Charging devices for charging batteries are commonly provided for digital cameras, mobile phones, personal digital assistants (PDAs) and a wide variety of other consumer electronic devices. Although not limited to such a configuration, a charging device may be provided as part of a docking station for a camera so that the batteries in the camera charge every time the camera is placed in the docking station.
During operation, the charging device provides a charging current to the batteries in the camera (or other electronic device) until the batteries are charged, and then provides a lower or “trickle” current to the batteries to maintain the charge until the camera is removed from the docking station. It is not uncommon, however, for consumers to use non-rechargeable batteries in the camera and then accidentally connect the camera to the docking station.
To prevent damage and other adverse effects that may occur when attempting to charge a non-rechargeable battery, charging devices typically include a chaise terminator for use in the event that a non-rechargeable battery is detected. Sometimes, a voltage comparator with hysteresis and a reference threshold voltage are used to detect the presence of a non-rechargable battery and stop charging. This method establishes a very low threshold voltage after charging is inhibited and requires that the detected non-rechargable battery voltage drop below the threshold in order for the comparator to turn the charging current on again.
Unfortunately, this low threshold voltage may never be reached and thus the comparator does not turn the charging current back on unless the charging device is power-cycled. Power-cycling the charging device in order to reactivate the charging device may be frustrating to the user. In addition, the low threshold is established using resistors with finite tolerances, which adds to the tolerance stack-up of the system.
Another potential problem can occur when the voltage generated by the non-rechargeable battery and detected by the comparator drops below its threshold after the charge current stops. In this case, the charging system oscillates between charging ON and charging OFF while potentially causing the non-rechargeable battery to exceed safe temperature limits, split open, and spill potentially harmful electrolyte.
Processor-driven shut-offs are also available. However, these typically operate on digital input, which are more expensive, may be as sensitive to small changes in the voltage, are more complex, and therefore may not accurately detect the difference between rechargeable and non-rechargeable batteries, or may fail more frequently.