U.S. patent application number 09/748627 was filed with the patent office on 2002-06-27 for charge termination circuit.
Invention is credited to Gaza, Brian Scott.
Application Number | 20020079868 09/748627 |
Document ID | / |
Family ID | 25010242 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079868 |
Kind Code |
A1 |
Gaza, Brian Scott |
June 27, 2002 |
Charge termination circuit
Abstract
A charge termination circuit which may be disposed within the
battery cell pack and used in conjunction with known battery safety
circuits to protect against charging conditions which reduce the
life of a battery cell. In particular, the charge termination
circuit is adapted to be used in conjunction with an off the shelf
battery safety circuit and one or more power FETs coupled between
the battery cell and the battery charger terminals on the battery
cell package. The charge termination circuit includes a
microcontroller and monitors the battery cell voltage by way of an
I/O part and activates a first timer anytime the charging current
to the battery is below a predefine threshold, for example,
indicative of a maintenance or trickle charge. When the first timer
times out after a predetermined time period, for example, two
hours, the charge termination current forces the one or more of the
serially coupled FETs switches to interrupt battery charging of the
battery cells. The charge termination circuit also monitors the
battery cell voltage and closes the FETs to allow charging when the
cell voltage falls below a predetermined level. As a second level
of protection, each time the microcontroller is powered up, a
second timer is activated. The second timer is set for a much
longer period of time than the first timer, for example, six hours,
and is used in the event that the charging current does not fall
below the threshold discussed above. The second timer is used to
terminate charging by switching the serially coupled power FETs off
when the second timer times out. The charge termination circuit is
adapted to work in conjunction with conventional battery safety
circuit and provide and additional level of functionality with a
battery cell package in order to provide protection against
operating conditions which reduce battery cell life, heretofore
unknown.
Inventors: |
Gaza, Brian Scott;
(Naperville, IL) |
Correspondence
Address: |
John S. Paniaguas
KATTEN MUCHIN ZAVINS
Suite 1600
525 West Monroe Street
Chicago
IL
60661
US
|
Family ID: |
25010242 |
Appl. No.: |
09/748627 |
Filed: |
December 22, 2000 |
Current U.S.
Class: |
320/155 |
Current CPC
Class: |
H02J 7/00306 20200101;
H02J 7/00304 20200101; H02J 7/0029 20130101; H02J 7/0031
20130101 |
Class at
Publication: |
320/155 |
International
Class: |
H02J 007/04; H02J
007/16 |
Claims
We claim:
1. A battery circuit for extending the life of a battery cell
comprising: a device for sensing the charging current to a battery
cell and determining when the charging current is at a level
indicative of a trickle charge; and a timer for measuring the time
period during which a trickle charging current is applied to the
battery cell and generating a termination signal for terminating
charging of the battery cell after a first predetermined time
period.
2. The battery circuit as recited in claim 1, wherein said circuit
also includes a device for measuring the battery cell voltage and
overriding said termination signal when said battery voltage drops
below a predetermined value.
3. The battery circuit as recited in claim 2, further including a
second timer which is enabled anytime power is applied to said
battery circuit and generates a termination signal after a second
predetermined time period.
4. The battery circuit as recited in claim 3, wherein said first
predetermined time period is shorter than said second predetermined
time period.
5. The battery circuit as recited in claim 4, wherein said first
predetermined time period is two hours.
6. The battery circuit as recited in claim 5, wherein said second
predetermined time period is 6 hours.
7. The battery circuit as recited in claim 1, wherein said battery
circuit includes a microcontroller.
8. A charge control circuit for one or more battery cells
comprising: a first and second battery terminal; a first and second
battery charger terminal; one or more switching devices connected
between one of said battery terminals and one of said battery
charger terminals; a battery safety circuit for controlling said
one or more switching devices under first predetermined conditions;
and a charge termination circuit for controlling said one or more
switching devices under second different predetermined
conditions.
9. The charge control circuit as recited in claim 8, wherein said
charger termination circuit includes a device for sensing the
charging current to a battery cell and determining when the
charging current is a level indicative of a trickle charge; and a
timer for measuring the time period during which a trickle charging
current is applied to the battery cell and generating a termination
signal for terminating charging of the battery cell after a first
predetermined time period.
10. The charge control circuit as recited in claim 8, wherein said
charge control circuit also includes a device for measuring the
battery cell voltage and overriding said termination signal when
said battery voltage drops below a predetermined value.
