WO1999050948A1

WO1999050948A1 - Rechargeable battery having overcharge protection circuit and method of charging rechargeable battery

Info

Publication number: WO1999050948A1
Application number: PCT/US1999/007107
Authority: WO
Original assignee: Aspen Electronics, Inc.
Priority date: 1998-03-31
Filing date: 1999-03-31
Publication date: 1999-10-07
Also published as: US5939865A

Abstract

A rechargeable battery is provided with an overcharge protection circuit which is particularly adapted for nickel metal hydride battery packs to be used in conventional charging adapters suitable for NI-CAD battery packs. The battery pack includes a microprocessor which measures the voltage across the electrical contacts of the battery pack. Upon reaching peak charge (29), the microprocessor increases the voltage (33) appearing across the battery pack contacts. After a predetermined time period (31) as measured by the microprocessor, the microprocessor decreases the voltage (35) appearing across the battery pack terminals to exceed the change in voltage to which the charging adapter responds to terminate a high rate of charging (37).

Description

-1-

RECHARGEABLE BATTERY HAVING

OVERCHARGE PROTECTION CIRCUIT AND

METHOD OF CHARGING RECHARGEABLE BATTERY

BACKGROUND OF THE INVENTION

The present invention relates to a rechargeable battery pack of the type

commonly used in video recorders, cellular telephones, power tools and the like.

Even more particularly, the present invention relates to an overcharged protection

circuit for a battery pack. The invention also relates to methods of recharging such

battery packs.

A battery capable of being recharged after a battery is discharged through

use, is commonly called a rechargeable battery. Presently, rechargeable batteries

are becoming ubiquitous in present day society, and it is advantageous that the rechargeable battery always be charged to maximum capacity. Conventionally, a

rechargeable battery is charged by using a base unit plugged into a wall outlet, or

for smaller rechargeable batteries, by using a vehicle mounted adapter. A charger

usually operates in either a standard mode, having a charging rate in coulombs (amps/second) dependant on the characteristics of the rechargeable battery, or in a

trickle mode at a charging rate substantially less than in the standard mode. Until

recently, most rechargeable batteries have included one or more rechargeable nickel -2- cadmium (NI-CAD) cells comprising a rechargeable battery pack. Each of these

battery packs have two external contacts for contacting the positive and negative

terminals of a charging adapter.

When the user inserts a rechargeable battery into a conventional charging

adapter system, the insertion of the battery is recognized and charging automatically

begins. Unfortunately, when the user inserts a fully charged battery into the

charging adapter, the battery voltage can actually decrease. (A decrease in battery voltage will be referred to herein as generating a -ΔV, and a voltage increase will

be referred to herein as generating a ΔV.) In addition, overcharging of a

rechargeable battery can also result in a decrease in battery voltage -ΔV.

Furtliermore, when a vehicle mounted adapter is used, a fully charged battery may be charged too frequently, for example each time the vehicle is started, also

resulting in a decrease in battery voltage. The decrease in voltage has a memory

effect in the rechargeable battery which is increased due to frequent charging, thereby reducing the lifetime of the battery.

As shown in Fig. 1 and 2, when a NI-CAD battery pack is charged by a

charging adapter, the voltage across the two contacts of the battery pack generally

rises until it reaches a peak voltage and then the voltage of the battery pack begins

to decrease. Meanwhile, when the NI-CAD battery pack is being charged, the temperature of the battery pack initially increases relatively slowly but increases

dramatically over time. There have been several attempts to produce charging -3- adapters which terminate the high rate of charging at the peak charge, prior to the

substantial voltage decrease stemming from overcharging. Early attempts were

directed to charging adapter systems which terminated the high rate of charging based upon the temperature of the rechargeable battery pack. These systems

included a thermostatic switch located in close proximity to the battery pack which

measured the temperature of the battery. Once the battery pack heated to a

predetermined temperature, the thermostatic switch would sense the battery temperature and terminate the high rate of charge to the battery pack.

More recent attempts to terminate the charging of a battery pack near the

peak charge are based on the detection of a decrease in voltage (-ΔV) in the battery pack. The high rate of charge is terminated when the voltage peaks at a maximum

and then drops by a predetermined voltage level, as shown in Fig. 1 as -ΔV.

