METHOD AND APPARATUS FOR CHARGING A RECHARGEABLE BATTERY

Abstract: A method and an apparatus for charging a battery are provided. During charging of the battery, the rate of change of the battery temperature is determined for at least part of the charging process. A reference value is determined based on the obtained values of the rate of change of the battery temperature, and further determined values of the rate of change of the battery temperature are compared to the reference value, whereby termination of the charging process is controlled based on this comparison. The reference value may be determined as a minimum value of obtained values of the rate of change of the battery temperature, and termination of the charging process may be initiated, when the rate of change of battery temperature exceeds the reference value by a predetermined amount.

Description: METHOD AND APPARATUS FOR CHARGING A RECHARGEABLE BATTERY TECHNICAL FIELD The present invention relates to a method and an apparatus for charging a rechargeable battery. More particularly, the present invention is directed to the control of the termination of the charging process. BACKGROUND Generally, when charging a rechargeable battery or a secondary battery such as NiCd (Nickel -Cadmium) , NiMH (Nickel-Metal- Hydride) , it is known to have a rapid charging process wherein a relatively high constant current is applied to the battery until a certain event occurs. A typical method of detecting this event is to measure the increase in battery temperature as a function of time in order to detect when the battery tempera- ture rate of change (ΔT/Δt) reaches a predetermined high limit, see for example the patent specifications US 3,852,652, US 5,329,219 and US 5,550,453. However, a common drawback of the above mentioned known charging processes is the use of a predetermined reference value to be reached for the measured battery temperature rate of change when terminating the charging process. In order to obtain an optimal charging process, such a reference value should be chosen as a function of the battery technology, the manufacturer of the battery, the number of cells within the battery and the size of the battery cells. Furthermore, the optimal reference value will change as a function of battery use and degradation. Hence, the use of a predetermined reference value which is constant throughout the battery life might result in undercharge of the battery leading to a poor battery capacity or overcharge of the battery leading to a decreased battery lifetime. Therefore, the need exists for a battery charging method and apparatus allowing a charging reference value to be determined during the charging process as a function of the measured battery temperature rate of change. Another drawback of known charging processes in which the values of a characteristic charging parameter is monitored for the detecting of a certain event, which indicates the termination of a rapid charging stage, is the appearance of battery parameter peak values at the initial stage of charging. In order to avoid a premature termination of the charging process due to a rise in such a characteristic parameter, the need exists for a battery charging method and apparatus avoiding the detection of the characteristic parameter during the initial charging stage, but which allows the detection of a fully charged battery in order to avoid overcharging of a battery, which has already been fully charged. SUMMARY OF THE INVENTION The present invention provides a method where termination of the charging process can be controlled in an optimum way by using a reference value which is determined during the charging process based on determined values of the rate of change of battery temperature, whereby a change in the reference value, for example as a function of battery life, is taken into account . According to a first aspect of the present invention this is achieved by a method of charging a rechargeable battery, said method comprising: connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery, determining values of the rate of change of battery temperature during at least part of the process of charging the battery, determining and storing a reference value based on the obtained values of the rate of change of battery temperature, comparing values of the rate of change of battery temperature with the stored reference value or a function thereof, and controlling termination of the charging process based on said comparison. When monitoring battery temperature during a charging process the temperature will increase when the battery approaches the fully charged state. Thus, it is preferred to use a threshold value for the battery temperature rate of change and having termination of the charging process based on such a threshold value. In order to have this threshold value being adaptive, it is preferred to determine the threshold value based on the determined reference value. Thus, for example, termination of the charging process may be initiated at the point in time at which the determined value of the rate of change of battery temperature exceeds the stored reference value by a predetermined amount. In order to avoid overcharging of the battery it is important to use the smallest possible value for the termination threshold value, and it is preferred that the determined and stored reference value represents a minimum of the obtained values of the rate of change of battery temperature. In such a case the threshold value may be calculated by adding a predetermined maximum allowed change in the rate of change of battery temperature to the obtained reference value. Such a predetermined maximum allowed change may be in the range of 0,25-2°C/min. and preferably about 0,5°C/min. However, the optimal value will vary for different battery capacities and different battery technologies. In order to control termination of the charging process, the power supplied to the battery needs to be reduced. Here, it is preferred that termination of the charging process comprises reducing the charging current. This reduction may be abrupt by turning the power supply off, but the termination of the charging process may alternatively comprise a final charging period, during which period the battery may be charged with a reduced current until the charging process is finally stopped. Here, the duration of the final charging period may be determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. As an example, the length of the final charging period may be in the range of 5-50%, preferably