stargazing/astrophotography; particularly during the winter. It's highly desirable for the system to be capable of this as we store the van about an hour's drive from home on a site without electricity and I'd rather not have to remove the batteries and take them home for charging.

So my thoughts are to fit a second panel (say 100W poly) in series with the first as the Victron controller needs a panel voltage of 15V or more to charge the leisure bank and fitting the second panel in series should ensure that the voltage is over 15V for more of each day.

The about is pretty much all I know on the subject and so the collective wisdom/advice of the community would be very welcome and gratefully received.

Thanks in advance, Geoff

 

[wildebus]

Just a bit of info on how the controller works ... you need a voltage of >5V more than the Battery Voltage to 'wake up' the controller - so the panel has to go over 17V ish to start it. Once the controller is working, the voltage of the panel has to remain >1V over the battery to keep it working (so say >13.5 to 15.4V depending on state of charge of the battery).

You are right, connecting a pair in series will pretty well guarantee the input to the controller will have a high enough Voltage to keep it alive throughout the daylight hours.
You ideally want to match the 2nd panel to the first in terms of spec I would say - so if the first is a poly, get a poly. if it is a mono, get a mono. match the voltage and the power rating.
Can you take the panel off to read the label on the back? If not, post if Mono or Poly, plus the measurements (L x W) - Panels generally seem to be the same W/M2, even when the shape and proportions vary, so should get a good idea from physical size.

 

[GeoffL]

Thanks @wildebus for the info. FWIW, I don't have enough roof real-estate to fit a second panel the exact same size as the one that's currently fitted and I'll probably need to remove the roofrack to make enough space to fit the second panel at the rear end of the roof behind the rear lounge skylight. Hopefully, I can read the label with a mirror rather than need to remove the panel as it's been on some years and I'm not sure how stubborn the mounting screws will be! The cable gland isn't under the panel, so I won't need to remove the panel to get at the cabling. BTW,. how can I tell whether I have mono or poly if I can't read the label?

Gut feeling suggests that the current panel will be a number of cells in series (i.e. an array 9 cells long by 4 cells wide = 36 cells). If so, I hope it should be OK if the cells of the second panel match the ones of the first, even if the number is different (e.g. I could use a panel that had fewer cells: whatever I have the space for). Gut feeling also suggests that all that should matter is that the current produced by the new panel is the same as or slightly greater than that produced by the current panel; the lower-current device should limit the flow rate with the voltages being additive? I also wonder how critical it is to match the technology (poly panels are considerably cheaper and I'm not sure that mono panels were readily available when my current panel was fitted -- more than five years ago). I hope that the MPPT controller would do a good job of changing whatever is available from the panels to whatever the leisure battery bank needs!

Thanks again, Geoff

 

[wildebus]

As a quick check, see this page ...http://www.onboardenergydirect.co.uk/shop/solar-pv-panels-regulators.html - to see the differnence in appearance between the two. Top Left IS Poly. the one below IS Mono.
The Monos usually have a distinct centre between the group of 4 cells, the polys look like long stripes (bad description). I think you have a Mono panel if you can so easily count the cells? Mono is generally the slightly better option, but the price difference between the two is not that great anymore.
performance-wise, it is reckoned after around 10 years you should still expect to get over 90% of the original performance.

This video I think is a good explanation of using different panels together - what can happen and the best way to do it...

 

[GeoffL]

@wildebus : I suspect that you're correct -- the panel I have looks more like the mono ones found on the site you linked and just about everywhere else. Looking futher, I suspect that I'll have to bite the bullet and lift the existing panel to check the label -- although I've no idea when I'm going to be able to do that. That said, the current arrangement will do until October at least, so I have plenty of time.

As an aside, the YouTube video you linked suggested that the best way to combine dissimilar panels was for each to have it's own controller. I thought that the two controllers might 'argue': as one brought the voltage accross the battery bank up, the second controller would switch off or go into 'float' mode. This is why I'm considering adding a second panel on the roof feeding the same controller rather than adding an additional system.

Thanks again.

 

[wildebus]

yup, multiple controllers are generally not a good idea either usually.
However .... something that is "coming soon" in the Victron world (date still TBA) is the ability to have multiple SmartSolar Controllers working as a team. So IF you were to decide to go for separate controllers, another Victron SmartSolar one would probably be the one to go for as sometime in the future they will talk to each other (This feature should be available in a new firmware before October as I understand it)

 

[RichardHelen262]

You could put your phone on a selfie stick and take photos of the underneath of your existing panel rathe than lifting it

 

[Joe express]

Preamble: I have a single solar panel on the roof of the van, which I assumed was 100W (but now suspect may be more -- I don't know how to find out what type or model I currently have). It was on the van when I bought it a couple of years ago and came with an 8A PWM controller. Earlier this year, I upgraded the controller to a Victron SmartSolar 75/15 because we were off to Australia for a couple of months and i needed to leave the solar system connected to maintain the leisure battery (the 8A controller would have 'overcooked' the leisure bank). The best this system has managed so far is 360Wh on the summer solstice, which was in full sun for most of the day. Daily peak panel voltage is just over 20V under load and the panel has a 'matrix' of 9 cells by 4 cells. As we use 20 to 30Ah most days, I suspect that the current system won't be able to recharge the leisure bank within a few days of a night's stargazing/astrophotography; particularly during the winter. It's highly desirable for the system to be capable of this as we store the van about an hour's drive from home on a site without electricity and I'd rather not have to remove the batteries and take them home for charging.

So my thoughts are to fit a second panel (say 100W poly) in series with the first as the Victron controller needs a panel voltage of 15V or more to charge the leisure bank and fitting the second panel in series should ensure that the voltage is over 15V for more of each day.

The about is pretty much all I know on the subject and so the collective wisdom/advice of the community would be very welcome and gratefully received.

Thanks in advance, Geoff

I have a clever gadget its a small endoscope with a light on the end that just plugs into my smartphone and it can get beneath the panel and read the label without having to remove it they are available fro amazon or ebay and really cheap.
Just an idea.

 

[GeoffL]

I've measured the panel -- it's looks like a monocrystalline panel 120 x 52 cm. I thought I'd lift the panel to check the label but it's not so easy until I have the correct tools to refit it as the panel is pop-riveted to two of the brackets! I know that @wildebus wrote it's possible to get a good idea of what I have from the type and dimensions, so I'm hoping someone can wave a magic calculator over the above (although I'll probably get one of those endoscopes that @Joe express mentioned. Thanks all.

 

[wildebus]

I would say it is a 100W panel as you suspected. Could just possibly be a 110W but that is an unusual and fairly rare size.
Helen262s idea with the selfie stick is a good idea
If you put a meter on the two outputs when off the controller you will get the Voc (Open Circuit Voltage) and that will help you find a 2nd panel that is a pretty good match. You have already determined the voltage peaks at around 20V which is pretty typical
This panel I think is a decent budget buy and works pretty well - https://amzn.to/2SDi2t3 . I have 4 of these on my Van (so 400W) and depending on weather can achieve >2kWh in a day up here in Scotland.
If you were to add the above panel to the one you currently had I think (but can't guarantee) they would work together and the pair of them would work with your existing Victron 75/15 Controller in either Parallel or Series configuration - Max voltage would be about 44V in Series and around 13.5A in Parallel mode, so either way you are good


I was speaking to someone last weekend who fitted his solar panels to his Westfalia at least two years ago and never got round to actually cabling them up! He even forgot what size they were! (fortunately they were raised up enough that I could see the label on the underside )