GB2379099A

GB2379099A - Battery charging apparatus and charging process

Info

Publication number: GB2379099A
Application number: GB0120590A
Authority: GB
Inventors: James Samuel Bowen; Mark Corbridge; Olivier Boireau
Original assignee: Sendo International Ltd
Current assignee: Sendo International Ltd
Priority date: 2001-08-24
Filing date: 2001-08-24
Publication date: 2003-02-26
Anticipated expiration: 2021-08-24
Also published as: GB2379099B; GB0120590D0

Abstract

A method of charging a re-chargeable battery (112) includes monitoring a rate of change of battery charge of said re-chargeable battery (112) during a charging process (400); and determining, in response to said step of monitoring, when said re-chargeable battery is charged. This provides the advantages that over-charging of the battery due to ambient temperature changes is avoided. Furthermore, the charging process is not dependent upon the battery being fully discharged before commencing the charging process.

Description

BATTERY CHARGING APPARATUS AND CHARGING PROCESS Field of the Invention This invention relates to a method of monitoring the charging of a battery, and particularly a method of monitoring a rate of change of battery charge (dV/dT) during a battery charging process of a wireless communication product.

Background of the Invention The focus of many electronic devices in today's high technology marketplace is to make them increasingly portable, lighter, more sophisticated with longer battery life. Such devices, for example, include cellular phones, portable or mobile radios, personal digital assistants, MP-3 players, laptop computers etc. In order to make these devices portable, it is necessary to design them to be battery-operated. There are currently two favoured approaches to the battery powering of portable devices: (i) Enable the portable device to be powered by standard off-the-shelf batteries such as'AA'or'AAA' batteries; or (ii) Enable the portable device to be powered by re-chargeable batteries, that can be re-charged in situ, or that can be temporarily disconnected from the portable

device and placed in a battery charger to effect a re- charge of the battery.

In the field of batteries, and in particular battery- charging apparatus and methods, it is known to measure a charging process parameter during a battery re-charging process, in order to prevent any over charging of the rechargeable battery. The known charging process monitoring method is to measure the time a battery has been charging. The charging time is then compared to pre-determined periods to determine how much the battery will/should be re-charged.

The process is flawed in a number of practical scenarios.

One scenario would be, for example, if the battery were not fully discharged before the commencement of charging operation. Consequently, the time it would take to effect a full charge would be less than the time taken to charge a fully discharged battery. Hence, over-charging of the battery will occur which would cause damage to the battery.

An alternative known method of monitoring a parameter in a battery charging operation is by measuring a temperature of the battery during charging. Throughout the charging process, it is known that the temperature of the battery remains predominantly constant until the battery reaches its capacity, or close thereto. At this point the temperature of the battery increases. By sensing this increase in temperature it is possible to

determine when the battery has reached its charging capacity.

However, this alternative method has the disadvantage that the ambient temperature in the battery environment can affect the temperature of the battery. Hence, if the ambient temperature changes suddenly, the temperature of the battery and battery monitoring terminals can change.

This can have the affect of distorting any temperature monitor into determining that the battery has reached its charging capacity.

A need therefore exists for an improved battery charging process wherein the abovementioned disadvantages may be alleviated.

Statement of Invention In accordance with a first aspect of the present invention, there is provided a method of charging a rechargeable battery, as claimed in claim 1.

In accordance with a second aspect of the present invention, there is provided a battery charging apparatus, as claimed in claim 12.

In accordance with a third aspect of the present invention, there is provided a communication device, as claimed in claim 21.

Further aspects of the invention are as claimed in the dependent claims.

In summary, the present invention proposes, inter-alia, a method of charging a re-chargeable battery that incorporates the monitoring of a rate of change of charge in the battery during a re-charging process.

Advantageously, the charging of the battery is directly related to the amount of charge held within the rechargeable battery. Hence, the method does not suffer from the problem of overcharging the battery of the timing-based method described above.

Furthermore, ambient variables such as temperature affect the monitoring of the battery charge substantially less than the known temperature measuring method.

Brief Description of the Drawings Exemplary embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: FIG. 1 shows a charging circuit diagram adapted to support the inventive concepts of the preferred embodiment of the invention.

FIG. 2 shows a graph illustrating the general relationship between the voltage measured across a

battery's terminals versus charging time of the charging circuit of FIG. 1.

