HK1035967B - Battery pack protection circuit and battery pack including a protection circuit - Google Patents

Description

Battery pack protection circuit and battery pack including the same

Background

The present invention relates generally to portable telephones and other electronic devices that are powered by an external battery pack, and more particularly, to a protection circuit for preventing accidental shorting of exposed contacts of an external battery pack when disconnected from the telephone or charging unit.

Background

The back of the portable phone typically has spring contacts for mounting to mate with flat contacts of an external battery pack when the external detachable battery pack is secured to the phone. Various releasable securing means have been used to secure such battery packs to portable telephones. When recharging is required, the battery pack is loaded into the charging device with similar spring contacts.

One problem with such external batteries is that they are occasionally short-circuited by the user when they are disconnected from the phone or charging unit. For example, most such battery packs, such as Li-ion battery packs, have built-in (build in) self-protection circuits for preventing overcharge or overdischarge of the battery pack. However, when the spare battery pack is carried around in a user's purse, pocket, or handbag, an accidental short circuit may occur in the event that a loose coin or key contacts the positive and negative exposed contacts. This situation does not start the built-in self-protection circuit, since the threshold of such a short circuit must be higher than the operating discharge current under normal conditions, i.e. if the peak current of the device is 1amp, the self-protection circuit of the battery must be switched off (trip) at a current higher than 1 amp. In such a battery pack, the current self-protection circuit cannot be activated.

Summary of The Invention

It is an object of the present invention to provide a novel and improved protection circuit for an external battery pack, in particular for a portable telephone battery pack.

According to the present invention there is provided a battery protection circuit comprising a switching component having an input for receiving a control signal, a detector for detecting an impedance at a load terminal, and a control component, wherein said switching component controls connection of a battery output to the load, a control component for determining whether said impedance has a value between a predetermined maximum value and a predetermined minimum value, and generating a control signal in response to said determination, said control signal causing the switch to close if said determination is positive and otherwise causing the switch to open, thereby connecting said battery output to the load only if said detected impedance has a value between said predetermined minimum and maximum values.

Preferably, the control assembly includes a first comparator for comparing the impedance with a predetermined maximum value and generating a first output signal if said impedance is above the predetermined maximum value and a second output signal if said impedance is below the predetermined maximum value; a second comparator for comparing the impedance with a predetermined minimum value and generating a third output signal if the impedance is below the predetermined minimum value and a fourth output signal if the impedance is above the predetermined minimum value; and a second detector connected to the outputs of the first and second comparators for generating a control signal for closing the switch when the impedance is between the minimum and maximum values.

Preferably, the battery powered load has positive and negative input contacts connected to the positive and negative outputs of the battery, respectively, and an ID output to which a predetermined load identifying impedance is applied. When the battery is connected to an appropriate load by a battery protection circuit, the ID output is connected to first and second comparators which compare its impedance to predetermined minimum and maximum values and permit power to be supplied from the battery only when the detected impedance is between the minimum and maximum values.

In a preferred embodiment of the invention the circuit further comprises a self-protection control unit for controlling the discharge of the battery, the control unit having an input, an over-discharge output and a switch assembly comprising a first switch controlling the series connection of the battery and the load in the discharge mode, and a second switch disabling the first switch if the load ID impedance is detected to be outside a predetermined range between a minimum and maximum value, wherein the first switch has a control input connected to the over-discharge output of the control unit. The second switch may be arranged to control the power input to the control unit so as to disable the control unit itself when the detected impedance is outside a predetermined range. Or between the overdischarge output of the control unit and the first switch to disable the control input to the switch in case the detected impedance is outside a predetermined range.

The control unit may further have an overcharge output, and a third switch for controlling charging of the battery is connected in series between the charge input and the battery input, the third switch having a control output connected to the overcharge output. The control unit is arranged to open the third switch when the battery is charged to a predetermined value. A fourth switch, also controlled by an output from the second detector, may be connected between the overcharge output and the third switch so that the battery cannot be charged or discharged if the detected ID impedance is outside a predetermined range.