Such conventional charging adapters work sufficiently well with NI-CAD

battery packs. However, these charging adapters, which terminate the high rate of charging based upon a -ΔV, do not prevent overcharging of more recently

developed forms of battery packs such as nickel metal hydride rechargeable battery

packs. As shown in Fig. 3, the voltage across the two contacts of a nickel metal hydride battery pack increases gradually. Immediately prior to peak voltage, the

rate of voltage increasing over time itself increases. This change in the rate of

voltage increase will hereinafter be referred to as Δ²V/Δt². As understood by those

skilled in the art, Δ²V/Δt² represents the second derivative of the measured voltage -4- across the contacts of the battery pack with respect to time. After the nickel metal

hydride battery reaches peak charge, the battery pack voltage drops relatively

slowly. As seen by a comparison of the plots of Figs.1 and 3, nickel metal hydride

battery packs do not tend to drop in voltage nearly as quickly as a NI-CAD battery

pack because typical nickel metal hydride battery packs take longer to drop in

voltage after obtaining peak charge. Thus, conventional adapters do not terminate

the high rate of charge quickly enough to prevent overcharging of the nickel metal

hydride battery. Furthermore, as shown in Fig. 3, the temperature of a nickel metal

hydride battery continues to rise steadily during charging. If a charging adapter

does not terminate its charge soon after a nickel metal hydride battery reaches peak

charge, the battery can be damaged from overheating. Accordingly, it has become generally preferred to terminate the high rate of charge to a nickel metal hydride

battery pack by sensing the battery pack temperature and terminating the high rate

of charge when the temperature reaches a threshold temperature. This necessitates

the need for separate charging adapters for NI-CAD battery packs and nickel metal hydride battery packs resulting in a duplicity of costs. Thus, it would be

advantageous if both NI-CAD battery packs and nickel metal hydride battery packs

could be charged from a single charging adapter.

To this end, U.S. Patent No. 5,708,350 discloses a nickel metal hydride

pack which may be charged from the conventional charging adapters originally

intended for charging NI-CAD battery packs. The nickel metal hydride battery

pack includes an overcharge protection circuit which artificially transmits an -5- increased ΔV to the charging adapter once the nickel metal hydride pack reaches a

threshold temperature. The overcharged protection circuit includes a temperature

responsive switch coupled between the battery pack contacts and the rechargeable

cells. Upon the temperature response switch reaching a threshold temperature, the

switch adjusts the voltage appearing across the contacts of the rechargeable battery

by an amount exceeding the predetermined change in voltage to which the adapter responds. The voltage across the contacts is increased by inserting a resistor into

the current path. The resistor acts as a current limiter which eventually drops the

battery temperature below the temperature which caused the switch to activate.

Once the battery temperature drops below this predetermined threshold level, the thermostatic switch removes the resistor from the circuit and the voltage appearing

across the contacts of the battery pack artificially drops. Thus, the overcharged

protection circuit of U.S. Patent No. 5,708,350 tricks a conventional charging adapter to terminate the high rate of charge even though the requisite voltage drop appearing across the battery contacts may not have occurred.

This system suffers from several drawbacks. First, where the battery pack

has been placed in a cold environment prior to insertion into the charging adapter, the thermostatic switch will not operate properly when the nickel metal hydride

battery reaches peak voltage. Furthermore, depending on the ambient conditions

surrounding the charging adapter, and the characteristics of the thermostatic switch,

it may take an unacceptably long time for the thermostatic switch to both artificially -6- increase the battery pack voltage and then artificially decrease the battery pack

voltage resulting in overheating and overcharging of the nickel metal hydride

battery pack.

Thus, there exists a need for a system enabling NI-CAD and nickel metal hydride battery packs to be charged in the same charging adapter. It would also be

highly desirable to provide a nickel metal hydride type battery pack which may be

effectively charged in conventional adapters which specialize in charging NI-CAD

battery packs.

It would also be highly desirable to provide a nickel metal hydride-type

battery pack including an overcharge protection circuit which has its high charge rate terminated upon reaching its peak charge.

