BATTERY PACK WITH TEMPERATURE ACTIVATED

BOOST CIRCUIT

BACKGROUND

TECHNICAL FIELD

This invention relates generally battery packs, and more specifically to a

battery pack having a temperature dependent boost circuit embedded therein.

BACKGROUND ART

Personal electronic devices are widely used in today's information age.

Cellular telephones, two-way radios, pagers, personal data assistants, portable

computers and multimedia players are only some of the devices commonly used by

people to stay organized and informed. Many individuals carry such devices wherever

they go, outdoors as well as indoors. Rechargeable batteries are the workhorses that

provide energy to these devices. Rechargeable batteries offer the user freedom of

movement without having to sacrifice functionality of such devices.

Lithium ion batteries are the most popular choice in rechargeable applications

due to their high energy storage to weight ratio. Lithium-based batteries, however,

tend to be temperature sensitive and may experience a shortened life span or reduced

energy capacity when exposed to cold temperatures. Nonetheless, many applications

demand that portable electronic devices be fully operational in cold environments. For

example, policemen on the beat in northern regions need their radios to be operational

regardless of the temperature. Additionally, construction workers need power tools to

work in cold environments as well.

Prior art solutions for keeping lithium batteries warm include coupling a

resistive heater to the battery pack. The resistive heater heats the battery to a warmer

temperature, thereby allowing the battery to provide its full energy capability, thus

returning some of the effective energy storage capacity. The problem with this

solution is that the resistive heater must have an energy source to generate heat. Since

some rechargeable cells have little capacity at low temperatures, a user must plug such

a heater into an alternate power source, like a wall outlet. This plug requirement

eliminates the portability of the device.

There is thus a need for a battery pack that remains operational at low

temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a battery pack in accordance

with the invention.

FIG. 2 illustrates a software flow chart suitable for operation in a microcontroller in a

battery pack in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail. Referring

to the drawings, like numbers indicate like parts throughout the views. As used in the

description herein and throughout the claims, the following terms take the meanings

explicitly associated herein, unless the context clearly dictates otherwise: the meaning

of "a," "an," and "the" includes plural reference, the meaning of "in" includes "in"

and "on."

This invention is a battery pack having a temperature dependent boost circuit

for increasing the cell voltage at cold temperatures. In other words, the available cell

voltage is multiplied by the boost circuit into a range usable by a host device. The

battery pack includes at least one electrochemical cell, like a high rate lithium-ion cell

for example. A boost circuit, which increases, or "boosts" the voltage of the cell, is

coupled in series with the electrochemical cell. A controller, like a microcontroller or

application specific pulse width modulator for example, is coupled to the boost

circuit. The controller is capable of actuating, disabling and regulating the boost

circuit.

In parallel with the boost circuit is a boost bypass circuit. The boost bypass

circuit, which maybe as simple as a transistor, is capable of either blocking current by

entering a high impedance state, or passing current by entering a low-impedance state.

The state of the bypass circuit is controlled by the controller.

A temperature sensor, like a thermistor or temperature sensitive resistor for

example, is coupled to the controller. The temperature sensor indicates relative

temperature to the controller by changing a characteristic like impedance.

When the controller senses that the temperature has fallen below a

predetermined minimum temperature threshold, the controller knows that the cell

voltage of the electrochemical cell is low due to the operational characteristics of the

cell. Note that the predetermined minimum temperature threshold will be determined

by, among other things, the discharge efficiency characteristics of the particular cell

used with the invention. As such, the controller actuates the boost circuit, thereby

making it operational so as to increase the output voltage of the battery pack. As

electronic devices typically only work when the voltage of the attached battery is

above a minimum threshold, this increased voltage keeps the attached electronic

device operational in cold environments. Concurrent with actuating the boost circuit,

the controller causes the boost bypass circuit to enter a high impedance state.

By way of background, as noted above, standard Li-ion cells, for example cells

with LiCoO ₂ cathodes and graphite anodes typically have little or no energy storage

capacity at low temperatures. For example, the typical LiCoCVgraphite cell stores less

than 1% of its full capacity at temperatures below -20 degrees centigrade.

However, recent developments in lithium technology have produced cells that

do have substantive energy storage capacities at low temperatures. For example,

experimental results have shown that cells manufactured by Sony, Inc. can deliver as

much as 20% of their full energy storage capacity below -20 degrees centigrade. These

types of cells, i.e. those that maintain substantive energy storage capacity at low

temperatures, will be referred to herein as "high rate" cells.