On the other hand, the function of the self-protection control unit can be completely separated from the switch assembly. In another embodiment, an additional switch is provided to control the series connection of the battery to the load independently of the first switch and the self-protection control unit. The additional switch is responsive to a control signal from the control component to open when the detected impedance is outside a predetermined range.

The battery protection circuit will prevent the battery from being accidentally short-circuited when not in use, such as by a key, coin, etc. The discharge is only possible when the circuit detects a connection to a load with a suitable ID impedance.

Drawings

The present invention will be better understood from the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts, and in which:

fig. 1 shows a battery protection circuit according to a first embodiment of the present invention;

FIG. 2 shows a modified battery protection circuit

Fig. 3 shows another modified battery protection circuit; and

fig. 4 illustrates a battery protection circuit according to another embodiment of the present invention.

Description of the preferred embodiments

Fig. 1 shows a battery protection circuit 10 according to a first embodiment of the present invention, which is connected between a load 12, such as a portable telephone or a battery charger, and a primary battery (cell)14 of the type commonly used to power portable telephones and other electronic devices, such as video game units. The battery pack housing has a series of exposed contacts 15, 16, 17, 18 which include a positive input terminal V +, a negative input terminal V-, an ID terminal, and a thermistor terminal, respectively. A set of flat contacts may be provided on the phone and charger unit so that the battery pack may be releasably connected to the portable phone or charger unit as desired.

The circuit 10 includes a first connection line 20 between the V + contact 15 and a positive battery output 22, and a second connection line 24 between the V _ contact 16 and a negative battery terminal 26. In the telephone and charger unit, a predetermined ID impedance is provided at the telephone or charger ID terminal 17 by connecting a resistor Rp between the V + contact 15 and the exposed ID contact 17 of the telephone and charger unit. Circuit 10 is mounted to prevent battery 14 from discharging unless the detected impedance is in a range between a predetermined minimum value of Rp-D and a maximum value of Rp + D, where D is determined from normal variations in the phone, charger and battery impedance values.

A battery temperature resistance (temperature resistance) or battery thermistor Rt is connected between the contact 18 and the wire 24. Many battery packs for portable telephones, portable electronic devices, and the like already have a built-in self-protection IC28 and a pair of switches 30, 32 in line 24 for controlling the discharge and charge, respectively, of the battery pack or cell 14 (which may be a Li-ion cell or the like). However, in the prior art arrangement, line 20 is connected directly to power input 34 of IC 28. The switches 30, 32 are preferably FETs. IC28 has an Overcharge (OCD) output 36 connected to the gate of switch 30 to open the switch and prevent further charging if the battery pack voltage exceeds a safe limit. The Overcharge (ODD) output 38 of the IC28 is connected to the gate of the FET32 to open a switch and prevent discharge of the battery pack when the current or voltage exceeds a safe limit. The usual operation of such a self-protection IC, which is typically provided in an external battery pack, is to allow the FETs 30 and 32 to allow charging and discharging of the battery pack when the battery pack current and voltage are within safe limits, as will be understood by those skilled in the art.

The prior art arrangement described above does not prevent accidental cell discharge in the event that some metal objects cause a short circuit between the battery contacts 15 and 16. The circuitry in the embodiment of fig. 1 additionally provides protection against such accidental battery discharge. In this embodiment, a third switch 40 is connected between the power line 20 and the power input 34. Such a switch is responsive to the battery enable input 42 from the phone/charger identification system 44 so as to turn on the IC28 only when the system 44 detects the connection of a load such as a portable phone or charger unit having an ID impedance Rp within a predetermined range, as explained in detail below. The switch 40 is also preferably a FET.

