Batteiy Charging System
by MICHAEL CHEIKY
AND
TE-CHIEN FELIX YANG
BACKGROUND OF THE INVENTION
l o FIELD OF THE INVENTION
The present invention relates generally to battery charging systems and more particularly to charging systems for preventing battery overcharge.
15
CROSS-REFERENCES
The present application is related to two copending applications, each entitled "Battery Charging Method and System," each by inventors Michael Cheiky and Te-Chien Felix 0 Yang, serial numbers to be determined, each filed December \ [ , 2001, which are included herein by this reference, and which are not admitted to be prior art with respect to the present invention.
BACKGROUND ART
Rechargeable batteries, for storing electrical energy, and battery chargers, for charging batteries and bringing the batteries back to a charged state, after the batteries have been depleted, have been known and are common. Typically, the batteries are charged after full or partial depletion by delivering energy to the batteries and reversing chemical processes within the batteries, by applying a voltage to the batteries, forcing current tlirough the batteries, and, thus, restoring charge. A common charging technique is to apply a voltage source to the battery to be charged, which is greater than the batteiy voltage of the battery, and stop charging when the battery ceases to accept additional current. This almost always results in deleterious effects on the battery, reduces performance and batteiy life.
Batteries generally consist of two or more galvanic cells. Two electrodes of dissimilar materials are isolated one form the other electronically, but placed in a common ionically conductive electrolyte. Overcharge of the battery can lead to complicated and undesirable side reactions, in particular as they pertain to the decomposition of electrolyte. The latter can lead to gas production, which in turn leads to increased battery internal impedance. The battery with this increased battery internal impedance can quickly stray from optimum operating conditions. Additionally, overcharging promotes the growth of dendrites, which in rum leads to battery shorting. On other
hand, present demands upon batteries call increasingly for greater power densities, so that undercharge is also to be avoided in any charging scheme.
Silver-based batteries typically have high energy densities, i.e., high energy to weight
and volume ratios, an ability to deliver energy at relatively high current drains, and high
reliability, making them excellent candidates for use in next generation technologies, as
well as meeting current day energy storage and delivery demands. Thus, there is a need
for a charging system that minimizes the deleterious effects of overcharging.
The charging of silver-based batteries is characterized by two plateaus, reflecting the
two active oxidation states of silver. The first plateau occurs as silver is transformed to monovalent silver oxide (Ag2O) while the second plateau reflects the formation of
divalent silver (AgO). Towards the end of charge, generally at approximately 90% of
maximum capacity, the plateau transforms into a steeply rising curve and the battery
begins to be overcharged. Consequently, a battery charging system that limits the
maximum charging voltage and charging current is needed. The battery charging
system should taper charge the battery, so as not to drive too much energy into the battery too fast, and, thus, prevent damage to the battery.
With the advent of more sophisticated and expensive battery systems, such as silver-
based batteries and other high impedance batteries, the need arises for more advanced
charging systems and methods, which prevent overcharging and damage to the
batteries. This need becomes more important, especially for silver-based batteries and
other high impedance batteries, which have high energy densities and require long term
reliability. Such batteries may be used in spacecraft and in other applications, requiring
no replacement or minimal replacement over extended periods of time. Thus, there is a
need for devices and methods to facilitate charging such batteries to their maximum
capabilities, with minimum or substantially no deleterious effects, and maximization of
life of such batteries. The charging system should be inexpensive, easy to manufacture
and use, small and light weight, durable, long lasting, reliable, and capable of being
used in aerospace and defense applications.
Different battery charging systems have heretofore been known. However, none of
these battery charging systems satisfies these aforementioned needs.
Different charging systems, using shunt regulators have been disclosed.
U.S. Patent Nos. 5,821,733 (Turnbull) and 5,747,964 (Turnbull) disclose
rechargeable batteries and battery charging systems for multiple series
connected batteiy cells which include a plurality of shunt regulators, adapted to
be connected in parallel with each of the cells. The voltage of each cell is
monitored during charging. When a cell is fully charged, excess charging
current is shunted around the fully charged cell to enable the remaining cells to
continue to charge.
