WO2008075246A2

WO2008075246A2 - Battery low indication

Info

Publication number: WO2008075246A2
Application number: PCT/IB2007/054984
Authority: WO
Inventors: Schelte Heeringa; Oedilius J. Bisschop
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-12-15
Filing date: 2007-12-10
Publication date: 2008-06-26
Also published as: WO2008075246A3

Abstract

An apparatus (A) comprises a battery (3), a load (5), a switch (4) arranged between the battery (3) and the load (5), and a battery charge indicator (1). The battery charge indicator (1) comprises a detector (10) which detects whether a battery voltage (VB) across the battery (3) drops below a threshold value (VTH), an indicator (15) which indicates to a user that the battery voltage (VB) dropped below the threshold value (VTH), and a controller (13) which determines whether the load (5) is connected to the battery (3) to activate the detector (10) for comparing the battery voltage (VB) with the threshold value (VTH) after the load (5) has been disconnected from the battery (3).

Description

Battery low indication

FIELD OF THE INVENTION

The invention relates to an apparatus comprising a battery, a battery charge indicator, and a method of indicating a battery charge.

BACKGROUND OF THE INVENTION

US4,536,757 discloses a device for signaling a specific charge condition of a rechargeable battery. The battery provides a supply- voltage to a motor of a shaving apparatus. When the shaver is switched on by connecting the motor across the battery also the charge-condition indicator is connected across the battery. Now, a comparator circuit compares the battery voltage with a reference voltage. If the battery voltage sensed is lower than the reference voltage, a counter starts up-counting clock pulses. After a predetermined number of clock pulses, and thus after a predetermined period of time, a set-reset flip-flop is set. After the shaver is switched off, the set of the set-reset flip-flop activates a LED (Light Emitting diode) and/or a speaker to produce a warning that the battery charge is too low to the user during a predetermined period of time. The same warning will be provided again when the shaver is switched on again without being charged. The set-reset flip-flop is reset as soon as a charger is connected to the battery.

This prior art battery charge indicator has the drawback that the warning for a too low charge is given too early.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a battery charge indicator which more precisely indicates a low battery charge.

A first aspect of the invention provides an apparatus comprising a battery as claimed in claim 1. A second aspect of the invention provides a battery charge indicator as claimed in claim 3. A third aspect of the invention provides a method of indicating a battery charge as claimed in claim 9. Advantageous embodiments are defined in the dependent claims. An apparatus in accordance with the first aspect of the invention comprises a battery, a load, a switch, and a battery charge indicator. The switch is arranged between the battery and the load to connect and disconnect the load from the battery. For example, if the apparatus is a shaver and the switch is a mechanical switch which is user operable to switch the shaver on and off. Alternatively, the switch may be an electronic switch which is indirectly operated by the user, for example by a small light touch sensor.

The battery charge indicator comprises a detector to detect whether a battery voltage across the battery drops below a threshold value. A controller determines whether the load is connected to the battery. For example, the controller detects whether the mechanical switch between the battery and the load is closed, or the controller implicitly knows the status of the electronic switch because the controller controls the switch, for example in response to a user input. The detector is activated at a sense instant or during a sense period occurring after the load has been disconnected from the battery. Thus it is determined whether the battery voltage is lower than the threshold value only after the switch has disconnected the load from the battery. At this sense instant, the load current through the load is not anymore flowing through the battery. Thus the voltage drop due to the internal resistance of the battery is not influencing the comparison of the battery voltage with the threshold value. If the battery voltage is compared with the threshold value when the switch connects the load to the battery, due to the voltage drop caused by the internal resistance of the battery and the load current flowing through the battery, the battery voltage sensed is too low and the warning for a too low battery charge is provided too early. Further, this voltage drop resulting from the internal resistance of the battery depends on the current drawn by the load. For example, the current drawn by a shaver motor may depend on how firm the user presses the shaver on the skin. This causes an uncertainty about the actual charge still present in the battery. This uncertainty in the load current makes it difficult to compensate for the voltage drop.

An indicator indicates to the user when the battery voltage drops below the threshold value. Due to the indication provided, the user knows that she or he should replace or charge the battery, or that she or he has only a limited time for using the apparatus. For example, the warning may be given that the charge in the battery is sufficient for one next shaving operation only.

