When referring to batteries, especially in the case of Lithium Ferro Phosphate (LiFePO4), the ingrained idea behind many people’s minds is to look at voltage. While this is an acceptable metric in some instances, the concept falls apart when referring to large enough energy storage systems.
What is SoC and voltage?
SoC or State of Charge is an indication of how “full” a battery bank is. This value is almost always given as a percentage. Therefore, to put it simply a battery that is at 100% SoC is a full battery, capable of providing its entire rated capacity to whatever electrical load it needs to provide it to.
Voltage on the other had can be thought of as a measurement of “electrical pressure” within a battery. Almost all battery systems have a positive (+) and negative (-) terminal on them. When a battery has a certain voltage, it means that there is that much “electrical pressure” between the two terminals. A given electrical load can be powered by this “electrical pressure” and conceptually, this is the DC current that is created when a battery is used to do this.
Charging and discharging
When comparing the above explanations of SoC and Voltage, we can see that both may be an accurate representation of the relative “fullness” of a given battery. However, there are important things to consider when analysing a battery under these two conditions.
SoC is a metric calculated by the BMS (Battery Management System) of a battery. It does this based on several factors, one of which is voltage. However, there are other factors that the BMS must consider as inputs when doing this.
The capacity of a battery is often measured in Ah (Amp hours) or Wh (Watt hours). A well designed BMS uses a shunt in addition to the voltage measurement to measure the number of current (Ah) that flows into the pack. With an understanding of time that the current flows, a BMS is also able to use this information to determine the Ah or Wh that is present in the battery. With knowledge of the nominal voltage of the battery, this is a much more accurate measure of capacity as one can understand the power that has been put into the battery, as opposed to a simple voltage measurement.
The voltage of LiFePO4 batteries can change depending on the load that is applied as well as how long it has remained at a steady-state rest condition. Two fully charged 16 cell LiFePO4 battery can be found to have almost 0.5V difference if the total current drawn from the pack differs by around 50A.
In conclusion,
We can see that SoC is a comparatively better metric in comparison to Voltage when identifying how full a given battery is. It is important to take this into consideration with system design, battery identification and usage.
REVOV batteries by default come with a BMS capable of accurately determining SoC as well as relaying this information to leading brand inverters. To learn more about REVOV products click here
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