Note: Descriptions are shown in the official language in which they were submitted.
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PCT PATENT APPLICATION
TITLE OF THE INVENTION
VOLTAGE CLAMP TO ALLOW LOW-TEMPERATURE RECHARGING OF
NICKEL-CADMIUM BATTERIES IN EMERGENCY LIGHTING FIXTURES AND
METHOD OF USING
INVENTOR:
David B. Crenshaw, a U.S. citizen, and a resident of Collierville, Tennessee
38017.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable
REFERENCE TO A "MICROFICHE APPENDIX"
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates charging of rechargeable batteries
such as nickel-
cadmium type batteries and more specifically to an apparatus to facilitate
charging of nickel-
cadmium type batteries at temperatures below 0 C without damaging the battery.
Still more
specifically, the application relates to the charging of batteries used to
power emergency lighting
in an environment in which the batteries will experience temperatures below 0
C.
2. General Background of the Invention
Batteries are electrochemical devices that are used to supply energy for
electrical and
electronic products. Chemical energy stored in the battery is converted into
electric current when
the battery is discharged. Batteries are classified as primary or secondary
types. Because the
chemical materials in a primary battery are irreversibly consumed during
discharge, a primary
battery may only be discharged once. On the other hand, since the chemical
reaction that
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produces electricity in a secondary battery is reversible, a secondary battery
can be repeatedly
recharged (i.e. electricity stored in it) so that it can be repeatedly
discharged.
One popular type of secondary (i.e. rechargeable) battery is the nickel-
cadmium type
("NiCd"). Many emergency lighting applications use NiCd batteries in emergency
lighting
ballasts, backup ballasts, or as backup power sources. NiCd batteries are well-
suited for these
applications because: (1) they are sealed, which means they can be used for a
long period of time
with little maintenance; (2) they are typically one of the most economic
choices since they exhibit
long service life-typically exceeding 500 charge/discharge cycles; (3) they
are capable of
providing high-rate and near constant discharge due to their low internal
resistance; (4) they are
not restricted on mounting or orientation; and (5) they are highly reliable,
rugged, and dependable
An additional benefit of NiCd batteries is the fact that their ambient
temperature
specification for discharge is from -20 C to +70 a relatively large range for
a secondarybattery.
While the ambient temperature specification for charging is also relatively
large, 0 C to +70 C, it
does not completely encompass the range available for discharge. The ambient
temperature range
for discharging is larger than that for charging because the internal gas
pressure createdby oxygen
gas that is generated during charging tends to increase as the ambient
temperature decreases,
especially when the temperature decreases below 10 C. Thus, as the temperature
decreases below
10 C, the charge current should be reduced to a safe level to reduce the rate
of production of
oxygen gas to avoid causing the battery to leak.
This presents a problem for applications in which the other benefits of NiCd
batteries are
desirable, but the ambient temperature is frequently or constantly below 0 C
such as outdoor
locations and cold storage facilities. While it is safe to use NiCd batteries
to supply power in
these conditions, problems arise when a constant current or quasi-constant
current charger is used
to maintain or recharge the NiCd batteries.
NiCd battery cells have the additional characteristic that the voltage across
aNiCdbattery
cell increases as the temperature decreases. While the exact number may vary
slightly by
manufacturer, the nominal voltage across a single NiCd battery cell is
typically 1.2V at 25 C and
the maximumvoltage across a single NiCd battery cell is typically 1. 6V. Thus
a NiCd batteiy cell
can be recharged using a trickle charge at temperatures below 0 C if the
voltage across the NiCd
battery cell could be clamped at a maximum of 1.6V.
In addition, many applications require the use ofmultiple NiCd battery cells
connectedin
series in a NiCd battery pack. Such a NiCd battery pack can similarly be
recharged using a trickle
charge at low temperatures. But in the case of a battery pack, the voltage
across the NiCdbattery
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pack would have to be clamped at the maximumvoltage for the NiCd batteiy
paclc, which is equal
to the number of cells connected in series multiplied by the maximum voltage
for each cell,
typically 1. 6V. For example, in a two-cell NiCd battery or battery pack, the
maximum charging
voltage would be 3.2V (1.6V maximum voltage/cell * 2 cells).
One known device designed to address this problem is the use of a heater in
the device to
keep the NiCd battery's temperature at or above 0 C in combinationwith a
sensor that allows the
battery to charge only when the temperature is above 0 C. (See U.S. Patent No.
6,753,651,
assigned to the assignee of the present application). While this device
prevents the battery from
being chacged when it is too cold, it consumes excess energy to keep the
battery at a temperature
above the ambient. Moreover, it does not allow the battery to be charged if
the ambient
temperature is low enough that the heater cannot maintain the battery at 0 C.
