Percentages and voltage of lifepo4

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nschizzano
nschizzano Registered Users, Users Awaiting Email Confirmation Posts: 24 ✭✭
What voltage is at what percent on a lifepo4? And what's the lowest I should never go on my new litime lifepo4 12v 200ah 100bms. Thanks

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  • Horsefly
    Horsefly Registered Users Posts: 472 ✭✭✭✭
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    LiFePO4 charge and discharge curves are very flat in roughly 80% of the SOC range, like between 10% full and 90% full.  Using voltage as a measure of state of charge is of no use.  You really need to coulombs (Amp hours) going in and out of the battery. Most good battery meters are equipped with a shunt that does this coulomb counting.

    Generally, you don't want to go below about 3.0V per cell. For your 4-cell 12V battery, that says don't go below around 12V. At 3V per cell you are probably below 10% state of charge. Your BMS will probably have a cutoff at 2.55V or 2.6V per cell, which would cut off all discharge if any one cell gets that low. The BMS is really a final line of defense, so you don't want to take a chance that one cell drifts lower than the others and the battery cuts out.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • AzSun
    AzSun Registered Users Posts: 13 ✭✭
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    The LIFEPO4 charge/discharge curve is pretty flat so percentages are only accurate when they are near full charge or discharge. If you were able to measure the voltage of each of the four cells in your battery, the
    best thing is to not let any cell go below 3 volts. The Li Time is sealed so you are not able to do that. The next best way is to use battery terminal voltage. At 12.00 volts, there is only around 10 percent remaining. At 13.60 volts, or approximately 3.4 volts per cell it is above 90 percent charged. Interestingly,
    50% charge is not necessarily 12.8 volts. The percentage to voltage relationship is not very linear during most of the charge discharge cycle except at the ends. Your battery BMS unit prevents overcharging or over discharging. It also balances out the cell voltages. It is not unusual for the cells to be out of balance when they are new. Due to this imbalance, it takes a few to many cycles for the battery to perform to specification. You will likely not be able to get the full 200AH for a while. It may actually shut down internally. Information on how to reset it is available online.
  • nschizzano
    nschizzano Registered Users, Users Awaiting Email Confirmation Posts: 24 ✭✭
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    Horsefly said:
    LiFePO4 charge and discharge curves are very flat in roughly 80% of the SOC range, like between 10% full and 90% full.  Using voltage as a measure of state of charge is of no use.  You really need to coulombs (Amp hours) going in and out of the battery. Most good battery meters are equipped with a shunt that does this coulomb counting.

    Generally, you don't want to go below about 3.0V per cell. For your 4-cell 12V battery, that says don't go below around 12V. At 3V per cell you are probably below 10% state of charge. Your BMS will probably have a cutoff at 2.55V or 2.6V per cell, which would cut off all discharge if any one cell gets that low. The BMS is really a final line of defense, so you don't want to take a chance that one cell drifts lower than the others and the battery cuts out.
    Okay so don't go below 12v. I also just asked LITIME Service. So the battery BMS will just cut me off when it gets too low anyways? But it's not a good idea to continuously let it do that? 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    BMS "cutoffs" are really intended to stop "catastrophic" battery damage (and risk of fire). Your system should not use the BMS "cufoffs" (voltage/temperature/current/SoC/etc.) for "normal operation".

    There are other secondary issues that can cause "problems"... For example, "turning off the battery bank" can damage/ruin your DC connected devices (solar battery chargers, electronics, etc.). The battery bank is a critical component in regulating your DC Bus Voltage (I.e., 10.5-16.0 volts or so--depending on battery chemistry, ratings, etc.). If the BMS (or you) turn off the battery, the Battery Bus Voltage can easily exceed the ~15.0 VDC not to exceed voltage for many "12 volt" devices (radios, AC inverters, and such).

    The BMS is the "circuit breaker" for protecting the battery bank. BMS/Circuit Breakers/Fuses as a "last resort" for system/wiring/battery safety. Not for normal operation(s).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset