question

bleon avatar image
bleon asked

MPPT 75/15 with 2x175W panel

Hello,

I've been using a Victron 175W solar panel + MPTT 75/15 for a few years, but need some more power now.
I thought I would add another 175W panel, which would double my power easily.
My battery is a 12v Lithium 280Ah.

Note: this is used in summer only no chance for below 5°C temp
The MPPT 75/15 max input voltage is 75V.
Panel Voc is 23.7 x 2 = 47.4V in series (mppt calc give 53.2 at -10°C)
Panel Impp x Vmpp = 9.03 * 19.4 = 175W

My understanding is that the MPPT job is to somehow convert the voltage/current to something suitable for the battery.
With two panels, my best case would be (19.4 x 2) * 9.03 = 350W
The MPPT would then convert that power to something suitable for the battery (around 14v), though limited by MPTT max current, 15A.

I know that I will waste some energy, but the oversizing is done so that the MPPT will give its maximum output current during a greater part of the day
than with a single panel.

So my question is why the MPPT 75/15 is indicated as max 220W (12v) while the calculation indicates it can accomodate 2x175W=350W panels without exceeding its 75V Voc.

Hope someone can help me understand this.
--
Bruno



MPPT Controllers
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4 Answers
Michelle Konzack avatar image
Michelle Konzack answered ·

Yes, you can use the panels in parallel..

The MPPT will simply cut any energy above 220W.

However, with oversizing you get a lot of more energy out and MUCH longer

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jetlag avatar image
jetlag answered ·

This 220W is roughly the power you can pull out of an 15A charger at 14,6V battery voltage.

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bleon avatar image
bleon answered ·

@Michelle Konzack my idea is to use them in series not parallel. So higher voltage but same current as a single panel.


@Jetlag Ok so 220W is just a maximum output power, but input is not limited to that as long as you don not got above MPPT max input voltage.


Thank you both

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kevgermany avatar image kevgermany ♦♦ commented ·
And Isc. But in series you'll see no change.

I take it that by summer only, you mean to disconnect the panels from the controller before the colder weather arrives.


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jetlag avatar image
jetlag answered ·

You find the values in the datasheet. There is written:

1a) If more PV power is connected, the controller will limit input power.

1b) The PV voltage must exceed Vbat + 5V for the controller to start. Thereafter the minimum PV voltage is Vbat + 1V

2) A PV array with a higher short circuit current may damage the controller


For you especially 1a) and 2) is interessting. So yes, the input power can be more then the 220W (in your case) but the short circuit current (Isc) should not exceed the 15A, like kevgermany already mentioned. In your series connection of both modules the power will increase, but the current will remain below 15A on the input side. So this should be fine.

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