jkrobbinssr avatar image
jkrobbinssr asked

400 watts of solar on a 75/15 mppt charge controller

Hello, I have a mppt 75/15 charge controller with 200 watts of panels on it right now. I know that is the max but read somewhere that you could add more panels but it would only pass the 200 watts at peak sun without damaging the controller. My problem is where I put the current panels is now not in direct sun so I would like to add two more 100 watt panels in parallel so I can try to get some more wattage during cloudy days and place these panels on the roof where the sun is shining now. There are alot of trees around the building. Any info is greatly appreciated.

battery charging
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3 Answers
JohnC avatar image
JohnC answered ·

Hi. That should be ok, and yes, the mppt will just limit output to 15A into whatever V you're charging. Say 14V x 15A = 210W.

But for best results the panels should be fairly closely matched, and strung correctly so not to damage the unit. There's a 75V limit on panel Voc, which will add with longer series strings. And a 15A Isc limit, which will add with parallel strings.

There's a calculator here: Try starting with '2S2P' stringing. But if you're not sure, just post your panel specs and we can check for you.

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jkrobbinssr avatar image jkrobbinssr commented ·

Thanks for the reply. I have 2 renogy 100 watt panels in parallel on one end of the garage and would like to add two more at the other end of the garage all paralleled together. The two sets of panels would be about 20ft apart.

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JohnC avatar image JohnC ♦ jkrobbinssr commented ·

Just to be clear here, by 'closely matched' I meant in specs, not physical location. 20ft isn't an issue, and 'solar wire' is normally provided in 4-6 mm2 sizes to match MC4 connectors. Either should do.

If your panels are '12V' jobs like these:, then you'll see better results with them wired in series, two panels per series string. 4x of those panels wired all parallel would exceed the permissible Isc of a single 75/15 anyway.

Justin is right too, but if you're anything like me, you'd try just a single mppt and step up if you need. If the new panel specs are far different to the old, then another 75/15 would surely be the way to roll straight off.

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jkrobbinssr avatar image jkrobbinssr JohnC ♦ commented ·

Thank you to both of you for the information. What will the benefit be of doing 2s2p. I know higher voltage coming into the controller but what is the gain? I also am a little confused about setting up the controller for regular 100ah lead acid batteries

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JohnC avatar image JohnC ♦ jkrobbinssr commented ·

The gain comes in poor light, like sunup, sundown, cloud, and in your case *shade*. The panels won't perform to their listed specs in those conditions, and the tracked Vmp may go as low as 50% of specs. Then shut down. Better some power than none.

Easier to wire in series too, and for you if just one 75/15, it'll be essential to go 2S2P.

Just pick a stock Pb charge algorithm to suit your batt type, which you could tweak if needed for special needs. Mostly wouldn't be necessary..

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Justin Cook avatar image
Justin Cook answered ·, for this type of setup I'd personally recommend adding a second 75/15 controller for your other two panels with both controllers paralleled to your battery bank. While the connection you describe would definitely work so long as the PV panels are correctly connected in series/parallel, you'll see a higher charge efficiency if you have a second controller on the panels more likely to see direct sun. This has the added benefit of not clipping your potential energy harvest by capping out at 15A. From two 100w panels in direct sunlight going into a 12v nominal battery bank, you could easily see 14.8A of current under ideal circumstances, so it'd be a shame to waste whatever additional energy is being produced from your two less-than-ideally-placed panels... this way you can get your near-15A from the direct-sun panels and also get the 5-10A you're likely seeing from your non-direct panels.

It's a bit of an additional investment, but considering the relatively low price-point of the 75/15s to begin with, I think you'll quickly find it worthwhile.

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petajoule avatar image
petajoule answered ·

Using the right panels, I have been able to find a configuration of operating max. 720W panel capacity on a 75/15, but I am operating right now 520Wp (2 x 260Wp)


there the section "Oversizing PV Array"

It's actually question number 1 in the MPPT FAQ ;-)

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