The phone identification circuit 44 has a phone ID input 46 connected to the phone ID terminal 17, wherein an impedance Rp should appear at the phone ID terminal 17 if a battery pack is connected to the portable phone or charger input. The phone or charger ID resistor Rp forms a voltage divider circuit with resistor R1 and provides a divided voltage to the upper limit comparator 50 as the first input 56 and a first input 48 to the lower limit comparator 54. The first voltage divider circuit, comprising resistors R2 and R3, sets the upper limit voltage (Rp + D) and provides a second input 52 to the upper limit comparator 50. The second voltage divider circuit, comprising resistors R4 and R5, sets the lower limit voltage (Rp-D) and provides a second input 58 to the lower limit comparator 54. The outputs RH, RL of comparators 50 and 54 are provided as inputs to or gate 60, which in turn provides the battery enable input or control signal 42 to the gate of FET 40.

By adjusting the resistors R1-R5, R of the telephone and charger units_icA limit Rp +/-D is set. These resistors should be made as high as possible while keeping the required accuracy at the highest and lowest thresholds, otherwise excessive current will be continuously drawn from the battery. Alternatively, the voltage divider circuit may be replaced by a two reference voltage (voltage reference), but this approach is more costly than using R2-R5, and the current may be high depending on the reference selected.

The following logic or truth table provides the possible states of OR gate 60, where Rd is the detected electrical at terminal 17The resistor value.

Resistor value	RH	RL	BATT.EN.	Evaluation of
					Rd≤R-D	0	1	1	The detected impedance is below a lower threshold. FET40 is on and IC28 is off.
Rp-D＜Rd＜Rp+D	0	0	0	The detected impedance is within range. FET40 turns off and allows the battery pack.
					Rd≥Rp+D	1	0	1	The detected impedance is above a high threshold. FET40 is turned on and IC is turned off.

From the above table, it can be seen that unless the detected impedance at input 46 is between the low (Rp-D) and high (Rp + D) impedance limits, the IC28 controlling the charging/discharging FETs 30, 32 will be disabled. Since IC28 is disabled, FETs 30 and 32 are turned off and the battery cannot be discharged or charged.

If the voltage across R1 is between the high and low limits, the output from OR circuit 60 will produce a voltage on battery enable input 42 that turns off FET40, thereby enabling IC 28. This turns off FETs 30 and 32, allowing current (voltage) to appear at the external negative terminal or contact 16 of the battery pack.

When the battery pack is not attached to the phone or charging unit, Rp will be infinite and there will be virtually no voltage across R1. Thus, the voltage at R1 will be out of range and IC28 will not be activated. FETs 30 and 32 remain open and little or no current flows from the current. Thus, the battery is not short-circuited.

The embodiment of fig. 1 may be used with any battery self-protection IC28 that does not have to be powered at all times. However, some manufacturers make battery pack overcharge/overdischarge protection ICs that require them to provide a voltage to the IC that remains above a predetermined voltage, otherwise the IC will not function properly after power is restored. These manufacturers' battery self-protect ICs have latch circuits (latches) that prevent the IC from turning on after power is restored (unless a predetermined voltage is reached). Thus, the circuit of fig. 1 cannot be used with a self-protected IC having a built-in latch circuit, and the circuit of fig. 2 or 3 can be used in this case.

Fig. 2 shows a modified battery protection circuit 100 that can be used with a battery self-protection IC280 that has a built-in latch circuit and must be powered at all times. Apart from this requirement, IC280 of fig. 2 functions similarly to IC28 of fig. 1. Many of the elements in fig. 2 are the same as in fig. 1 and like reference numerals are used to identify corresponding elements.

In the case ofIn an embodiment, a third switch or FET40 is located between the ODD output 38 of the IC280 and the gate of the second FET 32. As in the previous embodiment, the provision of the battery enable output 42 of the battery identification circuit 44 serves as a control input at the gate of the FET 40. The logic table for this embodiment is as follows:

resistor value	RH	RL	BATT.EN.	Evaluation of
					Rd≤Rp-D	0	1	1	The detected impedance is below a low threshold. FET40 is turned on and FET32 is disabled. Normal discharge is prohibited.
Rp-D＜Rd＜Rp+D	0	0	0	The detected impedance is within range. FET40 is off and FET32 is enabled. Battery +/-and discharge enable.