around 25%, of the total charging time lapsed up to the point in time at which termination of the charging process is initiated. However, the final charging period need not be a function of the charging time but may have a predetermined duration. In order to reduce the charging power supplied to the battery during the final charging period, the charging process may be controlled so as to reduce battery terminal voltage by at least a predetermined amount during initiation of the final charging period for avoiding overcharging. As an example, the battery terminal voltage may be reduced at least 100 mV, preferably at least 200 mV, at the initiation of the final charging period, with the battery terminal voltage preferably not being increased during this final period. However, the battery terminal voltage may also be reduced as a function of the number of cells within the battery. Such a reduction may be in the range of 10-lOOmV per cell, preferably in the range of 20-70mV per cell, and even more preferred about 40mV per cell. Alternatively, the charging power may be reduced by controlling the charging process so that the battery terminal voltage is not allowed to increase during the final charging period, for example by keeping the voltage constant during this final charging period. When charging a battery, the determined rate of change of battery temperature during upstart of the charging process may vary as a function of the initial battery temperature. Thus, for a battery which has been stored at a low temperature but which is being charged at a higher temperature, a high value of the rate of change of battery temperature can be observed during upstart of the charging process until the battery has reached the ambient temperature. It is therefore a goal to avoid the influence of such an initial high rate of change of battery temperature. Thus, it is preferred that the control of the termination of the charging process is based on values of the rate of change of battery temperature being determined after a predetermined time period has lapsed or after a point in time at which point in time the value of a characteristic start-up charging parameter measured during an initial stage of the charging process has reached a predetermined value. In a preferred embodiment the characteristic start-up charging parameter is the battery terminal voltage, which may be measured as an open circuit voltage. When the charging process has been stopped, the capacity of the battery may decrease due to self discharging. Such self discharge may vary for different batteries and for different technologies. However, if self discharging might be a problem, it is preferred that the state of charge of the battery is maintained after termination of the charging process by means of a pulsating trickle-charge current, or by means of a low maintenance current . The first aspect of the present invention also provides an apparatus for charging a rechargeable battery, said apparatus comprising: means for connecting the battery terminals to an electrical power source for the supply of a charging current to the battery, said battery having a battery terminal voltage and a temperature, means for measuring the rate of change of the battery temperature during at least part of the process of charging the battery, means for determining and storing a charging reference value based on measurement values of the rate of change of battery temperature, means for comparing measurement values of the rate of change of battery temperature with the stored reference value or a function thereof, and means for controlling termination of the charging process in response to such a comparison. It is preferred that the termination control means may be adapted to initiate termination of the charging process at the point in time at which the determined value of the rate of change of battery temperature reaches a threshold value which exceeds the stored reference value by a predetermined amount. In such a case the comparing means may be adapted to determine such a threshold value by adding a predetermined maximum allowed change in the rate of change of battery temperature to the obtained reference value. As already mentioned it is important to be able to determine the smallest possible threshold termination value. Thus, it is preferred that the means for determining and storing the reference value is adapted to determine and store a minimum of the obtained values of the rate of change of battery temperature. For terminating the charging process, it is preferred that the termination control means is adapted to reduce the charging current during the process of termination. The termination means may be adapted to determine a final charging period, and preferably being adapted to determine the duration of such final charging period as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. In a preferred embodiment the termination control means is adapted to reduce battery terminal voltage by at least a prede- termined amount during the final charging period for avoiding overcharging. Preferably, the termination control means is adapted to perform such reduction at the initial stage of the final charging period. According to an embodiment of the apparatus of the present invention it is preferred that this apparatus comprises means for measuring a characteristic start-up charging parameter, and that the termination control means is adapted to control termination of the charging process based on values of the rate of change of battery temperature being determined after a prede- termined time period has lapsed or after a point in time at which point in time the value of the characteristic start-up charging parameter measured during an initial stage of the charging process has reached a predetermined value. The characteristic start-up charging parameter may be the battery terminal voltage, and it is preferred that the means for measuring the start-up charging parameter is adapted to measure the battery terminal voltage as an open circuit voltage. When self discharging of the battery might be a problem, it is preferred that the apparatus further is provided with means for maintaining the state of charge of the battery after termination of the charging process. Such maintaining means may provide a pulsating trickle-charge current, or a low maintenance current . According to a second aspect of the present invention there is provided a method of charging a rechargeable battery, said method comprising: connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery, determining