BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a battery unit including at least one storage cell, two external connecting terminals, one of which is positive and the other of which is negative, and a safety circuit connected to prevent entry of charging current into the cell as soon as a voltage applied between said external terminals is greater than a particular maximum safety voltage.
2. Description of the Prior Art
This kind of battery unit is more particularly intended for powering portable devices such as mobile telephones, camcorders and other devices. Its safety circuit is designed to protect each cell against failure of the charger leading to an overvoltage between the external connecting terminals of the battery unit, for example. A safety circuit of the above kind is described in U.S. Pat. No. 5,547,775. Its role is therefore to prevent deep discharging and overcharging of each cell of the battery unit, for example to prevent a short circuit across the external connecting terminals destroying the cell. It primarily consists of a plurality of excessive overcharging and excessive discharging detector circuits, a plurality of discharge circuits and a plurality of switching circuits associated with the respective cells of the battery unit. Each cell of the battery unit is therefore connected to an overcharging and deep discharging detector circuit, a discharge circuit and a switching circuit. In this way the safety circuit described in the aforementioned document is capable of bringing about discharging of each cell of a battery unit if it detects that the cell has an excessive voltage at its terminals or preventing current being drawn from a cell if it has detected that the cell concerned is deeply discharged.
Battery units which include lithium ion cells have a significant advantage in terms of stored energy density and weight saving but lithium ion cells cannot withstand overvoltages at their terminals. The addition of a safety circuit as indicated above to a lithium ion cell battery unit improves the performance and reliability of the battery unit.
From the charging point of view, the various types of cell have different requirements, and at present the chargers employed must be matched to the type of cell that they are intended for charging. A charger for an alkaline battery unit, for example, modulates its output voltage to generate a constant charging current in the battery unit that it is charging, whereas a lithium ion battery unit charger maintains a constant voltage at the external connecting terminals of the battery unit it is charging.
If a lithium ion battery unit is charged with an alkaline charger, on approaching the end of charging the resistance of the lithium ion cells increases and the charger therefore increases the applied voltage to maintain a constant current, until the point is reached where the voltage applied is greater than the maximum safety voltage of the lithium ion battery unit. The safety circuit of the lithium ion battery unit will then open to isolate the cells from the charger, which represents the end of charging.
If a user who has an alkaline charger regularly uses it to charge a lithium ion battery unit, the safety circuit is activated during each charging cycle, although it is intended to operate only as a final resort. This mode of use compromises the reliability of the battery unit: the fact that the safety circuit is regularly activated reduces its reliability and consequently the reliability and safety of the battery unit are themselves reduced.
This kind of situation occurs frequently: in professional video batteries, for example, the interfaces between the battery unit and the camera are fixed and a lithium ion battery unit therefore has the same interface as an alkaline battery unit. A user can therefore always connect a lithium ion battery unit to an alkaline charger, which has the drawback of causing the battery unit to operate regularly in a safety mode.
The object of the invention is to remedy these drawbacks.
SUMMARY OF THE INVENTION
To this end, the invention provides a battery unit including at least one storage cell, two external connecting terminals, one of which is positive and the other of which is negative, and a safety circuit connected to prohibit entry of charging current into the cell as soon as a voltage applied between the external connecting terminals is greater than a particular maximum safety voltage, in which battery unit an additional protection circuit is connected between the external terminals and the safety circuit to prohibit the entry of charging current into the safety circuit as soon as the voltage applied between the external connecting terminals is greater than a particular maximum charging voltage which is less than the maximum safety voltage.
With the above arrangement, the safety circuit of a lithium ion battery unit is no longer activated each time that the battery unit is charged by an alkaline charger, which helps to maintain the reliability of the battery unit at a satisfactory level, whether it is used with an alkaline charger or with a lithium ion charger.
In one particular embodiment of a battery unit according to the invention, the additional protection circuit includes a MOSFET switch which is controlled to prohibit or authorize the entry of a charging current into the safety circuit. Hereinafter the protection circuit is referred to as closed or open according to whether this MOSFET is turned on or off. A problem that is frequently encountered in controlled switching is oscillation: because of slight disturbances to the voltage caused by opening the circuit, the protection circuit finds itself in a configuration in which it should close again. If precautions against this are not taken, the circuit begins to oscillate indefinitely at the time it should merely open. In one particular embodiment of the protection circuit according to the invention, the MOSFET switch is controlled by a comparator having a first input which receives the maximum charging voltage and a second input which receives the voltage applied to the external connecting terminals, the second input being connected to the external connecting terminals via a divider bridge having a division ratio that varies to introduce hysteresis into the control of the switch. The resulting hysteresis between opening and closing of the switch helps to eliminate the oscillation problem. The circuit closing voltage is made less than the maximum charging voltage (above which the circuit is opened). In this way, when the circuit is opened, the input voltage can fall to some degree (whilst remaining greater than the closing voltage) without closing the circuit. The circuit obtained in this way is stable and is not disturbed by voltage variations caused by its operation.