Inverter/Charger and MPPT Controller interaction

Hi, all!

As a technically skilled solar newbie I have a technical question.
I am planning to setup a hybrid system, in my case solar panels + MPPT + battery + inverter/charger. I do not plan to sell power back to the grid.
Solar energy will have priority over grid.

So, I have a MPPT and inverter/charger both connected to the batteries, meaning both are trying to manage the batteries.
If they are not communicating to each other (not from the same vendor, i.e. Midnite Classic 150 + SMA Sunny Island or Studer Xtender) their charge stages are not in sync, i.e. one might be in bulk while other in absorption mode. Sure I would try to set voltage configuration + battery temperature sensors on both, but they will not be perfectly in sync.
Majority just ignore this. Can this cause any trouble?

Moreover, if both in bulk mode they both put maximum (configured) DC current to the battery, since they do not know about each other. Some systems (SMA Sunny Island) do have shunt installed and they monitor current to the battery. Some system have only stage sync (Studer Xtender, but no current sensing). Ideally, I would limit the current (I will have small battery capacity) and make MPPT charger a priority. Inverter charger should just add what MPPT cannot provide.

Is there any solution to this? Or silently ignored? Should I care?

Thanks for all the input,
Matej

Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Interver/Charger and MPPT Controller interaction

    So your system will be constantly linked with the grid and charging your batteries through the inverter/charger? The solar panels and charge controller won't have much to do - - unless the grid happens to go down - - -
    Or do we misunderstand your description?
    Need lots more info as at present, the imagination runs in several directions.
    Loads; PV watts; CC make and model; Batteries, how many, voltage, type; inverter make and model?
  • matejS
    matejS Registered Users Posts: 2
    Re: Interver/Charger and MPPT Controller interaction

    Here is my planned design:

    500W PV (for a start; two BISOL panels), Midnite Classic 150, SMA Sunny Island 2224 (not decided yet about this one), battery (AGM 200Ah 24V) to:
    run a pump (100W) and lighting (max 200W) and some minor consumers (max 200W).

    During the day PV + inverter run the load. Excess is stored into the battery.
    When there is not enough power, inverter/charger switches to AC and also helps charging the battery.

    The idea is that PV is primary (priority), then batteries, then grid.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Interver/Charger and MPPT Controller interaction

    without doing any math for particulars here, know that your choice of agm battery can influence whether it'll work ok with multiple charge sources or not. some agm batteries can only take as much charge power as some fla type batteries, but the sunxtender agm batteries can have very high charge rates without harm, several times c in fact. check the charge rates for the batteries you are considering and see if you will exceed the battery ratings with the max amount of charge you intend to put to them. if you plan on sunxtenders then just go with it without worry about the 2 simultaneous charge sources to them.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Interver/Charger and MPPT Controller interaction

    In theory it should work as planned as long as you have the shunt installed for the sunny island. Regarding the out of sync charge phases there might be some complications with SI because it doesn't do a daily absorb charge like the classic. I'm not sure how the SI would handle the situation where it wants to float, but the classic is in bulk or absorb so the battery voltage is higher than what the SI expects. It _might_ try to backfeed the grid with the higher battery voltage, but you may be able to prevent this by setting the reverse power setting in the SI.

    Sorry, more questions than answers :) May be worthwhile giving SMA a call.

    EDIT: Regarding the maximum charging current into the batteries, the SI can regulate this if you install the shunt, i.e. if the batteries can accept 50A and the classic is producing 50A, then the SI will know that it can only add 10A.
  • Eric L
    Eric L Solar Expert Posts: 262 ✭✭
    Re: Interver/Charger and MPPT Controller interaction

    It seems like you have just enough solar capacity to give your 200 amp-hour battery bank the basic charge it needs to stay healthy -- roughly about 20 charging amps at the bank voltage.

    If this is right, why do you even want to charge the batteries from the grid? It's inefficient since the batteries never give as much back as it takes to charge them. And, since you have the grid available, it's unnecessary. If you were worried about the batteries getting enough charging amps, you could let the inverter/charger give the batteries an occasional harder charge at night (e.g., after a long cloudy spell).

    It could be that after loads you don't have quite enough charging amps from solar alone, but then the simpler thing would seem to be to add another panel or two.

    I have a grid-supported system like you're thinking of that flips to grid when the bank voltage gets too low. But I never use the charger function on my inverter -- it's turned off. The pv array does the job just fine as soon as the sun returns.
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Interver/Charger and MPPT Controller interaction

    I do not have a Sunny Island. But my system is similar using an Outback inverter and Midnite Classic CC. There is no reason for direct communication between the inverter's charger and the Midnite solar CCs. The inverter's CC is not conflicting with the Midnite CCs -it is rarely ever used. Normally it is only being used to maintain the batteries in float overnight. During the day the Midnite CCs are in bulk mode and the batteries are held at float voltage with all excess power being sold to the grid.

    The only scenario when both the inverter CC and the solar CC would both be trying to accomplish 3 stage charging would be if the grid was out and I was using a generator to try and charge via the inverter CC while also charging with PV. Even then there should be no conflicts since the absorb and float charge settings are the same and the measured battery voltages are offset to be identical. Also in that scenario the generator would likely only be used to do bulk charging.
  • solartechie
    solartechie Registered Users Posts: 1
    Re: Inverter/Charger and MPPT Controller interaction

    Hi Matej,
    There is some sort of control that has to be done to use the complete PV power. Even if you change the absorption charge/ float charge voltage settings, bulk mode creates the problem.
    I tried the same concept and finally i took a mppt charge controller from Energywin Technologies. They provide a MPPT controller with inbuilt logic which we can directly integrate to the existing UPS/inverter.