values of a first characteristic charging parameter during at least part of the charging process, and controlling termination of the charging process based on values of the first parameter being determined after a point in time at which point in time a second characteristic charging parameter measured during an initial stage of the charging process has reached a predetermined value or fulfills a predetermined criteria. Here, the first characteristic charging parameter may be any battery parameter which is suitable for the control of the charging process such as battery terminal voltage, charging current, battery temperature, the rate of change of any of these parameters or any combination of these parameters and/or their rate of change. Preferably, the first characteristic charging parameter is the rate of change of battery temperature calculated from measured values of the battery temperature. Similarly, the second characteristic charging parameter may also be selected from any of the above mentioned first characteristic charging parameters with the exception that it should not be the same parameter as the one chosen as the first characteristic parameter. However, it is preferred that the second characteristic charging parameter is the battery voltage. In order to terminate the charging process it is preferred that the obtained values of the first parameter are compared with a stored reference value or a function thereof, and the termina- tion of the charging process is based on said comparison. Preferably, the termination of the charging process is initiated when the measured values of the first charging parameter reaches a threshold value being a function of the stored refe- rence value. Thus, for example, termination of the charging process may be initiated at the point in time at which the obtained value of the first parameter exceeds the stored reference value by a predetermined amount . In a preferred embodiment the stored reference value is determined during the charging process based on obtained values of the first parameter. The reference value may be determined as a maximum of the obtained values, but it is preferred that the reference value represents a minimum value of the obtained first parameter values. When terminating the charging process, it is preferred that the termination comprises a final charging period, and it is preferred that the length of the final charging period is determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. Furthermore, it is preferred that termination of the charging process comprises reducing the charging current or charging with a reduced current during the final charging period. In accordance with the second aspect of the present invention there is also provided an apparatus for charging a rechargeable battery, said apparatus comprising: means for connecting the battery terminals to an electrical power source for the supply of a charging current to the battery, said battery having a battery terminal voltage and a temperature, means for determining values of a first characteristic charging parameter during at least part of the charging process, means for determining a second characteristic charging parameter during an initial stage of the charging process, and means for controlling termination of the charging process. Here, the termination control means is adapted to control the termination process based on values of the first parameter being determined after a point in time at which point in time the second characteristic charging parameter has reached a predetermined value or fulfills a predetermined criteria. In a preferred embodiment the means for determining the first characteristic parameter is adapted to determine the rate of change of battery temperature, and it is preferred that the means for determining the second characteristic parameter is adapted to determine the battery voltage. Preferably, the voltage is determined as an open circuit voltage. In order to terminate the charging process it is preferred that the apparatus according to the second aspect of the invention comprises means for storing a reference value and means for comparing determined values of the first parameter with the stored reference value or a function thereof, and that the termination control means is adapted to terminate the charging process based on this comparison. Preferably, the apparatus further comprises means for determining the reference value, said reference value determining means being adapted to determine the reference value during the charging process based on obtained values of the first parameter. In a preferred embodiment the reference value determining means is adapted to determine a minimum value of the obtained first parameter values. However, the reference value determining means may alternatively be adapted to determine a maximum value of the obtained first parameter values. For termination of the charging process it is preferred that the termination control means is adapted to initiate termina- tion of the charging process at the point in time at which the determined value of the first parameter exceeds the stored reference value by a predetermined amount . In order to terminate the charging process it is preferred that the termination control means is adapted to determine a final charging period, and that the duration of this final charging period preferably is determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. During such final charging period it is preferred that the termination control means is adapted to reduce the charging current. However, the termination control means may alternatively be adapted to terminate the charging process abruptly without any final charging period. If self discharging of the battery is considered a problem, it is preferred that the apparatus according to the second aspect of the invention further is provided with means for maintaining the state of charge of the battery after termination of the charging process. Such maintaining means may provide a pulsating trickle-charge current, or a low maintenance current. BRIEF DESCRIPTION OF THE DRAWINGS The operation of the invention, together with further objects and advantages thereof, may best be appreciated by reference to the following detailed description taken in conjunction with the accompanying drawing, in which: Fig. 1 is a schematic diagram of a battery charging apparatus according to an embodiment of the present invention, Fig. 2 is a flow diagram illustrating the operation of an exemplary embodiment