How to know Lithium battery charge percentage

PluckaPlucka Registered Users Posts: 61 ✭✭
I have 360amp/hr lithium battery and I run everything on 240 volts .After toast and a cup of tea in the morning the battery is 12.6 to 12.9 volts .What is the percentage of discharge? It is winter here with lots of cloud and the battery only gets up 13.3 volts after running fridge, freezer etc during the day .In the summer the solar input shuts down around lunch time as the battery was full @ 13.6 volts


  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    What is the exact brand/model/chemistry of your Li Ion pack (4x cells in series?)... For example LiFePO4 (lithium iron phosphate) batteries are commonly used for home and vehicle battery banks--More stable chemically and less likely to fail with fire--But they charge at a substantially lower voltage than many other Li Ion chemistries.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,067 ✭✭✭✭✭
    Li batteries have a nice simple voltage curve / state of charge.  It gets wild at the upper & lower ends, but you should never go there.   Your battery mfg should have a graph of what Voltage:SoC is.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • PluckaPlucka Registered Users Posts: 61 ✭✭
    My battery is LiFePO4  4 cells in series and I'm just after someone elses opinion  if I go below 12 volts .
  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    edited June 2018 #5
    The "generic" LiFePO4 cell should not be discharged below 2.8 volts per cell (4x2.8=11.2 volts)... If you look at the curves in my link above, there is not much capacity left below ~3 volts per cell (12.0 volts).

    Generally, the longest life for Li Ion chemistries is to run them between 20% and 80% state of charge. Running high or low voltages (high or low State of Charge) generally gives you a shorter life (high SoC gives both a shorter cycle and aging life).

    And remember that voltages are "per cell"... If you have once cell running below ~2.8 volts and the others are at 3.0 volts (=11.8 volt battery bank voltage)--It is that low cell that is "at risk".

    If you have a BMS (battery management system) that can measure/control per cell voltages/state of charge--That will help. Otherwise, you should measure (and log) the cell voltages (like once a month) to insure the batteries are "balanced" (there is a whole bunch of discussions about "top" and "bottom" balancing (full charge vs is discharged balancing).

    The closer you run to the "limits" (3.6 volts 100% and 2.8 volts 0%), the "more" cell balancing matters.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PluckaPlucka Registered Users Posts: 61 ✭✭
    edited June 2018 #6
    I have a battery managemant system. My new 3000 watt inverter arrived  and is performing great put out 242 volts. The cables that came with it aren't long enough so I had to join them but I'll put new ones on tomorrow. Thanks to those that responded to my question.
  • karrakkarrak Solar Expert Posts: 314 ✭✭✭✭
    Due to the very flat charge/discharge curves for LiFePO4 batteries it is not practical to use voltage to accurately determine the SOC. On the other hand due to their very high coulomb/current efficiency of around 99.5% you can get very accurate SOC readings by keeping track of the current going into and out of the battery. Victron make the BMV700 battery monitor or you can get some cheaper units on ebay . Things to look for are
    • Automatic reset to 100% when the battery voltage reaches the full voltage and the current has tapered off.
    • Ability to adjust the reading to take into account the battery efficiency.
    • Ability to calibrate the device
    • Devices using a current shunt will be more accurate than one that use a magnetic current sensor

    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
    Homemade BMS

  • PluckaPlucka Registered Users Posts: 61 ✭✭
  • jonrjonr Solar Expert Posts: 1,187 ✭✭✭✭
    The only reasonable SOC you can get from voltage is at the extremes - say > 90% and < 10%.  But best life is obtained when you avoid the extremes.
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