There are two methods of creating high rate cells. The first method is to use

lithium iron phosphate as the cathode material. This cathode material is then either

doped with metallic elements or coated with a highly electrically conductive material,

such as carbon. The second method is to use traditional lithium cobalt oxide,

manufactured in fine primary particles measuring less than 5 microns in diameter, and

to change the physical parameters of the cell so that the cell's internal impedance is

minimized. By way of example, one might design the cell with thick current

collectors, or apply a thin coating of active materials on the current collector, or

design the current collector material to be longer than in standard cells, or use a high

concentration of low viscosity solvents (for example, propylene carbonate,

ethylmethyl-carbonate, diethyl-carbonate) and high ionic conductivity lithium-based

salts for the electrolyte. In any event, high rate cells are commercially available from

companies like Sony.

The problem with high rate cells, however, is that the cell voltage is greatly

diminished at low temperatures, hi other words, the cell can deliver energy, but the

output voltage is considerably lower than it is at, for example, room temperature. This

causes a problem with electronic devices in that most devices have a low voltage limit

below which either they can not operate or they disable functions. Consequently, even

though there is energy in the cell, the electronic device still shuts off due to the low

cell voltage. The present invention solves this issue by incorporating a temperature

dependent boost circuit in the battery pack.

Turning now to FIG. 1, illustrated therein is a schematic diagram of one

preferred embodiment of the invention. A battery pack 100 is shown having a positive

terminal 101 and a return terminal 102 for coupling to a load, like a portable

electronic device. The pack 100 includes at least one cell 103, for example a

rechargeable electrochemical cell like a high rate lithium ion cell.

Coupled serially between the positive terminal 101 and the cell 103 is a boost

circuit 104. Boost circuits are well known to those of ordinary skill in the art and

generally include an inductor, switch, diode and capacitor. The switch periodically

couples an input voltage to ground through the inductor, thereby storing energy in the

inductor. When the switch is opened, the energy stored in the inductor passes through

the diode to the capacitor at a voltage higher than that of the input voltage.

In parallel with the boost circuit 104 is a boost bypass circuit 105. The boost

bypass circuit 105 may be as simple as a transistor coupled in parallel with the boost

circuit 104. The boost bypass circuit 105 is capable of allowing or stopping the flow

of current by switching between a low impedance state and a high impedance state.

Both the boost regulator 104 and the boost bypass circuit are controlled by the

controller 106, which maybe a microcontroller, a discrete circuit or an application

specific circuit that includes a pulse width modulator or equivalent switching signal.

The controller 106 can actuate, regulate or disable the boost circuit 104. This is done

with an on/off control line 111 and a regulation signal, like a pulse width modulated

signal 112, where required. Additionally, the controller 106 can cause the boost

bypass circuit 105 to enter either a high impedance state or a low impedance state by

way of the on/offline 111.

The controller 106 receives several inputs from the circuit. A first input is a

reference voltage provided by a voltage reference 107. The voltage reference 107 may

be integral to the controller 106, or may be a separate component as is shown in FIG.

1. The voltage reference provides a reference voltage against which the other inputs of

the circuit may be compared.

A second input is a scaled cell voltage 113 that is proportional to the cell

voltage. In the exemplary embodiment of FIG. 1, the voltage proportional to the cell

voltage is generated by a resistor divider 108. A third input is a scaled pack voltage

114. Again, in the exemplary embodiment of FIG. 1, this scaled pack voltage 114 is

generated by a second resistor divider 109.

A fourth input is from a temperature sensor 110. The temperature sensor 110

may be a thermistor, positive temperature coefficient device, negative temperature

coefficient device, temperature sensitive resistor, thermocouple, or other device. The

temperature sensor 110 generates a signal indicative of the temperature of either the

cell 103 or the overall pack 100, depending upon where it is positioned within the

pack 100.

As the cell voltage falls with temperature, and when the controller 106 sees

that the temperature has fallen below the predetermined minimum temperature

threshold, like -20 degrees centigrade for example, as indicated by the temperature

sensor 110, the controller 106 actuates the boost circuit 104. (Note that other, optional

steps may be taken prior to actuation of the boost circuit 104. For example, the

controller 106 may additionally check the pack output voltage or cell voltage to ensure

that actuation of the boost circuit 104 does not damage either the load or the cell 103.)

This actuation causes the voltage of the cell 103 to be "stepped up", or increased, such

that the voltage at the terminals 101,102 is higher than the voltage of the cell 103.

This ensures that the portable electronic device coupled to the pack 100 will remain

operational, even at low temperatures.

