WO1995000993A1

WO1995000993A1 - Cutoff circuit for battery recharger

Info

Publication number: WO1995000993A1
Application number: PCT/US1994/006838
Authority: WO
Inventors: Joseph A. Keating
Original assignee: Bolder Battery, Inc.
Priority date: 1993-06-17
Filing date: 1994-06-16
Publication date: 1995-01-05
Also published as: AU7208694A

Abstract

A method and apparatus for recharging a battery using a constant current source, in which a heat-producing element such as a zener diode (27) is in parallel with the battery (20) to be charged and is thermocoupled to a heat-sensitive switch such as a bimetallic switch (24) in series with the battery. Voltage across the zener diode in excess of a predetermined level causes the zener diode to conduct current, thereby heating it and opening the switch to cut off the current.

Description

CUTOFF CIRCUIT FOR BATTERY RECHARGER

FIELD OF THE INVENTION

The present invention relates to the field of battery rechargers. In particular, it relates to an electronic circuit and method to drop the current in a constant current charger from a full recharging current to a trickle charge current or to no current or to ^ome other recharging protocol to prevent overheating c a battery, such as a lead-acid battery.

BACKGROUND OF THE INVENTION The use of rechargeable batteries is steadily and rapidly increasing in recent years. With developments in rechargeable battery technology, rechargeable batteries are used not only in traditional areas such as flashlights and small electronic devices but also in heavy duty power tools, temporary power sources, small vehicles such as forks lifts, and even larger vehicles such as small automobiles. Many of the rechargeable batteries in use today are nickel-cadmium or lithium batteries. But there have recently been dramatic advances in lead-acid batteries as well, as demonstrated by the thin-plate lead-acid batteries disclosed in U.S. Patent No. 4,047,300 by Juergens and assigned to the assignee of the present invention.

A characteristic of rechargeable batteries is that the internal resistance of the batteries and the electrochemical reaction in the batteries during recharging tends to produce heat. This heat, in turn, further increases the internal resistance, which leads to still more heat. There is thus a cycle of heat production during recharging which if uncontrolled will damage or even explode the battery.

In some nickel-cadmium battery rechargers, excessive heat build-up is prevented by use of a heat sensitive switch such as a bimetallic switch. If the temperature of the switch exceeds a predetermined switch activation temperature, the switch is opened to reduce the recharging current from full current to a trickle charge current or to cut-off the recharging current altogether. The thermocouple switch approach is relatively imprecise, since the temperature of the switch will be affected somewhat by the proximity of the switch to the batteries being recharged and even by the ambient air temperature. In the case of nickel- cadmium batteries, however, these variables are not very important since nickel-cadmium batteries will withstand fairly high temperatures without being damaged.

Lead-acid batteries are less resistant to damage from high temperatures than are nickel-cadmium batteries. For example, placing a typical rechargeable lead-acid battery into the type of nickel-cadmium recharger that has a heat sensitive cutoff-switch would cause high-temperature damage to the battery even before the temperature reached a sufficient level to open the switch to convert from full current to trickle current or no current. One approach to avoid this is to use a heat sensitive switch with a lower switch-off threshold, so that the switch will open at a lower temperature before the battery is heat damaged. Such an approach is not very feasible, however, because the lower temperature would need to be so low that the switch would be affected by the temperature variables mentioned above such as the precise placement of the switch in relation to the battery and the ambient air temperature.

In recognition of the difficulty of using a heat sensitive switch to convert a lead-acid battery recharger from full current to trickle current, in the manner of a nickel-cadmium battery recharger, the prior art has taken a completely different approach to lead- acid battery rechargers. Instead of abruptly reducing the recharging current f full current to trickle current or no current based on battery temperature as sensed at a heat sensitive switch, many of the lead- acid battery rechargers of the prior art gradually reduce the current over time. Thus the recharging current starts at full charge and declines gradually until it reaches a trickle charge. Other rechargers in the prior art use timers to limit the duration of recharging, or voltage sensors to adjust or discontinue the recharging regimen in response to the sensed battery voltage. Such devices do not attempt to monitor the temperature of the battery or to adjust the recharging current as a function of battery temperature.

These prior art lead-acid battery rechargers have several drawbacks. First, they are quite expensive in comparison to the constant current tupe of rechargers commonly used for nickel-cadmium batteries. Second, they are not interchangeable with constant current rechargers. Therefore, users must be careful to recharge nickel-cadmium batteries in a nickel- cadmium battery recharger and to recharge lead-acid batteries in a lead-acid battery recharger.