An earlier attempt to solve a seemingly similar problem is described in U.S.
Pat. No.
4,719,401. This patent describes a special Zener diode shunted across
eachindividualbattery cell,
that in response to a cell failure (an open circuit), the Zener becomes a
permanent short that shorts
around the failed cell and all the charging cuirent is routed around the cell
through the special
diode looping element. Thereby, the failed battery cell is effectively removed
from the circuit.
However, one problem with this proposal is that the Zener diodes must be
placed across each
individual cell. Another problem with this proposal is that the special diode
becomes apei7mnent
short circuit, so that, if the battery cell were to return to a normal state,
the cell would still be
permanently removed from the circuit. Yet, another problem with this proposal
is that it relies on
the fact that the cell will fail permanently in the open circuit state,
leaving no response or
mitigation to the issue of increased battery voltage alone that may return to
a normal state once
environmental conditions are corrected. Those skilled in the art readily
comprehend that this
proposal is an attempt to solve a different problem other than the one
described in this application.
Another proposed solution is taught in U.S. Pat. No. 3,343,058. This patent
describes the
use of a tunnel diode device shunted across each battery cell. When the
battery voltage reaches
the tunnel diode voltage, the tunnel diode conducts thus limits the battery
voltage. However, one
problem with this proposal is that the tunnel diodes must be placed across
each individual cell.
Another problem with this proposal is that tunnel diodes have a voltage verses
current
characteristic curve that is valley shaped beyond the breakdown voltage, which
allows the voltage
to increase as the current in the diode decreases then beyond the valley
voltage, the current is
allowed to increase as voltage increases. This voltage-current characteristic
curve is unmatched
with the battery and the battery charger. Another problemwith this proposalis
that tunnel diodes
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have very low breakdown voltages, typically 200 mV, and very low valley
voltages, typically 300
mV to 500 mV, therefore they are unsuitable for higher voltage multi-cell
battery packs.
Another proposed solution is taught in U. S. Pat. No. 6,268,714. This patent
descnbes the
use of a voltage limiting circuit connected in parallel with a battery and
including a series-
connected forward biased diode and an impedance which permits linear
adjustments. Aproblem
with this proposal is that the series-connected forward biased diode and
impedance must be placed
across each individual cell. Another problem with this proposal is that there
are at least two
minimum components that must be used for each cell. Another problemwith this
proposal is that
the forwardbiased diode's foi-ward voltage is a fixedvalue, about 0.7 V,
whichmustbetakeninto
account for the series impedance. Selection of the impedance must be carefully
administered so as
to avoid significant current flow through the diode under normal operating
conditions.
In contrast to the aforementioned earlier proposals to the present invention,
the Zener
diode (or a series of Zener diodes) is placed reverse biased or anti-parallel
across the batterypack
which is a series airangement of a plurality of cells. This simple device is
employed so as to clamp
the battery voltage to a maximum allowed value when the total battery pack
terminal voltage
reaches a maximum value at low temperatures; furthermore, the charging current
in the battery is
reduced at these low temperatures. This method finds special application where
constant-current
battery chargers, or quasi-constant-current battery chargers are used. Since
the batteries are
finding increased popularity in temperatures below 0 C, and the battery
voltage must be clamped
to a maximum allowed value when charging at these low temperatures, and the
charge currents
must be reduced to a safe allowable level, this simple method is cost
justified when compared to
other methods.
Also related to the present invention is the use of a Zener diode known as a
Transient
Voltage Suppressor (TVS). Transient Voltage Suppressors (TVS) are
specializedZener diodes
intended to clamp the voltage appearing across their terminals, thereby
preventing transient spikes
from damaging sensitive components electrically also connected across the TVS
device terminals.
They accomplish this by conducting current in response to avoltage across the
TVS that exceeds
the Zener avalanche rating. Because transient voltages can be quite high,
these devices must be
able to handle large avalanche currents. The silicon TVS is designed to
operate in the avalanche
mode just as Zener diodes are. They use a large junction area to absorb large
transient currents.
The TVS is characterized by a fast response time, faster than the standard
Zener diode. Fromthis
point forward, the reference to "Zener diode" and its derivative terms, will
be understood to
incorporate all forms ofZener diodes, as well as, Transient Voltage Suppressor
type Zener diodes.
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What is needed is a device that will allow a NiCd battery, or NiCd battery
pack or similar
battery, to be recharged at temperatures below a specified critical
temperature using a constant or
quasi-constant current source and will allow the NiCd battery or battery pack
to be charged at a
lower rate than would otherwise be accepted by the battery (including
potential harm to the
battery) such as by being slow charged at low temperatures by clamping the
voltage across the
NiCd battery or NiCd battery pack at or below its maximum voltage.