FIG. 3 shows a graph illustrating a rate of change of voltage (dV/dT) or charge for the graph of FIG. 2, in accordance with the preferred embodiment of the invention.

FIG. 4 shows a flow chart illustrating a method of monitoring a battery charging process, according to the preferred embodiment of the present invention.

FIG. 5 shows a flow diagram illustrating a method of processing voltage information, according to the preferred embodiment of the present invention.

FIG. 6 shows a graphical illustration of a summation means used in the method of FIG. 5, according to the preferred embodiment of the present invention.

FIG. 7 shows a flow diagram illustrating a method of processing voltage information, according to an alternative embodiment of the present invention.

FIG. 8 shows a graphical illustration of a summation means used in the method of FIG. 7, according to the alternative embodiment of the present invention.

Description of Preferred Embodiments For the purposes of the foregoing description, the terms voltage and charge in relation to a battery are understood to be synonymous and interchangeable.

Referring first to FIG. 1, a charging circuit diagram 100, suitable for use in re-charging, for example, a nickel metal hydride (Ni-MH) battery 112, is shown in accordance with a preferred embodiment of the invention.

The battery 112 is connected to a battery charger 110.

The charging circuit comprises control logic 114, which comprises a monitor 106. In use, control logic 114 receives the voltage signal from the charger 110 at a voltage charge (VCHG) port 102 so that it can detect when a charger is present.

A positive field effect transistor (P-FET) 116 is controlled by the current control port (ICTL) 104 of the control logic 114. When the battery 112 is to be charged, the control logic 114 sets the gate and drain voltages of the P-FET 116 to allow current to pass through to a Schottky diode 118 and resistor 120 to the battery 112.

In operation, during the charging process, the control logic 114 monitors the battery charge 106 by measuring voltage/charge samples 132 across the battery 112 at point'A'130 and ground. When the battery 112 is determined by control logic 114 to be fully charged, the control logic 114 closes the P-FET 116 so that current

can no longer flow to the battery 112. In the context of the preferred embodiment of the present invention, the control logic functionality can be performed by one or more processors, such as a digital signal processor (DSP), a controller or any combination thereof.

Although the preferred embodiment of the present invention is described with respect to measuring voltage samples across resistor 120, a skilled artisan would recognise that many other ways of sampling the battery voltage would be applicable.

Referring now to FIG. 2, a graph 200'illustrating the general relationship between the voltage 210 across the battery 112 versus charging time 220. After an initial surge of voltage 230, the voltage increases at a steady rate 240. As a consequence, it is within the contemplation of the invention that the monitoring process according to the preferred embodiments of the invention can be initiated by, for example, starting the monitoring process after an initial surge and/or starting the monitoring process once the rate of change of battery charge is within say, a threshold range. When the battery becomes fully charged, a second surge 250 in the voltage is effected, caused by a rise in temperature, before the voltage level begins to drop off 260 at a time of t=X 270.

In accordance with the preferred embodiment of the present invention control logic 114 closes the P-FET 116,

at the commencement of the second surge 250 in the voltage measurement, or when the rate of change of voltage becomes negative (i. e. when the voltage actually drops), so that current can no longer flow to the battery 112. In this manner, over-charging of the battery is prevented.

In practice, the voltage change versus time curve of FIG 2 is not smooth, and fluctuations in the charging voltage, in the form of noise can occur. Furthermore, the device itself may produce some noise that affects the voltage line being monitored. Such noise effects may cause the battery charge level to temporarily drop.

Although such temporary drops in the voltage may be only for a short period, and only by a small amount, the drops do cause the dV/dT value to reduce or even become negative. Hence, if the monitoring process detects that temporarily dV/dT=zero, the control logic 114 may assume that the battery 112 is charged.

In order to counteract such noise distortion causing an incorrect assumption that a battery is charged, it is envisaged that the control logic 114 would incorporate: (i) Averaging circuitry, and/or (ii) Historical data of previous charging processes, in order to negate the effects of such noise'blips'. In this manner, the control logic 114 can determine whether the temporary drop is, or is likely to be, a result of

the battery 112 being fully charged or as a result of noise on the charge line.

Referring now to FIG. 3, a graph 300 is shown illustrating the rate of change of voltage (dV/dT), i. e. voltage 310 versus time 320, for the graph of FIG. 2.