Rd≥Rp+D

1

0

1

The detected impedance is above a high threshold. FET40 is turned on and FET32 is disabled. Normal discharge is disabled.

With this arrangement, the IC280 is no longer powered down, but the discharge FET32 is disabled when the battery pack is not connected to an impedance Rp within a predetermined range. This approach is less advantageous than that of fig. 1 because self-protected IC280 is often on and powered, but can be used in all cases where the IC must be powered on (power on) at all times.

The embodiment of fig. 2 allows the battery pack to charge if the charging voltage is within safe limits, regardless of whether the correct ID impedance Rp is present, but allows discharge only when the battery is connected to a phone or other predetermined load or charger unit. In contrast, the embodiment of fig. 1 does not allow charging or discharging unless the detected impedance Rp is within a predetermined range. In some cases, the embodiment of FIG. 2 has advantages over the embodiment of FIG. 1. If an abnormal phenomenon occurs such that the battery voltage is lower than the normal operation range, the battery can be charged even when the battery recognition circuit is not functioning. In this case, the cell may be charged through a parasitic diode connected across the source-drain of FET 30.

In one particular example of a battery protection circuit as shown in fig. 2, the battery self-protection IC280 is RM127C manufactured by Ricoh corporation. A dual comparator 50, 54, which is a MAX966EUA manufactured by Maxim, inc, is provided in the IC 70. The or gate 60, in this case TC7SL32FU manufactured by Toshiba corporation, is provided by IC 72. These ICs require additional resistors R6 and R7 as shown in fig. 2. Since some of the comparator outputs are open drains (open drains), R7 AND IC72 may also be removed AND the comparator outputs connected together to produce an AND for each comparator in IC 70. The impedance Rp is selected according to the nominal impedance value provided by the phone. In one particular example, Rp is 130Kohm in the nominal case, and D is chosen to be 6%. However, it should be understood that other impedance values may be used depending on the battery pack and phone used. In this particular example, the following impedance values are used in the protection circuit:

R1： 150kohm；

R2： 1.5Mohm；

R3： .5Mohm；

R4： 5Mohm；

R5： 1.5Mohm；

R6： 510kohm；

R7： 510kohm；

R_pi： 130kohm

it should be understood that the above values are only examples and that other components and other nominal impedance values with equivalent functionality may be used to meet certain requirements when the correct ID impedance occurs within predetermined limits.

Fig. 3 shows another embodiment that functions more similarly to fig. 1, but which allows the self-protected IC280 to be powered up at all times. Again, the same reference numerals are used for the same parts in this embodiment.

This embodiment is similar to that of fig. 2, with FET40 connected between the ODD output 38 of IC280 and the gate of discharge FET 32. However, in this embodiment, a fourth switch or FET62 is connected between the OCD output 36 of IC280 and the gate of charge FET 30. A battery load enable output 42, or a telephone identification circuit 44, is connected to the gates of FETs 40 and 62 to inhibit charging and discharging FE's if the detected impedance is outside a predetermined rangeT30 and 32. The logic table for this embodiment is as follows:

resistor value	RH	RL	BATT.EN.	Evaluation of
					Rd≤Rp-D	0	1	1	The detected impedance is below a low threshold. FETs 40 and 62 are turned on, prohibiting normal discharge and charging.
Rp-D＜Rd＜Rp+D	0	0	0	The detected impedance is within range. FETs 40 and 62 are turned off and the battery pack is enabled.
					Rd≥Rp+D	1	0	1	The detected impedance is above a high threshold. FETs 40 and 62 are openOn, normal discharge and charging are prohibited.

The embodiment of fig. 3, like the embodiment of fig. 1, switches off the charge FET30 and the discharge FET32 when the detected impedance Rp is outside a predetermined limit range. The ODD and OCD output pins of the IC280 operate in a normal manner to allow discharge and charge of the battery pack if the impedance Rp is within predetermined limits and when the current and voltage of the battery are within safe limits.