SUMMARY OF THE INVENTION

The present invention is a battery recharger circuit, primarily for lead-acid batteries. The circuit in the preferred embodiment is used with a constant-current source such as an ordinary nickel- cadmium battery recharger. The circuit includes a zener diode which is reverse biased so that when the breakdown voltage plus a predetermined diode voltage drop is exceeded, the zener diode begins to conduct current. The internal resistance of the diode causes the conducting diode to heat. The diode is thermocoupled to a bimetallic switch leading to the battery. When the temperature of the switch rises to a predetermined level due to the heat generated by the diode, the switch opens the circuit through the switch to the battery. A resistor is in another circuit to the battery, so that opening the switch in the switch circuit diverts the current flow through the resistor circuit, thereby reducing the current from full current to trickle-charge current. The circuit may also include a resistor between the zener diode and the battery to guard against surges in the constant current recharger to which the circuit is connected.

An important advantage of the invention is that the circuit is so small, simple and inexpensive that it can be incorporated into the battery with which it is used. The "battery" may then include a lead-acid electrochemical cell in combination with the recharging circuit of the invention. This allows the lead-acid "battery" to be used and recharged interchangeably with nickel cadmium batteries. It is then not necessary for the user to have one recharger for nickel-cadmium batteries and another recharger for lead-acid batteries, and to be careful not to put a battery into the wrong recharger; the user can simply own a single constant current recharger designed for nickel-cadmium batteries, and the user can use that recharger for both nickel-cadmium batteries and lead-acid batteries.

Other important advantages of the invention are its extremely low cost in components and assembly labor, high reliability and compact size. In addition, the device includes the safety aspect of a heat sensitive switch, which prevents a disastrous melting or explosion of the battery even if the device or the battery or recharger were to malfunction. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic electrical diagram of the present invention and a battery to be recharged.

DETAILED DESCRIPTION OF THE INVENTION

A schematic electrical diagram of the preferred embodiment of the present invention is shown in FIG. 1. The device is connected to the positive terminal 14 and negative terminal 16 of a consistent current source such as an ordinary nickel-cadmium battery recharger (not shown). The main current 18 runs from the positive terminal 14 of the recharger, to the battery to be recharged 20, and to the negative terminal 16 of the ^echarger through a bimetallic switch 24.

A diode circuit 26 runs from the positive terminal 14 of the recharger, to the bimetallic switch 24, bypassing the battery to be reci.arged 20. The diode circuit incudes a zener diode 27. There also may be a resistor 29 in this diode circuit, with the function described below. A resistor circuit 32 having a resis- r 34 (or perhaps some other resistive circuit if desired) bypasses the bimetallic switch 24 in the main circuit 18, and has the function described below. In a preferred embodiment, the recharger is a constant current type such as, for example, a SKIL brand "Top Gun" constant current charger which delivers a constant current of about 1.2 amps. The battery is a 12 volt lead-acid cell of the Juergens thin-plate design mentioned above, although other battery packs and other voltages could be accommodated as well. The zener diode 27 is 15V x 2W, resistor 28 is 8 ohm, resistor 34 is 8K ohm, and the bimetallic switch 24 opens at 45°C. Of course, the component values are illustrative only, and other values could be used instead. The elements within the dotted line 40, namely the zener diode 27, resistor 28, bimetallic switch 24 and resistor 34, are thermocoupled.

In operation, the current initially flows through the main circuit 18 due to the closed state of the bimetallic switch 24, to recharge the battery 20. The zener diode 27 is reverse biased so that when the zener breakdown voltage plus a diode drop (0.7V in the preferred embodiment) is exceeded, the zener diode 27 conducts, so that current flows through the parallel paths of the zener diode 27 and the battery 20. Resistor 29 protects the zener diode 27 against surges from the power supply or recharger, and may not be necessary depending on the variability of the power supply or recharger and the durability of the zener diode. The current flowing through the internal resistance of the zener diode 27 causes the zener diode 27 to heat. The heat produced by the conducting zener diode 27 is transferred to the bimetallic switch 24 through the thermocouple. The bimetallic switch 24 opens when it reaches a threshold temperature (about 45°C in the preferred embodiment).