Turnbull shows different embodiments of his shunt regulators, hi one of
Turn-bull's embodiments, Turnbull simply shows shunt regulators, each in
parallel with a battery cell. In another embodiment, Turnbull uses shunt
regulators and field effect transistors, whose drain and source terminals are
connected in parallel across each of the battery cells. Each shunt regulator is
under the control of a voltage sensing circuit, which includes a differential
amplifier which senses the actual cell voltage of the battery cell and compares it
with a reference voltage, elsewhere in the charging circuit. In yet another
embodiment, Turnbull uses a plurality of isolation switches to disconnect the
battery cells from the charging circuit to prevent the battery circuit from
discharging the cells when the battery charger is not being used.
U.S. Patent No. 5,982,144 (Johnson et al) discloses a rechargeable power
supply overcharge protection circuit with shunt circuits that shunt current about
a battery or batteiy cell of a string of battery cells, when it is charged to a
maximum charge limit. The shunt circuit includes shunt regulators connected across each battery cell.
U.S. Patent No. 6,025,696 (Lenliart et al) discloses a battery cell bypass module
having a sensor for detecting an operating condition of a battery cell, such as voltage or temperature, and a controller connected across the battery cell of a
lithium ion battery, the controller then being operable to change to the
conductive mode and thereby shunt current around the battery cell. The
controller includes a voltage limiting operational -amplifier operable for
transmitting a voltage excessive output signal, when the input thereto exceeds a
predetermined value, and a transistor having a predetermined gate voltage
allowing bypass current flow, the transistor being responsive to the voltage
excessive output signal from the voltage limiting operational amplifier to shunt
current around the batteiy cell.
U.S. Patent No. 4,719,401 (Altmejd) discloses zener diodes, each of which are
shunted across each cell in a series connected string of battery cells.
Different charging systems and methods, using plateaus and inflection points have been
disclosed.
U.S. Patent No. 5,642,031 (Brotto) discloses a battery recharging system with
state of charge detection, that initially detects whether a battery to be charged is
already at or near full charge to prevent overcharging. A state of charge test is
first performed on the battery, by applying a current pulse and then observing
the voltage decay characteristics which result, batteries which are initially
nearly fully charged exhibiting a larger voltage decay than batteries which are not as fully charged. The result of this initial state of charge test is used to
determine how to best terminate battery charging.
U.S. Patent No. 4,392,101 (Saar et al) and U.S. Patent No. 4,388,582 (Saar et
al) disclose a method and apparatus of fast charging batteries by means of
analysis of the profile of the variation with time of a characteristic of the
battery, which is indicative of the variation in stored chemical energy as the
battery is charged. The method comprises analyzing the profile for the
occurrence of a particular series of events, preferably including one or more
inflection points, which identify the point in time at which the application of a
fast charge rate should be discontinued. Additional methods of analysis provide
for termination or control of the charging current, upon the occurrence of other
events such as limiting values on time, voltage or voltage slope, or a negative
change in the level of stored energy. The variation of the characteristic with
time is analyzed, preferably by measuring successive values of the
characteristic, computing the slope and comparing successive slope values so as
to identify inflection points and other significant events in the variation of the
characteristic. Apparatus for performing these methods comprises a power
supply and a microcomputer for analyzing the profile and controlling the power
supply.
Saar and Brotto show a voltage-time curve, which can be separated into at least
four distinct regions. Region I represents the beginning of the charging sequence just after the batteiy is initially attached to the charger and the charging begins. After the charging sequence passes tlirough region I, the
charging curve will enter a more stable region II. Region II is generally the
longest region of the charging sequence, and is marked by most of the internal
chemical conversion within the battery itself. Because of this, the voltage of the
battery does not substantially increase over region II, and thus, this region
represents a plateau region in the charging curve. At the end of region II is an
inflection point in the curve, which represents a transition from region II to
region III, and is noted by a point where the slope of the curve changes from a
decreasing rate to an increasing rate. Region III is the region in which the
battery voltage begins to increase rapidly with respect to time, thus, representing
a region of rapid voltage rise. As the battery voltage increases tlirough region
III to its fully charged condition, the internal pressure and temperature of the
battery also increases. When the effects of temperature and pressure within the
battery begin to take over, the increase in battery voltage begins to taper off. This tapering off effect is noted as another inflection point and is also
characterized by the sharp fall in the voltage derivative curve dV/dt. Region IN
represents the fully charged region following the latter inflection point and
including the charge termination target. The charging voltage only stabilizes at
the charge termination target for a very short period of time. Consequently, if
charging continues, the additional heating within the battery will cause the
voltage of the battery to decrease and in addition may cause damage to the
battery.
