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valvetdk avatar image
valvetdk asked

The Size of MPPT

Hi

I’m new to all this solar system, and off grid, but just got my first setup up running this weekend, and everything seems to work.


I have a MPPT 100/30 with 2 solar panels ( longi solar LR5 405 watt) I used the calculator to figure out which MPPT I needed for my setup, and it said the MPPT 100/30 would be the best option with 92% undersized, but now I see if I want expand with one more panel in series I need the 150/35 or if I want 2 more In a string 100/50 so my question is - should I have thought more future-proof and bought a bigger mppt than what I would need now at the moment? I just thought I read somewhere that you shouldn't buy an excessively large mppt if you didn't have enough solar cells connected because then it wouldn't be able to start up because the voltage would be too small, is that correct? or have I made a fool of myself by not buying a bigger mppt to begin with?


And second, I don’t see anywhere in the data sheet what the minimum voltage is for the Mppt100/30 ?

MPPT SmartSolarvoltagesystem design
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1 Answer
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kevgermany answered ·

If you need upgrade to a bigger system, the new panels generally don't match the old ones. So solution is to add another controller for the new panels. Generally mixing different panels on one MPPT is inefficient and leads to lower output than expected.

There's no single minimum input voltage for the controllers. But it must be 5V higher than actual battery voltage before charging will start. So if you have a 12V battery, partly charged at (say)13.5V, the MPPT will start charging when panel voltage reaches 18.5V. For a 24V system at 27V, you need 32V from the panels. Once charging starts panel voltage must remain 1V higher than the batteries.

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Related Resources

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