TSMPPT correct charge time settings?

photon
photon Registered Users Posts: 2
Hello,
I have a PV system in an rv which consists of
- 6 kd140's in series
- tsmppt-45 charge controller
- 4 Sun Externer AGM 2580's in a 24V bank (2 24V strings in parallel)

Now I have been using the MSVIEW software to adjust voltages for absorb and float along with various other settings but am not comfortable with how long the system remains in Absorb and Float. These charge times seem long considering the only loads are a laptop computer and little else. Battery voltage in the morning is approximately 95% charged.

One conceptual issue I'm struggling with is the AH capacity of my bank and how that relates to charging. Two parallel strings should be about 500 AH (2 12V 258AH => 24V bank w/258AH in parallel with an identical string)? Would the charge controller then be charging a 500AH bank?

Considering the size of this bank if the above is correct, seeing charging current of 5A going into the bank would mean I have a charging rate of 1% of capacity?

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: TSMPPT correct charge time settings?
    photon wrote: »
    Considering the size of this bank if the above is correct, seeing charging current of 5A going into the bank would mean I have a charging rate of 1% of capacity?

    Welcome to the forum.
    Yes, 5 amps is a 1% charge rate. But when a battery is near full charge it won't take much current. You have a 840 watt array which should be able to push nearly 30 amps into the battery. That assumes perfect conditions (array tilt, noon, cold weather, discharged battery). Even if the battery was discharged enough to take 30 amps, in the morning when charging first starts you would have few amps because the sun is too low. By the time the sun is high enough to give high current, the batteries are well into absorb charge and the current is tapering off.

    BTW, when your loads are low and the battery is shallow cycled every day (your situation), AGM batteries are a better choice than flooded batteries. You could get by with half your battery capacity.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: TSMPPT correct charge time settings?

    Some observations.

    KD140's have a Voc of 22.1. Put six in series and you get 132.6. If you are in a cold climate the inverter will shut down when the temps cause the panel Voc to rise (total will exceed 150 Volt input limit). Array Vmp would be 106.2, which is well above the 35 Volts needed for a 24 Volt system. This will reduce conversion efficiency. Unless you have a long wire run from the array to the controller you'd be better off with two parallel strings of three panels: Voc 66.3 (cold max 86.19), Vmp 53.1, Imp 15.82.

    Expected peak current for 840 Watt array with MPPT controller: 840 * 0.77 / 24 Volts = 26.95. That is barely 5% for that size battery bank. If your loads are small, why so much battery? You could possibly run half the battery on the same panels and get better over-all performance. My 24 Volt system has 700 Watts of array and 232 Amp hours of battery and runs the whole place, including refrigerator, water pumps, and office equipment.

    The fixed Absorb time length on MS controllers (and some others) is one of their flaws. Outback and MidNite sensibly time the Bulk stage and adjust the Absorb length accordingly. Unfortunately with AGM batteries you can't check the SG and see if the batteries really are charged or not. I suggest you get a battery monitor, use it to determine full charge, and adjust the Absorb time accordingly. Float time can be whatever; it is not a critical number. The longer the panels can maintain Float the better.
  • photon
    photon Registered Users Posts: 2
    Re: TSMPPT correct charge time settings?

    I'm about to install a Battery Monitor.

    With respect to the batt capacity, the anticipated daily load will be 3kwh and by my calculations an AH draw of 138 AH for a 24V bank. If I did these calculations right it should give me about 2 days autonomy.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: TSMPPT correct charge time settings?

    I thought you said your loads were light? :confused: 3kW hours per day isn't "light". That's more than I use here. It's about 125 Amp hours on 24 VDC, so your 500 Amp hour bank would give you a 25% DOD rate with another 25% for Day Two before starting the generator (you always get some charging from panels, even if only an Amp or two).

    Battery monitors are nearly essential for AGM's. Although it's best to install them at the start when the batteries are new and fully charged (the monitor has to know "this is a fully charged battery of 'X' Amp hours" to begin with so it can judge SOC according to Amps in/out afterward).

    So staying with that size battery bank ... What's the phrase I always use? *Forum Chorus* "You need more panels!" :p