of the invention, Fig. 3 illustrates a charging process according to the present invention, Fig. 4 illustrates a charging process similar to the process of Fig. 3 but with the battery having a higher initial temperature, Fig. 5 shows the charging curves for the battery of Fig. 3 when the battery is holding 90% of its capacity at the start of the charging process, and Fig. 6 illustrates a charging process according to the present invention with a low initial battery temperature and a high ambient charging temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The basic operational principles of a battery charger according to the invention is illustrated in Fig. 1. Fig. 1 shows a battery pack 10 which is to be charged by a battery charger apparatus 20. The battery pack 10 comprises a number of series connected individual battery cells 11, a battery temperature sensing thermistor 12 (NTC thermistor) , a battery type resistor 13 and battery pack output terminals 14, 15, 16, 17. Battery output voltage is provided across terminals 14 and 17, and the charging current supplied to the battery is sensed by a sense resistor 23 connected to the battery terminal 17 and ground. The thermistor 12 has current supplied through a pull-up resistor 21 and senses battery temperature and provides a related output at the terminal 16. The type resistor 13 has current supplied through pull-up resistor 22 and provides a battery type related voltage at the terminal 15. The battery charger 20 comprises a power supply 24, a micro controller 25, and a signal conditioning circuitry 26. The power supply 24, which preferably is a switch mode power supply, has a power input 27 which is supplied with a DC voltage, preferably in the range of 12-15 Volts DC. The power supply has an output terminal 28 which provides the charging power to the battery terminal 14, preferably through a switch 29. The charg- ing output 28 of the power supply 24 is controlled from a control output 30 of the micro controller 25. When the power supply 24 is a switch mode power supply the control output 30 is preferably a PWM (pulse width modulated) signal, which may be fed to a filter for converting the PWM signal to a variable analogue voltage, which is then used for the control of the power supply 24. When using a PWM control signal 30, the micro controller 25 controls the power output to the battery 10 and terminal 28 by controlling the duration of on- and off-periods of the PWM signal. The signal conditioning circuitry 26 converts the voltage input signals representing the battery terminal voltage, the battery type, the battery temperature, and the charging current, to voltage output signals being suitable as input signals for analogue to digital, A/D, converter inputs 32 of the micro controller 25. Preferably, the current sense resistor 23 has a very low value which may be around 0,1 Ω, and the conditioning circuitry 26 may then comprise an operational amplifier in order to provide a suitable output. The supply voltage for the micro controller 25 is preferably around 5 Volts, and since the battery terminal voltage may exceed 5 Volts, the conditioning circuitry may also comprise a voltage divider for providing a suitable output signal for the battery terminal voltage. It is preferred that the micro controller 25 comprises a switch control output 31 for switching the switch 29 on and off. The switch 29 may be turned off at short time intervals during the charging process in order to measure an open circuit voltage of the battery, thereby avoiding the voltage drop from the internal loss resistance when measuring the battery terminal volt- age . The micro controller 25, which for example may be an COP 8ACC from National Semiconductor, is programmed to implement battery charging in accordance with the present invention. Thus, the micro controller 25 controls the power delivered from the power supply 24 to the battery 10 based upon the input signals from the conditioning circuitry 26, which input signals represent the battery type and the battery charging parameters: battery terminal voltage, battery temperature and charging current. The manner in which this is accomplished will now be discussed in connection with Figs. 2-4. Referring now to Fig. 2, a flow diagram is illustrated which starts at an initial step 40 and proceeds to process step 41. Step 40 is indicative of connection of the battery pack 10 to the charger 20 and initialising the micro controller 25. During the initialising process 40, the micro controller 25 reads the value of the battery type voltage, and this value is used as a reference for addressing battery specific predetermined charg- ing parameters stored in the micro controller 25. These predetermined parameters may represent a maximum charging current, Imax; an initial value for an end time period, End_Time, which initial value defines a safety time at which the charging process will be stopped unless a new value of End_Time is determined and stored during the charging process; a maximum change in the rate of change of battery temperature, ΔT/Δt_add, which defines a maximum allowed change in the rate of change of battery temperature compared to a determined minimum value of the battery temperature rate of change, Min_ΔT/Δt, which minimum value is a variable determined during the charging process; an initial value for Min_ΔT/Δt, which initial value preferably is set at a large value; an initial voltage limit, VoltLimit, which defines a minimum limit that the measured battery voltage should reach before updating the predetermined initial value of Min_ΔT/Δt; an initial time period, TimeLimit, defining a time period which shall expire before updating the predetermined initial value of Min_ΔT/Δt. Exemplary values of the initial value of End_Time, Min_ΔT/Δt and ΔT/Δt_add are given in connection with the description of Figs. 3-5. Here, the value of VoltLimit will typical be 1,4 Volts per battery cell, while the value of TimeLimit will typical be 5 minutes for NiCd or NiMH batteries. If the measured battery voltage reaches VoltLimit before TimeLimit has expired, then updating of the predetermined initial value of Min_ΔT/Δt will start at the point in time when VoltLimit has been reached, otherwise updating of the predetermined initial value of Min_ΔT/Δt will start when the TimeLimit period has expired. After initialisation at step 40 the charging process is started at process step 41. Here, the charging process is controlled based on measured values of the open circuit battery voltage, Vopen, the charging current, Char, and