A battery charge indicator in accordance with the second aspect of the invention comprises the indicator, the detector, and the controller of the apparatus.

In an embodiment, the controller activates the detector to compare the battery voltage with the threshold value within 0.5 second after the load has been disconnected from the battery. Although it is possible to compensate for the relaxation of the battery after the load has been disconnected, it is easier to activate the detector immediately after the load is disconnected. Immediately after the load is disconnected, the relaxation of the battery is minimal and no compensation for the relaxation is required. Usually, for most rechargeable batteries, the relaxation has not progressed too much within 0.5 seconds.

In an embodiment, if the detector has been activated and has detected that the battery voltage dropped below the threshold value, the detector controls the indicator to provide the indication after the load has been disconnected from the battery. Thus the user is warned about the low charge of the battery after the load has been switched off. This has the advantage that the user gets the indication after use of the apparatus. For example, during the shaving operation the user may not be able to see whether a LED is lit or may not be able to detect a warning sound because of the noise the shaver is producing due to the shaving action. After he removed the shaver from near his face and switched it off, the user can easily see or hear the indication. In an embodiment, the battery charge indicator further comprises a memory element for storing the event that the detector has detected that the battery voltage dropped below the threshold value. Now, if the memory element indicates that the detector has detected that the battery voltage was below the threshold value after the load was disconnected from the battery, the indicator is able to provide the indication when the load is connected again to the battery. This indication when again connecting the load to the battery reminds the user of the low battery situation. For example, the user who forgot to charge the battery of the shaver after the indication provided when the shaver is switched off, when switching on the shaver later is reminded that this is the last time a shaving operation can be performed before the battery has to be charged. In an embodiment, the battery charge indicator further comprises a charge detector to detect whether the battery is being charged. The memory element is a set-reset flip-flop which has a set input coupled to the detector to set the set-reset flip-flop if the detector has detected that the battery voltage dropped below the threshold value. The set-reset flip-flop has a reset input coupled to the charge detector to reset the set-reset flip-flop if is detected that the battery is being charged. Thus, only after detecting that a charging of the battery is started or performed, the detection whether the battery voltage is lower than the threshold voltage is again performed when the load is disconnected from the battery. Otherwise, once a battery voltage is detected lower than the threshold voltage, this is stored and the indication will be presented every time the load is connected to the battery. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 shows a schematic block diagram of a charger and an apparatus comprising a rechargeable battery and a battery charge indicator,

Figs. 2 A to 2D show schematically signals occurring in the block diagram of Fig. 1, Fig. 3 shows schematically a shaver and a mains adapter.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

DETAILED DESCRIPTION

Fig. 1 shows a block diagram of a charger and an apparatus comprising a rechargeable battery and a battery charge indicator.

The charger 6 comprises a transformer with a primary winding 60 coupled to the mains. A rectifier 62 is coupled to the secondary winding 61 of the transformer to supply a DC-output voltage suitable for charging the battery 3 of the apparatus. The charger 6 shown is an example only and any other construction which allows charging the battery 3 from the mains or another battery may be used. For example, the charger 6 may comprise a switch- mode power converter. The apparatus comprises the battery 3, the load 5 and a switch 4 arranged between the battery 3 and the load 5. The switch 4 is a mechanical or electronic switch for connecting or disconnecting the load 5 from the battery 3. For example, if the apparatus is a shaver wherein the load 5 comprises the shaver motor, the switch 4 may be a user activated mechanical switch or an electronic switch which is indirectly controlled by the user. It has to be noted that the present invention is not limited to a shaver with a rechargeable battery 3. The present invention is relevant to any apparatus with a battery 3 in which a load 5 is switched on and off. The battery 3 may be a rechargeable battery, a primary non-rechargeable battery, or a super-capacitor. The apparatus may for example be any battery operated apparatus in which it is relevant to be warned that the battery charge is low. Examples of such apparatuses are any apparatus for use in personal care or in hand-held applications like PDA's, mobile phones.

