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The Relationship between Voltage and Capacity of LFP Battery

Knowing the relevant voltage of the LFP battery helps to set the reasonable end voltages, so the LFP battery could be working in the most effective condition.

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LFP battery is the short name for LiFePO4, also as known Lithium Ferro(Iron) Phosphate Battery. An LFP battery refers to a li-ion battery using lithium iron phosphate as a positive electrode material.

Knowing the relevant voltage of the LFP battery helps to set the reasonable end voltages, so the LFP battery could be working in the most effective and reasonable condition, and offer a safe, stable, and long-lasting power supply.

As the safest lithium technology, the LFP battery has many advantages in energy storage applications.

  • l  Safe
  • l  Super Long Cycle Life
  • l  Light Weight for Easy Handling and Installation
  • l  Suitable for High Temperature
  • l  Environmentally friendly

LFP Battery Cell Voltage

Due to the difference in various raw materials used by each manufacturer(including positive, negative electrode materials, and electrolytes), the performance and voltage of the batteries are not exactly the same. In general conditions, the nominal voltage of the LFP battery cell is 3.2V, the high-end voltage is 3.6V, and the low-end voltage is 2.0V.

The charging voltage of the LFP battery cell is recommended as 3.65V, 14.6V+-0.2V for a 12.8V battery. 3.65V per cell is proven to be a reasonable charging voltage after long-time practical use.

The relevant voltage of the LFP battery is different in different conditions. Mainly in 3 cases, open-circuit voltage (OCV), charging condition, and discharging condition. In applications, the charging and discharging voltage are more useful, involving a reasonable set of end voltages to ensure that the battery works in a good and stable situation.

LFP Battery Voltage In Charging Condition(0.5C, 25℃)

Here, 0.5C means charging at a current of 0.5*(number of capacity)A. For example, a 12.8V100Ah LFP battery is charged at 14.6V with 50A current. SOC means the State Of Charge, which means the percentage of energy remaining.

SOCVoltage
100%14.60V
80%13.90V
60%13.70V
40%13.60V
20%13.40V
5%12.5V
Related Voltage in Charging Condition

The charging curve is shown below:

Charging Voltage Curve

The voltage will be different if charge with a different number of current. The bigger the current, the higher the voltage.

LFP Battery Voltage In Discharge Condition(0.5C, 25℃)

Just like the above condition in CHARGE, here for a 12.8V100Ah battery, it means discharge at 50A.

SOCVoltage
75%12.85V
50%12.75V
20%12.55V
10%12.24V
5%11.90V
Related Voltage in Discharge Condition

Note the related voltage will also be different if the discharge current is different.

For most energy storage applications, a DOD of 80% is reasonable and effective.

Based on the above data, the end voltage can be set at 12.55V, but please note that for smaller currents such as 10A, the discharge depth will still exceed 80% even when the voltage is higher than 12.55V.

And it is recommended to set the overcharge voltage at 15.60V to avoid overcharging the battery.

Notes for Voltage Difference

1.     The voltages of different lithium batteries from different materials are different.

2.     For the same LFP battery, same SOC (for example 20%), the voltage varies depending on the discharge current. The higher the discharge current, the lower the voltage.

3.     The effect of ambient temperature on the battery voltage. The lower the temperature, the lower the voltage.

4.     The impact after cycle use. After a long time of use, the voltage will also be different.

5.     Different manufacturers, the different capacities of LFP batteries, the discharge voltage is slightly different.

OK, hope this post will help you understand more about the LFP battery voltage and SOC/Capacity, then a better use of your precious LFP battery.

12V LFP Battery

12V LFP battery is an easy replacement for 12V lead-acid battery, only required to replace the battery, even no need to change the cables. Now with affordable prices and improved BMS technology, Sunon 12V LFP battery is perfect for energy storage applications such as solar & renewable industry, telecom base station, UPS & backup power, CCTV & security system, etc.

For more information, please click the button below.

Andy
Andy
Hi, I’m Andy. Since the year 2015, I’ve been working in SunOn Battery, a manufacturer in China that makes various batteries for 15 years now. The purpose of this article is to share with you the knowledge related to batteries and energy storage solutions from a Chinese supplier’s perspective.

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3 Responses

  1. I have a bank of 3-48 volt LFP batteries (11kw each) that I bought direct from a distributor in China.
    My problem is with the specs they gave me of 44v-54v.
    The initial states of charge when I received them were between 48.2 and 48.8 volts. 
    When I charged them up, the voltage increase started stalling significantly at around 49.7 volts, and very very slowly worked up to around 51.1 volts and stalled there.
    The batteries have inetgral BMS, so I’m thinking that the BMS may have started curtailing the charge at around 49.7 vols, indicating that I may be around 85% full there.
    Conversely, the batteries cut out at around 44.5 volts, which the BMS may have curtailed at around 15%.

    I ran the batteries through 2 succesive chage/discharge cycles, so this would seem indicative of a 5.2 volt spread between say 15% and 85% charge.

    The stall in charging at 51.1 volts seems to be very low from the data that you’ve published here.
    Is there any other means to derive a reasonable estimate of state of charge for these LFP’s?

    1. Hi Bob,

      The 48V LFP battery is always made up of 15S(15 pcs in series) or 16S.

      But, as your highest voltage is 51.1V while charging, I am still not sure how many pieces of cells are in series.
      LFP battery voltage curve is flat, so it is better to monitor the current if possible.

      Do you have a more detailed specification of your battery?

      By the way, a Victron SHUNT would be a good battery status monitor.
      https://www.victronenergy.com/battery-monitors/smart-battery-shunt

      Andy

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