U.S. Patent No. 6,215,312 (Hoenig et al) discloses a method and apparatus for
analyzing an AgZn battery, which diagnoses the status of the battery having
high and low voltage plateau states corresponding to its state of charge.
Other fast charging devices and methods have been disclosed, some of which are complicated and involved.
U.S. Patent No. 5,307,000 (Podrazhansky et al) discloses a method and
apparatus, which uses a sequence of charge and discharge pulses. The
discharging pulses preferably have a magnitude, which is approximately the
same as the magnitude of the charging pulses, but which have a duration which is substantially smaller than the duration of the charging pulses. The
discharging pulse causes a negative-going spike, which is measured and prompts the charging to stop.
U.S. Patent No. 6,097,172 (Podrazhansky et al) discloses an apparatus and
method for charging a battery in a technique wherein charge pulses are followed by discharge pulses and then first rest periods and other discharge pulses
followed by second rest periods. Selected ones of the second rest periods are
extended in time to enable a battery equilibrium to be established and the open
circuit voltage of the battery to settle down and reflect an overcharging
-condition of the battery. By comparing the open circuit voltages measured
during different extended second rest periods small voltage decreases are
detected and used to determine an overcharging condition, such as when gases are generated and affect the open circuit voltage. Once overcharging is detected the battery charging is stopped. U.S. Patent No. 6,232,750 (Podrazhansky et al) also discloses another battery charger, which rapidly charges a battery utilizing a bipolar waveform.
U.S. Patent No. 5,204,611 (Nor et al) and U.S. Patent No. 5,396,163 (Nor et al) disclose circuits in which rechargeable batteries and cells are fast charged by a controlled current, and substantially at a rate not exceeding the ability of the battery or cell to accept current. The resistance free terminal voltage of the battery or cell is detected during an interval when the charging current is interrupted, and compared against an independent reference voltage to control the charging current when a difference' between the reference voltage and the sensed resistance free terminal voltage exists.
Different charging systems and methods, using time as a factor in charging have been disclosed.
U.S. Patent Nos. 6,137,268 (Mitchell et al) discloses a battery charging system in which current is averaged over a long time period (seconds) to determine the maximum average charging rate. When the integral of charging current over this long time period reaches the programmed. maximum charge value for one period, current is simply cut off for the remainder of the fixed long period. .
U.S. Patent No. 6,215,291 (Mercer) discloses a control circuit, having a
bandgap reference circuit, which minimizes the charging cycle time of a battery
charging system, by maximizing the length of time that high constant charging
current is applied to a discharged battery.
Other charging devices, batteries, and methods have been disclosed, which still do not
satisfy the aforementioned needs.
U.S. Patent No. 5,166,596 (Goedken) discloses a battery charger having a
variable-magnitude charging current source. U.S. Patent No. 6,222,343 (Crisp
et al) discloses a battery charger, which is capable of charging different types of
batteries, a method for charging a batteiy, and a software program for operating
the battery charger.
U.S. Patent Nos. 5,387,857 (Honda et al); 5,438,250 (Retzlaff); 6,215,291
(Ostergaard et al); 6,037,751 (Klang); 5,089,765 (Yamaguchi); 4,113,921 (Goldstein et al); 5,049,803 (Palanisamy) 5,160,880 6,124,700 (Nagai et al);
(Palanisamy) 4,745,349 (Palanisamy); 5,721,688, (Bramwell); 6,252,373
(Stefansson); 5,270,635 (Hoffman et al); 6,104,167 (Bertness et al); 3,708,738
(Crawford et al); British Patent Nos. GB2178608A (Yu Zhiwei) and 892,954
(Wolff); World Patent Nos. WO00/14848 (Simmonds) and WO01/47086
(Gabehart et al); French Patent No. FR2683093-A1 (Michelle et al); and
European Patent Application No. EP1076397A1 (Klang) each disclose other
devices, batteries, and methods, which do not satisfy the aforementioned needs.
