Extending solar power

[Dr Bob]

I need some advice. We currently have 2 *100W solar panels on our nb, connected to our 660Ahr bank via a 30A MPPT controller.  Works great and we get up to 12A in - but not really sure of that number as when the sun is shinning I dont look at the meters (and the inverter is always on with a 240v fridge so a continuous discharge). I am looking to put another couple of panels on the roof to make the most of the summer sun. The original panels were put on by 'Onboard Solar' but I now notice their standard panels are higher Wattage. Looking at the internet I was thinking the following 150W panels might work

https://www.photonicuniverse.com/en/catalog/full/90-150W-12V-solar-panel-with-5m-cable-for-camper--caravan--boat-made-of-high-quality-German-solar-cells.html#tabs-2

They are rated at 8.5A max current so a couple of these plus the existing panels takes me to 30A (ish). I could go for 100W ones instead which are not quite as long. Two questions:

Firstly, What does the 30A for the MPPT mean?  Is this all the unit will supply and it is happy to have more V x A coming in from the panels, or if the VxA is too high due to too many panels, will it get fried? The panels are usually well off the max power so it would be good to go for the 150's for the less sunny conditions.

Secondly, can you mix different panels? It is almost certain the new panels will have different open circuit voltages or closed circuit currents. The existing panels are wired in series to the controller so I could put the 2 new ones in their own series and parallel them into the charger. If I do it this way, would it be better to have the two 'old' panels seriesed ditto the new ones, or should I put one old one and one new one in each series circuit. Would the charger get upset if it is seeing two different volts and amps from the different series circuit inputs? A contact has suggested the best way may be to wire the new panels in with a separter (new) 30A MPPT controller.

[MtB]

I suggest you just wire up all four panels in series. This gets around all panel incompatability problems. The downside is you need to be careful as the resulting DC voltgage is dangerously high is good ligh conditions so don't touch the wires!

All this is providing the sum of the the panel voltages is less than that the controller can cope with. I dunno the answers to your other questions. 

 

[Dr Bob]

53 minutes ago, Mike the Boilerman said:

I suggest you just wire up all four panels in series. This gets around all panel incompatability problems. The downside is you need to be careful as the resulting DC voltgage is dangerously high is good ligh conditions so don't touch the wires!

All this is providing the sum of the the panel voltages is less than that the controller can cope with. I dunno the answers to your other questions. 

 

Thanks Mike. I will check the manual for the controller - we are not back on the boat till tomorrow - to see the controller max voltage. I dont fancy 80 (ish)V with 30A comming down the wires hence the idea to parallel two panels each in series.  I supose then I could series up a new and an old so both circuits are the same?

[Bricksh]

I asked this question to midsummer energy when I installed two different sorts of panels earlier this year. The answer was basically if installed in parallel the voltage is restricted to that of the lowest panel voltage output and in series the current is limited to that of the lowest current output (ie throttling the larger output panels)

What I did therefore, was install two arrays of panels (No 1 - 3 x 100W mono's in series and No 2 - 4 x 68W amorphous in series). I then used two tracer MPPT's onto the same battery bank. This has worked great for me. 

[WotEver]

A few things. 

Within reason, solar controllers will only be damaged by excessive voltage. A controller with a max input voltage of 60V won’t live if fed with a 100V panel. 

A 30A controller will only output a maximum of 30A. If that’s at 14.4V then it’s a maximum output power of somewhere over 400W. If you had 1kW of panels connected on the input that wouldn’t bother the controller a bit as long as the voltage remained within limits, it would simply only ‘use’ 430W or thereabouts. 

A 150W panel will supply 50% more current than a 100W panel, as long as the two panels are nominally the same voltage. 

A 150W panel in series with a 100W panel will have its current limited by the smaller panel. A 150W panel in parallel with a 100W panel will not be limited by it (again, assuming they are nominally the same voltage). 

So, as you currently have two 100W in series I would suggest you put the two new 150W in series with each other and that pair then in parallel with the existing ones. 

