How to coordinate external MPPT with invwerter?

[Beat]

My friend has a Mecer 3kW low cost inverter installed as UPS. Now he plans to add solar PVs. But the inverter's solar controller (not MPPT) can only accept PV operating voltage 30 to 36V. Today's panels however run at around 40V. Therefor he considers to buy an external MPPT to accommodate more powerful panels. The question arises how to coordinate the external MPPT with the inverter's grid charger. We would like to give the the MPPT priority over grid charging when available. How to do this?

Edited May 17, 2023 by Beat

[Flouw]

connect the external MPPT directly to the battery. 

 

is that a 3kw model with only 36v voltage range? doesnt sound right. is it a 24V model inverter? 

[Scorp007]

53 minutes ago, Beat said:

My friend has a Mecer 3kW low cost inverter installed as UPS. Now he plans to add solar PVs. But the inverter's solar controller (not MPPT) can only accept PV operating voltage 30 to 36V. Today's panels however run at around 40V. Therefor he considers to buy an external MPPT to accommodate more powerful panels. The question arises how to coordinate the external MPPT with the inverter's grid charger. We would like to give the the MPPT priority over grid charging when available. How to do this?

Yes some early models/basic had a low voltage range. As the inverter will never see solar the external will do the sense of the battery SOC and charge when it needs to. 

If the setting is to charge from UTIL then the grid will charge when present. 

If one leaves the grid input off or switch it on when needed both will charge. Use PV and grid charge in setting 16.

One could also on good days set it to charge only from PV which means it will never charge from grid. The external will charge at all times when there is sun and the battery needs charge. 

Edited May 17, 2023 by Scorp007

[Modina]

I like giving people options.  MPPTs are not exactly cheap.  The cheaper models might fit the bill for now, but will have too low current and voltage limits to be usable at some future stage, should your friend want to upgrade his system.

Your friend could consider dropping the output voltage with a few diodes.  Small diodes have a voltage drop of about 0.7V but high current diodes usually much more, typically 0.9 to 1.2V.   Rather than buy separate diodes, I would use a bridge rectifier and use the + and - terminals that effectively has two diodes in series.  For instance, two of these rectifiers https://www.communica.co.za/products/fb3504 would cost less than R 100 and provide about 4.0 ... 4.8V drop, depending on the current.  You should mount these rectifiers on a small heatsink.  For PWM chargers it is better to drop more voltage and stay away from the peak input voltage.  The charger has anyway no ability to use the extra voltage headroom.

Should one do this?  I don't know.  You will waste some available solar power.  A PWM charger is anyway not very efficient.  One will likely loose 25 to 35% overall. The elegant way is to use a MPPT, no doubt about it.  Another option might be to sell the PWM inverter and replace it with a MPPT inverter.

Your friend should first decide how many panels he wants to use and then choose a MPPT that can handle those panels.  He might be very shocked how expensive a MPPT is if he goes for a system with more than 2 or maybe 4 panels.

PS.  Note that without a load, the voltage drop of diodes will be less than during high current loads.

[Beat]

20 hours ago, Modina said:

Your friend should first decide how many panels he wants to use and then choose a MPPT that can handle those panels.  He might be very shocked how expensive a MPPT is if he goes for a system with more than 2 or maybe 4 panels.

The elegant way is to use a MPPT, no doubt about it.  Another option might be to sell the PWM inverter and replace it with a MPPT inverter.

Thanks, Modina, for your considerations. We have come to almost the same conclusion. The question as mentioned above remains: How to coordinate an external MPPT with the inverter's grid charger.

[Chris_S]

1 hour ago, Beat said:

Thanks, Modina, for your considerations. We have come to almost the same conclusion. The question as mentioned above remains: How to coordinate an external MPPT with the inverter's grid charger.

If the battery voltage is low then the mppt can never raise the voltage that much. The inverter will see a low battery voltage and also charge the batteries. Both will just charge together 

[Scorp007]

3 hours ago, Beat said:

Thanks, Modina, for your considerations. We have come to almost the same conclusion. The question as mentioned above remains: How to coordinate an external MPPT with the inverter's grid charger.

As stated in my previous reply you cannot coordinate the AC charger with the external MPPT. 

Its like if 2 guys push a car. You will never know which guy is pushing harder. One can only control it in a way by taking turns to push or choose for both to stay pushing together. 

The inverter will not know about the external charging taking place. 

 

[BritishRacingGreen]

26 minutes ago, Scorp007 said:

As stated in my previous reply you cannot coordinate the AC charger with the external MPPT. 

Its like if 2 guys push a car. You will never know which guy is pushing harder. One can only control it in a way by taking turns to push or choose for both to stay pushing together. 

The inverter will not know about the external charging taking place. 

 

Hi @Beat very interesting question, and I for one has my head scratching around this. Listening everyones valuable input, It appears that the best solution would be to have a controller, eg. small Raspberry PI, to command  inverter to not charge from grid (charger source priority)  based on a timetable with an adhoc override facility as well.  

[JustinSchoeman]

If the max output current of both sources combined is less than the max charge current of the battery, then there is no real need to coordinate them. Just hook them both up in parallel and away you go.

If you need to restrict/share charge current then it is trickier.  I am not aware of any commercial solutions for this, but if either/both devices have some sort of Home Automation link, then you should be able to set something up (although you will need a shunt on the dumb device).