the battery temperature, Tbat. From the values of Tbat, values of the battery temperature rate of change ΔT/Δt are calculated. During the first stage of the charging process, it is preferred to increase the charging current Ichar until the predetermined Imax has been reached. When Imax has been reached, a second charging stage is entered, in which the output of the power supply is preferably controlled so as to charge at a constant charging current, i.e. the output of the power supply is controlled so that Ichar is close to Imax. In a preferred embodiment, the value of Imax is chosen to be within the range of 0,5-1,5 A for NiCd and NiMH batteries. At decision step 42, it is determined if the charging time has reached the stored value of TimeLimit and if ΔT/Δt is smaller than the stored value of Min_ΔT/Δt . If so, then at process step 43, the stored value of Min_ΔT/Δt is replaced with the measured value of ΔT/Δt, from where the process passes on to decision step 44. If the requirements at step 42 are not fulfilled, then the process passes on directly to decision step 44, where it is determined if the measured open circuit voltage Vopen has reached the stored value VoltLimit. If no, then the process passes on to decision step 49, where it is determined if the charging time has reached the stored value of End_Time. If the charging time has not reached End_Time, then the process returns to process step 41 for further charging. If End_Time has been reached, then the normal charging process is stopped and the charging current is reduced to a low maintenance or trickle charging current at process step 50 in order to maintain the charging status of the battery. The maintenance or trickle charge current is preferably set in the range of 0,05-0,lC, where 1C is equivalent to a charging current used to fully charge the battery in one hour. Here it should be noticed that passing directly from step 44 to 49 and then to 50 is not the route of a normal charging process. However, the battery to be charged might be a defect battery or there might be a bad connection to the battery terminals, leading to the result that measured battery voltage will not reach the stored VoltLimit value within the initial safety value of End_Time. Thus, the charger will automatically terminate the charging process at expiration of the initial End_Time period. If the measured battery voltage has reached VoltLimit at step 44, the process passes on to decision step 45, where it is determined if the measured value of ΔT/Δt is smaller than the stored value of Min_ΔT/Δt. If so, then at process step 46, the stored value of Min_ΔT/Δt is replaced with the measured value of ΔT/Δt, from where the process passes on to decision step 47. If the requirement at step 45 is not fulfilled, then the pro- cess passes on directly to decision step 47, where it is determined if the measured value of ΔT/Δt is larger than the combined value of the presently stored minimum value added to the predetermined maximum allowed value of the change in ΔT/Δt, i.e. if the measured value of ΔT/Δt has reached Min_ΔT/Δt plus ΔT/Δt_add. If the answer to the decision at step 47 is no, then the charging process has not yet reached the normal stage of termination and the process passes on to decision step 49 , from where the charging process proceeds or stops as described above. If the answer at decision step 47 is yes, the process passes on to process step 48, where a maximum allowed battery voltage, MaxVolt, is defined as fl (Voltage). When step 48 has been reached, a third stage of the charging process is entered where the remaining part of the charging process is controlled so that the measured battery voltage does not exceed MaxVolt. The value of MaxVolt is not a predetermined value but a function of the measured battery voltage V (ΔT/Δt) at the point in time where ΔT/Δt has reached Min_ΔT/Δt plus ΔT/Δt_add. In a preferred embodiment fl (Voltage) is defined as (kl x V (ΔT/Δt) - k2) , where the constant kl may be set to 1 and the k2 may be chosen in the range of O-lOOmV, preferably about 40mV, per battery cell. Thus, for a 4 cell battery the value of MaxVolt may preferably be set to V (ΔT/Δt) - 160mV. For NiMH batteries it is preferred to have such a reduction in the battery voltage in order to avoid overheating of the battery, since such overheating might damage the battery and decrease the battery lifetime. The constant k2 may alternative- ly be set to zero, which for example may work for NiCd batteries. However, it is also preferred to have k2 in the range of 50mV for NiCd batteries. At process step 48 the stored initial value of End_Time is replaced with a new End_Time value being a function of the total charging time f2 (Time) at the point in time, t (ΔT/Δt), where ΔT/Δt has reached Min_ΔT/Δt plus ΔT/Δt_add. In a preferred embodiment the new value of End Time is defined as (k3 x T8 t (ΔT/Δt) + k4) , where the constant k3 may be set to about 1,25 and k4 may be set to zero. However, k3 can also be chosen in the range of 1-2, and k4 may represent a fixed time period in the range of 0-20 minutes. At process step 48 it is determined how the charging process is to be terminated, i. e. a final charging period and the maximum battery voltage is determined. Here it should be mentioned that in a preferred embodiment the measured battery temperature is compensated for variations due to a change in the battery temperature. Using a maximum allowed voltage results in a decrease in the charging current during the final charging period. The termination of the charging process is illustrated by the loop comprising process step 51 and decision step 52. After the determination of MaxVolt and the new End_Time value, the charging process is continued as described above until the total charging time reaches the stored value of End_Time in step 52, at which point in time the charging process is stopped and the charging current is reduced to a low maintenance or trickle charging current at process step 50 in order to maintain the charging status of the battery. The variables, predetermined constants and functions of the above described preferred embodiment of the invention are shown Table I. Variables Constants: Functions : Fl (Vopen) Example: kl x Vopen - k2, typical kl = 1 and k2 = 40 mV for single cell batteries F2 (Time) Example: k3 x Time + k4, typical k3 = 1.25 and k4 = 0 Table I. Charging variables, constants and functions. For the process of Fig. 2 it is preferred that the measured battery voltage is the open circuit battery voltage, Vopen. This is indicated