WO2008064008A2

WO2008064008A2 - Voltage clamp to allow low-temperature recharging of nickel-cadmium batteries in emergency lighting fixtures and method of using

Info

Publication number: WO2008064008A2
Application number: PCT/US2007/084553
Authority: WO
Inventors: David B. Crenshaw
Original assignee: Crenshaw David B
Priority date: 2006-11-13
Filing date: 2007-11-13
Publication date: 2008-05-29
Also published as: MX2009005116A; EP2082477A2; AU2007323912A1; EP2082477A4; WO2008064008A3; BRPI0718845A2; CA2669713A1

Abstract

The present invention relates to emergency lighting ballasts having rechargeable batteries, usually of the nickel-cadmium type and more specifically to an apparatus to facilitate the charging of the battery pack at temperatures generally below normal operating ranges of the batteries (such as below 0° C) without damaging the battery. The apparatus includes such as a reverse-biased zener diode which has a breakdown voltage in the reverse mode which is at or below the maximum allowable charging voltage for the battery. Accordingly, when ambient conditions would otherwise allow the charging current of the battery to exceed the allowable level, the clamping zener diode breaks down shunting the excess current away from the battery.

Description

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 O⁰C 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 O⁰C.

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 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 (Ie. 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 -20⁰C to +70° a relatively large range for a secondary battery.

While the ambient temperature specification for charging is also relatively large, 0⁰C 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 created by oxygen gas that is generated during charging tends to increase as the ambient temperature decreases, especially when the temperature decreases below 10⁰C. Thus, as the temperature decreases below 10⁰C, 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 0⁰C 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. NiCdbattery cells have the additional characteristic that the voltage across a NiCd battery 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 25⁰C and the maximum voltage across a single NiCd battery cell is typically 1.6 V. Thus a NiCd batteiy cell can be recharged using a trickle charge at temperatures below O⁰C if the voltage across the NiCd battery cell could be clamped at a maximum of 1.6V.

In addition, many applications require the use of multiple NiCdbattery cells connected in 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 pack would have to be clamped at the maximum voltage for the NiCd battery pack, 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 0⁰C in combination with a sensor that allows the battery to charge only when the temperature is above 0⁰C. (See U.S. Patent No. 6,753,651, assigned to the assignee of the present application). While this device prevents the battery from being charged 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 0⁰C.

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 each individual battery 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 current 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 apermanent 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 itrehes 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 problem with this proposal is that tunnel diodes 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 describes 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 problem with this proposal is that the forward biased diode's forward voltage is a fixed value, about 0.7 V, whichmustbe taken into 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 battery pack which is a series arrangement of aplurality 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 0⁰C, 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 specialized Zener 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 a voltage 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. From this point forward, the reference to "Zener diode" and its derivative terms, will be understood to incorporate all forms of Zener diodes, as well as, Transient Voltage Suppressor type Zener diodes. 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 asp ect, the invention is a voltage clamping device that allows a NiCd battery or similar battery pack 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 maximum by 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 0⁰C and -20⁰C.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature, objects, and advantages of the 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 IA is circuit diagram of an embodiment of this invention.

Figure IB is circuit diagram of a second embodiment of this invention. Figure 1C is circuit diagram of a third embodiment of this invention.

Figure ID 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 IA, an embodiment of the current invention is shown. Current source C is electrically connected to NiCd batteiy pack B and is of the constant or quasi-constant type. NiCd battery pack B comprises one ormore NiCdbatteries connected in series, each NiCdbatteiy 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 10a comprises a single zener diode 12 electrically connected to current source C in parallel with NiCd batteiy pack B.

Zener diode 12 is selected to have a zener diode breakdown voltage equal to the maximum permissible charging voltage for NiCd battery pack B. Thus, while NiCdbatteiy pack B is above the temperature at which the battery terminal voltage 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 NiCd battery 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. 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 1 Oa to handle temperatures down to -20⁰C without being damaged, which is the point when the resistance in NiCd battery B typically increases to such 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 -20⁰C 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 IB, an alternate embodiment of the invention is shown. In this embodiment, voltage clamping device 10b 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 of yet another zener diode 22. The breakdown voltage of the 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 damaging any of zener diodes 22. This allows voltage clamping device 10b to handle temperatures down to -20⁰C 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 10b.

