False charge level after high inverter load?

mbladen
mbladen Registered Users Posts: 20 ✭✭
I have been running some unscientific tests on my 3000Va inverter which is connected to a 24v 360ah Battery bank (2x4 12/90ah) whilst watching the log screen (copy attached) i noticed that whenever a high load >2.2kw was applied to the inverter the charge level would drop quite fast, on the load being switched off (100w-400w standard load) the battery would increase in charge.

after running the load for  32 minutes within the hour of testing I was down to 35% charge (@Time 13:26), I did not feel comfortable going any lower as I could not estimate the real charge level (novice) as soon as I switched on the mains within a minute the charge level was at 65% 

I am only using my system for back-up power and so am happy with a DoD of 50% as I don't envisage many cycles but which figure do I use to estimate and accurate 50% charge as it seems that if there has been a high load present then it would be falsely low however if it was running the standard load then it could be accurate.

I also noticed the Amp rating exceeding the 100amp maximum that I thought the system would use however I guess this is due to the drop in battery voltage requiring additional current to supply the same power? the cables got a little warm but nothing that I thought I should worry about (famous last words)

Any advice or comments would be greatly appreciated as I stumble down this unfamiliar road of ON/OFF Grid tied/untied inverter living :-)

take care

Marc 



here is a xls in case of difficulties with the google link 

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited January 2017 #2
    The 2.2Kw load is pretty high for the battery capacity, the battery voltage will sag and the current rise, the capacity reading it seems, would be based on battery voltage, as soon as the load is disconnected the battery voltage recovers, hence the rise in capacity post load. Think of it as presure in a water line, open a valve, the pressure drops, close the valve, the pressure rises again, albeit if you depress the battery too far,  it may not recover to full voltage.  Factors which can improve this is having a larger capacity battery, and increasing the size of the battery to inverter conductors, the reason they get hot is because of the volt drop across them, wasting energy as heat, reflecting in the voltage sag at the inverter.

    Depending on on the distance between the two, the further you go, the larger the cross section, or gauge, conductors you will require, to reduce the volt drop. As current is inversely proportional to voltage, you will notice it rise in current as the voltage drops, there is a direct correlation between the two.

    What gauge, or square mm. are used?. They look to be around 16mm^2 from the pictures in your other post, 50mm^2 or larger would be better suited.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    4 strings of 2 - 12 volt 90 ah?  for a 360 ah 24 volt battery bank. Correct?

    Load is 2200 watts at 24 volts or 2200/24=92 amps. Correct?

    Batteries are typically rated at a load of 1/20th of their capacity. You are drawing at greater than 1/5th of their capacity. This is a huge load for your battery bank. As McGivor said you will have voltage sag, but you also have diminished capacity as the battery rating at a draw of 1/5th is likely 70-80% of the 360ah that they are rated at for a 1/20th load. In addition if this is the output of the inverter, it does not include the energy loss of the inverter another 10-15% load!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.