SUMMARY OF THE INVENTION

Briefly, in accordance with the invention, I provide a rechargeable battery

pack incorporating an overcharged protection circuit. The rechargeable battery pack and protection circuit is particularly adapted for nickel metal hydride battery cells.

However, the overprotection circuit of the present invention may be incorporated

into any type of battery pack, including but not limited to NI-CAD battery packs. -7-

The battery pack of the present invention includes one or more rechargeable

cells configured in parallel or in series depending on the properties required of the

rechargeable battery pack. The one or more battery cells are coupled to a positive

electrical contact and a negative electrical contact which are positioned on the

exterior of the battery pack as to permit the contacts to engage the positive and

negative charging terminals of a charging adapter when the rechargeable battery

pack is inserted into the charging adapter. The battery pack further includes an

overcharge protection circuit including a voltage adjusting circuit, voltage

measuring means, voltage controller and a timing means. Preferably, the voltage

adjusting circuit comprises a resistor in parallel to a switch, all of which is in series

with the positive poles of the rechargeable cells and the positive contact of the

battery pack. Furthermore, in a preferred embodiment, the voltage measuring means, voltage controller and timing means are combined into a single

microprocessor which is situated in parallel with the positive and negative contacts

of the battery pack.

In operation, the voltage measuring means measures the voltage appearing across the positive and negative contacts of the battery pack while the voltage

controller is coupled to the voltage adjusting circuit to control the operation of the

switch. The switch is a typical electric switch which permits current to flow across

the switch (closed circuit) or to obstruct the current flow across the switch (open circuit). While charging a rechargeable battery pack, the switch remains in the

closed position. The voltage measuring means measures the voltage across the -8- battery contacts to determine when the battery pack has reached peak charge. Upon

reaching peak charge, the voltage controller opens the switch of the voltage

adjusting circuit and informs the timing means as to when the switch was opened. In this manner, upon the opening of the switch, the resistence between the

rechargeable cells and the positive contact is increased resulting in an increase

voltage across the positive and negative contacts of the battery pack. After a

predetermined amount of time, which is measured by the timing means, the voltage controller automatically closes the switch of the voltage adjusting circuit thereby

resulting in a decrease in voltage (-ΔV) appearing across the positive and negative

contacts of the battery pack. This decrease in voltage appearing across the battery

pack contacts is by an amount exceeding the predetermined change in voltage to

which the charging adapter responds, resulting in the termination of the high rate of charge.

In an alternative embodiment, the microprocessor may determine peak

charge by means other than by measurement of the voltage appearing across the

contacts of the battery pack. For example, the microprocessor of the present

invention may include a temperature measurement means, timing means and voltage

controller wherein the switch of the voltage adjusting circuit is opened upon the

battery pack reaching a predetermined temperature, and thereafter closing after a

predetermined time period. However, in the preferred embodiments, the voltage

adjusting circuit is operated based upon measurements of voltage. -9-

In an additional preferred embodiment, the switch of the voltage adjusting

circuit is only opened and thereafter closed after a predetermined time once three

criteria are met. In particular, it is preferred that the microprocessor, comprising

the voltage measurement means, voltage controller and timing means, opens the

switch of the voltage adjusting circuit only after 1) the voltage appearing across the

positive and negative contacts of the battery pack first reaches a threshold voltage;

2) the microprocessor thereafter measures an increase in the change in voltage over time (Δ²V/Δt²) typical for nickel metal hydride batteries; and 3) the microprocessor

thereafter measures the change in voltage with respect to time (ΔN/Δt) being equal

to or less than zero. Once these three criteria are met, the voltage controller of the

microprocessor opens the switch of the voltage adjusting circuit thereby increasing the voltage appearing across the positive and negative contacts of the rechargeable

battery pack. After a predetermined time, as measured by the timing means of the

microprocessor, the voltage controller closes the switch, thereby decreasing the

voltage appearing across the positive and negative contacts of the battery pack. The

decrease in voltage is sufficient to which the charging adapter responds to terminate

the high rate of charge.

