GB2320142A

GB2320142A - Rapid re-charging of batteries

Info

Publication number: GB2320142A
Application number: GB9625196A
Authority: GB
Inventors: Jonathan Owen Hardie; Mark Anthony Wallis
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 1996-12-04
Filing date: 1996-12-04
Publication date: 1998-06-10
Anticipated expiration: 2016-12-04
Also published as: GB2320142B; GB9625196D0

Abstract

A battery (12, Fig.1) is charged from a constant current source (10) in charge cycles each having a fixed duration ON period, in which the battery (12) is connected to the source (10) by turning an FET switch (11) on, and an OFF period of variable duration dependent on the battery voltage and during which the switch (11) is off, whereby average charging current is varied. At a predetermined time t after the start of each ON period, the battery voltage is compared with a threshold voltage (step 21, Fig.2). The duration of the next OFF period is increased if the battery voltage exceeds the threshold voltage (step 24), and is decreased if the battery voltage is less than the threshold voltage (step 23). An off-time variable is incremented or decremented in steps 24, 23, and a "smoothed" off-time value is calculated from the new value of the off-time variable and the previous value of the smoothed off-time (step 25). Charging is terminated if the smoothed off-time exceeds a limit value Tm for more than N successive charge cycles (steps 26-29). A ROM (17, Fig.1) may store sets of values of the battery threshold voltage, limit time Tm and cycle number N for different types of battery, the appropriate set being selected according to sensing of a battery type identifier on a battery to be charged. The charging apparatus may be part of a mobile phone handset chargeable from an AC mains supply or from an automobile battery.

Description

RAPID RE-CHARGING OF BATTERIES This invention relates to the rapid charging of rechargeable battery, such as NiCd and NiMH batteries.

One of the problems involved in the rapid recharging of rechargeable batteries is that of preventing an excessive rise in the battery temperature during the final phase of re-charging, i.e. when the battery is nearly fully charged. Such excessive temperature rise can cause permanent damage to the battery.

It is an object of the present invention to a battery charging apparatus in which the above mentioned problem is solved.

A battery charging apparatus in accordance with the invention comprises a current source for supplying charging current to the battery, a switch for controlling connection of the current source to the battery, and control means for controlling said switch to vary the average current flowing to the battery in accordance with the battery voltage, said control means comprising an on timer for turning the switch on for a fixed duration period in each cycle of operation of the apparatus, comparison means for comparing the battery voltage with a predetermined threshold value at a predetermined time after commencement of said fixed duration period in each cycle, and an off timer for turning the switch off following each fixed duration period for a variable duration period dependent on the output of the comparison means in each fixed duration period.

The duration of the off period may vary from zero to a very long duration, and is preferably determined in time units of constant duration.

Preferably the duration of the off period is increased by one unit if the predetermined threshold value is exceeded and reduced by one unit if the predetermined threshold value is not reached.

To determine whether rapid charging should be terminated, the control means preferably includes "filter" means for generating a "smoothed" value determined by the value of the on-time in a plurality of successive cycles and means for determining when the smoothed value exceeds a limit value for a plurality of cycles of operation.

An example of the invention is shown diagrammatically in the accompanying drawings in which: Figure 1 is a diagram of the battery charging apparatus, and Figure 2 is a flow diagram showing software for controlling the operation of the apparatus.

The apparatus, which may be part of a mobile phone handset, includes a constant current source 10 which is connected in series with an FET switch 11 and the battery 12 to be charged across an external battery charging supply 13 which may be an ac mains device or a device for connection to an automobile battery. Charging of the battery is controlled by a control means 14 including a cpu 15 with RAM 16 and ROM 17. The cpu may be the same unit as that used for all the other phone functions.

The apparatus operates by turning the switch 11 on for a fixed duration period of duration T, for example 1s, in each operating cycle and sampling the battery voltage at a time t for example 100 ms, after the start of each on period. The battery voltage is converted to a digital value by an analog-to cligital converter 18 included in the control means 14 and the sample value is compared with a threshold value stored in the ROM 1 7. The cpu determines the duration of an immediately following off period Toffn in accordance with the result of this comparison. If the threshold voltage is exceeded, an off-time variable stored in RAM 16 is increased by one unit. If the threshold value is not reached the off-time variable is decreased by one unit. The switch is then turned off for a period of duration determined by the off-time variable, each unit being of duration T.