The two peaks 330,340 of the curve in FIG. 3 represent respectively the first voltage surge 230 and second voltage surge 250 in FIG. 2.

One embodiment of the present invention, using dV/dT to detect when the battery 112 reaches its capacity, detects when the dV/dT level goes below a predetermined threshold. In accordance with this embodiment, control logic 114 (of FIG. 1) closes the P-FET 116, at the commencement of the second surge in order to differentiate between the initial surge and the second surge that may exceed the predetermined threshold.

Once control logic 114 (of FIG. 1) closes the P-FET 116, the rate of change of voltage being allowed to pass to the battery 112 from the charger 110 falls off until dV/dTO at point X 360.

In the preferred embodiment, the predetermined threshold in the method of detecting when the charge of the battery reaches its capacity, is when dV/dT=zero. However, it is within the contemplation of the invention that the predetermined threshold may be a small positive value, to ensure that a subsequent steady trickle voltage increase,

once the battery is fully charged, does not cause overcharging. In this context, it is noteworthy that it is normal to overcharge a battery by, for example, 5-10%.

This small amount of overcharge is acceptable and does not cause damage to the battery. There is no actual recognised limit to the amount of overcharging, but it is known that if a battery is excessively overcharged, its lifetime is reduced.

Referring now to FIG. 4, a flow chart illustrating a method 400 of monitoring a battery charging process is shown, in accordance with a preferred embodiment of the present invention.

The method 400 includes a first step 402 whereby a voltage sample of the battery 112 is measured during a charging process. This voltage sample is then converted from an analogue level to a digital level in an analogue to digital conversion step 404, as known in the art.

The digital value for the voltage sample is subsequently processed, as in step 406. This voltage sample, at the particular time, is compared to one or more previous voltage samples at respective times, in order to calculate a rate of change of voltage (dV/dT), as shown in step 408. The result of this dV/dT calculation is used to determine, in step 410 whether charging should continue, as in step 412, or finish, as in step 414.

It is envisaged that the voltage processing in step 406 may encompass a mechanism for dampening any temporary fluctuations that may occur in the voltage supply from the battery charger 110.

Referring now to Figure 5, a flow diagram 500 illustrates a method of processing voltage information in accordance with the preferred embodiment of the present invention.

In the preferred embodiment of the present invention, the voltage processing is carried out within the control logic 114 of FIG. 1.

However, it is within the contemplation of the invention that such processing can be performed in any suitable means, for example the charger itself could contain the necessary functionality to determine the charge of the battery that is being charged. Alternatively, the battery may be charged in situ, i. e. whilst being attached to the portable device having the re-chargeable battery operably coupled thereto. Furthermore, it is within the contemplation of the invention that the portable device may be any battery-operated device, whether the battery requires disconnecting before charging or not. As such, the portable device may be a cellular phone, a portable or mobile radio, a personal digital assistant or a laptop computer.

In FIG. 5, conversion means 502 converts a digital value for the voltage into an integer representative of the voltage, say in millivolts (mV). Summation means 504

subsequently interpret the integer representative of the voltage to provide a sequence of measured voltage values per unit time that are used by the calculation means 506 to calculate dV/dT.

Referring now to FIG. 6, a graphical illustration 600 of the summation means 504 is shown in more detail. A buffer 610 is used to store the monitored voltage value.

In the preferred embodiment, the buffer 610 includes 2N sections. The sections in the buffer 610 are preferably divided into two blocks. The first block comprising sections 0 to N-l 630 contains the voltage values in mV for the most recent N voltage samples. The second block comprises sections N to 2N-1 620 and contains the voltage values in mV for the N voltage samples preceding those for which values are held in sections 0 to N-l.

The latest voltage value, taken at time t, is stored in section 0, with subsequent preceding values for previous voltage samples being stored chronologically in the other sections. For each section, the summation means 504 adds together the values in either block to provide two values:

These values are then passed by the summation means 504 to calculation means 506, which calculates dV/dT 640:

In this manner, the rate of change of charge is calculated using average voltage values. In this manner, the effect of any fluctuations or noise in the voltage provided by the charger is significantly reduced.

Referring now to Figure 7, a flow diagram 700 illustrates an alternative method of processing voltage information in accordance with the second embodiment of the present invention. In the second embodiment of the present invention, the voltage processing of step 406 of FIG. 4

is again carried out within the control logic 114 of FIG.

1.