A control circuit 13 determines whether the switch 4 is closed or not. For example, if the switch 4 is a user operated mechanical switch, the control circuit 13 may determine the voltage across the motor 5, or the current through the motor 5. Alternatively, if the switch 4 is an electronic switch, the user of the apparatus indicates indirectly that the switch 4 should be closed. For example by just picking up the apparatus or by pressing another switch (not shown) which need not be dimensioned to switch the relatively large current through the load 5. Now, the control circuit 13 controls the switch 4 in response to the user input and thus knows when the load 5 is connected to the battery 3. The control circuit 13 supplies a control signal VMI which indicates a sense instant or a sense time period. The control circuit further supplies a switch status signal SWS which is the signal controlling the switch or which is another signal indicating whether the switch 4 is closed or open.

The detector 10 compares the battery voltage VB across the battery 3 with a threshold value VTH. This comparison takes place at the sense instant or during the sense time period controlled by the control circuit 13 with the signal VMI. Thus, the detector 10 performs the comparison after the switch 4 is opened and the load current IL does not flow through the battery 3 anymore. This has the advantage that the voltage VB across the battery 3 is not disturbed by the internal resistance of the battery 3. If the battery voltage VB is sensed when the switch is closed, this would give rise to a too low voltage due to the voltage drop caused by the load current IL through the internal resistance of the battery 3. Consequently, it would be detected that the battery charge is below a particular level too early. In an embodiment, the level of the threshold value is selected such that sufficient charge is left in the battery 3 to be able to perform at least one operation cycle. For example, if the apparatus is a shaver, sufficient charge is left to be able to do one average shaving operation, for example of 3 to 5 minutes.

The voltage detector 10 supplies a set signal SET to a set input S of a set-reset flip-flop 11 (further also referred to as SR-FF 11). The output of the SR-FF 11 supplies an output signal Q to control an indicator 15 via an optional timing circuit 12. Thus, if the detector 10 detects after switch-off of the load 5 that the voltage VB across the battery 3 is below the threshold voltage VTH, the indicator 15 receives a signal IND which activates the indicator 15 to provide the battery low indication to the user of the apparatus. For example, the indicator 15 provides a visual and/or acoustic and/or tactile indication, for example by blinking a LED, generating a sound with a loudspeaker, or creating a movement sensation for example by interruptingly operating the motor, or with an eccentric weight like in a mobile phone.

The optional timing circuit 12 may take care that the indication is provided during a predetermined period in time only, for example for 2 seconds only, or repetitive, for example once every second or minute. In an embodiment, the timing circuit 12 controls the indicator 15 such that when the set-reset flip-flop 11 is set the indicator 15 is activated during a predetermined period in time after the switch 4 is opened. Thus, when during the closed switch 4 the battery voltage VB drops below the threshold VTH, the indicator 15 is activated during the predetermined period in time after the switch 4 is opened. Let it be assumed that the battery 3 is not connected to a charger during the period in time the switch 4 is opened. Now, when the switch 4 is closed again, the set-reset flip-flop 11 is still set and the indicator 15 will be activated during the complete on-time of the switch S4. When the switch 4 is opened again, the indicator 15 stays on during the predetermined period in time.

The SR-FF 11 acts as a memory which keeps its set state as long as no reset occurs. Thus, when the switch 4 is closed again, still the set state is present and again the indication will be provided to the user. Consequently, the user is remembered when using the apparatus again that this is the last use possible and that he or she should exchange or recharge the battery 3.

The SR-FF 11 has a reset input R which receives a reset signal when the battery 3 is exchanged or when the battery 3 is being charged. This reset signal causes the SR-FF 11 to be reset and no indication will be provided until, after switch off of the motor 5 by switch 4, is detected that the battery voltage VB is below the threshold value VTH. The reset signal may originate from a mechanical switch (not shown) which is activated when the battery 3 is removed or when a charger 6, such as for example a mains adapter, is connected to the apparatus. Alternatively, the reset signal may be generated electronically by a reset circuit 14 which, for example, detects the voltage VB across the battery 3. As soon as the voltage VB rises above a predetermined value it can be concluded that the charger 6 is connected or activated. Alternatively, it may be detected whether a charge current is flowing into the battery 3. Fig. 1 shows, by way of example, a mains adapter as the charger 6. The mains adapter comprises a transformer with a primary winding 60 connected to a mains plug (not shown) to receive the mains voltage when the mains plug is inserted into a mains outlet. The transformer further comprises a secondary winding 61. In the embodiment shown, the charger 6 further comprises an optional rectifier 62 coupled between the secondary winding to supply a DC- voltage to an output terminal of the charger 6. Usually, this DC voltage is connected via a cord and a plug to the apparatus. The cord, the plug on the cord, and the contra plug of the apparatus are not shown. Only the input terminals INl and IN2 where the apparatus receives the DC-voltage are shown. Although the rectifier 62 is shown to be a single diode 62, the rectifier 62 may be a bridge rectifier.