in Table I, where fl (Voltage) is given as Fl (Vopen) . However, it should be understood that the principles of the present invention may also be applied when the measured battery terminal voltage is the terminal voltage during charging. Referring now to Fig. 3 which shows the charging curves for a charging process controlled as described above with reference to the flow diagram of Fig. 2. The battery of Fig. 3 is fully discharged before the charging process is started and the battery is charged at room temperature with an initial battery temperature around 23 °C . In Fig. 3 the thick solid line waveform represents the measured open circuit battery voltage, the dashed waveform represents the measured charging current and the thin solid line waveform represents the measured battery temperature. The battery of Fig. 3 is a 1600 mAh NiMH battery with 6 cells. Imax is set to 900 mA, the initial End_Time value is set to 160 min., the initial value of Min_ΔT/Δt is set to a high value of 10°C/min., thereby disabling the effect of the rate of temperature change during the upstart of the charging process , the value of ΔT/Δt_add is set to 0,5°C/min., the value TimeLimit is set to 5 min. and the value of VoltLimit is set to 8,25 Volt. The function for MaxVolt is set to V (ΔT/Δt) - 240 mV, and the function for End_Time is set to 1,25 x t (ΔT/Δt). Here it should be mentioned that the optimal value of ΔT/Δt_add will vary as a function of battery capacity and the maximum charging current. Thus, the value of ΔT/Δt_add should be larger both for a smaller nominal battery capacity and for a higher charging current . In order to measure a relative change in ΔT/Δt of 0,5°C/min., it is necessary to measure changes in the battery temperature at a relatively high resolution. In a preferred embodiment the temperature is measured using a 10 bit AD converter resulting in a resolution in ΔT of about 0,1°C. If is preferred that the measured open circuit voltage Vopen is compensated for changes due to battery temperature variation when compared to the stored value of VoltLimit. Such a compensation might be 20 mV/°C subtracted from the measured values of Vopen for temperatures below 35°C, when comparing Vopen to VoltLimit. From Fig. 3 it is seen that the first stage of charging is rather short and the charging current reaches Imax within a short time period. During the second stage of charging the current is controlled to be approximately Imax and during the third stage of charging equivalent to the final charging period, the charging current is decreased. In Fig. 3 the value of TimeLimit is smaller than the time at which the compensated Vopen reaches VoltLimit. Thus, after TimeLimit has been reached new values of Min_ΔT/Δt are stored according to steps 42 and 43. However, in the preferred embodiment none of the new values of Min_ΔT/Δt are used for the control of termination of the charging process before VoltLimit has been reached according to step 44. Fig. 4 shows the charging curves corresponding to the curves of Fig. 3 for another sample of a 1600 mAh NiMH battery with 6 cells. The predetermined charging parameters and functions are the same as for the charging process of Fig. 3. The battery of Fig. 4 is charged at the same room temperature of about 23 °C as the battery of Fig. 3, but the battery of Fig. 4 has been stored at a higher temperature before being charged resulting in an initial battery temperature around 27°C. Thus, the temperature rate of change ΔT/Δt will be smaller for the charging curves of Fig. 4 than for the curves of Fig. 3. Hence, the ΔT/Δt termination value need to be smaller for the charging process of Fig. 4 when compared to the charging process of Fig. 3 in order to avoid overcharge of the battery. This problem is solved by a charging method according to the present invention. Since the determined and stored values of Min_ΔT/Δt are smaller for the warm battery of Fig. 4 than for colder battery of Fig. 3, the resulting maximum allowed value of ΔT/Δt, Min_ΔT/Δt + ΔT/Δt_add, will be smaller in Fig. 4 than in Fig. 3. For batteries having a lower initial temperature than the battery of Fig. 3 but being charged at the same ambient temperature, even higher values of ΔT/Δt will be observed during the initial stage of charging, which values might equal the desired termination value of ΔT/Δt. Hence, the charging process should be controlled so as to avoid termination based on an initial high value of ΔT/Δt. This might be done in a simple way by having a TimeLimit set at a high value, for example 15 min. However, this might cause overcharge when charging an almost fully charged battery. This problem is solved by a charging method according to the present invention. If a battery is almost fully charged, the battery voltage will reach a high value such as VoltLimit within a relatively short time period, whereas for an empty battery the battery voltage will increase much more slowly. This is illustrated in Fig. 5 which shows the charging curves for the battery of Fig. 3 when the battery is holding 90% of its capacity at the start of the charging process. Here the battery voltage reaches VoltLimit within 4 min. from start of the charging process compared to the 35 min. of the fully discharged battery of Fig. 3. Hence, the use of the parameter VoltLimit to determine, when the values of ΔT/Δt can be used to control termination of the charging process, brings a charging solution which avoids overcharging of almost fully charged batteries. Fig. 6 also illustrates a charging process for the battery of Fig. 3. Here, the battery is holding about 50% of its capacity at the start of the charging process. However, the initial battery temperature is very low, about -7°C, whereas the ambient charging temperature is high, about 35°C. Here, the battery is charged with a low maintenance current until the battery temperature reaches 5°C, from which point in time a normal charging process is started. From Fig. 6 it is seen that the normal charging process is initiated around Time equal to 9 min. and correspondingly TimeLimit is reached at Time equal to 14 min., when the initial value for TimeLimit is set to 5 min. Due to the temperature compensation and the low battery temperature, the measured values of Vopen have to reach a relatively high value before reaching the corresponding VoltLimit value. Thus, in Fig. 6 Vopen has a value about 8,45 Volt at Time equal to 27 min. and the battery temperature is about 25°C, resulting in a voltage compensation of 200 mV, which corresponds to Vopen fulfilling the VoltLimit