The second embodiment follows the flowchart of the preferred embodiment of FIG. 5, with an additional step of applying an attenuation function {4}, as shown in step 710.

The attenuation function in step 710 takes the voltage value in millivolts for the voltage sample V (t) and subtracts from it the result of the attenuation function

for the preceding voltage sample F (t-l), as shown in equation {4}.

In this manner, the emphasis for calculating an appropriate dV/dT value is made dependent upon the attenuation function and is therefore more easily controllable according to the charging circumstances.

Referring now to FIG. 8, a graphical illustration 800 of the summation means 504, when incorporating the attenuation function of the second embodiment, is shown in more detail. Again, a buffer is used to store the monitored voltage, where the buffer includes 2N sections.

The sections in the buffer are preferably divided into two blocks, although it is within the contemplation of the invention that any number of blocks or samples per block for averaging purposes can be used. The first block comprising sections 0 to N-l 830 contains the voltage values in mV for the most recent N voltage samples. The second block comprises sections N to 2N-1 820 and contains the voltage values in mV for the N voltage samples preceding those for which values are held in sections 0 to N-l.

As mentioned, the attenuation function takes the voltage value in millivolts for the voltage sample V (t) and subtracts from it the result of the attenuation function for the preceding voltage sample F (t-l), as shown in

x equation {4}. The result is subsequently dived by 2 , where k is a definable value that can be varied in order to vary the amount of attenuation required for the attenuation function in equation {4}.

Finally, the result of this division is added to the result of the attenuation function 810 for the preceding voltage sample F (t-l) to give the function value for the present voltage sample F (t).

Where 1, the effect of the attenuation function 810 is to take the difference between the attenuation function based on the voltage value in mV for the present voltage sample V (t) and the result of the attenuation function for the preceding voltage sample F (t-l) and to halve it.

This is then added to the result of the attenuation function of the preceding voltage sample F (t-l). In this manner, the effect of any fluctuations is substantially halved.

As previously indicated, by varying the value of k, the amount of attenuation can be varied. For example, for =2, the effect of any fluctuations would be substantially quartered.

The result of the attenuation function 810 F (t) is performed on each new sample and then used in replacement of the voltage sample V (t) for the first embodiment illustrated in FIGs 5 and 6, and passed on to the summation means 504.

The result of the attenuation function 810 F (t) is placed into section 0 of the first block of the buffer 830, with the results of the attenuation function 810 for the preceding voltage samples being stored chronologically in the other sections of the buffer 820.

For each section, the summation means 504 adds together the values in either block to provide two values:

These values are then passed by the summation means 504 to the calculation means 506, which calculates dV/dT 810:

In this manner, any fluctuations in the voltage from the charger are attenuated firstly by the attenuation function 710,810 and then by the summation means 504.

Due to the attenuation of the voltage processing in step 406, prior to the calculation step 506, there is a substantial reduction in the errors associated with the monitoring means assuming that the battery 110 has reached its capacity when in fact it is only a fluctuation in the voltage provided by the charger.

It will be understood that the monitoring and calculation of a dV/dT value during a charging process, as described above, provides at least the following advantages: (i) avoids over-charging of the battery due to ambient temperature changes; (ii) is not dependent upon the battery being fully discharged before commencing the charging process ; (iii) limits the effect of noise distortion on the accuracy of the measurement process; and (iv) the filtering process allows other applications to be running whilst charging is taking place, without the noise in the voltage caused by these applications causing premature ending of charging. Most of the noise is caused either from the mains supply or from applications on the phone being run at the same time. Such operations require power from the battery, and thus affect the voltage and current.

Whilst the specific, and preferred implementations of the embodiments of the present invention are described above,

it is clear that variations and modifications of such inventive concepts could be readily applied by one skilled in the art.

Thus, an improved battery charging process and apparatus has been described wherein the aforementioned disadvantages associated with prior art arrangements have been substantially alleviated.

Claims

Claims 1. A method of charging a re-chargeable battery, the method comprising the steps of: monitoring a rate of change of battery charge of said re-chargeable battery during a charging process; and determining, in response to said step of monitoring, when said re-chargeable battery is charged.

2. The method of charging a re-chargeable battery according to Claim 1, wherein the monitoring step includes the step of: sampling a voltage across said re-chargeable battery terminals or a battery terminal and a ground potential to monitor a rate of change of battery charge.

