The present invention relates to a method for predicting remaining charge of a battery, such as a rechargeable battery of a portable electronic device.
In general, chargeable lithium ion batteries are widely used for portable electronic devices, such as notebook personal computers. The lithium ion batteries have advantages such that the operational cost of portable electronic devices can be reduced and the capacity of the current which is instantaneously dischargeable is large. Normally, a machine which has a rechargeable battery so called secondary battery, such as a lithium ion battery, installed therein incorporates a charging circuit which is to be connected to an external power supply to charge the rechargeable battery. To meet the demands for higher performance and size reduction, recent portable devices require a compact charging circuit which can quickly charge a rechargeable battery to a full level. General portable electronic devices include a capability for predicting the remaining charge of a rechargeable battery, when they are in use, in order to avoid problems with data loss by notifying users of the consumption states of the batteries. The prediction of the remaining battery charge should be carried out accurately.
As shown in FIG. 1, an ordinary portable electronic device 101, such as a notebook personal computer (PC); is connected to a battery pack 102 which has a plurality of built-in rechargeable batteries (e.g., lithium ion batteries) 102a and 102b and operates on power from each rechargeable battery 102a or 102b. The portable device 101 is also operable on power which is supplied from an external power supply such as an AC adapter 103.
A power supply unit for the portable device 101 will be discussed below. The portable device 101 includes a power-supply microcomputer 104, a charger 105, a selection circuit 108, first and second DC—DC converters 109 and 110 and a Low Drop Out regulator (LDO) 111 as a switching regulator.
The charger 105, which is connected to the battery pack 102 and the AC adapter 103, supplies a charge voltage and charge current to the rechargeable batteries 102a and 102b in accordance with a control signal from the power-supply microcomputer 104 to charge the rechargeable batteries 102a and 102b with a constant voltage and a constant current.
The selection circuit 108 selects at least one of the battery pack 102 (rechargeable batteries 102a and 102b) and the AC adapter 103. The input voltage from the selected power supply is supplied to the first and second DC—DC converters 109 and 110 and the LDO 111.
The first DC—DC converter 109 generates a supply voltage to be supplied to a CPU (not shown) from the input voltage. The second DC—DC converter 110 generates a supply voltage to be supplied to peripheral circuits (not shown) from the input voltage. The LDO 111 generates a supply voltage for generating a clock signal (not shown) from the input voltage.
The portable device 101 has a remaining-charge predicting capability for predicting the remaining charge of the battery and notifying a user of the predicted remaining charge.
The remaining-charge predicting capability will be discussed below. Generally speaking, lithium ion batteries are susceptible to overdischarging. If a user erroneously overdischarges a lithium ion battery, therefore, the performance of the lithium ion battery may not be recovered if it is charged. To prevent such overdischarging, the battery pack 102 incorporates a protection circuit 112 which detects when the voltage of one of the rechargeable batteries 102a and 102b drops below a specified voltage and stops further discharging. When the protection circuit 112 functions, the supply of the power from the rechargeable battery 102a or 102b to the portable device 101 is stopped and the portable device 101 stops operating. At this time, the portable device 101 such as a notebook PC, may suffer loss of data which is being processed. To avoid such a problem, the portable device 101 predicts the remaining charge of each of the rechargeable battery 102a and 102b and notifies the user of the consumption state of the rechargeable battery.
The prediction of the remaining battery charge is executed in consideration of various characteristics of the rechargeable batteries 102a and 102b (lithium ion batteries) incorporated in the battery pack 102. The following will explain the characteristics of an ordinary lithium ion battery.
FIG. 2A shows a change in discharge characteristic caused by an increase in the number of times a battery set (rechargeable battery) comprising three cells is used (the number of charges/discharges which will be hereinafter called “cycle number”). The vertical scale shows the discharge voltage and the horizontal scale shows the discharge time. A curve A indicates the discharge characteristic with the cycle number being 1 (initial state), and curves B, C and D respectively indicate the discharge characteristics with the cycle number being about 250, about 400 and about 500. A curve E shows the discharge characteristic with the cycle number being about 650. It is apparent from FIG. 2A that as the cycle number increases, the dischargeable capacity of the rechargeable battery decreases, so that the discharge time (usable time) becomes shorter. This phenomenon is called the “cycle degradation characteristic”.