It may well be that the MPP of the new panels falls at a different voltage to the existing panels, so it will be a compromise. 

[MtB]

37 minutes ago, Dr Bob said:

I dont fancy 80 (ish)V with 30A comming down the wires hence the idea to parallel two panels each in series. 

Except it won't be 30A, not that the current affects the danger. It will be 14.4v x 30A / 80v. 5.4A, approx.

 80v DC however is probably more lethal than 240v AC if you inadvertantly grip it, as the DV voltage will make your muscles lock and stay gripping it. With AC, you get 100 chances a second to let go as the voltage passes through zero. 

[Dr Bob]

46 minutes ago, WotEver said:

A few things. 

Within reason, solar controllers will only be damaged by excessive voltage. A controller with a max input voltage of 60V won’t live if fed with a 100V panel. 

A 30A controller will only output a maximum of 30A. If that’s at 14.4V then it’s a maximum output power of somewhere over 400W. If you had 1kW of panels connected on the input that wouldn’t bother the controller a bit as long as the voltage remained within limits, it would simply only ‘use’ 430W or thereabouts. 

A 150W panel will supply 50% more current than a 100W panel, as long as the two panels are nominally the same voltage. 

A 150W panel in series with a 100W panel will have its current limited by the smaller panel. A 150W panel in parallel with a 100W panel will not be limited by it (again, assuming they are nominally the same voltage). 

So, as you currently have two 100W in series I would suggest you put the two new 150W in series with each other and that pair then in parallel with the existing ones. 

It may well be that the MPP of the new panels falls at a different voltage to the existing panels, so it will be a compromise. 

Thanks Tony. That makes sense (and thanks to Brickish for the summary).

I understand the current limitation of mixing a 100W panel with a 150W one - hence no point mixing two different power panels on the same series circutie.

I am not sure I understand what the voltage limitation is if the the 2*100W are in one series circuit and the 2*150W are in another and these two circuits are parralled into the MPPT. If one series circuit has a voltage of say 18V and the other 20V, then the higher one will be limted to the 18V and so the power will drop (ie the amps reduce)? If so, it does seem important to get new panels that are within 10% of voltage of the old ones otherwise the extra power will be limited by up to 10%.  I can see the compromise then.

Brickish put a second MMPT controller which gets round the compromise but at a premium of £100.

 

52 minutes ago, Mike the Boilerman said:

Except it won't be 30A, not that the current affects the danger. It will be 14.4v x 30A / 80v. 5.4A, approx.

 80v DC however is probably more lethal than 240v AC if you inadvertantly grip it, as the DV voltage will make your muscles lock and stay gripping it. With AC, you get 100 chances a second to let go as the voltage passes through zero. 

Yes, my mistake. Dont fancy 80V DC. I manged to shake hands with one of the live bulbs in our inflating father christmas, 24V, 10 bulbs in the string - 240v supply, and the AC got up to my elbows!! The 100 chances per sec to let go works.

Also, as above I would then be current limited as above and not get the 150W benefit. Thanks for the input.

[WotEver]

58 minutes ago, Dr Bob said:

I am not sure I understand what the voltage limitation is if the the 2*100W are in one series circuit and the 2*150W are in another and these two circuits are parralled into the MPPT. If one series circuit has a voltage of say 18V and the other 20V, then the higher one will be limted to the 18V and so the power will drop (ie the amps reduce)? If so, it does seem important to get new panels that are within 10% of voltage of the old ones otherwise the extra power will be limited by up to 10%.  I can see the compromise then.

Yes, you have three options. 

1. Select new panels of nominally the same voltage as the existing. 

2. Select new panels of a higher (not lower) voltage than the originals and accept that they’ll never achieve their full potential. 

3. Fit a second controller. 

The compromise I was talking about isn’t simply that the array will be pulled down to the lower voltage, it’s that the MPP of the two sets of panels at any one time might also be different. So the controller ends up at a compromise voltage somewhere between the two. 

