The present invention relates to a state-of-charge measuring method, and more particularly, to a state-of-charge measuring method using multilevel Peukert's equation for battery-powered products such as electric vehicles.
Accurate battery state-of-charge information is very important for the driver of an electric vehicle since, by knowing the exact amount of energy available from the batteries, the driver of the vehicle can travel without the fear of running out of energy before reaching the destination.
The state-of-charge of a battery is a function of used capacity and available energy. The used capacity can be easily calculated by simply multiplying the average discharge current and total time. On the other hand, determining the available energy of a battery is not straightforward because the available energy depends on the magnitude of the average discharge current. Low average discharge current increases the available energy, and high average discharge current decreases the available energy. And therefore, to calculate the available energy accurately, a sophisticated algorithm such as Peukert's equation must be used. In other words, to measure the state-of-charge of a battery accurately, the usage pattern such as driving profile must be taken into account.
There exist many methods for measuring battery state-of-charge. They include open-circuit voltage check, measurement of electrolyte's specific gravity, measurement of current under load and current accumulation (ampere-hour or Ah) method to name a few. However, all of the above methods do not take into account the usage pattern, and therefore, are susceptible to a large degree of error.
As mentioned earlier, one method that takes into account the usage pattern in determining the state-of-charge of a battery is Peukert's equation. Peukert's equation is used in conjunction with Ah method to reduce error. By using Peukert's equation, the available energy of a battery can be calculated with good accuracy, and consequently, the state-of-charge can be determined more precisely.
In a conventional method using Peukert's equation, the usage range of a battery is established and the maximum (I.sub.H) and minimum currents (I.sub.L) are set according to the battery's usage range, and Peukert's constants (n and K) are obtained by using the maximum and minimum currents. Peukert's equation calculates the available capacity (Ah.sub.-- available) of a battery as follows: EQU Ah.sub.-- available=KI.sub.avg .sup.(1-n)
where n and K are Peukert's constants and I.sub.avg is the average discharge current. The constants n and K are found as follows: ##EQU1## and EQU K=I.sub.H.sup.n t.sub.H =I.sub.L.sup.n t.sub.L
where I.sub.H and I.sub.L represent upper and lower current values, respectively, of the current range in which a battery is normally operated and t.sub.H and t.sub.L are discharge times at I.sub.H and I.sub.L, respectively. Using Peukert's equation, the state-of-charge of a battery is calculated as illustrated in FIG. 1. Referring to FIG. 1, in step 101, discharge current I(t) is read. In step 102, the average discharge current (I.sub.avg ) is calculated by using equation (1). ##EQU2##
In step 103, the capacity drawn from the battery (Ah.sub.-- used) is calculated by using equation (2) and in step 104, the available capacity (Ah available) is calculated by using equation (3) EQU Ah.sub.-- used=I.sub.avg .times.t (2) EQU Ah.sub.-- available=KI.sub.avg .sup.(1-n) (3)
Finally in step 105, the state-of-charge is calculated by using equation (4) and the process returns to step 101. ##EQU3##
Peukert's equation is a preferred method for applications having widely fluctuating loads such as electric vehicles since it can determine the available capacity under any discharge conditions. However, since the conventional Peukert's equation uses a maximum current I.sub.H and a minimum current I.sub.L as only references, its accuracy deteriorates when the average discharge current is faraway from either I.sub.H or I.sub.L as shown in FIG. 3. Therefore, to improve the accuracy over the entire range, it is necessary to introduce another reference point, i.e., an intermediate I.sub.M, between the maximum current I.sub.H and the minimum current I.sub.L as shown in FIG. 3.