In the embodiment of fig. 1 to 3, the protection circuit works in conjunction with the battery pack self-protection circuit, i.e., the battery self-protection IC28 or 280, and the charge and discharge FETs 30 and 32. However, it has been found that in many cases, it is desirable that the function of the interlock or battery protection circuit be separated from the self-protection function of the IC28 or 280. In some cases, in a circuit as arranged in fig. 1-3, a self-protecting IC may falsely detect a circuit condition if an interlock or self-protecting circuit overrides the control of FET32 or if the Vdd input to the self-protecting IC is opened or closed by the interlock or protecting circuit. Fig. 4 shows another embodiment of the invention in which the potential problem is overcome. Again, in this embodiment, the same reference numerals are used for the same parts. In the embodiment of fig. 4, the problem of potentially improper functioning of the self-protected IC28 is overcome by adding a third FET80 in series with FETs 30 and 32, and connecting the battery enable output 42 from the telephone identification circuit 44 to the gate of FET 80. FET80 is preferably an n-channel power FET.

The phone identification or interlock circuit 44 of fig. 4 is similar to that of the previous embodiment except that the or gate 60 of the previous embodiment is replaced by an and gate 82 and the positive and negative inputs of the two comparators 50 and 54 in IC70 are reversed to allow the interlock circuit to control an n-channel FET instead of the p-channel FET used in the previous embodiment. The phone ID input 46 to the circuit 44 is connected to the phone ID terminal 17 where the impedance Rp appears when the phone or charger unit is properly connected to the protection circuit. Input 46 is connected to the positive and negative inputs of comparators 50 and 54, respectively. As in the previous embodiment, the first voltage divider circuit R2, R3 sets the upper limit voltage (Rp + D) and provides it as a second input 52 to the upper limit comparator 50. The second voltage divider circuit R4, R5 sets the lower limit voltage (Rp-D) and provides a second input 58 to the lower limit comparator 54. The outputs RH and RL of comparators 50 and 54 are then provided as inputs to and gate 82, which in turn provides battery enable signal 42 to the gate of FET 80.

The circuit of fig. 4 also includes two additional FETs 84 and 86 that are used to disable the interlock circuit 44 until the battery pack is connected to the phone or charger, which provides a threshold at the phone ID input 88 that is high enough to enable the FET 84. FET86 has a control output 90 connected to the power supply inputs of IC70 and gate 80 so that if the gate-source voltage of FET86 is zero, interlock circuit 44 is turned off and the battery pack output is disabled. When the phone ID impedance is present, FET84 is turned on, which in turn turns on FET86, supplying power to the power supply inputs of the interlock circuit elements 70 and 80 so that they operate to determine whether the impedance is within the desired range. This arrangement allows the circuit to be powered down when not needed, reducing power consumption.

R_icThe phone is allowed to effectively measure the ID voltage so that the phone will recognize that a battery is present, over-discharge and allow it to be charged again. R is a handle_icSet to a value low enough not to sufficiently affect the measurement of the ID resistor and yet high enough to limit current if FET32 is turned on. Of course a similar R can be used in the circuits of fig. 1-3_icWherein R is shown with broken lines in the figure_icTo the circuit.

Optionally, a resistor R9 may be connected across the drain-source of FET80, as shown in dashed lines in fig. 4. Resistor R9 allows the phone to enable and disable the battery pack by turning Rp in and out of the circuit with a transistor. Resistor R9 allows a small amount of current to flow from the battery to the circuitry in the phone, which allows and disables the Rp connection between the ID and the V + pin of the battery pack. Resistor R9 provides a ground reference between the battery pack and the phone for this function. The addition of resistor R9 keeps current consumption to a minimum when the phone is turned off and the battery is connected to the phone.