[Chewbacka]

You could use your existing mppt controller for the new (higher power panels) and use a cheap pwm controller for the existing panels.

[Dr Bob]

13 minutes ago, Chewbacka said:

You could use your existing mppt controller for the new (higher power panels) and use a cheap pwm controller for the existing panels.

Now there's some good thinking. I'd only need a 20A one. Not thought of that one. Have a greenie.

[Richard10002]

I’d get a 2nd MPPT rather than a PWM, in order to get as close to their max of about 14A.

[smileypete]

Haven't read all replies but 500W of panels should play OK on a decent MPPT.

I'd put the existing 100W in a series pair, the new 150W in a series pair, then put the pairs in parallel to the MPPT.

I take it the existing and new panels have a similar Vmp (voltage at max power point) though.

ETA: did a diagram for someone a while back, see the diagram on the left:

Edited December 31, 2017 by smileypete

[Dr Bob]

1 hour ago, Richard10002 said:

I’d get a 2nd MPPT rather than a PWM, in order to get as close to their max of about 14A.

Yes, I'd missed the pwm bit. I will stick to MPPT.  Thanks.

1 hour ago, smileypete said:

Haven't read all replies but 500W of panels should play OK on a decent MPPT.

I'd put the existing 100W in a series pair, the new 150W in a series pair, then put the pairs in parallel to the MPPT.

I take it the existing and new panels have a similar Vmp (voltage at max power point) though.

ETA: did a diagram for someone a while back, see the diagram on the left:

Thanks Smileypete, that's pretty much what WotEver said in one of the posts if I can get them with similar Vmps. That's what I will do if the Vmps are similar. Need to find out what they current panels are.

[WotEver]

1 hour ago, Dr Bob said:

Thanks Smileypete, that's pretty much what WotEver said in one of the posts if I can get them with similar Vmps. That's what I will do if the Vmps are similar. Need to find out what they current panels are.

Actually it’s exactly what I said

[Dr Bob]

17 minutes ago, WotEver said:

Actually it’s exactly what I said

No, you didn't say "Haven't read all the replies". Happy new year!!

[WotEver]

1 hour ago, Dr Bob said:

No, you didn't say "Haven't read all the replies". Happy new year!!

Happy new year!

[Dr Bob]

On 31/12/2017 at 19:34, smileypete said:

Haven't read all replies but 500W of panels should play OK on a decent MPPT.

I'd put the existing 100W in a series pair, the new 150W in a series pair, then put the pairs in parallel to the MPPT.

I take it the existing and new panels have a similar Vmp (voltage at max power point) though.

ETA: did a diagram for someone a while back, see the diagram on the left:

Just bumping this thread again with one final question. I am just about to order a couple of panels and thinking of the option on the left (thanks smileypete and Wotever). The panels are the ones linked in the first post so should be similar VOC to the originals - or at least within 10% so if I go with the 150W panels I end up with 500W. Will the 30A MPPT work with this? I never seem to get more than 12A in summer IIRC from the 200W I have and so the extra 300W will take me to just under 30A - although Wotevers calcs at 14.4V say a max of 430W from the 30A controller.

I am therefore tempted to try it with the 30A MPPT and if it is suffering in the summer then I can always buy a 20A one as the wiring is then in place and an easy retrofit. I will be happy only to get 430W rather than 500W of power as it will be 30A which is the key thing.....crikey, I will be happy with 20A ......... I will check the volatage on the current MPPT first.

 

[WotEver]

32 minutes ago, Dr Bob said:

Wotevers calcs at 14.4V say a max of 430W from the 30A controller.

You’d have a max theoretical charging current of around 35A from 500W of panels. Point 1 is that you’re very unlikely to see such perfect conditions in the UK as to achieve that. Point 2 is that the 30A controller couldn’t care less even if you did; it’ll simply limit the charge to 30A. 

