bryanmccoy57 avatar image
bryanmccoy57 asked

I want to feed my MPPT150/100 with a dc power supply (115VAC to 60VDC 20Amps) so during rainy low solar output weather I can charge my batteries. Open circuit is 70VDC on the power supply.

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3 Answers
markus avatar image
markus answered ·

Hi, yes this should work. But it is not an official supported solution though.

Keep an eye on your power supply to not overload it. Adjust proper amp limit on your MPPT in case, to avoid overloading. I would also disconnect the solar panels when connecting a power supply.



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benl avatar image benl commented ·

Hi @Markus; can you advise on why the MPPT FAQ states "no" with regard to

"Can I use the MPPT Solar Charger as DC-DC charger?" ?

The answer to that question adds an example of a 24V battery as the DC source. This would seem to be quite correct for a battery source; it would be a very bad idea to power the MPPT from a battery with respect to the lack of any short-circuit protection should the MPPT shunt the PV input per error conditions 38 and 39, as discussed in this thread.

If that's the only concern, it seems that question should be expanded to something like "Yes, but only where the DC power source is current-limited to less than the MPPT rating. For example, it is not suitable to charge a 12 Volt battery from a 24V battery bank."

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pokerfjes avatar image pokerfjes benl commented ·

I think the reason the FAQ says "no" is just because they don't want to see people fry their hardware and/or deal with support or warranty related to this.
I have powered several SmartSolar MPPTs with my lab power supply, but that has short circuit protection and should be good even if the error conditions 38/39 occurs.

Most important task is to avoid the two error conditions entirely. That is why I only had one SmartSolar (with dc lab power) connected alongside my Multiplus when charging 16.4V - making sure there was no other charging source that could exceed the SmartSolar.

Wasn't there an update just recently that would allow two SmartSolar MPPT to synchronize the charging? Would this help if say you had one SmartSolar for PV and one for a CC/CV power supply connected at the same time?

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espen avatar image
espen answered ·


If you get an overvoltage error, and the PV input short protection is triggered, you will damage the MPPT charger.

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markus avatar image markus ♦♦ commented ·

Hi, @Espen thank you.

I don't exactly get what you mean. Can you further describe this please?

You mean overvoltage on the PV input?


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espen avatar image espen markus ♦♦ commented ·


An overvoltage error on the battery will short the PV input.

See Error codes 38 and 39 for some of the situation that can cause it:

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markus avatar image markus ♦♦ espen commented ·

Good point. Thank you. The maximum short circuit current the charger would be able to handle according to its data sheet is:

70A (max 30A per MC4 conn.)

so it should be unlikely to get destroyed by a 20A power supply.

I'd worry more, if the power supply would survive in this case...


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bryanmccoy57 avatar image bryanmccoy57 markus ♦♦ commented ·

Thank You Markus. I plan on using diodes to isolate the solar panels from the power supply - and vice versa. The power supply will run off a medium-sized inverter generator during those rainy days when I have little or no solar power input available.

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pokerfjes avatar image pokerfjes espen commented ·

@Espen very valuable info, just what I was looking for.
I have a follow up on this @Espen and @Markus - as I'm working on something similar to what the OP is doing. I plan on using a 100/50 MPPT outputting 50A@16.5V to charge a 4S LI-Ion battery bank. PSU will be capable of 50A@24V to avoid overloading it.

What if a 50A fuse is placed between the power supply and the MPPT. If the MPPT shorts it's input - the breaker trips without any harm being done to either of the two components?


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bluejeans avatar image
bluejeans answered ·

Old post. But I am thinking of using a laptop power supply at 19.5v and 4.6amps for occasionally charging a 100ah LiFePO4 batt through a 75/15 MPPT. (the batt will be primarily solar charged)

Can I fuse the DC side of the power supply before the PV connection and be safe?

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kevgermany avatar image kevgermany ♦♦ commented ·


It's voltage that does the damage. Your charger will get nowhere near the 75V limit of the MPPT.

Not sure this will work, quite apart from the non-standard feed to the MPPT and possible warranty issues. The MPPT needs to see an input that is 5V over battery voltage before it kicks in. With a battery at 14V, say, there's little headroom.

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