Battery Pack Having Under-Voltage and Over-Voltage Protection

This invention relates in general to batteries and in particular to battery packs having disconnect switches.

As electronics have become increasingly portable and sophisticated, consumers have come to expect quality power sources for these products. Typically the best solution is to use a rechargeable battery or battery pack. Such battery packs must be very robust in order to provide as many charge/discharge cycles as possible. In addition, since battery packs are energy storage devices, they must be safe. This is especially true of some of the newer varieties of batteries that have recently become available, for example, lithium ion batteries which have a significantly higher energy density compared to established battery systems such as nickel-cadmium. In order to achieve both safety and reliability, it is necessary for designers of such battery packs to assume a certain level of mistreatment, either intentionally or otherwise, once in the hands of the end user.

One event that decreases reliability, and has occurred with increasing frequency is accidental deep discharge of the battery pack. This occurs when a device is left on for an extended period of time, such as when the user forgets about it, and the battery is discharged far beyond its normal discharge level. For newer systems such as nickel metal hydride, lithium ion, and lithium polymer, this could cause a battery failure. At best, the cycle life of the battery is significantly shortened; at an extreme, it can create a safety risk when recharged.

Another potential problem unique to newer battery systems, particularly lithium ion and lithium polymer, is overcharge. It is well known that lithium metal is highly reactive. In lithium ion cells, there is initially no lithium metal available in the cell. So long as the battery voltage does not exceed a threshold voltage, none will form. If the battery is charged at a voltage above this threshold level, then the battery cells experience what is referred to as an over-voltage condition. This over-voltage condition causes lithium metal to precipitate out of solution in a reaction similar to an electrochemical plating process. Over time, a safety risk could develop as more lithium metal builds up. Lithium polymer cells likewise experience a similar phenomena upon overcharge.

In a typical system, the charger is designed to avoid over-voltage conditions. A variety of failure modes can, however, cause a charger to continue charging once the threshold voltage is reached. This type of failure is commonly referred to as a runaway charger. In order to protect against such a failure, some sort of battery disconnect switch is needed to keep the battery cells from experiencing any over-voltage charging. Ideally such a switch, once activated should still allow the battery pack to be subsequently discharged.

Battery disconnect switches have been advantageously employed in the past to address both under-voltage and over-voltage conditions. However, their cost and complexity often offset any advantage a newer battery system may offer, which is typically already more expensive than conventional systems. For example, there are battery pack resident low voltage, or more commonly, under-voltage, switch circuits in use for lithium ion batteries. However, these circuits, once activated, require a specific signal from a charger to "wake up" the battery switch circuit and allow use of the battery. This requires a certain level of complexity on the part of the charger. Similarly, battery pack resident over-voltage disconnect circuits have been used in a number of lithium ion battery packs. However, they typically use multiple comparators, precision voltage references, and in many cases, custom made integrated circuits.

Accordingly, there is a need for a simple means by which a battery can be disconnected from a load when the battery has been normally discharged, i.e., prior to any deep discharge, and still be rechargeable. Likewise, there is a need for a simple means to disconnect a battery before an overcharge condition occurs so as to avoid any safety risk. Ideally, such requirements can be met by a single means.

FIG. 1 is a block diagram of a battery system with under-voltage protection means in accordance with the invention;

FIG. 2 is a block diagram of a battery system as in FIG. 1 and incorporating over-voltage protection means in accordance with the invention; and

FIG. 3 is a schematic diagram of a circuit in accordance with the invention .

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.

Referring now to FIG. 1, where there is illustrated therein a block diagram of a battery system in accordance with the instant invention. There is shown battery pack 10 and load 12, such as a cellular phone, portable two way radio, portable computer, or any such device requiring a portable power source. Load 12 draws a discharge current from battery pack 10 in the direction of arrow 14. A battery voltage V1 is applied between positive battery terminal 16 and negative battery terminal 18.