If you don’t find that adding 300W to your existing 200W more than doubles your average charging current in the summer then it’s a fair bet that the MPP of the two sets of panels differs and you’d do better with a second controller. I think this is unlikely to be the situation in your case. 

[peterboat]

We have been through this before, when I had two identical MPPT Tracer controllers to the bank they caused overcharging! If the makers of solar controllers make them so that they can be paired [not all makers] their is a problem in my mind of going down the 2 controller route.

I in the end bought the right MPPT controller for the job a Midnite jobby from the USA problems sorted and no more boiling batteries

[Dr Bob]

6 hours ago, WotEver said:

You’d have a max theoretical charging current of around 35A from 500W of panels. Point 1 is that you’re very unlikely to see such perfect conditions in the UK as to achieve that. Point 2 is that the 30A controller couldn’t care less even if you did; it’ll simply limit the charge to 30A. 

If you don’t find that adding 300W to your existing 200W more than doubles your average charging current in the summer then it’s a fair bet that the MPP of the two sets of panels differs and you’d do better with a second controller. I think this is unlikely to be the situation in your case. 

Thanks Tony. That's what I was thinking. 30A will be fine. I will try this first.

4 hours ago, peterboat said:

We have been through this before, when I had two identical MPPT Tracer controllers to the bank they caused overcharging! If the makers of solar controllers make them so that they can be paired [not all makers] their is a problem in my mind of going down the 2 controller route.

I in the end bought the right MPPT controller for the job a Midnite jobby from the USA problems sorted and no more boiling batteries

I was thinking this could be an issue with two controllers both fighting each other.

You have both convinced me.

[WotEver]

6 hours ago, peterboat said:

when I had two identical MPPT Tracer controllers to the bank they caused overcharging!

How?

[peterboat]

2 minutes ago, WotEver said:

How?

Tony I have no idea but it did, plus after checking various controller suppliers they do have to be linked so that one controller does the controlling, so if Midnite, vitron and others think its necessary I believe them and my own eyes

[Dr Bob]

11 minutes ago, peterboat said:

Tony I have no idea but it did, plus after checking various controller suppliers they do have to be linked so that one controller does the controlling, so if Midnite, vitron and others think its necessary I believe them and my own eyes

I have no idea how two controllers would interact with each other in charging a battery bank - but keen to understand how they would interact. I was thinking more along the lines of one controller charging at say 14.8v and the other then reducing its charge to a float charge voltage due to it thinking the battery is fully charged. Isnt this similar to say the alternator putting in a significant charge at the same time as the solar bank charging? I am not sure how that works either.

[peterboat]

4 minutes ago, Dr Bob said:

I have no idea how two controllers would interact with each other in charging a battery bank - but keen to understand how they would interact. I was thinking more along the lines of one controller charging at say 14.8v and the other then reducing its charge to a float charge voltage due to it thinking the battery is fully charged. Isnt this similar to say the alternator putting in a significant charge at the same time as the solar bank charging? I am not sure how that works either.

I have that red light on sometimes when solar is at full chat clearly it can do a higher voltage than the alternator, never causes any issues though

[WotEver]

14 hours ago, peterboat said:

Tony I have no idea but it did, plus after checking various controller suppliers they do have to be linked so that one controller does the controlling, so if Midnite, vitron and others think its necessary I believe them and my own eyes

Sorry but no, they don’t ‘have to’ be linked. Some controllers have that facility, which may give extra control over simply paralleling up the outputs. 

I was going to write an explanation as to why it works just fine with two (or more) controllers in parallel but I just came across this website which explains it well enough and even gives a video demonstration. 

https://www.altestore.com/blog/2016/08/wiring-solar-charge-controllers-in-parallel/

13 hours ago, Dr Bob said:

I have no idea how two controllers would interact with each other in charging a battery bank - but keen to understand how they would interact

All they will do is to sum their outputs just like having two alternators in parallel. Once the charging is out of the bulk phase and into absorption then one controller might take over the bulk of the charge but that’s not a huge problem if the batteries are already up to 80%-ish.