For the foregoing reasons, there is a need for a charging system that minimizes the
deleterious effects of overcharging. The charging system should limit the maximum
charging voltage and charging current applied to the batter}', and should taper charge
the battery, so as not to drive too much energy into the battery too fast, and, thus,
prevent damage to the batteiy. With the advent of more sophisticated and expensive
battery systems, such as silver-based batteries and other high impedance batteries, the
need arises for more advanced charging systems and methods, which prevent
overcharging and damage to the batteries. This need becomes more important,
especially for silver-based batteries and other high impedance batteries, which have
high energy densities and require long term reliability. Such batteries may be used in
spacecraft and in other applications, requiring no replacement or minimal replacement
over extended periods of time. Thus, there is a need for devices and methods to
facilitate charging such batteries to their maximum capabilities, with minimum or
substantially no deleterious effects, and maximization of life of such batteries. The charging system should be inexpensive, easy to manufacture and use, small and light
weight, durable, long lasting, reliable, and capable of being used in aerospace and
defense applications.
SUMMARY
The present invention is directed a battery charging system that minimizes the
deleterious effects of overcharging. The charging system limits the maximum charging
voltage and charging current applied to the batteiy, and taper charges the battery, so as
not to drive too much energy into the batteiy too fast and, thus, prevent damage to the
battery. With the advent of more sophisticated and expensive battery systems, such as
silver-based batteries and other high impedance batteries, the need arises for more
advanced charging systems and methods, which prevent overcharging and damage to
the batteries. This need becomes more important, especially for silver-based batteries
and other high impedance batteries, which have high energy densities and require long
term reliability. Such batteries may be used in spacecraft and in other applications, requiring no replacement or minimal replacement over extended periods of time. Thus,
there is a need for devices and methods to facilitate charging such batteries to their
maximum capabilities, with minimum or substantially no deleterious effects, and
maximization of life of such batteries. The charging system of the present invention
limits the maximum charging voltage and charging current applied to the battery, and
taper charges the battery, is additionally inexpensive, easy to manufacture and use,
small and light weight, durable, long lasting, reliable, and capable of being used in
aerospace and defense applications, and satisfies the aforementioned needs.
A battery charging system having features of the present invention comprises: a current
source; a battery; and a voltage and current regulator, which regulates voltage applied to
the battery and current supplied to the battery. The batteiy charging system shapes the
current supplied to the battery, and may be used to taper the current supplied to the
batteiy. The voltage and current regulator may comprise: an adjustable band-gap
voltage reference diode, a potentiometer, a resistor, and a transistor, or an adjustable
shunt regulator, and a transistor operating in conjunction with the adjustable shunt
regulator, or other suitable voltage and current regulator means. The battery charging
system regulates the current flow supplied to the battery, winch originates from a
constant charging current source. As the battery voltage exceeds a predefined terminal
voltage, the batteiy charging system diverts charging current through a transistor, thus
clamping the battery at the terminal voltage and shaping the current supplied to the
batteiy.
DRAWINGS
These and other features, aspects, and advantages of the present invention will become
better understood with regard to the following description, appended claims, and
accompanying drawings where:
FIG. 1 is a schematic diagram of a battery charging system, constructed in
accordance with the present invention;
FIG. 2 is a block diagram of the battery charging system of FIG. 1 ;
FIG. 3 is a more detailed block diagram of the battery charging system of FIG. 1 ;
FIG. 4 is a schematic diagram of an alternate embodiment of a battery charging
system, constructed in accordance with the present invention;
FIG. 5 is a schematic diagram of a timing circuit, which may optionally be used
with the battery charging systems of FIGS. 1 and 4; FIG. 6 is a schematic diagram of an alternate embodiment of a battery charging
system, constructed in accordance with the present invention;
FIG. 7 is a block diagram of an alternate embodiment of a battery charging system, constructed in accordance with the present invention;
FIG. 8 is a schematic diagram of an alternate embodiment of a battery charging
system, constructed in accordance with the present invention; and
FIG. 9 is a schematic diagram of an alternate embodiment of a battery charging
system, constructed in accordance with the present invention.
DESCRIPTION
The preferred embodiments of the present invention will be described with reference to
FIGS. 1-9 of the drawings. Identical elements in the various figures are identified with
the same reference numbers.
FIGS. 1 shows a circuit diagram of an embodiment of the present invention, a battery
charging system 10 having a current source Ic (20), resistor Rl (22), potentiometer R2
(24), transistor Ql (26), and an adjustable band-gap voltage reference diode Ul (28),
switch SI (30), and battery Bl (32). The battery charging system 10 is directed
particularly to charging silver-based batteries and other high impedance batteries, but
may be used with other suitable batteries, as well.
