A battery generates electric energy by oxidation and reduction reactions and is widely used in various ways. For example, a battery is applied to portable devices such as cellular phones, laptops, digital cameras, video cameras, tablet computers, and electric tools; electric-driven apparatuses such as electric bikes, motor cycles, electric vehicles, hybrid vehicles, electric ships, and electric airplanes; power storage devices used for storing power generated by new regeneration energy or surplus energy; uninterrupted power supplies for stably supplying power to various information communication devices such as server computers and base stations for communication, and so on.
A battery includes three basic components: an anode containing material which is oxidized while emitting electrons during discharge, a cathode containing material which is reduced while accepting electrons during discharge, and an electrolyte allowing the transfer of operating ions between the anode and the cathode. Batteries may be classified into primary batteries which are not reusable after discharge, and secondary batteries which allow repeated charge and discharge since their electrochemical reaction is at least partially reversible.
The secondary batteries include lead-acid batteries, nickel-cadmium batteries, nickel-zinc batteries, nickel-iron batteries, silver oxide batteries, nickel metal hydride batteries, zinc-manganese oxide batteries, zinc-bromide batteries, metal-air batteries, lithium secondary batteries and so on, as well known in the art. Among them, lithium secondary batteries are drawing the most attention due to their high energy density, high battery voltage and long life cycle in comparison to other secondary batteries.
Meanwhile, a depth of discharge (DoD) of a secondary battery relatively shows a discharged capacity of the secondary battery in a range of 0 to 1 based on an original capacity of the secondary battery.
Here, the original capacity means a value obtained by integrating an amount of current flowing out of a secondary battery in a state of Beginning Of Life (BOL) while discharging from a maximum charging voltage to a final discharge voltage.
For example, if a secondary battery has an original capacity of 1000 mAh and an integrated value of discharge current discharging from the secondary battery after the secondary battery is fully charged is 700 mAh, the depth of discharge (DoD) will be calculated to be 0.7.
Chemical substances (for example, lithium ions) participating in electrochemical reactions of a secondary battery irreversibly disappear as charging and discharging cycles increase. The lost chemical substances cause a capacity degradation of the secondary battery, and if the secondary battery suffers from capacity degradation, the voltage of the secondary battery reaches a final discharge voltage before the depth of discharge (DoD) reaches 1. Therefore, the depth of discharge (DoD) when reaching the final discharge voltage decreases in proportion to the capacity degradation.
For example, if the secondary battery has a final discharge voltage of 3.0V and the capacity degradation of the secondary battery has progressed by 20%, when the depth of discharge (DoD) of the secondary battery increases to 0.80, the voltage of the secondary battery decreases to 3.0V which is a final discharge voltage.
The capacity degradation of the secondary battery is a parameter required for accurately calculating a state of charge (SOC) of the secondary battery. The SOC is a parameter relatively representing a presently remaining capacity of a secondary battery in the range of 0 to 1 based on the entire capacity of the secondary battery on which capacity degradation is reflected.
The SOC of the secondary battery may be calculated by using a depth of discharge (DoD) of the secondary battery as in Equation 1 below.SOC=(DODmax−DoD)/DODmax DoDmax=1−ΔCapa   Equation 1
Here, SOC is a parameter representing present state of charge of the secondary battery, DoDmax is a parameter representing a depth of discharge (DoD) when the secondary battery reaches a final discharge voltage, DoD is a parameter representing a present depth of discharge (DoD), and ΔCapa is a parameter representing capacity degradation of the secondary battery in the range of 0 to 1.
In Equation 1, since DoD is a measurable parameter, the SOC of the secondary battery is resultantly determined according to capacity degradation of the secondary battery, expressed by ΔCapa.
The capacity degradation of the secondary battery may be accurately calculated by integrating an amount of current drawn from the secondary battery while the secondary battery charged to a maximum charging voltage is fully discharged to a final discharge voltage, and then comparing the integrated current amount with the original capacity.
However, in a circumstance where the secondary battery is actually used, a full discharge event rarely occurs at which the capacity degradation of the secondary battery can be accurately calculated. Therefore, in the related art, a method for indirectly estimating capacity degradation of a secondary battery is used.
For example, since an internal resistance of a secondary battery has a relation with capacity degradation, the internal resistance of the secondary battery may be estimated by sampling voltage and current of the secondary battery, and the capacity degradation of the secondary battery may be estimated according to the estimated internal resistance.
However, the capacity degradation of the secondary battery may not be exactly measured in this way, and furthermore the accuracy in estimation of the capacity degradation deteriorates according to a temperature change of a secondary battery.