Charge Controller Current Limit Setting question

Hello All,
I am new to the forum. I have been contemplating/ planning a Grid-Tie with Battery Backup System for a while now and have a question related to charge controller output current limiting. First some background on the system I am planning. Here is a summary of the major components: 18 x Kyocera 255 watt (4590 watt total) solar panels combined as 6 parallel groups of 3 in series, a Midnite Classic 150 charge controller ( I have used the Classic sizing tool), (eight) Trojan L16H 6V (435Ah) Lead Acid batteries and a Schneider XW+5548 inverter. I also have an existing 2.52Kw grid tie system which I will likely AC couple into the new system.
My battery bank selection places me near the 100Ah/Kw solar panel rule of thumb which I have seen discussed on more than one thread. Another calculation I have often seen is where the Array wattage for a Minimum (5%), Nominal (10%), and cost effective Maximum (13%) charge rate is determined. If I back calculate the charging rate based on my array size I come up with 13.7%.
Now to my question, my system will likely be used as a Grid-Tie system better than 95% of the time and I will likely use the Battery backup aspect of it perhaps less than 5% of the time. How should the output current limit of the charge controller be set? Should it be set to the 13% rate limit only when I am using in the backup mode and normally leave it set to its max setting thus making the full solar output available for sell? Is there a ‘rule of thumb’ or ‘best practice’ for this setting? I welcome all comments on this particular issue or on any aspect my component selection/design.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,319 admin
    For a starting point--My only concern would be the battery bank. It is on the small side--But could work for you for the near term. Then later, once you have the system wrung out and know what you want--You can adjust the battery bank size and other components as needed.

    One of the issues to watch for is "micro cycling" of the battery bank. More or less, you have DC current coming in from the charge controllers at a constant DC current. Another is when the battery bank gets over voltage from too much charging current (before the charge controller can dial back the current--My guess to what happens).

    The Hybrid inverter in GT mode is taking power from the battery bank and selling it to the grid (giant interactive dump controller).

    With single phase AC inverters, the DC current is basically a 120 Hz sine squared waveform (draws maximum current during voltage/current peaks and draws near zero current at the voltage/current zero crossings). So--The battery bank needs to be large enough that the 120 Hz ripple current does not drop to "discharging" 120 times a second and rise again to float/charging the other phase of 120 times a second.

    If the ripple voltage / current is too high for the battery bank, you can end up "micro cycling" the battery and also the ripple current can cause the battery bank to run hot (just like high ripple current does in a capacitor).

    Another issue that has been seen here (although rare)--Is that when the battery bank is "full", sometimes during MPPT Sweeps of the MPPT Charge controller (I think this is what is happening or something similar), that the MPPT controller to measure Vmp/Imp of the array takes 100% of the available energy from the solar array and dumps it to the battery bank. It does this to make a Vmp*Imp power curve from the solar array--And then picks Vmp for the next X minutes of operation. And or the MPPT charge controllers are a bit slow in changing their output current (cloud goes by and sun is exposed, takes a few seconds for the charge controller to throttle back on output charging current)...

    In any cases, with older Xantrex Hybrid inverter systems, there have been times when the battery bank has faulted the controller with >72 VDC on the battery bus. This has happened (as I recall) on installations with a relatively large wattage solar array and a ~1/2 size or smaller battery bank (i.e., not the 1kW per 100 AH but 1kW per 50 AH or smaller). The solar array simply can supply so much current that the battery bank cannot keep the voltage in the desired range (batteries themselves to the short term voltage regulation in DC power systems (instant to seconds)... The battery chargers are sort of longer term regulators--(seconds to minutes to hours).

    Again, not to say that your system will not work--It appears to be on the small side of comfortable for the battery bank to Inverter and battery bank to solar array size.

    I don't work for Schneider and I have no experience with their equipment--Just an observation from reading here. It probably will work OK--But the battery bank life may be on the shorter end of the life scale (3-8 year battery life--This system may be closer to the 3-5 year battery life--But this is all guess work on my side--Each system is almost unique--Battery Brand/Models/size of array/size of wiring/size of array/etc.).

    Hopefully some others here with XW Hybrid installations can chime in.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PLC GUYPLC GUY Registered Users Posts: 4
    Bill,
    Thank you for the useful comments. I understand my battery bank is on the small side. However, given that this will be the first time having to maintain a battery bank I think it best I start simple. One series string of batteries as opposed to 2 parallel strings. I'me sure there will be a learning curve. One other aspect of my planned system I did not mention in my first post is the shading issues I will have. My panels will be installed on the south side of my Cape which has dormers. So the full 6 groups of 3 series panels (4590 watts) will be getting maximum sunshine approximately 2 hours per day, most of the time I should be producing on 4 of the 6 groups (3060 watts).
  • BB.BB. Super Moderators, Administrators Posts: 32,319 admin
    Shading is a big thing with solar panels/arrays. Pretty much, a shaded panel in a string (even partial shading or even a shadow from an overhead power line) can be enough to kill the output of that string of panels. As long as you have planned for that (reduction in output).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JimboJimbo Registered Users Posts: 1
    Hi PLC Guy,


    I have an xw 4058 a couple things to consider, using a schnieder electric charger (instead of morning star) not that it is better but it will allow you to divert solar harvest to local loads, it does work, does not show up on the combox as solar harvest (don't know why) but you can see local loads reduced, this will help you max use of pv harvest, it is called enhanced interactive mode. I would also reccomend the combox I like using it for making program changes and checking what is going on. You may also want to consider relays for shutting your battery bank charging on and off if your not going to use it alot, you can use the aux output of the inverter or charger to shift 12v relays to do automated operations. there are also high voltage dc relays available to turn your pv load to your charger or bank on or off depending on voltage or other conditions.
  • PLC GUYPLC GUY Registered Users Posts: 4
    Jimbo, Thank you for your input! A combox sounds like a good idea. At one time I was considering a setup similar to what you describe, I was even considering doing part battery back-up and part micro-inverter (for my panels which will have shading issues). I decided against the micro-inverter option since the thought of having to replace a failed unit did not leave me at peace. Once I get off that 45 deg roof I don't want to have to go back! Thanks again.
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