Inverter shutting off under load

PluckaPlucka Registered Users Posts: 69 ✭✭
At 12.5 volts my 3000 watt inverter shuts down when I switch my kettle [900 watt ] on -same with the toaster but will run the fridge or freezer and continue to a lower voltage on my lithium batteries.Is this normal or is there a problem ?

Comments

  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    edited June 2018 #2
    A no load voltage of ~12.5V for a 4S LFP (LiFePO4) battery equates to an SOC of ~11%[edit:original 5% was a mistake] . At this low SOC the output voltage will drop very quickly under a high load.

    Is it the Inverter or the BMS shutting the inverter down? Could you give me more details about your BMS and the LV cutoff for the inverter

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • PhotowhitPhotowhit Solar Expert Posts: 4,800 ✭✭✭✭
    We have often noted that 3000 watt inverter or anything over 1500 watts is difficult since you must use very large cables due to the voltage drop on smaller cables. How far is your inverter from the battery bank and what size cables are you using?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • PluckaPlucka Registered Users Posts: 69 ✭✭
    Under light load I've seen the voltage at 11.9 volts.I think it's the bms that is doing the shut down.The whole 360 amp/hr lithium setup is very compact and the cables don't get warm.I don't know the cutoff for the inverter as the manual gives no figures for anything
  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    I hope this graph is of some use.


    11.9V at a load of 0.05C (~200W for your system) for a 4s 12V LFP battery equates to 2975mV/cell which equates to an SOC ~3%, a voltage of 12.5V (3125mV/cell) with a load of ~900W (~0.2C) equates to an SOC of ~11% (I made a mistake in my first post). Any variation it the internal resistance/impedance of the individual cells will result in a larger voltage difference between the cells so with no load we can get a situation where all the cells are at the same voltage but at a load of 0.5C the difference between the cells could be 0.050V or even higher. In this case the average cell voltage could be ~3.1V but the lowest cell could be ~2.8V or lower. Any mismatch in the balance of the cells will also add to this voltage difference.

    It is a good idea to keep the cell voltage greater than 3.2V (12.8V for a 12V battery) or SOC greater than 20% to extend the life of the battery and use the last 20% as an reserve. If you are getting down to these low voltages frequently I would look at increasing the number of solar panels you have on your system.

    Simon

    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • PluckaPlucka Registered Users Posts: 69 ✭✭
    Thank you Simon,I'm looking at getting another 200 watt panel to put on the side of the van so it can be adjusted to the sun rather than having them flat on the roof.
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