alexander-benner avatar image
alexander-benner asked

MPPT RS 450/100 never reaches OFF state

Hello everyone,

I have a MPPT RS 450/100 chargecontroller where my eastroof and westroof string is connected to. They both are 3800Wp and have a OCV of around 400V

I have a streetlight which shines on the solar panels all the night and generates about 80V. not enough current for the MPPT to do something, but constant 80V

The chargecontroller works fine together with my DIY LFP battery, besides of one little problem.

I have set a fixed absorption time of around 1,5h. The problem is, that the chargecontroller often dosn´t go into absorption mode, it simply skips the absorption and goes direkt into float mode.

I have searched around this Forum and found :

There I have found that this could have something to do with the off state of the MPPT... it never goes into absorption because it thinks the absorption time is already over.

And this is exactly my problem, I have tried to shut down the east roof string over night to confirm if the MPPT goes into OFF state, and if it enters absorption mode at the next day, and this is exactly what it does.

If I shut down the East roof string which has constant 80V because of the streetlight it goes into OFF state after about 3 seconds. After turning the string back on at the next morning it jumps into Bulk mode and enters Absorption correctly

If I left the East Roof running over night, the MPPT doesn´t turn off at all (it stays in Float or Bulk, wherever it was in the evenig) and if it should switch to absorption mode it skips it and jumps direkt into float mode.

Now my question:

It is very unpractical to switch of the east string every night, so is there an other way to tell the MPPT to go into off state?

(i think normaly it turns off if the voltage at the MPPT tracker falls below Battery voltage, but this is impossible for that string near the streetlight)

Maybe some other Ideas ?



MPPT Controllersmppt rs
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nesswill avatar image nesswill commented ·

Interesting, mine does go to an off state but i still have 48v to 36v on each tracker, they go through all stages of charging, not a great deal of sun apart for the 26th. (no lighting apart from the moon)



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4 Answers
JohnC avatar image
JohnC answered ·

Hi @Alexander Benner

Take a look at the 'Rebulk Voltage Offset' in the mppt settings. This might be a little tricky to set up with Lithiums, but it should reapply an Absorb target once exceeded. Not to say it will give you the full Abs period once you get there, but it's a start.

You don't mention a GX device, but I would have thought that (and VRM) would pass on the time of day, but that's just my supposition.

The heavy hit would be to 'short' the panels with a time based relay. High V etc, my last resort.

Just thinking..

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alexander-benner avatar image
alexander-benner answered ·

Hi @JohnC

thanks for your reply, all thoughts are wellcome.

I have set the Rebulk Voltage Offset to 0,6V this works fine for me and the MPPT switches from Float back to Bulk, but then also skips the Absorption phase. I thought this would start the whole cycle again, but it seems not to do so.

The whole Bulk, Absorption, Float cycle is only completed if the MPPT was in OFF state.

Regarding the GX device, Yes there is one. I have a Raspberry PI running Venus OS.

But I think the GX device don´t pass the time and date to the MPPT, I think this is just for visualisation at the VRM

Yes, I also have thougt about to "short" the East roof string during the night. This maybe also could be done by a "pull down" resistor and a relay (and maybe could be done via Node Red), because I don´t think the streetlingt is able to push any power.

But I thought I try to ask here, maybe I haven´t found the right setting or missed something.



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JohnC avatar image JohnC ♦ commented ·
@Alexander Benner

An option might be to work in the RS Remote port with a signal from the RPi. This should turn it off, but it may take some time in that state to register a new day, due to protections against transient weather effects.

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astrayan avatar image
astrayan answered ·

I've been using MPPT 100 50, and I have suffered it going into float prematurely, because it gets tricked by clouds or something, so I have set the float voltage high, so it doesn't matter.

I know you are able to charge LFP pretty high without reducing lifespan, but you could still improve the life of the battery by setting the charge lower

I have wondered how these controllers are supposed to work in the arctic circle.

Recently, they have added Tail current to the low price MPPTs, which is incompatible with aborption time. I assume nothing is being drawn overnight, or this would call the bluff of the street lights, and the voltage would drop. If the tail current option is there, it could be set low. On my application, it's not much use, because I'm running a computer and fridge all day.

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alexander-benner avatar image
alexander-benner answered ·


sorry for the late answer, I have spend my time with rebuilding my whole system to a complete ESS with 3 Multiplus 2 5000

I think this changes the whole situation, because the Multiplus inverters now are controling the chargecycle.

But the problem with the MPPT RS which never reaches off state is still existing.

@nesswill I think your voltage is close enough to the 48V battery voltage + drop for the Trackers / buck stage to work, and the MPPT RS is shutting off.

I think if I could lower my 80V to about 48V mine would shut off too.

@JohnC : Yes, feeding in a signal from the RPi to the MPPT RS Remote port could be an option.

@Astrayan, yes I have the option with tail current, but I would not use that, because I think an fixed absorption time is better for the balancer to do it´s work.

The MPPT draws no current from the 80V street light string, it tries but then the voltage drops and it can´t do something with that voltage, then the MPPT releases the current draw from that string and the voltage rises again (this cycle repeats all the time, measured with an multimeter at the string)

My next steps would be to try out Node Red, and see if I can do something similar with that.

The last step to force the MPPT into OFF state could be to short the string with an relay out of the RPI, but this is realy the last thing I want to do.

I will try what I can do with Node-Red (maybe shutdown the MPPT at night at a specific time and back on at another time)

Thanks for all your answers so far :)

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