The following logic or truth table mentionsThe enabled state of supply and gate 82, where Rd is the resistor value detected at input 46 of interlock circuit 44, and Rp is the expected phone or charger ID impedance:

resistor value	RH	RL	BATT.EN.	Evaluation of
					Rd≤Rp-D	1	0	0	Telephone ID resistor less than low threshold, disabling normal battery pack output (FET80 gate-source ═ 0)
Rp-D＜Rd＜Rp+D	1	1	1	The detected impedance is within range. FET80 gate-source-battery voltage.
					Rd≥Rp+D	0	0	0	The detected phone ID impedance is greater than the high threshold, disabling normal battery pack output (FET80 gate-source 0).
Absence of Rp	0	0	0	The interlock circuit 44 is turned off by FETs 84 and 86, disabling normal battery pack output, with FET80 gate-source equal to 0.

As can be seen from the above table, unless the detected impedance at input 46 is between the low and high impedance limits or thresholds, FET80 will be turned off and the battery pack output will be disabled. If the detected impedance falls within a range between the low and high thresholds, FET80 will be turned on or off so that the battery voltage appears at the battery pack output. When there is no phone or charger ID present, the interlock circuit 44 is opened via FETs 84 and 86.

In one particular example of a battery protection circuit as shown in fig. 4, IC70 has a MAX966EUA made by Maxim and the and gate 82 is a Toshiba TC7SL08FU and gate. FETs 30, 32 and 80 are N-channel power FETs, while FETs 84 and 86 are Toshiba 2SK2825 and 2SJ347 FETs, respectively. The resistor values are as follows:

R1-150kohm

R2-1.5Mohm

R3-.5Mohm

R4-1.5-1.74Mohm

R5-1.5Mohm

R6-510kohm

R7-510kohm

R8-510kohm

R9-100kohm

R_ic-10kohm

it will be appreciated that the above values are examples only and that other components and other nominal impedance values may be used with equivalent functionality to meet the determination requirements when the correct ID resistance occurs within predetermined limits.

The above example of fig. 4 allows the interlock or detector circuit 44 and switch 80 to operate completely independently of the self-protection feature of IC28, avoiding the risk of falsely detecting an over-current condition by IC 28. This circuit also has the ability to be turned off when not needed because the FETs 84 and 86 of interlock circuit 44 are turned on only when the phone or charger is present. The use of the additional resistor R9 also reduces power consumption by disabling the battery pack if the phone to the battery pack is disconnected.

In each of the above-described embodiments, the protection circuit additionally allows the battery to discharge only when it is detected that the battery pack is connected to an output load such as a portable telephone or a charger unit having an ID impedance Rp within a predetermined range, in addition to providing normal discharge and charge control according to the current, voltage and temperature of the battery pack. This prevents a high discharge rate of the battery in the event of an accidental short circuit between the battery contacts 15 and 16. If the circuit is open and no load is connected, the slowly detected impedance Rd is infinite, i.e. above a predetermined maximum value. If a short circuit occurs, the detected impedance is zero, i.e. below a predetermined minimum value. Thus, normal discharge occurs only when the battery pack is mated with a portable telephone, a charger unit, or other electronic device intended for use.

Note that each FET in fig. 1 to 4 has a parasitic diode connected across the source-drain connection. The FETs 40 and 62 of fig. 1-3 are replaced with PNP transistors or other electronic switching devices, but the p-channel FETs shown in the drawings provide the lowest voltage drop for this application and are preferred. Similarly, although n-channel FETs are preferred because they also provide the lowest possible voltage drop, other electronic switching devices may be substituted for FETs 30 and 32. Although the FETs provide the lowest voltage drop, other electronic switching methods may be substituted for FETs 30, 32, and 80 of fig. 4.