The current source Ic (20) supplies current to voltage and current regulator (34), having
voltage regulator means (35) and current regulator means (36), comprising the resistor
Rl (22), the potentiometer R2 (24), the transistor Ql (26), and the adjustable band-gap
voltage reference diode Ul (28). The voltage and current regulator (34) regulates
battery voltage VB I (37) and battery current IBI (38) supplied to the battery Bl (32),
when the switch S 1 (30) is closed, and, thus, completes a circuit. The voltage and
current regulator (34) is also shown in FIGS. 2 and 3.
The voltage regulator means (35), which regulates the battery voltage VBI (37) applied
to the battery Bl (32), comprises the resistor Rl (22), the potentiometer R2 (24), and
the adjustable band-gap voltage reference diode Ul (28), but may be a shunt regulator
and preferably an adjustable shunt regulator or any suitable voltage regulator. The
current regulator means (36), which regulates the battery current IBI (38) supplied to the
battery Bl (32), comprises the resistor Rl (22), the transistor Ql (26), and the
adjustable band-gap voltage reference diode Ul (28), but may be any suitable current
regulator. The current regulator means (36) allows the battery Bl (32) to be taper
charged, i.e., the battery current IBI (38) supplied to the battery Bl (32) is tapered, and
the current regulator means (36) may be used to shape the battery current IBI (38)
supplied to the batteiy B 1 (32). Components which perform more than one function
and operate in conjunction with one another in the present embodiment of the battery
charging system 10 help keep costs down and simplifies circuitry, but other suitable
voltage and current regulator circuits may be used. The voltage regulator means (35)
and the current regulator means (36) operate in conjunction with one another in the
present embodiment of the battery charging system 10; however, other suitable embodiments may be used.
At the start of charging, when the battery Bl (32) is substantially discharged, the battery
B 1 (32) has substantially the lowest impedance in the battery charging system 10, and
source current I0| (40) flows almost entirely through the battery Bl (32). Under these
circumstances, the following conditions are satisfied: Iiu, IRI, IUI, IQI « IBI ~ Ici,
where: Ir (42) is the current tlirough the potentiometer R2 (24); IR1 (44) is the current
through the resistor Rl (22); Iui (46) is the current tlirough the adjustable band-gap
voltage reference diode Ul (28); and IQ I (48) is the current through the transistor Ql
(26).
Typical values of IQI (48) at the start of charging, which is the current through the
transistor Ql (26), range from 10 to 100 microamperes, while those of the source
current I0ι (40) and the battery current IBI (38) range from 150 milliamperes to 2
amperes for a typical silver-based battery, during this period, although other values are
possible depending upon the type of batteiy being charged.
As the battery Bl (32) is being charged, the battery voltage VBI (37) increases, and the
impedance of the battery Bl (32) rises. More current IRI (44) starts to flow through the
resistor Rl (22) and the adjustable band-gap voltage reference diode Ul (28),
prompting the transistor Ql (26) to turn on and increase the current IQI (48) through the
transistor Ql (26), shunting the battery current IB I (38) from the battery Bl (32), and
decreasing the battery current IBI (38), as the battery Bl (32) approaches a substantially
fully charged condition. The charging system 10 of the present invention limits the
maximum charging voltage and charging current applied to the battery Bl (32) and
taper charges the battery B 1 (32).
The battery Bl (32) is taper charged during the battery Bl (32) final charging stage with
the battery voltage VBι (37) tightly regulated. Turn-on voltage VQ I (50) at base 52 of
the transistor Ql (26) is adjustable, by adjusting reference voltage Vui (54) of the
adjustable band-gap voltage reference diode Ul (28) with the potentiometer R2 (24),
which provides control of regulation and charging.
The battery charging system 10, thus, regulates the battery current IBI (38) supplied to
the battery Bl (32), which originates from the current source Ic (20), which is a
constant charging current source. As the battery voltage VBI (37) exceeds a predefined
terminal voltage, the battery charging system 10 diverts the battery current IBI (38) from
the battery Bl (32), tlirough the transistor Ql (26), thus clamping the battery voltage
VB I (37) at the terminal voltage and shaping the current supplied to the battery Bl (32).