Battery pack 10 is comprised of at least one battery cell 20, under-voltage switch 22, and voltage controlled switch 24. Under-voltage switch 22 has a one way action in that current in the direction of arrow 23 can always flow, regardless of whether under-voltage switch 22 is actuated or not. Current flowing in the opposite sense of arrow 23 will be blocked if under-voltage switch 22 is actuated, but will flow otherwise.

The switch state, i.e., on or off, of under-voltage switch 22 is controlled by control input 25. Specifically, under-voltage switch 22 is responsive to a switch signal generated by voltage controlled switch 24 and carried on line 26 to control input 25. Voltage controlled switch 24 is responsive to battery voltage V1 and provides a switch signal on line 26 so as to maintain under-voltage switch 22 in the on state, that is, conductive in both directions, until such time as battery voltage V1 falls below a predetermined under-voltage level. When the battery voltage V1 falls below the under-voltage level, voltage controlled switch 24 turns off and causes under-voltage switch 22 to block further discharge, thereby limiting the depth of discharge that battery cells 20 experience. When under-voltage switch 22 turns off, battery voltage V1 drops to about zero volts, which causes voltage controlled switch 24 to stay in the off state, even when the voltage across battery cells 20 rises above the under-voltage level. Since under-voltage switch 22 is a one way blocking switch the battery can be charged by simply applying recharge current. This reverses the voltage across under-voltage switch 22, raises battery voltage V1 above the under-voltage level, and turns on voltage controlled switch 24.

Referring now to FIG. 2, where there is illustrated therein a block diagram of a battery pack 10 as in FIG. 1, and further including over-voltage switch 28 and second voltage controlled switch 30. However, in this case, load 12 may discharge battery pack 10, as before, or supply a recharge current in the direction of arrow 32. Over-voltage switch 28, when activated, will block recharge current, but will allow discharge current to pass in the direction of arrow 29. In this sense, over-voltage switch 28 acts as a one way blocking switch, like under-voltage switch 22. When recharge current flows into the battery, the battery voltage rises. Once battery voltage V1 reaches a second predetermined level, second voltage controlled switch 30 turns on and provides a switch signal on line 34, causing over-voltage switch 28 to block any further charging. Typically, voltage across battery cells 20 would drop slightly, causing battery voltage V1 to drop accordingly.

However, since battery chargers typically regulate current, when over-voltage switch 28 opens, the charger voltage climbs to its maximum level in an attempt to maintain current regulation. While the charger output is connected parallel to battery pack 10, the charger output voltage becomes the battery voltage V1, and voltage controlled switch 30 maintains the switch signal on line 34, and over-voltage switch 28 continues to block recharge current. The cells can still be discharged since under-voltage switch 22 will not be engaged, and over-voltage switch 28 only blocks recharge current.

Referring now to FIG. 3, where there is illustrated therein a schematic circuit diagram of a circuit in accordance with the invention. There is shown a battery pack 10 connected to a charger or load 12 through positive battery terminal 16 and negative battery terminal 18, across which battery voltage V1 is evident. Also included is at least one battery cell 20, under-voltage switch 22, first voltage controlled switch 24, over-voltage switch 28, second voltage controlled switch 30, and polyswitch 36. Polyswitch 36 is a current interrupt device which responds to high current being passed through it. If an excessive level of current is applied, either in the charge or discharge direction, polyswitch 36 will change from a very low electrical resistance to a very high electrical resistance, providing a switch action. The use of a polyswitch in this manner is well known in the art

Under-voltage switch 22 and over-voltage switch 28 achieve a one way blocking operation by coupling a diode in parallel with a switch. In one preferred embodiment, N-channel MOSFETs are used since these devices provide excellent switch actions and have an intrinsic diode as a result of the manufacturing process used to fabricate them. Under-voltage switch 22 has a source terminal 38, drain terminal 40, and gate terminal 42. The intrinsic diode 43 is connected parallel to the MOSFET with anode 44 connected to source terminal 38. This orientation allows recharge current to pass through battery cell 20 regardless of the switch state of under voltage switch 22. Source terminal 38 is also connected to battery cell 20, while gate terminal 42 is a control input.