In accordance with the present invention, it is a principal object to provide

an improved rechargeable battery pack and method of charging the same. -10-

It is another object of the invention to provide a rechargeable battery pack

incorporating an overcharged protection circuit permitting nickel metal hydride

batteries to be charged in conventional charging adapters typically used for charging

NI-CAD batteries.

It is still another object of the invention to provide a battery pack

incorporating an overcharged protection circuit which terminates the high rate of

charge to a battery pack upon the battery pack reaching peak charge.

These and other and more specific objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph showing charging voltage versus time for a typical NI-CAD

battery pack;

Fig. 2 is a graph showing battery pack temperature versus time for a typical

NI-CAD battery pack;

Fig. 3 is a graph showing both charging voltage versus time and battery pack

temperature versus time for a typical nickel metal hydride type battery pack; -11-

Fig. 4 is an electrical circuit diagram of a battery pack including the

overcharge protection circuit of the present invention in which the switch of the

voltage adjusting circuit is in an open position;

Fig. 5 is an electrical diagram of a battery pack including the overcharge protection circuit of the present invention in which the switch of the voltage

adjusting circuit is in a closed position; and

Fig. 6 is a graph showing both charging voltage versus time and battery pack temperature versus time for a nickel metal hydride-type battery pack including the overcharge protection circuit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is susceptible of embodiment in various forms,

as shown in the drawings, hereinafter will be described the presently preferred

embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the

invention to the specific embodiments illustrated.

Referring to Figs. 4 and 5, the rechargeable battery pack 1 of the present

invention includes one or more rechargeable cells 3a - 3d which are preferably a

nickel metal hydride cells, such as nickel metal hydride. The rechargeable cells -12- may be in series or in parallel depending on the electrical properties required of the

battery pack. As shown in Figs. 4 and 5, a first cell 3a is in series with second cell

3b, having the positive terminal of 3b connected to the negative terminal of cell 3a.

Similarly, third and fourth cells 3c and 3d are connected electrically in series, with

the positive terminal of 3d connected to the negative terminal of cell 3c. Cells 3a

and 3b are configured in parallel with cells 3c and 3d forming positive pole 5 and

negative pole 7 which are electrically coupled to the battery pack's positive contact

and negative contact, respectively. The positive and negative contacts are formed on the exterior of the battery pack for contacting the positive and negative terminals of a charging adapter (not shown) when a user inserts a battery pack into the

charging adapter.

The battery pack of the present invention further includes an overcharge protection circuit 9 including a voltage adjusting circuit 10 and a microprocessor 15

or the like for determining the battery pack's peak charge. In a preferred

embodiment, the voltage adjusting circuit includes a resistor 11 in parallel to a

switch 13 located between the positive pole of the rechargeable cells and the positive contact of the battery pack. In operation, the switch 13 of the voltage

adjusting circuit 10 is positioned to be closed to permit current to flow there

through. Upon the battery pack reaching peak charge, the microprocessor 15 opens

the switch 13 of the voltage adjusting circuit thereby increasing the resistence

between the rechargeable cells and the positive contact of the battery pack and

increasing the voltage appearing across the positive and negative contacts of the -13- battery packs. After a predetermined amount of time, as measured by the

microprocessor, the microprocessor automatically closes the switch of the voltage

adjusting circuit thereby resulting in a decrease in voltage appearing across the

positive contact and negative contact. The resistor 11 of the voltage adjusting circuit is sufficiently great as to first, increase the voltage appearing across the

battery pack contacts, and to secondly, decrease the voltage appearing across the

battery pack contacts as to exceed the predetermined change in voltage to which conventional charging adapters for use with NI-CAD batteries would respond, resulting in the termination of the high rate of charge of the battery pack.

In another preferred embodiment, the voltage adjusting circuit is a transistor

which can be operated as an electronically controlled switch. For example, certain transistors, known as field effect transistors (FET), can be used to open or close or

cause the resistance of a circuit. These field effect transistors have low resistance

drain to source when the circuit is closed. This low resistance drain results in less

voltage drop across the transistor, less power dissipation, and less resulting production of heat. Furthermore, present day field effect transistors have

advantages of requiring very little current to turn "on" and being very small in size.