Figure 2 shows the algorithm executed by the cpu 15 in controlling the switch.

Initially the off-time variable is set to zero. The cpu starts the on timer in step 20 and after an interval t causes a sample of the battery voltage to be taken and converted into digital format (step 21). In step 22 the cpu compares the value with the threshold value. Step 23 in which the offtime variable is decremented by one is executed only if the battery voltage sample value does not exceed the threshold value. Step 24 is executed only if the battery voltage sample value exceeds the threshold value and results in the off-time variable being incremented. In step 25 a smoothed off-time value variable is calculated. The smoothed off time value Toff (smoothed)n is calculated as Toff(smoothed)n = KxToffn + (I- K) x Toff (smoothed) n-1 when k is a value between 0 and 1. In step 26 this smoothed off-time variable is compared with a limit value Tm. If the smoothed off-time value exceeds Tm step 27 is executed in which a count representing the number of successive cycles in which the smoothed off-time value has been exceeded is incremented. In step 28 this count is compared with a predetermined value N (e.g. 10) and rapid charging is terminated 29 if N is exceeded. If the smoothed off-time variable is less than Tm the count value is reset to zero in step 30. Step 31 starts the off timer and the program then returns to step 20.

The threshold value and the values of Tm and N may require to be set differently according to the exact type of battery to be charged. The different types of battery are tested experimentally to determine the optimum values, these values are stored in ROM 1 7 during manufacture of the apparatus. Each different type of battery will be provided with an identifier which can be read by one or more sensors 32 (Figure 1) and signals supplied to the control unit thereby are used by the control means 14 to determine which set of values is used by the charging control program.

It will be appreciated that the battery charging apparatus described above is advantageous in that the reduction in average charging current which occurs as the battery nears its fully charged condition ensures that battery temperature rise is minimised and the risk of permanent damage to the battery is thereby avoided. By comparison with some known battery charging apparatus, the accuracy of the analog-to-digital conversion can be reduced as small variations to battery voltage do not need to be detected. As sampling occurs only once per cycle, very little time is required for analog-to-digital conversion and calculation.

Claims

1. A battery charging apparatus comprising a current source for supply charging current to the battery, a switch for controlling the connection of the current source to the battery, and control means for controlling said switch to vary the average current flowing to the battery in accordance with the battery voltage, said control means comprising an on timer for turning the switch on for a fixed duration period in each cycle of operation of the apparatus, comparison means for comparing the battery voltage with a predetermined threshold value at a predetermined time after commencement of said fixed duration period in each cycle, and an off timer for turning the switch off following each fixed duration period for a variable duration period dependent on the output of the comparison means in each fixed duration period.

2. A battery charging apparatus as claimed in Claim 1 in which the duration of the off period is determined by the control means in units of constant duration.

3. A battery charging apparatus as claimed in Claim 2, in which the duration of the off period is increased by one unit each time the comparison means detects that the battery voltage exceeds said predetermined threshold value.

4. A battery charging apparatus as claimed in Claim 2 or Claim 3, in which the duration of the off period is decreased by one unit each time the comparison means detects that the battery voltage is less than said predetermined threshold value.

5. A battery charging apparatus as claimed in Claim 2, 3 or 4 in which said units are of duration equal to the duration of said fixed duration period.

6. A battery charging apparatus as claimed in Claim 2, 3, 4 or 5, in which said control means includes means for generating a "smoothed" value of the off period duration, and means for terminating charging when such "smoothed" value exceeds a predetermined value for more than a predetermined number of cycles.

7. A battery charging apparatus as claimed in any preceding claim in which said predetermined threshold value is stored in a read only memory forming a part of said control means.

8. A battery charging apparatus as claimed in Claim 6, in which a plurality of predetermined threshold values are stored in said read only memory for different battery types and a particular one of said values is selected by the control means according to the type of battery being charged.

9. A battery charging apparatus as claimed in Claim 7, including sensors for reading identifying features provided on a battery, the selection of the predetermined threshold value by the control means being determined by inputs thereto from said sensors.

10. A battery charging apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.