Referring now to Figures 1C and ID, alternative embodiments of the present invention are 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 10c 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 ID, voltage clamping device lOdcomprises 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 1Od for the reasons discussed above as to why standard diode 14 was included in voltage clamping device 10c. While not shown, a voltage clamping device according to the present invention may include a single zener diode in series with a series of standard diodes or it may include a series of zener 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.

Referring 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 current 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 embo diment, 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 ofNiCdbattery 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.

Thus, battery charger 100 functions as a constant current charger when NiCd battery pack B has a temperature above approximately 10⁰C and as a constant voltage charger when NiCd battery pack B's temperature falls below approximately 1O⁰C.

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 18.2 V, and the voltage clamping device includes two zener diodes 210, each having a zener breakdown voltage of 9. IV.

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 Jl -4 and neutral to Jl -5, and if the AC power source is 277 VAC 60Hz, it is connected to Jl-3 instead of Jl-4. For 120 VAC 60 Hz, current flow is through C2 and bridge rectifier D 1 and the parallel branch consisting ofKlB, K2B, K3B, and the R3/LED branch, and battery B then back though bridge rectifier and then to the AC mains neutral connected at Jl -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 of bridge rectifier D 1. 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 current source from a voltage source as viewed by bridge rectifier Dl and the load that is connected to the output of bridge rectifier Dl.

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.

Claims

I claim: 1. An apparatus for charging a battery pack in an emergency lighting system at a temperature in the range -20⁰C to + 70⁰C comprising: a means for generating a constant current; a battery pack connected electrically to said means for generating a constant current; and a voltage clamping device electrically connected to said means for generating a constant current in parallel with said battery pack.

2. The apparatus of claim 1 wherein said voltage clamping device further comprises a zener diode.

3. The apparatus of claim 1 wherein said voltage clamping device further comprises a series of zener diodes.

4. The apparatus of claim 1 wherein said voltage clamping device further comprises a zener diode in series with a standard diode.

5. The apparatus of claim 1 wherein said voltage clamping device further comprises a transient voltage suppressor diode.

6. The apparatus of claim 1 wherein said battery pack further comprises a plurality ofnickel- cadmium b atteries connected in s eries .

7. An emergency lighting systemballast for use at any temperature below 7O⁰C comprising: a means for generating a constant current having first and second electrical connection terminals; a rechargeable battery pack electrically connected between said first and second terminals; and a voltage clamping device electrically connected between said first and second terminals in parallel with said battery pack.

8. The apparatus of claim 7 wherein said voltage clamping device further comprises a zener diode, said zener diode having a breakdown voltage equal to the maximum charging voltage of said battery pack and having a sufficient power dissipation rating to conduct the constant current generated by said means for generating a constant current.

9. The apparatus of claim 8 wherein said voltage clamping device further comprises a standard diode in series with said zener diode.

10. The apparatus of claim 7 wherein said voltage clamping device further comprises a series of zener diodes, said zener diodes in said series of zener diodes having a collective breakdown voltage equal to the maximum charging voltage of said battery pack and each zener diode in said series having a sufficient power rating to conduct the constant current generated by said means for generating a constant current.

11. The apparatus of claim 10 wherein said voltage clamping device further comprises a series of standard diodes in series with said series of zener diodes.

12. The apparatus of claim 7 wherein said voltage clamping device further comprises a transient voltage suppressor diode.

13. The apparatus of claim 7 wherein saidbatterypack further comprises a plurality ofnickel- cadmium batteries connected in series.

14. An apparatus for charging a rechargeable battery pack in an emergency lighting fixture at any temperature in the range -2O⁰C to +70⁰C comprising: a means for generating a constant current; a first output terminal electrically connected to said means for generating a constant current; a second output terminal electrically connected to said means for generating a constant current; and a voltage clamping device electrically connected between said first output terminal and said second output terminal; said voltage clamping device having a breakdown voltage equal to the maximum charging voltage of said battery pack and having a sufficient power dissipation rating to conduct the constant current generated by said means for generating a constant current.

15. A method for adapting a battery charging circuit in an emergency lighting fixture for use in low temperature conditions, said charging circuit comprising a constant-current source battery charger and a batteiy pack, comprising the steps of: determining the maximum allowable charging voltage for said battery pack and selecting a voltage clamping device to be connected in parallel with said battery pack having a breakdown voltage equal to said maximum charging voltage and a power dissipation rating sufficient to carry the constant-current supplied by said battery charger.