Over-voltage switch 28 also has a source terminal 46, drain terminal 48, and gate terminal 50. Drain terminal 48 is connected to drain terminal 40 of under-voltage switch 22, source terminal 46 is connected to negative battery terminal 18, and is coupled in parallel with intrinsic diode 49 similar to under-voltage switch 22, but oriented in a reverse sense so as to block recharge current through battery pack 10. Gate terminal 50 acts as a control input and receives a switch signal from line 51.

First voltage controlled switch 24 comprises first divider resistor 52, second divider resistor 54, bipolar transistor 56, coupling resistor 58, and bias resistor 60. First and second divider resistors 52 and 54 are connected in series between the positive and negative battery terminals 16 and 18 respectively, and form midpoint node 62. The voltage at midpoint node 62 is proportional to battery voltage V1 and dependent on the values of divider resistors 52 and 54. Bipolar transistor 56 acts as a voltage controlled switch element and has emitter, collector and base terminals 64, 66, and 68, respectively. Bipolar transistor 56 conducts current from emitter terminal 64 to collector terminal 66 when a bias voltage is applied across the emitter-base junction.

Base terminal 68 is coupled to midpoint node 62 by coupling resistor 58. Coupling resistor 58 acts to limit current through the emitter-base junction of bipolar transistor 56. Divider resistors 52 and 54 are selected such that while battery voltage V1 is above the first predetermined level, a low voltage cutoff level, the voltage at midpoint node 62 is equal to or greater than the minimum bias voltage of bipolar transistor 56. As a result, bipolar transistor 56 conducts a current to bias resistor 60, thereby producing a bias voltage. This bias voltage maintains under-voltage switch 22 in an on state. If battery voltage V1 drops below the under-voltage level, the voltage at midpoint node will not be sufficient to bias bipolar transistor 56, and bipolar transistor will not conduct current to bias resistor 60. This results in under-voltage switch 22 shutting off. When battery voltage V1 falls below the under-voltage level, current can not be drawn from battery pack 10 and battery voltage V1 drops to about zero volts.

Second voltage controlled switch 30 operates in a similar fashion. It is comprised of divider resistors 70 and 72, which are connected in series and form midpoint node 74. In one preferred embodiment, thermistor 76 is included, and is a negative temperature coefficient type. If thermistor 76 is not required, then divider resistor 72 would be connected directly to negative battery terminal 18. Bipolar transistor 78 acts as a switch element, and is connected to midpoint node 74 through coupling resistor 75. The voltage on line 51 is pulled up to a high level through bias resistor 80. This voltage level is applied to over-voltage switch 28 at control input 50 such that over-voltage switch 28 is switched on. Third and fourth divider resistors 70 and 72, and thermistor 76, if included, are selected such that when battery voltage V1 is above the over-voltage level, a second predetermined level, bipolar transistor is biased on and the voltage on line 51 drops to about 0 volts. This switch signal causes over-voltage switch 28 to switch off and block further recharge.

The purpose of thermistor 76 is to compensate the thermal bias voltage error of bipolar transistor 78 in a manner known in the art. It is well known that bipolar transistors have a thermal bias voltage error of about -2 milliVolts/.degree.C. That is, as the temperature of the transistor increases, less voltage is required to bias it to an on state, and more voltage is required as the temperature decreases. Since the over-voltage level is chosen for safety reasons, it is imperative that the over-voltage switch acts at the same battery voltage level independent of temperature. Although shown here connected in series with divider resistors 70 and 72, it could just as easily be connected in parallel to divider resistor 72, or in a resistive network connected between midpoint node 74 and negative battery terminal 18. All of which are choices dependent on the specific design and the elements available.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.