criteria at Time equal to 27 min. (VoltLimit + 200 mV = Vopen) . Hence, for the process of Fig. 6, the value of Min_ΔT/Δt is not updated before Time equals 14 min. , but only the reference values stored after Time equal to 27 min. is used for the termination process. Since the value of ΔT/Δt decreases through the charging process until the almost fully charged state is reached, new reference values are stored until this almost fully charged state is reached. Thus, the value of ΔT/Δt does not have to reach a very high value in order to start the termination process, thereby avoiding overcharge of the battery. What has been described with reference to the present embodiments illustrates how a reference value is used for the rate of change of battery temperature, which reference value is determined and stored during the charging process . The obtained reference value is used when determining a threshold value for control of termination of the charging process, whereby this embodiment of the present invention implements adaptive battery charging. This allows the present embodiments to account for changes or differences in the threshold value of the battery temperature rate of change due to aging or manufacturing tolerances. Furthermore, it allows the present embodiments to account for variations in the threshold value due to differences in ambient temperatures . The above described embodiments for a charging process also bring a solution which accounts for differences in the initial battery temperature, whereby a premature termination of the charging process due to a high initial temperature rate of change is avoided. It should be understood that the above described embodiments of the present invention are not valid only for a single type of batteries such as NiMH (Nickel Metal Hydride) batteries but should also be applicable for other types of rechargeable batteries such as, for example, NiCd (Nickel Cadmium) and Lithium batteries. The foregoing description of preferred exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the present invention to those skilled in the art. All such modifications which retain the basic underlying principles disclosed and claimed herein are within the scope of this invention.

Claims: CLAIMS 1. A method for charging a rechargeable battery, said method comprising: connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery, determining values of the rate of change of battery temperature during at least part of the process of charging the battery, determining and storing a reference value based on the obtained values of the rate of change of battery temperature, comparing values of the rate of change of battery temperature with the stored reference value or a function thereof, and controlling termination of the charging process based on said comparison. 2. A method according to claim 1, wherein termination of the charging process is initiated at the point in time at which the determined value of the rate of change of battery temperature exceeds the stored reference value by a predetermined amount. 3. A method according to claim 1 or 2 , wherein the determined and stored reference value represents a minimum of the obtained values of the rate of change of battery temperature. 4. A method according to any of the claims 1-3, wherein termination of the charging process comprises reducing the charging current. 5. A method according to any of the claims 1-4, wherein termination of the charging process comprises a final charging period. 6. A method according to claim 5, wherein the duration of the final charging period is determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. 7. A method according to claim 5, wherein the final charging period has a predetermined duration. 8. A method according to claim 6, wherein the length of the final charging period is in the range of 5-50%, preferably around 25%, of the total charging time lapsed up to the point in time at which termination of the charging process is initiated. 9. A method according to any of the claims 5-8, wherein the charging process is controlled so as to reduce battery terminal voltage by at least a predetermined amount during initiation of the final charging period for avoiding overcharging. 10. A method according to claim 9, wherein the battery terminal voltage is reduced at least 100 mV, preferably at least 200 mV, at the initiation of the final charging period, and said battery terminal voltage preferably not being increased during this final period. 11. A method according to any of the preceding claims, wherein the control of the termination of the charging process is based on values of the rate of change of battery temperature being determined after a predetermined time period has lapsed or after a point in time at which point in time the value of a characteristic start-up charging parameter measured during an initial stage of the charging process has reached a predetermined value . 12. A method according to claim 11, wherein the characteristic start-up charging parameter is the battery terminal voltage. 13. A method according to any of the claims 9-12, wherein the battery terminal voltage is measured as an open- circuit voltage. 14. A method according to any of the preceding claims, wherein the state of charge of the battery is maintained after termination of the charging process by means of a pulsating trickle- charge current . 15. An apparatus for charging a rechargeable battery, said apparatus comprising: means for connecting the battery terminals to an electrical power source for the supply of a charging current to the battery, said battery having a battery terminal voltage and a temperature, means for measuring the rate of change of the battery temperature during at least part of the process of charging the battery, means for determining and storing a charging reference value based on measurement values of the rate of change of battery temperature, means for comparing measurement values of the rate of change of battery temperature with the stored reference value or a function thereof , and means for controlling termination of the charging process in response to such a comparison. 16. An apparatus according to claim 15, wherein the termination control means is adapted to initiate termination of the charging process at the point in time at which the measured or determined value of the rate of change of battery temperature exceeds the stored reference value by a predetermined amount. 