11. The battery circuit as recited in claim 8, further including a
second timer which is enabled anytime power is applied to said
battery circuit and generates a termination signal after a second
predetermined time period.
12. The battery circuit as recited in claim 8, wherein said first
predetermined time period is shorter than said second predetermined
time period.
13. The battery circuit as recited in claim 8, wherein said first
predetermined time period is 2 hours.
14. The battery circuit as recited in claim 8, wherein said second
predetermined time period is 6 hours.
15. The battery circuit as recited in claim 8, wherein said battery
circuit includes a microcontroller.
16. The charge control circuit as recited in claim 8, wherein said
switching devices include one or more FETs.
17. The charge control circuit as recited in claim 8, wherein said
first predetermined conditions include one or more of the following
conditions; overcharge, over-discharge or overcurrent.
18. The charge control circuit as recited in claim 17, wherein said
second predetermined conditions include extended trickle charging.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a battery charger
termination circuit and more particularly to a battery charger
termination circuit that functions in conjunction with known so
called battery safety circuits which terminate battery charging
after predetermined time periods under predetermined conditions to
prevent battery cell damage resulting from extended battery
charging cycles.
[0003] 2. Description of the Prior Art
[0004] Various portable devices and appliances, such as cellular
phones, require rechargeable batteries. Various types of
rechargeable batteries are known to be used in such applications.
For example, nickel-cadmium (NiCd), nickel metal hydride (NiMH), as
well as lithium ion batteries are known to be used. Because of the
different charging characteristics of such batteries, different
battery chargers are required. For example, both nickel-cadmium
(NiCd), as well as nickel metal hydride (NiMH) batteries, require
constant current charging. On the other hand, lithium ion batteries
require constant current charging up to a certain voltage value and
constant voltage charging thereafter. Because of the different
charging characteristics of the various battery types, different
charging circuits are required. Examples of charging circuits for
different battery types are disclosed in commonly-owned U.S. Pat.
Nos. 5,764,030; 5,998,966 and 6,002,237, hereby incorporated by
reference.
[0005] Various problems are known which can result in battery
damage during battery charging. One problem is known as gassing.
Gassing is a condition that occurs when a battery is charged below
room temperature. More particularly, gassing relates to a build up
of oxygen resulting from the chemical reaction that occurs within a
battery cell during charging. When a battery is being charged below
room temperature, the oxygen pressure can increase within the
battery housing and exceed the limits of the housing thus causing
damage to the battery cell. However, at higher temperatures, the
oxygen recombines, thus reducing the risk of excessive pressure
within the battery housing.
[0006] Various battery circuits have been developed to address this
problem. For example, U.S. Pat. No. 4,667,143 discloses a battery
charger which includes a thermistor for sensing battery
temperature. The thermistor is connected in parallel with a
temperature stable resistor to provide a temperature compensated
battery voltage signal. The temperature compensated voltage signal
is used to control the charging of the battery as a function of
temperature.
[0007] Commercially available monolithic battery circuits are also
known which control battery charging as a function of the battery
temperature. For example, a Texas Instrument Model No. bq2057
advanced lithium ion linear charge management (IC) integrated
circuit is available. This IC requires an external thermistor and
is used to inhibit battery charging until the temperature of the
battery is within user-defined thresholds. Thus the safety circuit
can prevent charging of the battery when the temperature of the
battery is below a predetermined threshold in order to prevent
gassing.
[0008] Another known problem that occurs during certain charging
conditions relates to overheating. Overheating occurs as a result
of prolonged charging of a battery causing the temperature of the
battery to increase to an unacceptable level, possibly causing
damage. In order to address this problem, various battery charger
circuits have been developed which limit charging times in order to
reduce the possibility of overheating of the battery cell. For
example, U.S. Pat. No. 4,035,709 discloses a battery charging
circuit which monitors the battery voltage and terminates fast
charging when the battery voltage reaches a predetermined level,
for example, 80% of the desired voltage level. Once the battery
reaches 80% of the desired voltage, rapid charging is terminated
and a timer is enabled which allows trickle charging for a fixed
period of time, for example, six hours. U.S. Pat. No. 5,727,232
also relates to a battery charger circuit which uses predetermined
time periods to control charging cycles.