To ensure that all of the energy in the cell 103 passes through the boost circuit

104, upon actuation of the boost circuit 104, the controller 106 causes the boost

bypass circuit 105 to enter a high impedance state. This causes current to flow through

the boost circuit 104 rather than through the boost bypass circuit 105.

In similar fashion, when the temperature sensor 110 indicates that the

temperature is above the predetermined threshold, the controller 106 disables the

boost circuit 104. Additionally, when the temperature sensor 110 indicates that the

temperature is above the predetermined threshold, the controller 106 causes the bypass

circuit 105 to enter a low impedance state. This disabling of the boost circuit 104 and

enabling of the boost bypass circuit 105 reduces the overall current drain of the

circuitry internal to the battery pack 100 when the pack 100 is operating at normal

temperatures. Additionally, the overall pack efficiency is increased by disabling the

boost circuit 104 at temperatures above the predetermined minimum threshold. In one

embodiment, a preferred range of temperatures in which it is desirable to have the

boost circuit 104 operational is 0 to -40 degrees centigrade.

hi one preferred embodiment, the controller 106 is a microcontroller running

operational software. Turning now to FIG. 2, illustrated therein is a fundamental flow diagram of a set of operating steps that may comprise steps within that operational

software. For example, the steps of FIG. 2 may constitute a subroutine in a software

program running on the microcontroller. Alternately, the flow chart could be

indicative of the operation of an equivalent hardware circuit.

The steps begin at the starting point 200. The software initially instructs the

microcontroller to determine the temperature of the battery pack at step 201. The

voltage of the cell is then determined at step 202.

At step 203, the microcontroller determines whether the temperature is below

the predetermined minimum temperature threshold, like -20 degrees centigrade, for

example. If the temperature is above this threshold, the boost circuit is disabled at step

207. This may be due to the fact that the efficiency of the cell discharge by itself is

greater than the efficiency of the boost circuit.

At optional step 204, the microcontroller determines whether the cell voltage

is above a predetermined minimum operational voltage threshold. This ensures that

the battery pack is not operating at a temperature and voltage that is below the

recommended operational limits of the cell.

At optional step 205, the microcontroller determines whether the output

voltage of the battery pack is less than a minimum operational voltage threshold of the

attached electronic device. If the answer is yes, this is indicative of both the cell

voltage being above the predetermined minimum operational voltage threshold and

the temperature is below the predetermined minimum temperature threshold. In this

case, the boost circuit is enabled at step 206. Concurrently, the microcontroller would

cause the boost bypass circuit to enter a high impedance state.

Had the voltage been below the predetermined minimum operational threshold

at step 204, or had the temperature been above the minimum temperature threshold at

step 203, the microcontroller would have disabled the boost circuit at step 207.

Concurrently, the microcontroller would cause the boost bypass circuit to enter a low

impedance state.

Where both the temperature is below the predetermined minimum threshold

and the cell voltage is above the predetermined minimum operating threshold, the

microcontroller may check the pack output voltage at step 205 to determine whether

the pack output voltage is sufficient to operate the attached electronic device. If the

pack voltage is too low, the microcontroller will actuate the boost circuit at step 206.

At step 205, if the pack voltage is sufficient to operate the attached electronic

device, the microcontroller moves to step 208. At step 208, the microcontroller may

monitor either the cell voltage or the pack voltage to determine whether that voltage

remains within the acceptable limits for the desired operational mode of the attached

electronic device. If so, and if the boost circuit is actuated, the pack may be able to

become more efficient by reducing the amount of boost at step 209.

To summarize, one embodiment of this invention comprises a rechargeable

battery pack having at least one rechargeable electrochemical cell, a microcontroller, a

reference voltage coupled to the microcontroller, and a boost circuit coupled serially

with the at least one rechargeable electrochemical cell. A bypass circuit is coupled in

parallel with the boost circuit, and both a reference voltage and a temperature sensor is

coupled to the microcontroller.

In one embodiment, when a voltage proportional to a voltage across the at

least one rechargeable cell falls below a predetermined minimum, and when the

temperature sensor indicates that temperature has fallen below a predetermined

minimum temperature threshold, the controller actuates the boost circuit. Concurrently

the microcontroller causes the bypass circuit to enter a high impedance state.

When either the voltage proportional to the voltage across the at least one

rechargeable cell exceeds the predetermined minimum or when the temperature sensor

indicates that the temperature exceeds the predetermined minimum temperature

threshold, the microcontroller disables the boost circuit. Concurrently, the

microcontroller causes the bypass circuit to enter a low impedance state.

I l

While the preferred embodiments of the invention have been illustrated and

described, it is 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

following claims.