So the OCV is only suited for lower SOC accuracy request systems like a vehicle’s lead acid battery or uninterruptible power supply.įigure 6. The figure below shows that the SOC variation is still big in the discharge condition under different loads. The battery voltage under charge and discharge conditions It means SOC estimation by static voltage measurement should be done with separated charge and discharge look-up table.įigure 5. The following figure shows that the SOC error is huge when battery is under charge and discharge conditions, because the I-R drop is caused by battery’s impedance and charge/discharge current. As a result, SOC error is big if it is only estimated by OCV. The SOC estimation cannot rely on the look up table. The battery voltage curve varies according to different loads, temperatures, ages, etc, so a static OCV table can’t represent the SOC. The general definition of open circuit voltage is only if battery relaxed for approximately more than 30 minutes. The open circuit voltage algorithm of fuel gauge is easy to implement by mapping open circuit voltage to the SOC look-up table, and using several points to fit the OCV curve. The other method is dynamic voltage-based algorithm designed by RICHTEK. There are two classic methods to do the SOC estimation which are Open Circuit Voltage (OCV) and Coulomb Counter, respectively.
#Coulomb counter chip full#
The basic function of fuel gauge is to monitor the voltage, charge/discharge current and battery temperature, and to estimate the battery’s SOC and Full Charge Capacity (FCC) of battery. The fuel gauge is responsible to estimate the capacity of battery in the domain of battery management. Capacity retention at different temperaturesīattery management is part of power measurement. A Li-ion battery has a self-discharge of 1~2% per month, Nickel-based has 10~15% per month.įigure 4. The self-discharge rate is typically doubled with every 10☌ increase. However, poor manufacturing practices and improper handling may increase this effect. Self-discharge is basically not a manufacturing defect but a battery characteristic. The self-discharge of all battery chemistries increases at higher temperature. Usually, the full charged capacity reduce 10%~20% after 500 cycles.įigure 3. When the accumulated discharge capacity is equal to design capacity, the Cycle Count increases. It’s estimated by the discharge capacity and design capacity. The Cycle Count is a reference number of how many charging/discharging cycles a battery has experienced as aging. Normally, the bigger the C-rate, the smaller the available capacity. The available capacity changes per different C-rates.
For example, when discharging with 1C, the battery will be fully discharged in 1 hour ideally. The C-Rate is the expression of Charging / Discharging Current which is normalized with battery capacity. The voltage is not fixed and will be charged according to different loads, temperatures, aging effects or user requirements.Ī battery is considered fully discharged when battery voltage is equal to or less than Mini Discharging Voltage. Usually, the Mini Discharging Voltage represents the voltage of SOC equal to 0%. The Mini Discharging Voltage could also be defined as terminal voltage. The charging curve of a typical lithium battery When the battery voltage is equal to Max Charge Voltage and charging current is less than C/10, the battery is considered fully charged.įigure 2. The diagram below shows the charging curve of a typical lithium battery. The condition may change according to different battery characteristics The battery is considered fully charged when the difference between battery voltage and charging voltage is within 100mV and charging current is less than C/10. It could be different if the anode/cathode material changes Typically, the Charging Voltages of a lithium-ion battery are 4.2V and 4.35V. The Max Charging Voltage is decided by the chemistry component and characteristic of the battery. Capacity at different C-Rates and temperatures Lower temperatures result in lower output capacity.įigure 1. The higher C-rate loading will have lower output capacity. The following diagram is an example of Voltage vs. A fully charged new battery will have 100% ASOC, but a fully charged aging battery could be less than 100% because of different charge/discharge conditions. The ASOC is a reference calculated by Design Capacity which is a fixed capacity from when the battery is manufactured. Typically, the range of RSOC is from 0% to 100%, a fully charged battery’s RSOC is always 100% and a fully discharged battery has 0% RSOC. Because the available energy change depends on different charging/discharging currents, temperatures and aging effects, the SOC could be defined more clearly as ASOC (Absolute State-Of-Charge) and RSOC (Relative State-Of-Charge). SOC is defined as the status of available energy in the battery and usually expressed as percentages.
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