[0009] Due to the differences in battery charger circuits, battery
manufactures are known to incorporate battery safety circuits
directly into the battery cell packages. More particularly, such
battery cell packages are known to include one or more battery
cells, serially connected to one or more switching devices, such as
field effect transistors (FETs), which in turn, are serially
coupled between the battery cell and the power supply terminals in
order to interrupt charging of the battery under certain operating
conditions. The FETs are under the control of a so-called battery
safety circuit, usually an integrated circuit (IC), also disposed
within the battery cell package. Such safety circuits are known to
provide overcharge, over discharge and overcurrent protection.
[0010] Overcharge protection relates to a condition when a
relatively large voltage is impressed upon the battery cell for an
excessive time period. Over-discharge protection relates to a
condition when the discharge current from the battery is excessive
resulting in the battery voltage dropping below a predetermined
voltage during a discharge mode (i.e. charger off mode). The
overcurrent mode relates to a condition when the discharge current
from batteries exceeds a level, indicative of a short circuit. The
so called battery safety circuits monitor the discharge current and
voltage in order to protect against the various conditions
mentioned above. When one of the conditions, such as overcharge,
over-discharge or overcurrent is sensed by the safety circuit, the
safety circuit interrupts battery charging from within the battery
cell, independent of the battery charger, by turning off one or
more of the serially coupled FETs.
[0011] Various battery safety circuits are known. Examples of such
battery safety circuits are available from Mitsumi Corporation. For
example, Mitsumi Model No. MM1412 and MM1491 are battery safety
circuits for use with lithium ion batteries. These devices are
described in data sheets entitled: "Protection of Lithium-Ion
Batteries MM1412" and "Lithium-Ion Battery Protection (for 1-cell
in series) MM1491", published by Mitsumi Corporation, hereby
incorporated by reference.
[0012] Use of the battery safety circuits inside of the battery
cell packages thus insures a certain level of battery protection
irrespective and independent of the battery charger used to charge
the battery cell packages. Although the battery charger circuits
and safety circuits discussed above provide adequate protection for
the various operating conditions, those circuits do not address
battery charging conditions which can result in reduced battery
cell life. In particular, battery cells are known to loose capacity
if a maintenance of trickle charge is continued for hours or even
days following a full charge. As such, appliances which utilize,
for example, lithium-ion batteries which have a reduces life are
more expensive to use. Thus there is a need for a battery circuit
to protect against battery charging conditions that are detrimental
to the battery cell life.
SUMMARY OF THE INVENTION
[0013] Briefly, the present invention relates to a charge
termination circuit which may be disposed within the battery cell
pack and used in conjunction with known battery safety circuits to
protect against charging conditions which reduce the life of a
battery cell. In particular, the charge termination circuit is
adopted to be used in conjunction with an off the shelf battery
safety, safety circuit implies the FET circuit which includes one
or more power FETs coupled between the battery cell and the battery
charger terminals on the battery cell package. The charge
termination circuit includes a microcontroller and monitors the
battery cell voltage and current by way of an I/O port configured
as an A/D input and activates a first timer anytime the charging
current to the battery is below a predefined threshold, for
example, indicative of a maintenance or trickle charge. The
charging current to the battery is measured across one or more of
the FETs in order to avoid introducing an additional series
impedance. When the first timer times out after a predetermined
time period, for example, two hours, the charge termination circuit
forces the one or more of the serially coupled FETs switches to
interrupt battery charging of the battery cells. The charge
termination circuit also monitors the battery cell voltage and
closes the FETs to allow charging when the cell voltage falls below
a predetermined level. As a second level of protection, each time
the microcontroller is powered up, a second timer is activated. The
second timer is set for a much longer period of time than the first
timer, for example, six hours, and is used in the event that the
charging current does not fall below the threshold discussed above.
The second timer is used to terminate charging by switching one of
the serially coupled power FETs off when the second timer times
out. The charge termination circuit is adapted to work in
conjunction with conventional battery safety circuit and provide
and additional level of functionality with a battery cell package
in order to provide protection against operating conditions which
reduce battery cell life, heretofore unknown.
DESCRIPTION OF THE DRAWINGS
[0014] These and other objects of the present invention will be
readily understood with reference to the following specification
and attached drawing:
[0015] FIG. 1 is a block diagram of a charge termination circuit in
an exemplary embodiment shown within a battery package.
[0016] FIG. 2 is a schematic diagram of a first exemplary
embodiment of the charge termination circuit shown coupled to an
off the battery safety circuit and a pair of serially coupled
FETs.