In FIG. 2B, the horizontal scale of FIG. 2A has been normalized. The horizontal scale indicates the discharge capacity and shows the end time at 100% discharge. FIG. 2B shows that the rate of voltage reduction of the rechargeable battery is nearly constant irrespective of the cycle number.
The cycle life characteristic of the rechargeable battery will be discussed below.
FIG. 3 shows the relationship between the cycle number and the discharge capacity which have been measured for three kinds of rechargeable batteries. The discharge capacity of the rechargeable battery decreases as the cycle number increases. For example, a curve F indicates that the discharge capacity when the cycle number is 600 (the capacity at the time of full charge) has dropped to about 30 to 40% of the maximum capacity.
The following will discuss the degradation characteristic of the rechargeable battery which varies according to the environment of use.
A curve H in FIG. 4 shows the characteristic of the rechargeable battery that has been left out for one month at 45° C., and a curve G shows the characteristic of the rechargeable battery before it has been subjected to the foregoing treatment. The discharge time (use time) of the rechargeable battery varies also depending on the temperature at which it has been used.
FIG. 5 is an explanatory diagram showing the relationship between the discharge power and dischargeable capacity at different temperatures of use for two kinds of rechargeable batteries. Curves I to K respectively show the characteristics when one type of rechargeable battery is used at 5° C., 25° C. and 45° C. Curves L to N respectively show the characteristics when the other rechargeable battery is used at 5° C., 25° C. and 45° C.
The rechargeable battery that is indicated by the curve I for use temperature of 5° C. can be used for about 2.8 hours with a discharge power of 10 W. The rechargeable battery of the same kind that is indicated by the curve K for use temperature of 45° C. can be used for about 3.2 hours with a discharge power of 10 W. The rechargeable battery of the other kind that is indicated by the curve L for use temperature of 5° C. can be used for about 2.8 hours with a discharge power of 10 W. The rechargeable battery of the same kind that is indicated by the curve N for use temperature of 45° C. can be used for about 3.1 hours with a discharge power of 10 W. Apparently, the dischargeable capacity of the rechargeable battery varies depending on the type, the use temperature and the discharge power.
The prediction of the remaining charge of a rechargeable battery has been carried out conventionally in consideration of the aforementioned various characteristics. The remaining charge predicting methods include a method for predicting the remaining charge based on, for example, the battery voltage of the rechargeable battery and a method for predicting the remaining charge based on the integrated values of the charge current and discharge current of the rechargeable battery.
FIG. 6 is a schematic diagram of a portable device 121 and a battery pack 122 according to a first prior art system that is equipped with a remaining charge predicting function.
The portable device 121 is, for example, a notebook PC. The portable device 121 has a built-in battery pack 122 called a smart battery or an intelligent battery.
The battery pack 122 includes plural (three) rechargeable batteries 122a to 122c, a protection circuit 123, a discharge control switch 124, a charge control switch 125, a remaining charge meter 126 as a remaining charge predicting device, an electrically erasable and programmable read only memory (EEPROM) 127 and a first sense resistor 128. FIG. 6 shows only a part of the portable device 121 that actually includes a second sense resistor 129, a charger 130 and a microcomputer 131, for example, a keyboard, which is one type of microcomputer.
The rechargeable batteries 122a to 122c, each of which is, for example, a lithium ion battery, are connected in series to one another to form a battery set. The positive terminal of the rechargeable battery 122a is connected to the positive terminal, t1, of the battery pack 122 via the discharge control switch 124, the charge control switch 125 and the first sense resistor 128, and the negative terminal of the rechargeable battery 122c is connected to the negative terminal, t2, of the battery pack 122. The discharge control switch 124 and the charge control switch 125 are formed by first and second P channel MOS transistors. The source of the discharge control switch 124 is connected to the positive terminal of the rechargeable battery 122a. The drains of both switches 124 and 125 are connected together. The source of the charge control switch 125 is connected to the positive terminal t1 of the battery pack 122 via the first sense resistor 128. The transistors of both switches 124 and 125 are connected in such a way that the back gates constitute a forward-biased diode with respect to the charge current and the discharge current.
