Will my solar controller take another panel. (1 Viewer)

[Adrianlle]

My 2x 100w solar panels are working again on my James Cook and I am wondering if I can add a third( there is space on the roof) as I hardly ever use ehu and charge the bike batteries from an inverter. 2x 90amp gel batteries. The VictronConnect app shows that that on a good day combined they produce a 200w yield and about 20v. The controller is a Victron blue solar mppt 75/15. So max input voltage of 75v and max input current of 15 amps. An additional 100w panel would be rated at say 20v max and say 5/6 amps. So can anyone assist on whether I could get away with the extra panel using the existing controller ?The combined voltage of the panels seems ok for 75v but the combined short circuit current of say 18 amps is a bit over the rated 15 amps of the Controller. On the other hand the 2 existing panels are only producing about 20v combined so the controller is not getting its full load from them. As always any help much welcomed! Adrian

 

[tonyidle]

Assuming the panels are in parallel (they will be) the voltage stays the same. 3 x100W panels on a clear day at the Equator should be able to output 25A but under more normal circumstances about half that. Normal advice is to ensure you regulator will cope with max panel output but I'd be tempted to use the one you have. Most decent regulators tell they're overloaded and shut down rather than fry so if it doesn't ever shut down for that reason you're OK. I'd give it a go.

 

[hilldweller]

The combined voltage of the panels seems ok for 75v but the combined short circuit current of say 18 amps

You are doing two calculations at once there.

You choice is 3 in parallel which would be 20V and ( maybe ) 18A
OR 3 in series which would be 60V and 6A.

20 x 18 = 360 = 60 x 6

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[Lenny HB]

You can just fit another regulator to the additional panel but my prefered way would be to change the regulator, as Tony says it is possible for them to peak at 25 amps and quite easy to get 22 amp in mid June in southern Europe.
Victron regulators will work with a higher input current that they are rated at but it just gets wasted, so not much point in fitting another panel if you don't change the regulator.

 

[Adrianlle]

Thank you all for the contributions: really helpful as ever. As a bigger Victron controller is going to be expensive ( and I would like to stay with Victron because the App is very useful) I will try the third panel with my existing controller and see how I get on! Adrian.

 

[Robert Clark]

which Controller do you have?
If you look on the Victron website it will show the maximum wattage and current that it will handle

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[TheCaller]

In the designation 75/15, these figures refer to the maximum voltage & current respectively.
The 75v maximum occurs on the input side & happens when panels are wired in series. Realistically, you could wire up to three 100W panels in series before overloading this controller.

As the OP clearly has the existing two panels wired in parallel, it makes sense to add a third the same way. As pointed out above, this doesn't affect the overall voltage, so the input voltage will remain at 20v,or thereabouts.

However, the maximum current occurs on the output side of the controller & will happen during the bulk stage of charging. At this point the output voltage from the controller will vary from around 13v (if the batteries are pretty flat) to about 14.5v before moving to the absorption phase.

So if the panel array was producing its theoretical maximum 300w & the controller converted that with 100% efficiency to a 13v charging current, the current would be 300w/13v=23A.

In practice, that is never going to be achieved. In the UK, you would typically get about half that. Unless you regularly spend time in North Africa in the summer, you will be unlikely to exceed the rated 15A maximum charge rate.

The next question is - what would happen if there was more than 15A available? My understanding is that for reasonable over-capacity, the excess is simply wasted. The controller won't be damaged by the full 300w being fed into it. I've read that somewhere & I know it wasn't on Victron's website, so I can't say for certain that it is correct. However, the data sheet says: Automatic load disconnect - Yes, maximum load 15A. To me, that suggests that the controller will simply shut down.

If it was me, I'd add the third panel & keep an eye on the app. You will soon see if you are getting anywhere near the maximum current. If you are so lucky, then you could either replace your existing controller with a bigger one, or buy a 2nd 75/15 & just feed the ouput of one panel through it. That would a) be cheaper & b) would reduce shading effects.

 

[Adrianlle]

Thanks for this. A very thoughtful reply. We are not going to Morocco in high Summer. More likely Scotland in the grey! So will give it a go as I too suspect the Victron will just shut down if and when there is too much current! Adrian.

 

[autorouter]

I guess you are adding the extra 100W panel to give you a reasonable output in cloudy weather or when the sun is low. If so, you could add a switch, so that when the sun is really bright you can switch off the extra panel. That would work if you were wiring them in parallel.

Or wire it through a fuse, and pull the fuse if the current gets too high.

Otherwise I would go with @TheCaller's suggestion and rewire the three of them in series. The current would be the same as the current from a single panel, that is about 5 or 6 amps.

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[TheCaller]

I guess you are adding the extra 100W panel to give you a reasonable output in cloudy weather or when the sun is low. If so, you could add a switch, so that when the sun is really bright you can switch off the extra panel. That would work if you were wiring them in parallel.

Or wire it through a fuse, and pull the fuse if the current gets too high.

Otherwise I would go with @TheCaller's suggestion and rewire the three of them in series. The current would be the same as the current from a single panel, that is about 5 or 6 amps.

I didn't suggest wiring them in series! I did say that the 75/15 controller will handle them in that configuration, but it's the output current from the controller that matters, not the input current.

Which ever way you wire them, the output voltage & current will be the same. Series or parallel, the input current is never going to exceed 15A, but it's theoretically possible that the output current might.

There are theoretical advantages of wiring in series, but I doubt the gains are sufficient to warrant rewiring the existing two - although the OP did subsequently suggest that Scotland was more likely than Morocco & Scotland is where the advantages of series wiring would show to better advantage, because it keeps charging (just) for longer at low light levels. It also rules out your sensible suggestion of a switch. Any gain from wiring in series can be negated by shading problems - get shading on one panel & it affects them all if they are wired in series. By the time you have filled every bit of roof space with panels, it's easy to get shading from other bits of roof furniture, never mind from external objects.

The other consideration is that once you get to three panels in series, you are talking 65+ volts, which is bordering on moving out of the comfortable low voltage range. Hardly likely to be fatal, but could make you jump a bit.

 

[autorouter]

I didn't suggest wiring them in series! I did say that the 75/15 controller will handle them in that configuration, but it's the output current from the controller that matters, not the input current.

Yes you're right about that. When I read the data sheet I didn't notice that the output current limit was 15 amps. It also says up to 200 watts of panel if charging 12 volt batteries.

 

[DBK]

I've got the same controller and 200W and also considering fitting an extra 100W panel. If I do I've decided to buy another 75/15 controller mainly to reduce the effects of shading and also because Victron give a maximum wattage rating for the controller which from memory is less than 75 times 15. I think the controller can take more watts but it limits its output but I don't think it is worth the risk. There's always the danger of a fire and if you have over-loaded the controller the insurance company may not be too sympathetic.

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[tonyidle]

I've got the same controller and 200W and also considering fitting an extra 100W panel. If I do I've decided to buy another 75/15 controller mainly to reduce the effects of shading and also because Victron give a maximum wattage rating for the controller which from memory is less than 75 times 15. I think the controller can take more watts but it limits its output but I don't think it is worth the risk. There's always the danger of a fire and if you have over-loaded the controller the insurance company may not be too sympathetic.

If anyone can achieve more than 200W charge from 300W of flat mounted panels I'd be very interested in how they did it. Other than being near the Equator on a clear day with a totally discharged battery.