[0017] FIG. 3 is a flow diagram for the charge termination circuits
illustrated in FIG. 2.
DETAILED DESCRIPTION
[0018] The present invention relates to a charge termination
circuit which monitors the charging current and terminates battery
charging, for example, within the battery cell package to prevent
extended maintenance or trickle charging after a battery has been
fully charged in order to extend the life or capacity of the
battery cells within the pack. The charge termination circuit in
accordance with the present invention is adapted to work in
conjunction with a known battery safety circuits known to be
incorporated into battery cell packages. As will be discussed in
more detail below, the charge termination circuit monitors the
charging current and cell voltage to prevent extended or charging
after a battery has been fully charged in order to extend the
battery cell life while allowing charge resumption when the battery
voltage drops below a predetermined level, for example due to self
discharge. As such, additional functionality may be provided within
the battery cell package independent of external battery
chargers.
[0019] Referring to FIG. 1, an exemplary battery cell package is
shown and generally identified with the reference numeral 20. Such
battery cell packages 20 are known to be used in a wide variety of
portable appliances, such as cellular phones and the like. Such
battery cell packages normally include two battery terminals 22 and
24, identified as "BH" and "BG". These terminals 22 and 24 are used
to connect the battery cell package 20 to the portable appliance
(not shown) for providing a source of DC power to the portable
device in a portable mode of operation. The battery cell package 20
also includes a pair of battery charger terminal 26 and 28,
identified as "PH" and "PL". The terminals 26 and 28 are used to
connect the battery cell package 20 to an external battery charger
(not shown) during a battery charger mode of operation.
[0020] The battery cell package 20 is normally configured as a
disposable device and includes one or more battery cells, generally
identified with the reference numeral 30, connected between the
battery terminals 22 and 24. One or more battery cells may be
provided in series or parallel depending on the voltage and current
requirements of the portable appliance in which the battery cell
package 20 is intended to be used. The embodiments illustrated in
FIGS. 2-4 relate to exemplary battery cell packages 20 which
utilize a single lithium ion battery cell. However, the principles
of the present invention relate to multiple cells connected either
in series and/or parallel and to virtually all battery types,
including nickel cadmium (NiCd) and nickel metal hydride (NiMh) as
well.
[0021] As mentioned above, many known battery cell manufactures are
known to include switching devices 32 within the battery cell
package 20 in order to interrupt battery charging of the battery
cells 30 during certain charging conditions, which include
overcharging, over-discharging and overcurrent. As such, one or
more switching devices 32 are normally provided between the battery
terminals 22 and 24 the battery charger terminals 26 and 28 on the
battery cell package 20 in order to interrupt battery charging
under specified conditions as mentioned above. The switching
devices 30 are known to be under the control of a battery safety
circuit 34 which monitors the battery voltage and current
conditions in order to control the switching devices 32.
[0022] In accordance with the invention, a charge termination
circuit 36 is provided which provides protection independent of an
external battery charger against charging conditions which tend to
reduce the cell life of the battery cell 30. As mentioned above,
these conditions include maintaining a trickle or maintenance
charge on a battery for an extended period of time (i.e. over two
hours) after the battery cells have been fully charged. As such,
the charge termination circuit 36 provides additional functionality
heretofore unknown, independent of external battery chargers. The
charge termination circuit 36, as will be discussed in more detail
below, works in independently of the battery safety circuit 34 to
inhibit charging by turning off the switching devices 32, when such
a battery life diminishing charging condition is detected while
enabling the full functionality of the battery safety circuit
34.
[0023] Turning to FIG. 2, a schematic diagram of an exemplary
battery cell package internal circuit is illustrated. As discussed
above, one or more battery cells 30 are typically coupled between
the battery terminals 22 and 24, either in series and/or parallel.
The battery cell package circuit includes a battery safety circuit
34 and a pair of serially coupled switching devices 32. In this
exemplary embodiment, the battery safety circuit IC 34 is a Mitsumi
Model No. MM1491, normally used for a signal cell lithium-ion
battery cell. The battery safety circuit 34 is described in detail
in Mitsumi data sheets entitled; "Lithium-Ion Battery Protection
(for 1 cell in series) Monolithic IC MM1491", herein incorporated
by reference.
