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Am I over-sizing the MPPT... and should I?

Aphers

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Nov 17, 2020
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OK so my plan is to install two 320w panels, feeding a 4s LFP bank.
Crude maths (640/12=53) leads me to spec a 60A MPPT.
Real life numbers are different, though. I've never seen more than about 80% of rated output from a panel (admittedly, this is in not-so-sunny Scotland); further, my bank will not be charging as low as 12v (I hope).
Using say 85% output from the panels, and a charge voltage in excess of 13v, drops my current to 42A. So I would spec a 40A MPPT instead, and just accept the lost 2A. It's a pretty big cost saving, depending on what brand you're looking at.

But am I being penny wise and pound foolish. Will the bigger MPPT run cooler and have a longer, happier life?
 
If you oversize your SCC with good planning on placement, you can go back and add panels that will be controlled by the same inverter Without purchasing a new one. Would just need to add something to combine the extra panels and a fuse if putting them in parallel.

I tried that with my RV roof panels, but when I measured for space, there was not enough space for the exact same panels, so I had to add an additional solar charge controller for the additional Panels. That is an example of not the best planning, but a work through.

Also, with the SCC, see if it can take extra voltage. My 100 volt, 50 amp Victron Charge controller can take 700 + watts for a 12 volt battery system or 1400 + watts for a 24 volt system.
 
I oversized my controller. It wasn’t terribly expensive to do so and gave me a level of easy expansion if needed. As you can see above, there are lots of options. This was a case where I felt extra was much better than not enough.
 
Thanks all.
I will likely go with the smaller controller (actually given the prices involved I may go with two 20A).
My experience of trying to add panels to an existing setup has been that a couple of years down the line I could no longer get an exact matching panel (in my case, thanks to Brexit the German supplier I bought the first panel from would no longer deliver to me in the UK).
So I think that assuming that any future panels would need their own controllers is not a bad move.

Haven't chosen the SCC yet, but probably Renogy or EPever.
 
Thanks all.
I will likely go with the smaller controller (actually given the prices involved I may go with two 20A).
My experience of trying to add panels to an existing setup has been that a couple of years down the line I could no longer get an exact matching panel (in my case, thanks to Brexit the German supplier I bought the first panel from would no longer deliver to me in the UK).
So I think that assuming that any future panels would need their own controllers is not a bad move.

Haven't chosen the SCC yet, but probably Renogy or EPever.

I have three controllers in my RV trailer. Two of them are Victron 100/50 MPPT. If I changed panels (very unlikely), they have some room for more watts/volts. I would be more likely to add a new controller as any panels I added would never sync well with the existing panels.
 
First off, I will suggest EPEver and please IGNORE Renogy, just do a search on problems with Renogy and that should answer that. EPEver is a good Value Grade line of gear that does exactly what it is supposed to and with little fuss.

Consider things ahead a bit. You may have one 12V Battery Pack and a 20A charger will charge it BUT is it enough ? Lot of folks trip on this point and start sounding like Drunken Sailors on a Saturday Night after having been at sea for 6 months. For example, a 12V/100AH LFP battery can take 50A Charge and will require 2 full hours to hit full from 0%SOC, which translates into 4 hours @ 25A charge rate, or 5 hours @ 20A. As you say "Sunny Scotland" is not known for long clear sunny days all the time and 5 hours of pure sunlight is fine in June but not happening in December.

I personally would suggest a more capable SCC that can do 50A or even 75A charge rate that can be dialed down via the SCC interface / software. Then as time moves along you can add more panels if needed for compensating for reduced Sunhours (we do this as a matter of course in Canada) and if you decide to add another battery pack you'll already be able to handle it with getting another SCC (plus breakers/fuses, witing etc) which tacks more onto the costs.

Hope it helps, Good Luck.
 
Agreeing with Steve on Renogy. A lot of their units seem wimpy and have no room for multiple solar panels or room for expansion.
 
I’m also in Scotland and have 530w and a 50a mppt. You’ve nae chance o it ever reaching max power even in summer. Most I’ve ever seen out of mine was around 30amps. That was down south in the middle of summer 2 years ago
 
I’m normally up north between Aberdeen and Inverness, the big worrie I’ve got is I still get a little power out of my solar at midnight in mid summer. I’m going to set my restart value at 12.5v for summer and 13.5v in winter. My mppt has a lithium and a user setting both can be programmed with charge setting but not with restart value. I will just change between the two about when the clocks change
 
Thanks again for the replies. I wasn't aware of problems with Renogy, I recall Will giving them a decent review and saying they were a bit more user friendly than EPever.

Just for context, the plan is to take the boat out of Scotland for a few years of full time cruising. Certainly down to Spain and perhaps across the Atlantic. Also, the 640w is just what I am adding, there is already 230w on the boat, plus a towed hydro generator.

So the actual total is 870w, and I could add a third 320w panel without too much difficulty, but it would be under the boom and hard to keep unshaded.

Personally I would rather size the SCC correctly now, and not worry about expansion. If I don't expand, then I've wasted money. And if I do expand, running a separate SCC for the new panels gives me more options, e.g. different panel voltages, parallel and series strings, and of course if any of the equipment fails I haven't put all of my eggs in one basket.
 
an epever 40amp mppt will accept an array of 780w with a 12v system if that's useful info for you
After one of my MPPT's went bad I'm in the same boat... can I do similar to OP and safely overpanel a Renogy Rover Li 40a to 600 watts?

Also if I AM ok to do that, I have a 3-to-1 connector on each 3-panel grouping (all same 100w 12v)...is there any problem adding a second 2-to-one connector?
 
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