The protection circuit 123 includes an overcharge preventing circuit and overdischarge preventing circuit (neither shown). The protection circuit 123 detects the terminal voltages (cell voltages) of the rechargeable batteries 122a to 122c and turns off the discharge control switch 124 to inhibit discharging when at least one of the cell voltages decreases to or below the specified voltage or reaches an overdischarge state. When at least one of the cell voltages rises above the specified voltage or reaches an overcharge state, on the other hand, the protection circuit 123 turns off the charge control switch 125 to inhibit charging.
At the time of charging, the charge current is supplied to the rechargeable batteries 122a to 122c via the charge control switch 125 which has been turned on and the discharge control switch 124. The charger 130 of the portable device 121 is connected to an AC adapter 132 connected to an external power supply. The charger 130 controls the charge current based on the value of the current that flows across the second sense resistor 129.
At the time of discharging, each of the rechargeable batteries 122a to 122c supplies the discharge current to the portable device 121 via the discharge control switch 124 which has been turned on and the charge control switch 125.
The remaining charge meter 126 includes a microcomputer (not shown) which measures the charge current/discharge current that flows across the first sense resistor 128 and predicts the remaining charge based on the integral value of that measured current and each cell voltage detected by the protection circuit 123. The remaining charge meter 126 stores the predicted remaining charge in the EEPROM 127 and supplies the predicted remaining charge to the keyboard microcomputer 131 provided in the portable device 121. When receiving the predicted value for the remaining charge from the remaining charge meter 126, the keyboard microcomputer 131 displays the remaining battery charge on an unillustrated display unit.
FIG. 7 shows a second prior art system equipped with a remaining charge predicting function.
A portable device 141 is, for example, a notebook PC. The portable device 141 has a built-in battery pack 142. According to the second prior art, the battery pack 142 differs from the battery pack in FIG. 6 in that it has a integrating current meter 143 as a remaining charge predicting device.
The integrating current meter 143 measures the charge current/discharge current that flows across the first sense resistor 128 and supplies a current integrated value to a power management microcomputer 144 of the portable device 141. Then, the power management microcomputer 144 calculates a remaining charge predicted value based on the current integrated value output from the integrating current meter 143 and displays the remaining battery charge on an unillustrated display unit based on the predicted value.
These prior art systems have the following shortcomings:
1. Shortcoming Pertaining to Prediction of Remaining Charge
In the first prior art system shown in FIG. 6, the remaining charge meter 126 calculates a remaining charge predicted value based on the cell voltage of each of the rechargeable batteries 122a to 122c and the integrated values of the charge current and discharge current and supplies the predicted value to the portable device 121. While this ensures highly precise prediction of the remaining charge, the manufacturing cost for the battery pack 122 increases due to the microcomputer provided in the remaining charge meter 126. This makes the battery pack 122 expensive.
In the second prior art system shown in FIG. 7, on the other hand, because the battery pack 142 is not equipped with a microcomputer, an increase in the manufacturing cost for the battery pack 142 is nit incurred. However, in the battery pack 142, the integrating current meter 143 predicts the remaining charge of the rechargeable battery only by detecting the current value. As shown in FIG. 3, when the cycle number of the rechargeable battery increases, the discharge capacity decreases, so that mere prediction of the remaining charge based on current integration would result in inaccurate prediction of the remaining charge. In a case where the battery pack 142 incorporates plural rechargeable batteries 122a to 122c, particularly, their capacities (terminal voltages) vary, thus lowering the precision of the prediction of the remaining charge.
2. Charge-Oriented Shortcoming
In the first prior art system, the charger 130 detects the charge current that flows across the second sense resistor 129 and charges the rechargeable batteries 122a to 122c with the constant voltage and constant current based on the detection result. To precisely perform such charging with the constant voltage and constant current, it is necessary to improve the precision of the current detection done by the charger 130. In this respect, normally, a sense resistor of a high precision type is used as the second sense resistor 129 that is provided to detect the current. This disadvantageously increases the manufacturing cost for the charger 130. Further, such a high precision type resistor is large in size, which undesirably enlarges the charger 130. This shortcoming also arises in the second prior art system.