By means of the above-described protection circuit it is possible for the phone/charger to distinguish between a broken battery pack and no battery pack connection. When connecting the battery pack, under normal conditions, the ID voltages are equal: R1/(R1+ rP) (V + -V-), and it cannot be determined if there is no external battery present or if the protection circuit prohibits the battery pack connection (i.e., if the charging voltage exceeds a safety limit). The phone/charger may be arranged to measure the impedance between the V-pin and the transistor terminal 18, either directly to the battery value or indirectly to the battery value by changing the voltage divider to a fixed voltage reference. The phone/charger can then distinguish between breaking the external battery protection circuit and not having any battery connections according to the following logic or truth table:

battery ID pin voltage	Battery TEMP impedance	Battery status to telephone/charger
			Effective range	Effective range	Connected and functional
Effective range	Invalid range	Connected and out of operating temperature/range
			Invalid range	Effective range	If steady stateConnected and not functioning
Invalid range	Invalid range	Without battery connection

During normal connection, after powering up the interlock circuit, the battery pack first sees 130kohm between the ID and V + pins. The time required to allow the battery output pins (V + and V-) is not apparent to the user. Once the phone senses that V + and V are allowed, the phone will switch to an external battery pack to discharge if the external battery is in the operating range. If not, the phone continues to discharge from the internal battery. If the phone is in charge mode, external battery charging is only initiated after the external phone senses that V + and V-are allowed.

Thus, the battery pack protection circuit as described above prevents the exposed battery pack contacts from being accidentally shorted, such as when the battery pack is not in use when placed in a purse, or pocket at will. The circuit does not allow for activation of exposed battery contacts on the battery pack unless the battery pack is determined to be compatible with the telephone or charger unit.

Although the present invention has been described by way of example only, it will be appreciated by those skilled in the art that modifications to the disclosed embodiments may be made without departing from the scope of the invention as defined by the appended claims.

Claims (26)

1. A battery protection circuit, comprising

A switch assembly for controlling connection of the battery output to a load;

a detector for detecting an impedance at a load terminal;

a first comparator for comparing said detected impedance with a predetermined maximum value and generating a first output signal if said impedance is above said predetermined maximum value and generating a second output signal if said impedance is below said predetermined maximum value;

a second comparator for comparing said detected impedance with a predetermined minimum value and generating a third output signal if said impedance is below said predetermined minimum value and generating a fourth output signal if said impedance is above said predetermined minimum value; and

a second detector connected to the outputs of said first and second comparators for generating a control signal for closing said switching assembly if said detected impedance is within a range between said minimum and maximum values and for opening said switching assembly if said impedance is outside said predetermined range to connect said battery to said load only if said detected load impedance is between said predetermined minimum and maximum values.

2. The circuit of claim 1, comprising a self-protection control unit for controlling discharge of the battery, the control unit having an input, an overdischarge output and the switch assembly comprising a first switch controlling series connection of the battery and a load in a discharge mode, wherein the first switch has a control input connected to the overdischarge output of the control unit, and a second switch disabling the first switch if the load ID impedance is detected to be outside the predetermined range between the minimum and maximum values.

3. The circuit of claim 2, wherein the self-protection control unit has a power input, and the second switch is arranged to control the power input to the control unit to turn off the control unit when the detected impedance is outside the predetermined range.

4. A circuit as claimed in claim 2, in which the second switch is connected between the overdischarge output of the control unit and the first switch to disable the control input to the switch if the detected impedance is outside the predetermined range.

5. A circuit according to claim 2, wherein said control unit has an overcharge output and a third switch for controlling the charging of said battery is connected in series between a charge input and a battery input, said third switch having a control input connected to said overcharge output.

6. The circuit of claim 5 wherein a fourth switch is connected between said overcharge output and said third switch for disabling said third switch if said detected impedance is within said predetermined range.

7. The circuit of claim 1 including a self-protection control unit for controlling discharge of the battery, the control unit having an input, an overdischarge output and an overdischarge switch controlling the series connection of the battery and a load, the overdischarge switch having a control input connected to the overdischarge output of the control unit, and the switch assembly including an interlock switch connected in series with the overdischarge switch and responsive to the control signal to open if the detected impedance is outside the predetermined range.

8. The circuit of claim 7, wherein the interlock switch comprises a FET having a gate, a source, and a drain, and wherein the control signal is coupled to the gate.