SUMMARY OF THE INVENTION
In one aspect, the invention is a voltage clamping device that allows a NiCd
battery
or similar battery paclc to be charged at temperatures below those critical
temperature at which
the charging voltage across the NiCd battery or battery pack is clamped to a
voltage acceptable to
the battery at a temperature otherwise unacceptable for charging. At the same
time, the device
allows the NiCd batteiy or battery pack to be slow or trickle charged at
temperatures below the
temperature at which its voltage reaches its maximumby clamping the voltage
across it by means
of a reverse biased zener diode.
In another aspect, the invention is a constant or quasi-constant current
battery charger that
incorporates a voltage clamping device that allows the constant or quasi-
constant current battery
charger to automatically convert to a constant or quasi-constant voltage type
battery charger
when the battery or battery pack connected to the charger approaches its
maximum permissible
charging voltage.
In yet another aspect, the invention can be incorporated into an emergency
lighting system
in order to allow the NiCd batteiy backup power source to be charged at
temperatures between
0 C and -20 C.
BRIEF DESCRIPTION OF THE DRAWINGS
For a further understanding of the nature, objects, and advantages ofthe
present invention,
reference should be made to the following detailed description, read in
conjunction with the
following drawings, wherein like reference numerals denote like elements and
wherein:
Figure 1A is circuit diagram of an embodiment of this invention.
Figure 1B is circuit diagram of a second embodiment of this invention.
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Figure 1 C is circuit diagram of a third embodiment of this invention.
Figure 1D is circuit diagram of a fourth embodiment of this invention.
Figure 2 is a circuit diagram of a battery charger employing an embodiment of
this
invention.
Figure 3 is a graph illustrating the effect the current invention has on the
voltage across a
NiCd batteiy as the temperature decreases.
Figure 4 is a graph illustrating the relationship of the current flow through
the NiCd
battery being charged and the present invention as the ambient temperature of
the battery
increases.
Figure 5 is a circuit diagram illustrating the use of an embodiment of the
invention
integrated into the ballast for an emergency lighting fixture.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure lA, an embodiment of the current invention is shown.
Current source
C is electrically comiected to NiCd batteiy pack B and is of the constant or
quasi-constant type.
NiCdbatterypackB comprises one ormore NiCdbatteries connectedinseries,
eachNiCdbattery
comprising one or more NiCd cells connected in series. In parallel with
battery pack B is the
present invention, voltage clamping device 10a. In this embodiment, voltage
clamping device l0a
comprises a single zener diode 12 electrically connected to current source C
in parallel withNiCd
batteiy pack B.
Zener diode 12 is selected to have a zener diode breakdown voltage equal to
themaximum
permissible charging voltage for NiCd battery pack B. Thus, while NiCd battery
pack B is above
the temperature at which the battery terminalvoltage is below its maximum
charge voltage, zener
diode 12 does not allow current to flow through it, directing the entire
constant or quasi-constant
current from current source C though NiCd battery pack B. But as the ambient
temperature
decreases and the voltage of NiCd battery pack B reaches its
maximumpermissible voltage, zener
diode 12 reaches its breakdown voltage and allows current to flow though it.
This causes the
voltage across NiCd battery pack B and zener diode 12 to be maintained at a
constant acceptable
voltage. Thus, as the temperature continues to drop causing the resistance in
NiCdbattery pack B
to increase, the current flow through zener diode 12 increases and the current
flow through NiCd
battery pack B decreases in order to keep the voltage across NiCd battery pack
B at or below its
maximum acceptable voltage. A graph of this relationship is shown in Figures 3
and 4.
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Zener diode 12 is also selected with a power dissipation rating that will
allow it to handle
the entire current supplied by current source C. This allows voltage clamping
device 10a to
handle temperatures down to -20 C without being damaged, which is the point
when the
resistance in NiCd battery B typically increases to sucll an extent that NiCd
battery pack B
effectively becomes an open circuit relative to voltage clamping device 10a.
Those skilled in the
art will recognize that device 10a will operate below -20 C with other
batteries.
Once the voltage across NiCd battery pack B drops below its maximum voltage,
the
voltage across voltage clamping device 10a falls below the zener breakdown
voltage of zener
diode 12, which causes zener diode 12 to prevent current flow through itself.
Referring now to Figure 1B, an alternate embodiment of the invention is shown.