17. An apparatus according to claim 15 or 16, wherein the means for determining and storing a reference value is adapted to determine and store a minimum of the obtained values of the rate of change of battery temperature. 18. An apparatus according to any of the claims 15-17, wherein the termination control means is adapted to reduce the charging current during the process of termination. 19. An apparatus according to any of the claims 15-18, wherein the termination control means is adapted to determine a final charging period, the duration of said final charging period preferably being determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. 20. An apparatus according to claim 19, wherein the termination control means is adapted to reduce battery terminal voltage by at least a predetermined amount during initiation of the final charging period for avoiding overcharging. 21. An apparatus according to any of the claims 15-20 further comprising means for measuring a characteristic start-up charging parameter, wherein the termination control means is adapted to control termination of the charging process based on values of the rate of change of battery temperature being determined after a predetermined time period has lapsed or after a point in time at which point in time the value of the characteristic start-up charging parameter measured during an initial stage of the charging process has reached a predetermined value. 22. An apparatus according to claim 21, wherein the character- istic start-up charging parameter is the battery terminal voltage. 23. An apparatus according to claim 21 or 22, wherein the means for measuring the start-up charging parameter is adapted to measure the battery terminal voltage as an open- circuit voltage. 24. A method of charging a rechargeable battery, said method comprising: connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery, determining values of a first characteristic charging parameter during at least part of the charging process, and controlling termination of the charging process based on values of the first parameter being determined after a point in time at which point in time a second characteristic charging parameter measured during an initial stage of the charging process has reached a predetermined value or fulfills a predetermined criteria. 25. A method according to claim 24, wherein the first characteristic charging parameter is the rate of change of battery temperature calculated from measured values of the battery temperature . 26. A method according to claim 24 or 25, wherein the second characteristic charging parameter is the battery voltage. 27. A method according to any of the claims 24-26, wherein obtained values of the first parameter are compared with a stored reference value, and the termination of the charging process is based on said comparison. 28. A method according to claim 27, wherein the stored reference value is determined during the charging process based on obtained values of the first parameter. 29. A method according to claim 28, wherein the reference value represents a minimum value of the obtained first parameter values . 30. A method according to any of the claims 27-29, wherein termination of the charging process is initiated at the point in time at which the determined value of the first parameter exceeds the stored reference value by a predetermined amount. 31. A method according to any of the claims 24-30, wherein termination of the charging process comprises reducing the charging current. 32. A method according to any of the claims 24-31, wherein termination of the charging process comprises a final charging period. 33. A method according to claim 32, wherein the length of the final charging period is determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated 34. An apparatus for charging a rechargeable battery, said apparatus comprising: means for connecting the battery terminals to an electrical power source for the supply of a charging current to the battery, said battery having a battery terminal voltage and a temperature, means for determining values of a first characteristic charging parameter during at least part of the charging process, means for determining a second characteristic charging parameter during an initial stage of the charging process, and means for controlling termination of the charging process based on values of the first parameter being determined after a point in time at which point in time the second characteristic charging parameter has reached a predetermined value or fulfills a predetermined criteria. 35. An apparatus according to claim 34, wherein the first characteristic charging parameter is the rate of change of battery temperature. 36. An apparatus according to claim 34 or 35, wherein the second characteristic charging parameter is the battery voltage. 37. An apparatus according to any of the claims 34-36 further comprising means for storing a reference value and means for comparing determined values of the first parameter with the stored reference value, wherein the termination control means is adapted to terminate the charging process based on said comparison. 38. An apparatus according to claim 37 further comprising means for determining the reference value, said reference value determining means being adapted to determine the reference value during the charging process based on obtained values of the first parameter. 39. An apparatus according to claim 38, wherein the reference value is determined as a minimum value of the obtained first parameter values. 40. An apparatus according to any of the claims 37-39, wherein the termination control means is adapted to initiate termina- tion of the charging process at the point in time at which the determined value of the first parameter exceeds the stored reference value by a predetermined amount . 41. An apparatus according to any of the claims 34-40, wherein the termination control means is adapted to reduce the charging current during the process of termination. 42. An apparatus according to any of the claims 34-41, wherein the termination control means is adapted to determine a final charging period, the duration of said final charging period preferably being determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated.

Inventors: ANDERSEN KIM ARTHUR STUECK, OESTERGAARD KIM, RASMUSSEN KIM, ANDERSEN, KIM, ARTHUR, STUECK, OESTERGAARD, KIM, RASMUSSEN, KIM
Applicants: ANDERSEN KIM ARTHUR STUECK, CHARTEC LAB AS, OESTERGAARD KIM, RASMUSSEN KIM, ANDERSEN, KIM, ARTHUR, STUECK, CHARTEC LABORATORIES A/S, OESTERGAARD, KIM, RASMUSSEN, KIM
Application year: 1998
Main classification: H02J7
IPC codes: H02J7