Preferred field effect transistors can be purchased from International Rectifier under

the designation IRF7416. -14- In a preferred embodiment, the microprocessor 15 determines the peak

charge of the battery pack based upon measurement of the battery pack's

temperature over time. For this embodiment, the microprocessor is connected to a thermometer or temperature measure device (not shown) which measures the

temperature of the battery pack. Upon the battery pack reaching a predetermined

temperature increase over time, the microprocessor 15 opens the switch 13 of the

voltage adjusting circuit. After a predetermined amount of time, the microprocessor

then closes the switch of the voltage adjusting circuit thereby terminating the high rate of charge to the battery pack 1.

In an additional preferred embodiment, the microprocessor 15 includes a voltage measuring means, a voltage controller and a timing means (all not shown).

The voltage measuring means measures the voltage across the positive and negative

contacts of the battery pack to determine when the battery pack has reached peak

charge. Upon reaching peak charge, the voltage measuring means initiates the

voltage controller to open the switch 13 of the voltage adjusting circuit 10 and initiates the operation of the timing means. Once the timing means has recorded

that a predetermined amount of time has passed, the timing means initiates the

voltage controller to automatically close the switch of the voltage adjusting circuit to

decrease the voltage (-ΔN) appearing across the positive and negative contacts of the

battery pack. Since the resistence is sufficiently great in resistor 11, the decrease in

voltage appearing across the battery pack contacts operates to terminate the high rate

of charging by the charging adapter. -15-

In still another preferred embodiment, the microprocessor 15, comprising

the voltage measurement means, voltage controller and timing means, opens the switch of the voltage adjusting circuit 10 only after three criteria have been met. As

shown in Fig. 6, the temperature 23 of a typical nickel metal hydride battery

increases steadily over time. During charge, the voltage 21 of a nickel metal

hydride battery typically increases steadily until reaching a point 27 where the

change in voltage over time increases (Δ²V/Δt²) typical for nickel metal hydride batteries. Soon after undergoing this increase in the change of voltage over time,

the charge rate of the battery pack reaches its peak charge 29 where the change in

voltage with respect to time is equal to zero. After reaching the peak voltage, the

voltage 21 would continue to drop across the battery pack contacts if the high rate of charge were allowed to continue. In the preferred embodiment, the

microprocessor opens the switch of the voltage adjusting circuit only after: 1) the

voltage appearing across the positive and negative contacts of the battery pack, as

measured by the microprocessor, reaches a threshold voltage 25; 2) the microprocessor thereafter measures an increase in the change of voltage over time

(Δ²V/Δt²) typical for nickel metal hydride battery 27; and 3) the microprocessor

thereafter measures the change in voltage with respect to time (ΔV/Δt) being equal

to or less than zero 29. Once these three criteria are met, the voltage controller of

the microprocessor 15 opens the switch 13 of the voltage adjusting circuit 10

thereby increasing the voltage across the positive and negative contacts of the

rechargeable battery pack. As shown in Fig. 6, this increase in voltage (ΔV) is

represented by numeral 33. After a predetermined time 31 , the voltage controller -16- closes the switch thereby decreasing the voltage 35 appearing across the positive and negative contacts of the battery pack. Since the charging adapter is responsive to

terminate charging upon a decrease in voltage, charging terminates at time

represented by numeral 37.

Having described my invention in such terms as to enable those skilled in the

art to make and use it, and having identified the presently preferred embodiment

thereof, I claim:

Claims

-17- 1. A rechargeable battery pack for use with a charging adapter having first and second electrical terminals for charging the battery pack, the charging

adaptor including means for changing between high and low charging rates in

response to a voltage change detected at the first and second contacts, the battery pack comprising:

at least one rechargeable cell having first and second poles;

a first electrical contact positioned for contacting the first terminal of

the charging adapter, said first electrical contact being electrically coupled to said first pole of said rechargeable cell;

a second electrical contact positioned for contacting the second terminal of the charging adapter, said second electrical contact being

electrically coupled to said second pole of said rechargeable cell;

a voltage adjusting circuit for increasing the voltage across the first

and second electrical contacts upon receipt of a first command and for

decreasing the voltage across the first and second electrical contacts upon

receipt of a second command; -18- a timing means for measuring time; and

a control means coupled to said timing means and said voltage

adjusting circuit for communicating said first command to said voltage adjusting circuit, and for communicating said second command to said

voltage adjusting circuit after a predetermined amount of time.

