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What size solar panel?


david lloyd

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My motorhome is now equipped with 2 X 110ah lithium batteries monitored, controlled and charged by Victron BMV, Victron 12v-12v charger, Victron smart charger, Victron 12/24v 100A battery protect and a Victron 75/15 MPPT solar controller. So far this has proved an excellent investment but one element that could be improved is the solar charging.

 

My Autosleeper Bourton still has the original 80W panel fitted that is not producing much at all even today in Brittany in full sun it has only yielded 50wh and is likely not to go above 100wh. I accept the limitations of time of year but feel a larger panel would improve matters a little. The question is - how big a panel will the 75/15 controller take? The space where the existing panel sits is 150cm X 100cm and the best option might be to simply replace with a Victron panel. The alternative is to add a second panel but the only other roof space available measures 100cm X 60cm and may present difficulties in trying to match the two panels.

 

I see that Victron 12v panels go up to 175W then the next largest is 215W but 24v - is there any problem fitting a 24v panel to the 75/15 controller? And would that indeed be a better prospect? Sorry for all the questions but I am not as technically advanced as May first appear.

 

Your thoughts and advice would be welcome.

 

David

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Driventemple - 2020-11-28 5:00 PM

 

The Victron 75/15 Contoller will handle solar up to 220W so you could fit a 215W 24V panel.

 

Thank you for that information. I need to measure up accurately but think the space is available to accommodate the 215W panel where the existing 80W panel is and, common sense tells me, the bigger the solar panel you can accommodate the better off you are so if it does fit that’s the direction I would go.

 

Not sure though what effect, if any, having a 24v panel has rather than a 12v. All I know is the 24v panel produces a much higher voltage output but not sure how this affects charging?

 

David

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The 'purists' on here will probably jump all over me for this but, here goes:

 

A MPPT Controller uses any spare voltage over the 14v or so required to charge the batteries to sort of 'fold it back' into increasing the current available to increase the charge to the battery.

For example. I have the Victron 75/15 Controller and a Solar Panel that outputs up to 28V and I regularly see these typical figures:

Panel Output 28V 6A

Controller Output to Battery: 14.4V 8.3A

To gain maximum advantage of an MPPT Controller the panel Voltage should be as high as possble.

 

Your 12V panel can't really be as low as 12v as it wouldn't charge the battery. It's probably 18V - most 100W panels are.

 

Stands back and waits for 'incoming' !

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75/15 will accept input voltage up to 75V. As voltage increases with lower temperatures, you need some wiggle room so let's call it max 60-65V under normal conditions just to be safe. To know if your controller can handle whatever panel combination you're thinking of, Victron provides this online calculator https://www.victronenergy.com/mppt-calculator. Just input specs of the panels you have/want.

 

That said, I prefer the excel version as the online one has some limitations and sometimes produces weird results.

 

One common approach is to slightly oversize the panels in terms of max current (but not voltage!) as you rarely have conditions that will produce more than the regulator can output (15A) and should that happen on the odd occasion, it will simply turn to heat. A combo of 2x 8A panels would be just fine.

 

As far as how big... Depends how much you rely on solar really. And how big panels you can fit on the roof. I would avoid mixing different panels as you will be wasting the bigger one.

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Good information. As far as space is concerned I could fit a second (smaller) panel but that wouldn’t be as efficient as replacing the 80W panel with the bigger Victron 215W which will fit in the space left when the 80W is removed. From the answers so far it would also be better fitting this 24v panel instead of the 175W 12v version.

 

As to dependency, the new lithium system allows me to charge the batteries from the alternator at around 30ah so usually enough to replenish as we move around. But it would be nice, when we stay longer in one place, to maximise the solar input rather than run the engine.

 

Thanks everyone

David

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Driventemple - 2020-11-29 12:33 PM

 

Running an engine not under load is not recomended. You should start up and drive off straight away.

 

Yes, under normal circumstance, as I say it would not be a problem moving on regularly. However, slightly more difficult if you are locked down in France and rules are a little more stricter in that you have to complete a declaration form attesting to the fact that (each of) you are moving around for a very specific reason allowed under the lockdown - no ‘pleasure cruising’ I’m afraid especially as the police do actively enforce the restrictions by random stops and checks.

 

I agree entirely that letting the engine tickover for any length of time is not recommended and could lead to carbon build up and DPF/EGR problems - hence the reason to find a more effective solar addition.

 

David

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Our experience of solar panels and extra battery capacity was that when you don't need warmth and light there is plenty and when you do need it there is pecious little. Fine for three seasons camping but not a lot of fun off site in the winter!

 

My view is that to opt for the most solar and battery capacity that your van's existing system will accept without creating more problems than it solves with incompatible items that should work together as intended but somehow never do makes sense?

 

PS We always carried a spare battery secured close to the leisure batteries, wired but not connected and it was a simle and quick job to isolate the leisure battery(ies) and replace them with the temporary spare small capacity but fully charged battery if all else failed and it was cold and dark!.

 

 

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