The battery charging system 10, shown in FIG. 1, comprises: the current source Ic (20);
the battery Bl (32); the adjustable band-gap voltage reference diode Ul (28) in series
with the resistor Rl (22), the series resistor Rl (22) and the adjustable band-gap voltage
reference diode Ul (28) connected across the current source Ic (20) and the batteiy Bl
(32), the adjustable band-gap voltage reference diode Ul (2S) having a reference input
55; the potentiometer R2 (24), which may be a voltage divider, connected across the
current source Ic (20) and the battery Bl (32), the potentiometer R2 (24) having an
output 56 connected to the reference input 55 of the adjustable band-gap voltage
reference diode Ul (28), and providing the reference voltage Vui (54) at the reference
input 55 of the adjustable band-gap voltage reference diode Ul (28); and the transistor
Q 1 (26) having an emitter 58 and a collector 62 connected across the current source Ic
(20); the battery Bl (32), the transistor Ql (26) having the base 52 connected to a
junction 64 between the series resistor Rl (22) and the adjustable band-gap voltage
reference diode Ul (28), the adjustable band-gap voltage reference diode Ul (28), the
resistor Rl (22), and the transistor Ql (26) operating in conjunction with one another to
regulate voltage applied to the battery Bl (32) and current supplied to the battery Bl
(32). The switch SI (30), which is in series with the current source Ic (20), may be used
to control current supplied to the battery Bl (32) by switching the current source Ic (20)
on or off. Although the circuitry of the present invention of the battery charging system
10, shown in FIG 1, has been described in considerable detail, other versions are possible, and may be used, as well.
The voltage drop VRI (57) across the resistor Rl (22) should be sufficiently large to
prevent the current lui (46) through the adjustable band-gap voltage reference diode Ul
(28) from being too large, but the current IRI (44) the resistor Rl (22) should be large
enough to drive the transistor Ql (26). Typically, the resistor Rl (22) is in the range of
0.7 to 1.2 K and preferably IK, but may be another suitable value, depending on the
type of battery being charged. The transistor Q 1 (26) may be any suitable p-n-p
transistor able to sustain the source current Icι (40), such as Zetex ZTX751, or other
suitable transistor. The adjustable band-gap voltage reference diode Ul (28) should
have a wide enough voltage range to cover the voltages VBI (37) achieved by the battery
Bl (32). Typically, for silver-zinc batteries, for example, the relevant voltage range of
the battery Bl (32) is between 1.6V and 2.1V, but may have other values for different
types of batteries. Elements such as National Semiconductor LM3856Z or Zetex
ZR431LC01 or other suitable adjustable-band gap voltage diode may be used for the
adjustable band-gap voltage reference diode Ul (28). The potentiometer R2 (24)
adjusts the operating value of the adjustable band-gap voltage reference diode Ul (28),
and should have a range of 100 to 300K, but may have other values, depending upon
the type of battery B 1 (32) being charged. A voltage divider or series resistors may be
used alternatively to the potentiometer R2 (24). The switch SI (30) may be any suitable
switch, and may be mechanical, a transistor or a microcontroller-controlled switch or
other suitable switch.
A non-obvious aspect of the battery charging system 10 of the present invention relates
to the end voltage of a silver-based batteiy. The minimum reference voltage Vui (54)
of the adjustable band-gap voltage reference diode Ul (28) is typically in the range of
1.24 volts. The ensuing voltage drop VRI (57) across the resistor Rl (22) and the
minimum reference voltage Vui (54) of the adjustable band-gap voltage reference diode
Ul (28) set the minimum cutoff voltage of the battery charging system 10 in the range
of 1.2 to 1.4 volts. Fortuitously, this cutoff voltage is below the first plateau of a
charged silver-zinc battery or silver-cadmium batteiy, which is 1.60 volts for the silver-
zinc battery and 1.25 volts for the silver-cadmium battery, respectively. This condition
might not be applicable for some other batteries, such as nickel-metal hydride, where the open circuit full-charge voltage is 1.2 volts.