2. The rechargeable battery pack of Claim 1 wherein said voltage

adjusting circuit includes a resistor in parallel with a switch for opening or closing

an electrical path.

3. The rechargeable battery pack of Claim 1 further comprising:

a voltage measuring means for measuring the voltage of said battery pack.

4. The rechargeable battery pack of Claim 3 wherein said voltage

measuring means is coupled to said control means, and said control means

communicates said first command to said voltage adjusting circuit upon said voltage

measuring means measuring a predetermined threshold voltage. -19-

5. The rechargeable battery pack of Claim 3 wherein said voltage

measuring means is coupled to said control means, and said control means

communicates said first command to said voltage adjusting circuit upon a

measurement of the change of battery voltage over a predetermined time period (╬öV/╬öt) being less than or equal to zero.

6. The rechargeable battery pack of Claim 3 wherein said voltage

measuring means is coupled to said control means, and said control means

measuring means measuring an increase in the change in voltage over time

(╬ö²V/╬öt²) being greater than a predetermined value.

7. The rechargeable battery pack of Claim 1 wherein said voltage adjusting circuit is a transistor.

8. The rechargeable battery pack of Claim 1 wherein said timing means

and control means are provided in a single microprocessor.

9. The rechargeable battery pack of Claim 3 wherein said timing

means, control means and voltage measuring means are provided in a single

microprocessor. -20-

10. The rechargeable battery pack of Claim 1 further comprising:

a temperature measuring means for measuring the temperature of said

battery pack.

11. The rechargeable battery pack of Claim 10 wherein said temperature

measuring means is coupled to said control means, and said control means

communicates said first command to said voltage adjusting circuit upon said

temperature measuring means measuring a predetermined increase in temperature

over time (╬öT/╬öt).

12. A method of recharging a battery having first and second electrical

contacts using a charging adaptor having first and second electrical terminals for

charging the battery pack, the charging adaptor including means for changing

between high and low charging rates in response to a voltage change detected at the

first and second contacts, the charging adapter also being responsive to detect the voltage across the first and second electrical contacts of the battery pack to change

between first and second charging rates for charging the battery pack, the method

comprising the steps of: -21- charging the battery pack at the first charging rate;

increasing the voltage across the first and second electrical contacts of

the battery pack;

measuring the commencement of time after the voltage is increased

across the first and second electrical contacts of the battery pack;

decreasing the voltage across the first and second electrical contacts

of the battery pack a predetermined amount of time after first increasing the

voltage appearing across the first and second electrical terminals of the battery pack; and

charging the battery pack at the second charging rate in response to

the decrease in voltage across the first and second electrical contacts.

13. The method of recharging a battery of Claim 12 further comprising

the step of:

measuring the voltage across the electrical contacts of the battery

pack. -22-

14. The method of recharging a battery of Claim 13 further comprising

the steps of:

detecting a predetermined threshold voltage, and

the step of increasing the voltage across the first and second electrical contacts is in response to the detection of the threshold voltage.

15. The method of recharging a battery of Claim 13 further comprising

the steps of:

detecting an increase in the change in battery voltage over time

(╬ö²V/╬öt²) being greater than a predetermined value, and

the step of increasing the voltage across the first and second electrical

contacts is in response to the detection of an increase in the change in battery

voltage over time (╬ö²V/╬öt²) being greater than a predetermined value.

-23-

16. The method of recharging a battery of Claim 13 further comprising

the step of:

detecting a change in battery voltage over a predetermined time

period (╬öV/╬öt) being less than a predetermined value, and

the step of increasing the voltage across the first and second electrical

contacts is in response to the detection of a change in battery voltage over a predetermined time period (╬öV/╬öt) being less than a predetermined value.

17. The method of recharging a battery of Claim 12 further comprising

the step of:

measuring the temperature of the battery pack, and

the step of increasing the voltage across the first and second electrical contacts is in response to the detection of an increase in temperature over time (╬öT/╬öt) being greater than a predetermined value.