In this
embodiment, voltage clamping device lOb comprises a series of zener diodes 22
with the anode of
one zener diode 22 being connected to the cathode of a following zener diode
22 which has its
anode connected to the anode ofyet another zener diode 22. The breakdown
voltage ofthe series
of zener diodes 22 is equal to the sum of the breakdown voltages of the zener
diodes 22 forming
the series. The zener diodes 22 are selected such that the zener breakdown
voltage of the series is
equal the maximum charging voltage of NiCd battery pack B that the series of
zener diodes is
parallel to.
Zener diodes 22 in the series are also selected to have a sufficient power
dissipation rating
to enable the series of zener diodes to handle the entire current provided by
current source C
without dama.ging any of zener diodes 22. This allows voltage clamping device
lOb to handle
temperatures down to -20 C without being damaged when the resistance in NiCd
battery pack B
increases to such an extent that NiCd battery pack B effectively becomes an
open circuit relative
to voltage clamping device lOb.
Referringnow to Figures 1C and 1D, alternative embodiments
ofthepresentinventionare
shown. In Figure 1C, voltage clamping device 10c comprises a single zener
diode 12 in series
with a standard diode 14. Standard diode 14 is included in voltage clamping
device lOc to
prevent reverse leakage current through zener diode 12 and to allow the
breakdown voltage of
voltage clamping device 10c to be fine-tuned. Zener diode 12 is selected as
discussed above in
the description of voltage clamping device 10a taking into account any
additional voltage drop
across standard diode 14. Similarly, in Figure 1D, voltage clamping device l
Od comprises aseries
of zener diodes 22 in series with a series of standard diodes 24. The series
of standard diodes 24
are included in voltage clamping device l Od for the reasons discussed above
as to why standard
diode 14 was included in voltage clamping device l Oc. While not shown, a
voltage clamping
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device according to the pTesent invention may include a single zener diode in
series with a series
of standard diodes or it may include a series ofzener diodes in series with a
single standard diode.
And in still another embodiment, which is not shown, a voltage clamping device
according
to the current invention utilizes a silicon transient voltage suppressor diode
in place of a standard
zener diode.
Refeiring now to Figure 2, battery charger 100 incorporating the present
invention is
shown. The dashed line represents the boundaries of battery charger 100.
Battery charger 100
comprises cuiTent source C, which is a constant or quasi-constant current
source, and voltage
clamping device 110. Voltage clamping device 110 is connected across the
terminals current
source C. In this embodiment, voltage clamping device 110 further comprises a
single zener diode
112. Zener diode 112 is selected to have a breakdown voltage equal to the
maximum charging
voltage of NiCd battery B that battery charger 100 is designed to charge.
Zener diode 112 further
has a power dissipation rating sufficient to handle the entire current
provided by current source C.
Tllus, battery charger 100 functions as a constant current charger when NiCd
battery pack B has
a temperature above approximately 10 C and as a constant voltage charger when
NiCd battery
pack B's temperature falls below approximately 10 C.
Referring now to Figure 5, an embodiment of the present invention 200 is shown
integrated into a ballast for an emergency lighting fixture. In this
embodiment, NiCd battery pack
B has a maximum charging voltage of 18.2V, and the voltage clamping device
includes two zener
diodes 210, each having a zener breakdown voltage of 9. 1V.
In this particular embodiment, current source C is a quasi-constant current
source
powered by an AC mains power source of either 120 VAC 60 Hz or 277 VAC 60Hz
(not
shown). If the AC power source is 120 VAC 60 Hz, it is connected to J1-4 and
neutral to J1-5,
and if the AC power source is 277 VAC 60Hz, it is connected to J1-3 instead
ofJl-4. For 120
VAC 60 Hz, current flow is througli C2 and bridge rectifier D 1 and the
parallel branch consisting
of K1 B, K2B, K3B, and the R3/LBD branch, and battery B then back though
bridge rectifier and
then to the AC mains neutral connected at J1-4. Resistor R2 is on the order of
10 MOhm, which
is provided as a safety feature to discharge capacitor C2 so that C2 does not
remain with a high
voltage charge. Resistor Rl provides the same function to capacitor Cl for the
same reason.
Capacitor C3 is provided to filter the output voltage waveform ofbridge
rectifier Dl. Capacitor
C2 is sized to offer sufficient impedance to the 60 Hz source so that
approximately 80% of the
applied voltage is dropped across C2. This series capacitor arrangement forms
a quasi-constant
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current source from a voltage source as viewed by bridge rectifier D1 and the
load that is
connected to the output o f bridge rectifier D 1.
While the above describes the illustrated embodiments, those skilled in the
art may
appreciate that certain modifications may be made to the apparatus and
methodology herein
disclosed, without departing from the scope and spirit of the invention. Thus,
it should be
understood that the invention may be adapted to numerous rearrangements,
modifications, and
alterations and that all such are intended to be within the scope of the
appended claims.
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