Alternatively, the charger 6 may comprise a switched-mode power converter. The charger 6 may be an integral part of the apparatus. If the charger and the apparatus are separate entities, usually, the apparatus comprises a diode 2 which is connected between one of the input terminals INl and the battery 3. The diode 2 is poled to enable the charger 6 to charge the battery 3 and to prevent discharging the battery when the input terminals INl and IN2 are short circuited.

Figs. 2 A to 2D schematically show signals occurring in the block diagram of Fig. 1. In all Figs. 2 the time t is increasing in the right direction along the horizontal axis. Fig. 2A shows the status of the switch 4, Fig. 2B shows the battery voltage VB, Fig. 2C shows the SET state of the SR-FF 11, and Fig. 2D shows the signal IND supplied to the indicator 15.

Fig. 2A shows the status of the switch 4. A high level indicates that switch 4 is closed, a low level indicates that switch 4 is open. Thus, the switch 4 is being closed at the instants tl and t5 and is being opened at the instant t2. The switch 4 may be directly or indirectly user operated like, for example in an electrical shaver, an electrical tooth brush, a PDA, or a laptop.

Fig. 2B shows the battery voltage VB. At the instant tl when the switch 4 is being closed, the load 5 is being connected to the battery 3 and the load current IL starting to flow through the battery 3 causes an immediate voltage drop determined by the internal resistance of the battery 3 and the load current IL flowing. The switch 4 is opened at the instant t2 and the load current IL stops flowing. The voltage drop due to the internal resistance of the battery 3 disappears immediately, the battery 3 starts relaxation and the battery voltage VB rises.

During the period in time lasting from instant tl to t2 the battery 3 is discharged by the load current IL and thus the battery voltage VB drops. At the instant t3, later than the instant t2, the detector 10 compares the battery voltage VB with the threshold value VTH. It has to be noted that after the instant t2, the battery voltage VB is not anymore disturbed by the voltage drop due to the internal resistance of the battery 3. In the example shown, it is detected at instant t3 that the battery voltage VB is below the threshold value VTH and the SR-FF 11 is set. The set of the SR-FF 11 is indicated by the active SET state in Fig. 2C which SET state changes its low level at instant t3 to the high level. The set of the SR-FF 11 causes the indication to become active at instant t3 as shown by the change of the signal IND to the high level at this instant in Fig. 2D. In Fig. 2D, the timing circuit 12 provides in response to the set state of the SR-FF 11 an active indication during the active period TA which lasts from the instant t3 to the instant t4. The SR-FF 11 stays in the set state until a reset is received.

At the instant t5, as shown by the high level in Fig. 2A, the switch 4 is closed again. Fig. 2B shows that the battery voltage VB at the instant t5 behaves similar as at the instant tl . Due to the internal resistance of the battery 3, immediately a voltage drop occurs which depends on this resistance and the load current IL. Further, the battery voltage VB starts decreasing due to the discharge of the battery 3 by the load current IL.

Because the SR-FF 11 is in the set state at the instant t3 while no reset occurred, it is not detected that the battery voltage VB rises above the threshold value VTH during the period in time lasting from the instant t3 to the instant t5. This is an important issue, because only at, or during a limited time period after, the instant t2 the exact charge state of the battery 3 is known. After relaxation, the battery voltage VB is not anymore an accurate indication of the actual amount of charge left in the battery 3.

It has to be noted that the sense instant t3 need not coincide with the instant t2. Usually the relaxation of the battery 3 is a slow process and it suffices to select the sense instant t3 not later than 0.5 seconds after the switch-off instant t2. Although, if the relaxation effect can be compensated, for example because the relaxation behavior of the battery 3 is well known, it is possible to select the sense instant t3 later than 0.5 seconds after the switch- off instant t2. But, of course, in the simplest implementation, such compensation is not attractive.

