The use of rechargeable nickel-cadmium (nicad) batteries for consumer products is well established. Such rechargeable batteries are frequently used in portable power tools, such as cordless power drills, saws and the like. Additionally, rechargeable batteries also find application in shavers, photographic equipment and other products.
Unlike disposable batteries, however, the nicad batteries require recharging upon dissipation of the electrical energy stored therein. The recharging period of the nicad batteries, if too long, may thus diminish the effectiveness of the power tools which incorporate the batteries. There have thus been prior art attempts to speed up the charge rate in order more quickly to restore the batteries to full capacity.
The normal recommended continuous charge rate for nicad batteries is C/10 where C is the battery capacity in ampere-hours. The normal charge rate thus results in a time of 12 hours or more to recharge a battery pack. Such a time requirement is excessive, however. If the batteries powering a product are discharged prior to completion of the desired task, it is necessary for the operator either to wait for a recharge or to replace the battery pack with a fully charged replacement pack. The first approach, as above noted, is typically highly time consumptive while the second is expensive.
Accordingly, the prior art has developed several approaches to reducing the recharge time for rechargeable batteries, including various techniques to avoid overcharging the units.
In one approach to the problem, battery manufacturers have conducted research into battery characteristics under charge and have developed special cells. Thus, some newer cells are characterized by a charge rate of C/3. These cells are capable of withstanding the higher charge rate indefinitely. The time required for fully charging such cells has thus been reduced to approximately 4 hours. However, even this amount of time may be too long for some applications.
Research by the battery manufacturers has also determined that properly designed cells may be charged at a rate of C/1, so that a cell may be recharged in approximately 1.2 hours, for the popular sub C (Cs) cell size. However, this approach can only be used if the high charge rate is terminated before the cells enter a destructive overcharge condition. For such cells, a maintenance, or "trickle" charge rate of C/10 is provided after the C/1 charge rate is complete. The trickle charge rate effectively "tops off" the battery charge and maintains the cell at full capacity until used.
It is thus necessary accurately to determine the particular point at which the permissible charge rate drops from the fast, C/1, rate to the trickle, C/10, rate in order to use the newly developed cells. Moreover, it is necessary to develop a control device which can accurately detect the changeover point and vary the charge rate accordingly.
Research into various cell characteristics which can be used for detecting the proper termination point for the C/1 charge rate has centered on voltage profiles, temperature changes, and internal pressure changes responsive to the charged state of the cell. Some prior art attempts have been directed to the use of internal cell pressure as the charging criterion. However, a special cell construction is required for sensing the internal pressure of the cell, involving access to the interior of the cell. The pressure sensing approach has thus not been widespread and is generally considered expensive.
Other cell characteristics which have been considered as the criteria for determining the permissible charging state of the cell have included voltage and temperature.
Sensing the voltage alone, however, has generally not been found useful, since the voltage change from a discharged state to a fully charged state of the cell is small and is hard to detect accurately. More specifically, the change in voltage is typically of the same order of magnitude as the variation in voltage which may be found between cells of a battery. Such a variation, when within established tolerance levels, is small relative to the total cell voltage. Prior charging circuit designs have thus combined voltage sensing with temperature sensing, usually by placing a thermistor into intimate contact with the battery pack. However, the prior circuits, while generally effective, were complicated and expensive.
More recent improvements in cell design have made it possible to sense only the cell temperature as the criterion for terminating a C/1 charge rate. It is considered acceptable in the battery art to protect the cells from temperatures in excess of 45.degree. C. Thus, in known circuits thermostatically controlled switches are provided in intimate contact with the batteries. The thermostat is designed to open the associated switch at a temperature of 45.degree. C.
A simple approach is used in one temperature sensitive arrangement of the prior art. Therein, the thermostatically controlled switch itself is used to break the fast charge current directly. A limiting resistor is provided in parallel with, and in close proximity to, the thermostat for supplying the C/10 maintenance charge current. In such a circuit, it is necessary to prevent further rapid charging of the battery cell once the trickle charge state has been entered.
