Lithium battery charge and discharge state analysis and fuel gauge design

1. Introduction of lithium-ion battery

1.1 State-Of-Charge (SOC)

State of charge can be defined as the state of available electrical energy in a battery, usually expressed as a percentage. Because the available electrical energy varies depending on the charge and discharge current, temperature and aging phenomena, the definition of the state of charge is also divided into two types: Absolute State-Of-Charge (ASOC) and Relative State of Charge (Relative State-of-Charge; ASOC) State-Of-Charge; RSOC). Usually the relative state of charge ranges from 0% – 100%, while the battery is 100% when fully charged and 0% when fully discharged. The absolute state of charge is a reference value calculated from the designed fixed capacity value when the battery is manufactured. A brand new fully charged battery has an absolute state of charge of 100%; an aged battery, even fully charged, will never reach 100% under different charge and discharge conditions.

The graph below shows the relationship between voltage and battery capacity at different discharge rates. The higher the discharge rate, the lower the battery capacity. When the temperature is low, the battery capacity will also decrease.  Figure 1. The relationship between voltage and capacity at different discharge rates and temperatures

1.2 Max Charging Voltage

Maximum charge voltage and battery chemistry are related to characteristics. The charging voltage of lithium batteries is usually 4.2V and 4.35V, and the voltage value will be different if the cathode and anode materials are different.

1.3 Fully Charged

When the difference between the battery voltage and the highest charging voltage is less than 100mV, and the charging current is reduced to C/10, the battery can be regarded as fully charged. Battery characteristics vary, as do full charge conditions.

The figure below shows a typical lithium battery charging characteristic curve. The battery is considered fully charged when the battery voltage is equal to the highest charging voltage and the charging current is reduced to C/10. Figure 2. Lithium battery charging characteristic curve

1.4 Mini Discharging Voltage

The minimum discharge voltage can be defined by the cut-off discharge voltage, which is usually the voltage at which the state of charge is 0%. This voltage value is not a fixed value, but varies with load, temperature, degree of aging, or others.

1.5 Fully Discharge

When the battery voltage is less than or equal to the minimum discharge voltage, it can be called full discharge.

1.6 Charge and discharge rate (C-Rate)

The charge-discharge rate is a representation of the charge-discharge current relative to the battery capacity. For example, after an hour of discharge at 1C, ideally, the battery will be fully discharged. Different charge and discharge rates will result in different usable capacities. Generally, the higher the charge-discharge rate, the smaller the usable capacity.

1.7 Cycle life

The number of cycles is the number of times a battery has undergone a complete charge and discharge, which can be estimated from the actual discharge capacity and the design capacity. Every time the accumulated discharge capacity is equal to the design capacity, the cycle number is once. Usually after 500 charge-discharge cycles, the capacity of a fully charged battery will drop by about 10% to 20%. Figure 3. Relationship between cycle times and battery capacity

1.8 Self-Discharge

The self-discharge of all batteries increases with temperature. Self-discharge is basically not a manufacturing flaw, but a characteristic of the battery itself. However, improper handling during the manufacturing process can also lead to an increase in self-discharge. Typically, the self-discharge rate doubles for every 10°C increase in battery temperature. The monthly self-discharge of lithium-ion batteries is about 1~2%, while the monthly self-discharge of various nickel-based batteries is 10~15%. Figure 4. Performance of lithium battery self-discharge rate at different temperatures