At instant t5, the indication is activated because the SR-FF 11 is still set. Consequently, the user is immediately alerted that the battery 3 has only a limited amount of charge left and should be (re-)charged. It has to be noted that the warning is issued independent on the actual battery voltage VB at the instant the switch 4 is closed. The SR-FF 11 is reset (not shown) whenever it is detected that the battery 3 is being charged, for example by detecting that a charger 6 is connected to the apparatus. After the battery charge indicator has been reset, its state is identical to that before the instant tl . Fig. 3 shows schematically a shaver SVR and a mains adapter 6. The mains adapter 6 has a mains plug PLG which fits in a mains socket, a cord CRD for transferring the charge voltage, and a connector OTL cooperating with a connector (not shown) of the shaver SVR which is coupled to the battery 3 of the shaver SVR. The shaver SVR has a user operated mechanical switch 4 which is arranged to connect or disconnect the motor 5 to or from the battery 3. The motor 5 drives the shave heads SH. The electronics required to perform the functions of the circuits shown in Fig. 1 may be provided on the printed circuit board PCB.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, although in the embodiment shown in Fig. 1 separate functional blocks are shown, the operations described may be performed all or partly with a suitable programmed (micro)processor. The memory function need not be a set-reset flip-flop, but may alternatively, for example, be a register of a processor.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. An apparatus (A) comprising a battery (3), a load (5), a switch (4) arranged between the battery (3) and the load (5), and a battery charge indicator (1) comprising: a detector (10) for detecting whether a battery voltage (VB) across the battery (3) drops below a threshold value (VTH), an indicator (15) for indicating to a user that the battery voltage (VB) dropped below the threshold value (VTH), and a controller (13) for determining whether the load (5) is connected to the battery (3) to activate the detector (10) for comparing the battery voltage (VB) with the threshold value (VTH) after the load (5) has been disconnected from the battery (3).

2. An apparatus as claimed in claim 1 being a shaver, wherein the load is a shaver motor.

3. A battery charge indicator for use in the apparatus of claim 1 and comprising: the indicator (15) for indicating the charge present in the battery (3) to the user, the detector (10) for detecting whether the battery voltage (VB) across the battery (3) drops below the threshold value (VTH), and being coupled to the indicator (15) for indicating that the battery voltage (VB) dropped below the threshold value (VTH), and the controller (13) for determining whether the load (5) is connected to the battery (3) to activate the detector (10) for comparing the battery voltage (VB) with the threshold value (VTH) after the load (5) has been disconnected from the battery (3).

4. A battery charge indicator as claimed in claim 3, wherein the controller (13) is constructed for activating the detector (10) for comparing the battery voltage (VB) with the threshold value (VTH) within 0.5 second after the load (5) has been disconnected from the battery (3).

5. A battery charge indicator as claimed in claim 3, wherein the detector (10) is constructed for controlling the indicator (15) to provide the indication after the load (5) has been disconnected from the battery (3), and the detector (10) has been activated and has detected that the battery voltage (VB) dropped below the threshold value (VTH).

6. A battery charge indicator as claimed in claim 3, further comprising a memory element (11) for storing that the detector (10) has detected that the battery voltage (VB) dropped below the threshold value (VTH), and wherein the indicator (15) is arranged to provide the indication when the load (5) is connected again to the battery (3) if the memory element (11) indicates that the detector (10) has detected that the battery voltage (VB) dropped below the threshold value (VTH).

7. A battery charge indicator as claimed in claim 3, wherein the detector (10) is a comparator, and the indicator (15) is constructed to provide a visual and/or acoustic and-or tactile indication.

8. A battery charge indicator as claimed in claim 6 and 7, further comprising a charge detector (14) for detecting whether the battery (3) is being charged, and wherein the memory element (11) has a set input (S) coupled to the detector (10) for setting the memory element (11) if the detector (10) has detected that the battery voltage (VB) dropped below the threshold value (VTH), and a reset input (R) coupled to the charge detector (14) for resetting the memory element (11) if is detected that the battery (3) is being charged.

9. A method of indicating a battery charge, the method comprising: detecting (10) whether a battery voltage (VB) across the battery (3) drops below a threshold value (VTH), indicating (15) to a user that the battery voltage (VB) dropped below the threshold value (VTH), and determining (13) whether the load (5) is connected to the battery (3) to activate the detector (10) for comparing the battery voltage (VB) with the threshold value (VTH) after the load (5) has been disconnected from the battery (3).