When the bimetallic switch 24 opens, the current is diverted through resistor 32. Diverting the current from the bimetallic switch which has virtually no resistance to this high ohm resistor, causes the current through the main circuit 18 to drop from full charge to trickle charge. Because resistor 34 is part of the thermocouple to the bimetallic switch 24, the heat in the resistor 34 produced by the current flowing through it will act to keep the bimetallic switch 24 open. It can be appreciated that the resistor 32 may be replaced with some other circuit, such as a timing circuit, an indicator light or some other desired function. It can also be appreciated that other circuit configurations are possible and that the zener diode described above could be replaced with some other component or set of components, so long as there is a system for heating the bimetallic switch when the battery voltage increases.

The very small size and very low cost of the device allows it to be made integral with the battery 20. Thus the cell may include the battery 20 plus the rest of the circuit shown in FIG. 1. This makes the "cell" of the battery plus the circuit virtually indistinguishable from ordinary nickel-cadmium batteries as far as the recharging operation is concerned. The user can own just a single constant current nickel-cadmium recharger, and use it without modification both for nickel-cadmium batteries and for "cells" of lead-acid batteries with integral circuits of the type described above.

Alternatively, the circuit may be physically separate from the battery. In that event, the circuit may be built into a constant current recharger of the type used with nickel-cadmium batteries to allow that type of recharger to be used for lead-acid cells, or it may be a stand-alone adaptor which is connected and disconnected to such a recharger depending on whether the recharger is to be used with lead-acid batteries or nickel-cadmium batteries.

Claims

CLAIMSWhat is claimed is:

1. A device for use with a recharger for a battery, comprising: a circuit connecting the battery to the recharger, the circuit including a heat- sensitive switch to open the circuit if the temperature of the switch exceeds a predetermined temperature; and heating means in parallel with the battery for conducting current to heat the heating means when the voltage across said heating means exceeds a predetermined minimum, the heating means being thermocoupled to the heat sensitive switch so that when said voltage exceeds said predetermined minimum the heating of the heating means opens the switch.

2. The device of claim 1, further comprising resistive means in parallel with the switch, so that the circuit conducts a first current through the switch when the switch is closed and a second current through the resistive means when the switch is open, the second current being less than the first current.

3. The device of claim 2, wherein the resistive means produces heat when it conducts current, and the resistive means is thermocoupled to the switch, so that heat produced by the conducting resistive means keeps the switch open.

4. The device of claim 3, wherein the second current is at a level that will maintain said battery in a charged state.

5. The device of claim 1, wherein said heat-producing means is a zener diode.

6. The device of claim 5, further comprising a resistor in series with the zener diode.

7. The device of claim 1, further comprising: a battery, the battery and the device being integrally attached to one another.

8. A method of recharging a battery, comprising: establishing a circuit of said battery and a battery recharger, the circuit including a heat- sensitive switch and heating means in parallel with the battery for conducting current to heat the heating means when the voltage across the heating means exceeds a predetermined minimum, the heating means being thermocoupled to the heat sensitive switch; and applying a current through the circuit to recharge the battery until the heating means heats the heat sensitive switch to open the switch to interrupt the current through the circuit.

9. The method of claim 8, wherein the circuit includes resistive means in parallel with the heat sensitive switch, and further comprising diverting current through the resistive means when the switch is opened.

10. The method of claim 9, wherein the resistance of the resistive means is greater than the resistance of the switch, so that the current through the resistive means is less than the current through the switch.

11. The method of claim 10, wherein the resistive means is thermocoupled to the switch and further comprising heating the switch with the resistive means after the switch is opened to maintain the switch open.

12. The method of claim 11, further comprising keeping the battery charged by flowing a current through the resistive means.

13. The method of claim 8, wherein the heating means is a zener diode.

14. The method of claim 13, wherein there is a resistor in series with the zener diode to protect the zener diode from surges in the recharger.

15. The method of claim 11, wherein the switch is a bimetallic switch.

16. A battery recharger, comprising: a constant current source; and a circuit to establish electrical communication between the constant current source and a battery, the circuit including a heat sensitive switch, and a heating means in parallel with and thermocoupled to the switch which heats when the voltage across the heating means exceeds a predetermined minimum.

17. The recharger of claim 16, wherein the heating means is a zener diode.

18. The recharger of claim 17, further comprising a resistor in series with the zener diode.

19. The recharger of claim 17, further comprising a resistive means in parallel with the switch so that when the switch is closed a first current flows through the switch and when the switch is open a second current less than the first current flows through the resistive means.

20. The recharger of claim 19, wherein the resistive means is thermocoupled to the switch so that after the switch opens, heat produced by current flowing through the resistive means keeps the switch open.