The battery charging system 10 regulates, shapes current supplied to the battery Bl (32), and shunts current away from the battery Bl (32), as the battery Bl (32)
approaches full charge. The battery charging system 10 is particularly useful for batteries having large internal impedances (greater than 100 milliohms) and multiple plateaus, but may be used for other types of batteries, as well. Silver-based batteries, such as, for example, silver-zinc, silver-cadmium, and silver-nickel metal hydride batteries have relatively large internal impedances, of the order of 100 to 200 milliohms. The battery charging system 10 of the present embodiment regulates, shapes, and shunts current at an appropriate predetermined voltage via the use of the adjustable band-gap voltage reference diode Ul (28) and a transistor which acts in parallel with the battery Bl (32).
FIG. 4 shows an alternate embodiment of a battery charging system 100, which is substantially the same as the batteiy charging system 10, except that the charging
system 100 has an operational amplifier 120, which is used to amplify voltage V I i (122), and provide amplified and buffered output 124 for use with auxiliary devices. The amplified and buffered output 124 may be used to feed auxiliary devices, such as a microcontroller or indicator device, such as a light emitting diode (LED), or other suitable device. Such auxiliary devices may be programmed in conjunction with other
control functions and/or methods, and indicate cutoff voltage. The operational
amplifier 120 may be National Semiconductor LM2902 or other suitable operational
amplifier.
The amplified and buffered output 124, which is indicated as VsenSe below, is simply:
Vsense = R3/R4 * VR11
The resistors should obey the relationship
R3/R4 = R5/R6 , where R3, R4, R5, and R6, are the values of the resistors R3,
R4, R5, and R6, respectively.
For proper current flow into the operational amplifier 120, R4 and R6 should preferably
have values above 10K, although other values may be possible. All resistors should
preferably have tolerances better than 1%, although higher tolerances may be used,
depending upon the requirements of the battery charging system 100, the types of
batteries being charged, microcontroller, and program requirements.
FIG. 5 shows a timing circuit 200, which may optionally be used with the battery
charging system 10 or the battery charging system 100, as a switch that controls the
current flow supplied to the battery Bl (32) of the batteiy charging system 10 or the
battery Bl 1 (126) of the battery charging system 100, respectively, by switching the
current from the current source on or off. The timing circuit 200 may be used in place
of switch SI (30) of the battery charging system 10 or the switch SI 1 128 of the battery charging system 100, respectively, or in another appropriate place to control the current
flow to the battery Bl (32) of the battery charging system 10 or the batteiy Bl 1 (126) of the battery charging system 100, respectively. The timing circuit 200 may alternatively
be a timing relay, microcontroller timer, or other suitable timer. A microcontroller with a clock speed higher than 1 MHz is preferable, but microcontrollers with other clock speeds may be used. A typical timer, element microcontroller that may be used is Microchip PIC16C505 operating at 4 MHz, or other suitable microcontroller.
FIG. 6 shows an alternate embodiment of a battery charging system 300 in which a plurality of batteries B20 (310) may be charged by connecting a plurality of battery voltage and current regulators (312), which are substantially the same as the voltage and current regulators (34) of the battery charging system 10, and driving the battery voltage
and current regulators (312) with a single current source IC10 (314) through a switch S 10 (316). Each of the battery voltage and cuπ-ent regulators (312) regulates voltage applied to said respective battery B20 (310) and current supplied to said respective battery B20 (310). Each of the battery voltage and current regulators (312) are connected across a respective one of the batteries B20 (310) which are in series, and which are' also in series with the cuπ-ent source IC10 (314) and the switch S 10 (316), each of the voltage and cuπent regulators (312) also being in series.
Each of the batteries B20 (310) can, thus, be individually charged in series without the
necessity of using a plurality of cwrent sources. Battery packs often typically have
batteries in series. Thus, all batteries in a batteiy pack may be individually and
independently charged in series to their respective cutoff voltages, thus ensuring a
balanced batteiy back.
FIG. 7 shows an alternate embodiment of a battery charging system 400, which is
substantially the same as the battery charging system 300, except that the battery
charging system 400 has a plurality of programmable voltage and current regulators
412, and a timer controlled switch 414, which is controlled by microcontroller 416.
The timer controlled switch 414 is in series with current source Ic30 (418) and a
plurality of batteries B30 (420), which are also in series with the timer controlled switch
414 and the cuπ-ent source Ic 0 (418). Each of the plurality of programmable voltage
and current regulators 412, are connected in series and across a respective one of the
batteries B30 (420), each of the voltage and cuπent regulators 412 regulating voltage
applied to each of the respective batteries B30 (420) and current supplied to each of the
respective batteries B30 (420). Each of the programmable voltage and cuιτent
regulators 412 may be individually programmed to accept a variety of charging methods
and processes.