More particularly, once the thermostatic switch has opened the rapid charge circuit the battery cell will cool, tending to reclose the thermostat and to reinitiate the process. Thus, it is necessary to latch the thermostatically controlled switch to an open condition once the fully charged state has been reached. The above described prior art approach utilizes the maintenance charge current to heat the thermostat, thereby to keep the thermostatic switch open for so long as the maintenance charge is continued. More specifically, in this approach the maintenance charge current is used to heat the limiting resistor for the trickle charging current. The close proximity between the limiting resistor and the thermostat provides a heat transfer therebetween, causing a temperature increase at the thermostat and opening the switch controlled thereby.
Although the above concept is low in cost, such an approach requires continued heating of the thermostat by the C/10 limiting resistor. Since the thermostat is in intimate contact with the battery cells, however, the above described approach provides continued heating of the battery cells during the maintenance charging state. Such heating can shorten battery life. Moreover, the above described circuit leads to reduced reliability of the thermostatic switch, since the thermostat itself is required to break the large rapid-charge current at each termination of the rapid charge condition.
In another example of this approach, wherein the thermostat is required to break large currents in the rapid-charge mode, a gate of an SCR is biased by a capacitor and the thermostatic switch is in series with the SCR. It is thus necessary to control precisely the voltage on the capacitor in order to assure proper biasing of the SCR gate. Reliability of this approach suffers still further because of possible variations in capacitor parameters, and because of the difficulty of providing a more precise point at which to turn on the SCR.
A more reliable concept has been to use the thermostat as a sensor only. In this approach, the thermostat is used to control associated electronics which, in turn, regulate the current. As with the previous approach, however, it is necessary to avoid overcharging the battery by a condition in which the fast charge rate is restarted once the batteries cool in the maintenance charge condition and the thermostat closes.
The major advantage of such an arrangement is that it is not necessary to heat the thermostat (hence the batteries) to latch the charger out of the fast charge mode while continuing a maintenance charge, since the charge rate is electronically controlled. Moreover, the thermostat is only required to switch a very low level sensing current rather than the full fast charge current.
Low cost circuits utilizing the above approach are sensitive to one or more variables, however, such as battery or electronic component tolerance or battery impedance, which affects the reliability of latching the circuit out of the fast charge mode. In one such circuit the collector-emitter junction of a transistor is used to clamp across a gate-cathode junction of a power SCR. Such an arrangement does not necessarily keep the SCR off and is subject to variations in junction voltages of the transistor. Under particular circumstances it is thus possible that the SCR, supplying a high rapid charge current, may not be fully turned off and may overcharge the battery. Other circuits, using integrated circuits, controlled tolerance electronic devices, or other special techniques have been used to increase the reliability of the above described approach. However, such circuits are more expensive and thus are less desirable.
There is thus a need in the prior art for an inexpensive circuit, providing reliable recharging of battery cells and including reliable, low cost, control circuitry to avoid overheating and overcharging the battery.
It is accordingly an object of the invention to overcome the difficulties of the prior art and to provide a battery charging apparatus for rapid charging and maintenance charging of a battery.
It is a more specific object to provide a low cost battery charging apparatus wherein a thermostatic switch detects an appropriate transition point for terminating rapid charging and for initiating maintenance, or trickle charging of a battery.
It is still another object to provide a low cost battery charging apparatus utilizing a thermostatic switch to switch a low level sensing current rather than the full charging current.
Yet another object of the invention is the provision of a dual mode battery charging apparatus wherein a thermostatic switch senses an increased temperature to transfer charging from a rapid mode to a trickle mode and wherein a voltage providing circuit is used to latch the apparatus to the trickle mode when the thermostatic switch returns to a low temperature status.
Still a more specific object of the invention is the provision of a voltage storage device for triggering a gate controlled SCR for rapidly charging a battery, including a circuit arrangement for changing the voltage level provided to the storage device in response to a temperature condition of the battery.
Yet a more particular object is an arrangement wherein an inverting structure is interposed between a voltage storage device and a gate controlled device triggered thereby, so that when a thermostatically controlled switch responds to a high temperature, fully charged, condition of a battery and increases the voltage of the storage device the reduced voltage of the inverting structure maintains the gate controlled device inactive even after the thermostatically controlled switch returns to a low temperature condition.
It is still a further object of the invention to provide a triggering device for a gate controlled device in a battery charging apparatus wherein a separate switch is required to be activated, in addition to activation of a thermostatically controlled switch, in order to cause a rapid charging operation and wherein reactivation of the thermostatically controlled switch, alone, will not reinitiate the rapid charging operation.