[0024] In order to fully understand the charge termination of
circuit 36, a brief description of the operation of the battery
safety circuit 34 and switching devices 32 is provided. In
particular, a pair of switching devices 32, implemented as a pair
of power field effect transistors (FETs), are serially connected
between the battery terminal 24 and the battery charger terminal
28. The power FETs 32 may be, for example, Sanyo model FTD2011,
described in Sanyo data sheets entitled, "FTD 2011 Load Switching
Applications", hereby incorporated by reference. The power FETs 32
are under the control of the safety circuit 34. Referring to FIG.
2, two FETs are provided in series and identified with the
reference numerals 33 and 35. One FET 33 is used for discharge
control while the other FET 35 issued for charge control.
[0025] The safety circuit IC 34 monitors the battery voltage by way
of its VCC pin 2. In particular, the battery terminal 22 is
connected to a VCC pin 2 on the safety circuit 34 by way of a
resistor R1. When the voltage at the VCC pin exceeds a
predetermined value, for example 4.35.noteq.0.025 volts, the CO pin
on the battery safety circuit 34 turns off the discharge FET 35 by
way of the resistor R3. The resistor R3 is connected between the
gate and source terminals of the FET 35 in order to reduce the cut
off time of the FET 35, resulting from gate to source
capacitance.
[0026] The battery safety circuit 34 also provides for
over-discharge protection by monitoring the voltage of a battery
terminal by way of the VCC pin. During this condition, the battery
safety circuit 34 monitors the battery voltage level and if it
drops below a predetermined threshold for example 2.4 volts, the DO
pin on the battery safety circuit 34 turns off the FET 33. This
protection is accomplished by monitoring the discharge current of
the battery. The discharge current is sensed by measuring the
voltage on the VM pin on the fall safety circuit. The discharge
current is equal to the battery voltage divided by the on
resistance of the two power FETs 33 and 35. The over-discharge
protection is activated anytime the discharge current is, for
example, 200.noteq.26 millivolts.
[0027] Lastly, the safety circuit IC provides over current
protection. When an over current condition is detected, the DO pin
on the safety circuit IC is activated to turn off one of the power
FETs in order to interrupt discharging.
[0028] The charge termination circuit 36 in accordance with the
present invention is shown in FIG. 2, within the dashed box,
identified with the reference numeral 36. In accordance with an
important aspect of the invention, the charge termination circuit
36 is adapted to work in conjunction with the battery safety
circuit 34 to provide additional functionality within a battery
cell package 20. The charge termination circuit 36 includes a
microcontroller 38, for example, an 8-bit microcontroller with a
reduced instruction set (RISK) architecture Various
microcontrollers are suitable for this application. An ATMEL, Model
No. ATtiny 15L is illustrated. The ATMEL microcontroller is
described in detail in an ATMEL publication entitled; "8 bit AVR
Microcontroller with 1K Bytes Flash Advance information, revision
1187B-03/00, pages 1-54, hereby incorporated by reference. The
microcontroller 38 includes a 6-bit input/output (I/O) port an
on-board analog to digital converter (ADC) and two timers. The
micrcocontroller 38 is used to monitor current and cell voltage. In
particular, port PB4 is used for measuring the charge of the
battery by way of the power FETs 32. In particular, port bit PB4 is
connected to the source terminal 6 and 7 of the power FET 35. The
voltage of the source terminals 6 and 7 of the FET 35, divided by
the on resistance for the two serially coupled FETs 32, represents
the charging current to the battery. In case of the power FETs 32
discussed above, each has an on resistance of 30 milliohms for a
total of 60 milliohms. Assuming a trickle charge current of about
80 milliamps, the voltage will be about 4.8 millivolts. Thus, 4.8
millivolts may be used as a threshold value for determining when
the external battery charger is providing a trickle charge to the
battery cells 30. As such, when the voltage at port bit PB4 drops
below 4.8 millivolts, the microcontroller 38 assumes a trickle
charge condition and enables first internal timer on the
microcontroller 38.
[0029] The internal timer is used to control the amount of time the
external battery charger applies a trickle charge to the battery 30
after the battery cell has been fully charged. Thus, the first
internal timer may be set for example for two hours. In accordance
with an important aspect of the invention, anytime the voltage at
port bit PB4 drops below, for example, 4.8 millivolts, indicative
of a trickle charge being supplied to the battery cell 30, the
first internal timer this condition and at the end of a time out
period, for example, two hours, the microcontroller 38 generates a
termination signal, i.e. applies a logical one to the port bit PB1
by way of a resistor R7, which, in turn, turns on a transistor Q1.