Each of the batteries B30 (414) can, thus, be individually charged in series without the
necessity of using a plurality of current sources. Thus, all batteries in a series battery
pack may be individually and independently charged in series to their respective cutoff
voltages, thus ensuring a balanced battery back.
The battery charging system 300 of the alternate embodiment of the battery charging
system 300 of FIG. 6 and the alternate embodiment of the battery charging system 400
of FIG. 7 regulates, shapes, and shunts current at appropriate voltages via the use of the
voltage and cun-ent regulators 312 and the programmable voltage and current regulators
412, respectively, while in series and without disconnecting the batteries B20 (310) and
the batteries (420), respectively, from the battery charging system 300 and the battery
charging system 400.
FIG. 8 shows an alternate embodiment of a battery charging system 500, which is
substantially the same as the battery charging system 10, except that the battery
charging system 500 may optionally have two end voltages, by the use of an
optoisolator U50 (510), which provides a two-level cutoff system. A microcontroller-
controlled Control 1 voltage (512) may be set, for example, to either 0 or voltage Ncc
(514). The current that arises from a voltage difference between the Control 1 voltage
(512) and the voltage Vcc (514) flows through limiting resistor R4 (516), which
activates the optoisolator U50 (510), and results in potentiometer resistor R3 (520)
being in parallel with upper portion 522 of potentiometer resistor R2 (523). The
potentiometer resistor R3 (520). has a large resistance compared with the resistance of
the. upper portion 522 of the resistor R2 (523). The effective resistance coupled to
adjustable band-gap voltage reference diode Ul (524) is reduced, thus providing an
offset to zener reference voltage VREF (526) of the adjustable band-gap voltage
reference diode Ul (524). Consequently, depending on the value of the Control 1
voltage (512), two cutoff voltages may be used in the battery charging system 500. The
cutoff voltages may be programmed to change as a function of time or may be changed,
as a result of other instructions, may have fixed values, or may be changed manually,
depending upon the needs of the battery charging system 500.
This process for setting up two cutoff voltages may be, for example, implemented as
follows: switch SI (528) is turned on to allow cuιτent to flow without battery Bl (530)
in the battery charging system 500; the Control 1 voltage (512) is then set, for example,
to the voltage Vcc (514); the potentiometer R2 (523) is then adjusted to achieve a high
cutoff voltage, across where the battery Bl (530) is to be connected; the Control 1
voltage (512) is then set, for example, to ground, and the potentiometer resistor R3
(520) is then adjusted to achieve a low cutoff voltage, across where the battery Bl (530)
will be connected.
The value of the limiting resistor R4 (516) should be sufficient to allow enough current
flow to activate the optoisolator U50 (510), without damaging the optoisolator U50
(510). Typical resistance values for the limiting resistor R4 (516) range from approximately 500 to 1,500 ohms, although other suitable values may be used,
depending on the components used and demands placed on the battery charging system
500. The value of the potentiometer resistor R3 (520) is governed by the desired
voltage offset, but for the battery-systems considered, the value of the potentiometer
resistor R3 (520) is typically between 10 times to 30 times the value of the
potentiometer resistor R2 (523), but other ratios may be used depending upon the types
of batteries to be charged in the battery charging system 500. Typical values of the
potentiometer resistor R2 (523) and the potentiometer resistor R3 (520) are 100K and
2M, respectively, although other values may be used depending upon the types of
batteries to be charged in the battery charging system 500.
FIG. 9 shows an alternate embodiment of a battery charging system 600, which is
substantially the same as the battery charging system 10, except that the battery
charging system 600 may optionally have a plurality of end voltages, by adding
additional optoisolators U60 (610). The number of cutoff voltages is one more than the
number of optoisolators U60 (610) used in the battery charging system 600. FIG. 9
shows the battery charging system 600 as a 3-level cutoff system. The procedures for
setting the cutoff voltages of the battery charging system 600 are substantially similar to
setting the cutoff voltages of the battery charging system 500.
Although the present invention has been described in considerable detail with reference
to certain preferred versions thereof, otlier versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the
preferred versions contained herein.