3. The method of charging a re-chargeable battery according to Claim 2, the method further comprising the step of: storing said voltage samples in a memory element; and summing a number of said sampled values to provide an indication of a battery charge.

4. The method of charging a re-chargeable battery according to Claim 2 or Claim 3, the method further comprising the step of: averaging a plurality of voltage samples for compensating for effects of noise.

<Desc/Clms Page number 20>

5. The method of charging a re-chargeable battery according to any preceding Claim, the method further comprising the step of: using historical data of previous rate of change of battery charge measurements to determine when said charging process is finished.

6. The method of charging a re-chargeable battery according to any preceding Claim, the method further comprising the step of: calculating when said rate of change of battery charge is less than or equal to zero to determine when said charging process is finished.

7. The method of charging a re-chargeable battery according to any of preceding Claims 2 to 6, the method further comprising the step of: applying an attenuation function to said sampled value in a first time period to provide an attenuated sampled value; and subtracting said attenuated sampled value from a preceding attenuated sampled value related to a preceding time period to provide an attenuated rate of change of battery charge indication during the period between the preceding time period and said first time period.

8. The method of charging a re-chargeable battery according to Claim 7, the method further comprising the step of: replacing said voltage samples by said attenuated rate of change of battery charge sampled values.

<Desc/Clms Page number 21>

9. The method of charging a re-chargeable battery according to Claim 2, the method further comprising the step of: starting said sampling step after an initial surge in said monitored rate of change of battery charge.

10. The method of charging a re-chargeable battery according to Claim 2, the method further comprising the step of: starting said sampling step once the rate of change of battery charge is within a predefined range of values.

11. The method of charging a re-chargeable battery according to Claim 2, the method further comprising the step of: stopping said sampling step after a second surge in said monitored rate of change of battery charge, and in response to said second surge determining that said re-chargeable battery is charged.

<Desc/Clms Page number 22>

12. A battery charging apparatus comprising: a charger for operably coupling to and recharging a re-chargeable battery; monitoring means, operably coupled to said rechargeable battery, to monitor a rate of change of battery charge of said re-chargeable battery during a charging process; and a processor, operably coupled to said monitoring means to determine when said re-chargeable battery is charged.

13. The battery charging apparatus according to Claim 12, wherein said monitoring means includes sampling means to sample a voltage across terminals of said rechargeable battery or a battery terminal and a ground potential to monitor a rate of change of battery charge.

14. The battery charging apparatus according to Claim 13, the apparatus further comprising: a memory element, operably coupled to said sampling means to store said voltage samples; and summing means, operably coupled to said memory element to sum a number of said sampled values to provide an indication of a battery charge of said re-chargeable battery.

15. The battery charging apparatus according to Claim 12 or Claim 13, wherein said processor averages a plurality of samples in order to compensate for noise effects in the monitoring process.

<Desc/Clms Page number 23>

16. The battery charging apparatus according to Claim 14, wherein the memory element stores rate of change of battery charge measurements to allow historical data of previous battery charge measurements to be used to determine when said charging process is finished.

17. The battery charging apparatus according to any of preceding Claims 12 to 16, wherein said processor determines when said rate of change of battery charge is below a pre-determined threshold, for example less than or equal to zero, that said charging process is finished.

18. The battery charging apparatus according to any of preceding Claims 12 to 17, wherein said processor comprises: an attenuator function to attenuate said sampled charge value in a first time period to provide an attenuated sampled value; and subtracting means to subtract said attenuated sampled value from a preceding attenuated sampled value stored in a memory element from a preceding time period to provide an attenuated rate of change of battery charge indication during the period between the preceding time period and said first time period.

19. The battery charging apparatus according to any of preceding Claims 12 to 18, wherein said charger contains said processor and said monitoring means.

21. A communication device comprising are-chargeable battery wherein the communication device or re-chargeable

<Desc/Clms Page number 24>

battery have been adapted for operation in any of method claims 1 to 11 or for use in the apparatus of any of claims 12 to 20.

22. The communication device according to Claim 21, wherein the communication device is one of: a cellular phone, a portable or mobile radio, a personal digital assistant or a laptop computer.

23. A battery charging apparatus substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 1 of the accompanying drawings.

24. A method of charging a re-chargeable battery substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 4 or FIG. 5 or FIG. 6 or FIG. 7 or FIG. 8 of the accompanying drawings.