The transistor Q1 may be a bipolar transistor with its emitter
connected to ground and collector connected to the CO pin 34 on the
battery safety circuit 34. When the transistor Q1 turns on, it
forces the CO pin to go low, which in, turn, turns off the charging
FET 35, thus interrupting the charging from the external battery
charger during this condition. During this time, the battery cell
voltage at terminal 22 is continuously monitored by the port bit
PB4 by way of the resistor R5. Thus, when this voltage drops below
a pre-set level, for example 3.9 volts, due to self-discharge, the
transistor Q1 is turned off by way of the port PB1 to allow
charging of the battery cells 30 to resume.
[0030] As mentioned above, the microcontroller 38 includes an
on-board ADC Port PB0 serves as a reference for the internal ADC
within the microcontroller 38. As such, the port bit PB0 is
connected to the battery charger terminal 26 by way of the resistor
R5 and a resistor R6.
[0031] For single cell batteries, an unregulated DC supply, such as
from an automobile cigarette lighter, is sufficient as a power
supply. As such, the unregulated supply may be connected directly
to the VCC pin on the microcontroller 38. In this way, portable
appliances can be charged in an automobile with a simply a
cigarette lighter adapter thus obviating the need for a separate
battery charger. For multiple cell batteries, a regulated supply
may be required.
[0032] In accordance with another aspect of the invention, the
charge termination circuit 36 provides a second level of protection
which interrupts charging of the battery cells 30 after an extended
period of time, for example 6 hours. In particular, the power
supply VCC which, as discussed above, may simply be a cigarette
lighter adapter in a vehicle, is connected to a reset port pin PB5
on the microcontroller 38. In that case, the port pin PB5 may be
connected directly to the terminal 26. Thus, each time the
microcontroller 38 is powered up and voltage is sensed on the reset
port pin PB5, a second internal timer within the microcontroller 38
is enabled. The second timer may be provided with a second
relatively longer time out period, for example, 6 hours. Thus,
battery charging will be interrupted independent of any external
battery charger 6 hours after charging is initiated. When the
second timer times out, a termination signal is v generated, i.e.
the port pin B1 drives the transistor Q1 to an on condition, which
turns off the charging FET 35. The second level of protection may
be used for the condition when the charging current to the battery
cells 30 does not fall below the 80 milliamp threshold, as
discussed above.
[0033] The discharge FET 33 may be under the control of the battery
safety circuit 34. The discharge FET 33 is thus turned off during
over-discharge and overcurrent conditions as discussed above.
[0034] A flow chart for the microcontroller 38 is illustrated in
FIG. 3. Initially, prior to the power supply voltage being applied
to the battery cell 30, the system waits for the application of
power supply voltage. As discussed above, once a power supply
voltage is supplied to the battery charger terminals 26 and 28 the
microcontroller 38 resets as indicated in step 40. After the
microcontroller 38 resets the a second internal timer two is
enabled in step 42. In addition, the system monitors the battery
charging current in step 44 and the battery cell voltage in step
46. The system then checks in step 48 whether the second timer has
timed out. As indicated above, the second timer may be set at a
relatively long time period, for example 6 hours. Once the timer
two times out, the transistor Q.sub.1. (FIG. 2) is turned on which
causes the power FET 35 to turn off and interrupt charging of the
battery cells 30 (FIG. 1).
[0035] As mentioned above, while the second timer is timing, the
system monitors the charging current and the battery cell voltage
in steps 44 and 46. Should the charging current drop below a
threshold, for example, 80 milliamps, as determined in step 50, the
system initiates a first timer. The 80 milliamp level is set as a
threshold level for determining when a trickle voltage has been
applied to a lithium-ion battery cell after full charge. The time
out period for the first timer is set at a relatively short time
period, for example, two hours. Once the first timer times out, as
indicated in step 54, the system precedes to step 49 and turns the
transistor Q1 on to interrupt battery charging. Should the charging
current not drop below 80 milliamps, the system checks in step 56
to determine if the battery cell voltage has dropped below a
predetermined threshold value, for example 3.9 volts, due to
self-discharge. Should this condition occur, the system turns off
transistor Q1 in step 58.
[0036] The system can also be configured to operate without the
safety circuit 34 (FIG. 2). In such an embodiment, a port PB1 on
pin 6 of the microcontroller 38 can simply be used to control the
power FETs 33 and 35 directly.
[0037] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
[0038] What is desired to be secured by a Letters Patent is as
follows:
* * * * *