9. The circuit of claim 8, comprising a resistor connected across the source and drain of the FET.

10. The circuit of claim 8, wherein the FET is an n-channel power FET.

11. The circuit of claim 1 wherein the comparator and the second detector both have power supply inputs, the circuit including a second switching component for detecting impedance at the load terminals and controlling the power supply inputs to the comparator and the second detector in response to the detected impedance, thereby disabling the comparator and the detector if no impedance is present.

12. The circuit of claim 2, wherein the first switch comprises an n-channel FET.

13. The circuit of claim 2, wherein the second switch comprises a FET.

14. The circuit of claim 13, wherein the second switch comprises a p-channel FET.

15. The circuit of claim 1 wherein said first and second comparators each have first and second inputs, said first input of each comparator being connected to said detected impedance, said circuit further comprising a first reference voltage connected to said second input of said first comparator, and a second reference voltage connected to said second input of said second comparator.

16. The circuit of claim 15, wherein the first and second reference voltages are from a voltage divider circuit.

17. The circuit of claim 15, wherein the second detector comprises an or gate.

18. An external battery pack for connection to an electronic unit or charger, the battery pack comprising:

a battery cell having a positive terminal and a negative terminal;

at least three external contacts on the battery pack for mating with corresponding contacts on an electronic unit or charger when mated with the battery pack;

a first of the external contacts is connected to the positive terminal of the battery pack;

a second of the external contacts is connected to the negative terminal of the battery cell;

a third one of the external contacts includes an ID contact for connecting to a predetermined ID impedance in an electronic unit or a charger;

a first switch located between said second external contact and a terminal of the battery for controlling the discharge of said battery;

a voltage divider circuit connected to the third external contact for generating an ID voltage based on impedance at the third external contact;

defining a first reference voltage of a predetermined maximum value;

a second reference voltage defining a predetermined minimum value;

first and second comparators each having a first input connected to the first ID voltage, the first comparator having a second input connected to the first reference voltage, the second comparator having a second input connected to the second reference voltage;

said first comparator has a first output if said ID voltage is higher than said first reference voltage and a second output if said ID voltage is lower than said first reference voltage;

the second comparator has a first output if the ID voltage is lower than the second reference voltage, and a second output if the ID voltage is higher than the second reference voltage;

a gate circuit connected to said outputs of said first and second comparators for generating a battery enable control signal when a second output from said first comparator and said second comparator is detected and for generating a battery disable signal when said first output from said first and said second comparators is detected; and

the first switch is operatively connected to the output signal of the gate circuit so that the switch connects the battery to the output contact in response to the battery enable and disable signals only when the detected load impedance is within a range between the predetermined minimum and maximum values.

19. The battery pack of claim 18, further comprising a battery self-protection unit connected between the first and second contacts, the control unit having an input, an overdischarge output, and an overdischarge switch controlling series connection of the battery and a load in a discharge mode, and further having a control input connected to the overdischarge output of the control unit.

20. The battery pack of claim 19 wherein said first switch is connected to said overdischarge switch for disabling said overdischarge switch if said load ID impedance is detected to be outside a predetermined range of said minimum and said maximum components, said first switch having a control input connected to an output of said gate circuit.

21. The battery pack of claim 19, wherein the first switch is connected in series with the overdischarge switch and to the gate circuit output signal so as to open if the detected load impedance is outside the range.

22. The battery pack of claim 20 wherein said self-protection control unit has a power input connected to a battery terminal, and said first switch is connected between said battery terminal and said power input to said control unit to turn off said control unit when said detected impedance is outside of said predetermined range.

23. The battery pack of claim 20, wherein the first switch is connected between the overdischarge output and the control output of the overdischarge switch.

24. The battery pack of claim 21, wherein the switch is a FET.

25. The battery pack of claim 24 wherein the first switch and overdischarge switch comprise FET switches having a source, a drain and a gate, and each FET switch has a parasitic diode connected across the source and drain.

26. The battery pack of claim 18, wherein the first and second reference voltages are from a voltage divider circuit.