PV input to CC plateaus in Grid Tied Battery Backup system

kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
Seeking expert comments on an issue I am facing. Thanks in advance.

I have a grid tied battery backup system. I recently changed charger settings on MPPT and XW to match with the Battery manufacturer's recommendations. 

Since then the PV Input plateaus at 2KW from 10AM to 5PM even on a clear sky day. 
Same with the DC Energy In and Out from the MPPTs. 
The Grid Output also almost plateaus. 
The battery summary chart shows battery current fluctuating between +3 and -3 Amps during night and maintaining at the grid support voltage. 

P.S. I haven't yet figured out how to insert images in a discussion. Once I figure it out, I'll insert the charts here. 

System configuration follows:
20 panels; two parallel strings of 10 panels each
Two MPPT 80 600 Charge Controllers - each one connected to a string
One XW-6848-01 Inverter/Charger
48V Battery Bank 190Ah capacity; 16A charge current; 4 12V batteries in series; Mfr: Enersys
Per manufacturer, battery Bulk, Absorption and Float voltages are the same at 54.53V at 20 Celsius; Battery temp correction is 3mv/C per cell at 20 Celsius; Total 24 cells.

MPPT settings:
Battery type: Custom (Old Flooded)
Charge cycle: 3 stage (No change)
Nominal voltage: 48V (No change)
Bulk voltage: 54.53 (Old 57.6V)
Absorption voltage:54.53 (Old 57.6V)
Float voltage: 54.53 (Old 54V) )
Recharge voltage: 51V (Old 50V)  
Maximum charge rate: 23% (Old 100%)
Equalize voltage: 57.6 (Old 64V)
Battery bank capacity: 190Ah (Old 440Ah)
Battery temp coefficient: -72mv/c (Old -108mv/c)

XW settings:
Battery type: Custom (Old Flooded)
Charge cycle: 2 stage (No change)
Nominal voltage: 48V
Bulk voltage: 54.53 (Old 57.6V)
Bulk termination voltage: 53.53V (Old 54V)
Absorption voltage:54.53 (Old 57.6V)
Float voltage: 54.53 (Not found because of 2 stage cycle)
Recharge voltage: 50V (No change) 
Maximum charge rate: 13% (Old 100%)
Equalize voltage: 57.6 (Old 64V)
High battery cutout: 59.53 (Old 65)
Low battery cutout: 39.2 (Old 44)
Battery bank capacity: 190Ah (Old 440Ah)
Battery temp coefficient: -72mv/c (Old -108mv/C)
AC Coupling: Disable (Old Enabled)
Search mode: Disable (Old Enabled)
Grid support voltage: 52 (Old 53) 


Comments

  • Wheelman55Wheelman55 Registered Users Posts: 192 ✭✭✭
    What are the brand, model and specs for your 20 panels?

    VOC, ISC, VMP, IMP
    Building Off-Grid in Terlingua, TX
    14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 3 - Discover AES 42-48-6650 48 volt 130ah LiFePO4 batteries
  • kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    Solarworld SW Plus300M, 300 Watt modules.
    Voc = 40.1Vdc
    Vpm = 31.6Vdc
    Isc = 10.23A
    Ipm = 9.57A
  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    Are the battery bank settings correct (are these AGM or GEL batteries)?

    If you use the math from your MPPT settings:
    • 54.5 volts charging * 190 AH * 0.23 max charge rate = 2,382 Watts
    Pretty close to your 2,000 Watt plateau charging rate...

    190 AH @ 48 volts seems to be a pretty small battery bank for a 6.8 kWatt XW system... For lead acid, typically would be around 680 AH suggested minimum battery bank capacity using our typical rules of thumbs.

    I do not know how to program an XW system--But it would appear that your MPPT programmed output charging current would limit both charging and output to the grid.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    These are AGM batteries.

    Last night I configured XW to utilize 'Enhanced Grid Support with Conext MPPT CC' by setting Grid support volts to 64. That resolved one of my issues which was, CC staying always in Bulk mode; now the CC is transitioning from Bulk to Absorption to Float. This also stopped the XW's charger from charging and discharging all night long; XW stayed in AC Pass Through. 

    Regarding the issue with solar power plateauing at 2K: I was thinking that the 'Maximum Charge Rate' setting is only for charging batteries and not used when CC is in Float mode. I assumed that during float mode, CC will automatically pass surplus PV power to the Inverter for powering AC loads and selling to grid. 

    I guess the installer designed my system for 190AH battery capacity just for my critical load which is only a fridge, a freezer, well pump and network gear. I think he sized the battery, primarily as a backup source during power outages. 

    Grid is pretty stable here and we lose power rarely and not for long. Hence, is it possible to configure MPPT and XW to sell surplus PV generation to the grid without having to invest more into higher capacity batteries?
  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    I don't know... I have zero detailed knowledge on these systems.

    Perhaps, somebody else here can help...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    I have reluctantly increased the 'Max charge rate' from 23% to 94%. It is not plateauing anymore. It was initially set to 23% because the max charge current for the battery is 19A and MPPT is rated at 80A. Reluctant because, I don't know how the 'DC Out Current' from the controller is divided between charging the batteries and simultaneously supporting grid and selling by the inverter. I am just hoping that the batteries are not given way more than 19A which is the maximum charge current for them. 

    I am assuming that at any given moment the batteries take in necessary current and leave the rest to the inverter for grid support and selling. 

    At peak, the Inverter showed, 'DC Power' of -6258W at 53.6V (Discharging batteries or perhaps discharging batteries and also passing the excess from MPPT that was used for simultaneously charging). 'AC1 Active Power' was 5033W at 251.1V (Selling to grid because it is positive). AC Load Active Power 52W at 126.2V.

    If my assumptions are correct:
    Amount of excess power from MPPTs used for charging batteries would be: 6258 - 5033 - 52 = 1173W
    At 53.6V of Battery voltage the charging current would be 1173/53.6 = 22A which is not way too much from 19A. If I factor in other power losses such as power utilized by the Inverter itself, then it would be less than 22A. 

    The battery temperature was not way too high at this time or anytime of the day. It hovers around 75 degrees.

    Are my assumptions correct? If these are correct, I don't have to size up my batteries. I should be careful not to put sell block during daylight hours.

    I may have exposed my ignorance on some of these calculations. So, bear with me please. 
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,296 ✭✭✭✭✭
    You should either hire the person who installed and has the knowledge,  or learn it yourself.
    It is not something you read for an hour BTW. It can take years for a novice to get up to speed on a battery based Hybrid power system.
    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    Having dealt with my installer, I am less sanguine about his knowledge on this topic. Hence, I am learning by myself however long it takes. In the process, I am seeking advice from gurus in this forum who may have the bandwidth to answer my questions.

  • kikokiko Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    The maximum charging current for the batteries is way more than 19A. I'm content with the settings I have made. Problem solved. 
Sign In or Register to comment.