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gareth avatar image
gareth asked

Checking my maths, A / V / W and a SmartSolar 75/10

Hi All,

I've got a couple of 300w panels with Voc of 39.98 and Isc of 9.66 a Smart Solar 75/10 and some 12v AGM batteries.

If I take the max values then I don't believe that I can use two panels on the one controller as in series the voltage will be over max at 80v and in parallel the current will be over at 20A so just the one panel in use currently.

But... 75V at 10A is 750w and I am rarely if ever seeing more than 135W of output from my existing panel. By my maths at 40v 135w is <4 A and I'm not getting volts above 36 so at worst two panels would be below all of those limits.

So, not withstanding the fact that if they did both go max volts and current my controller would blow up, would I get any benefit from dropping two panels onto the one SmartSolar?

I've got a load of around 90w and am currently struggling to keep it going overnight.

smart solar set-up help
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3 Answers
kevgermany avatar image
kevgermany answered ·

The two numbers of the 75/10 are input volts and output current. Generally as long as you don't overdo it, putting more panels on the MPPT is fine, it will throttle the panels to max output.

However make sure the total Isc of the panels is below to the limit in the controller specifications.

Assuming you have a 12V battery system. If you have set absorption voltage to the normal 14.4V, max power out of the controller is 14.4V*10A=144W, until the batteries start limiting charge.

Check max charge rate for your batteries as one of the factors in upgrading the controller. A good starting point is C10 or current to charge in 10 hours. Also if you intend to connect the panels in series, check panel voltage increase in winter.


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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

10A is the output current at battery voltage.

10A x 14V = 140W

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gareth avatar image
gareth answered ·

Vielen Dank @kevgermany and @Matthias Lange - DE.

sorry, I have now found and RTFM....

So to make full use of my 300w panel with a 12v system I would need more like a 25A output current so a 100/20 maybe (to retain the load connections) or better a 100/30 (but I loose the load connections).

Or given that I currently use 2x12 volt batteries in parallel I could switch to 24v with the existing controller and get up to 290w.

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kevgermany avatar image kevgermany ♦♦ commented ·
Yes, but if your loads are 12V, you need a 24/12 DC:DC converter.
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