Portable electronic devices, such as digital cameras, generally depend on a battery-based power supply for their operational power. A battery is an energy converting apparatus designed to transform chemical energy into electrical energy when discharging and to transform electrical energy back to chemical energy when recharged. Batteries, or battery packs, are used to supply power to those devices such as PDA, notebook, and digital camera by converting chemical energy into electrical power, making thereby the portability possible. There are a primary battery as represented by a dry cell and a secondary battery as represented by a lithium ion battery. In either case, the battery of the battery-based power supply provides the device with operational power without requiring a continuous connection to a fixed power source, such as an alternating current (AC) electrical outlet, thus facilitating portable operation. When depleted, the battery is either replaced with a fully charged, replacement or the battery is recharged if a rechargeable battery is being used. The secondary battery, which has a large capacity and can be used repeatedly through charging, is ordinarily used. The easily available primary battery is used in emergency, such as a time when the secondary battery is completely consumed. The rechargeable battery, so called a secondary battery, or a storage battery is produced and sold in various types and sizes. The typical types of the rechargeable battery includes a nickel cadmium (NiCd) type, a lead-acid type, a nickel metal hydride (NiMH) type, a lithium ion (Li-ion) type, a lithium polymer type, and an alkaline type battery. With increasing needs for use of portable appliances in the field of consumer electronics, miniaturization and weight reduction of appliances are underway, so that development of higher energy density batteries, particularly secondary batteries, has been keenly desired. A lithium secondary battery is one of candidates for secondary batteries satisfying such requirements. A lithium ion battery is characterized by its anode and cathode active materials made of a substance capable of occluding and releasing lithium ions. Lithium ion secondary batteries are small and lightweight, and can be charged and discharged more than 300 cycles repeatedly, and are hence used widely in laptop computers, cellular phones and many other electronic appliances, and the demand is increasing. The weight energy density of the lithium ion secondary battery (LIB) is higher than in the nickel-cadmium secondary battery (NiCd) and nickel mercury secondary battery (NiMH). Lithium secondary battery has a high voltage and a high energy density and also a light weight, as compared with a nickel-cadmium battery, a lead storage battery and a nickel-hydrogen battery.

Rechargeable battery packs comprising one or more rechargeable battery cells are widely used to supply power to a variety of portable electronic devices. Rechargeable battery packs save the purchaser of an electronic device the cost of continuously purchasing replacement batteries over the useful life of the device. Additionally, environmental concerns relating to disposal of non-rechargeable batteries points to an increasing use of rechargeable battery packs in the future. A battery pack is used in a state where it is built in the digital camera or in a removable state. In a state where the battery pack is attached to a digital camera, the battery pack can be charged. In addition to the secondary battery, a protecting circuit is provided for the battery pack. A circuit for detecting a voltage, a current, and a temperature of the secondary battery and the like are provided for the protecting circuit. Li-ion battery cells have proven to be very efficient in terms of watt-hours per unit volume of the cell and high output voltage generated by the cell. Thus, battery packs comprising one or more Li-Ion battery cells have found increasing use in digital cameras. In typical applications, two or more Li-Ion battery cells are disposed in a casing and electrically coupled in series and parallel to form a rechargeable Li-ion battery pack. The battery pack has an aggregate maximum charging current based on the number of cells in parallel and an aggregate recommended charge voltage based on the number of cells in series. The battery pack is sized to fit in a battery pack receiving opening or bay of a digital camera. During use of the digital camera, the battery pack gradually discharges as it supplies power to the electronics of the device. When the power supplied by the pack falls below a threshold value, the pack is either recharged in the device itself or removed from the device and recharged using a recharging apparatus. The rechargeable battery pack typically lasts only a few hours. Once the rechargeable battery pack is exhausted, the digital camera has to be turned off. In order to continue its operation, a user can bring along a back-up battery pack, which is identical in shape and design with the pack installed in the battery pack. Once the first installed battery pack is discharged to a low level after few hours of operation, the backup battery pack can be installed to replace the first installed battery pack for continuous operation.

Digital cameras and camcorders and other digital imaging devices are becoming smaller in size. Digital cameras typically are provided with a rechargeable battery cartridge or power pack such as a rechargeable lithium ion cartridge. The rechargeable cartridge has a housing designed to fit and lock in place within a receiving cavity within the imaging device to be powered. The cartridge is normally designed to be removed from the device being powered so that it can be recharged and then reinserted or else it can be recharged while locked in place within the device. In a digital camera, the power consumptive elements of a digital camera include a charging circuit of the flash, a back-light driving circuit of LCD display and a driving circuit of lens motor. These power consumptive elements require large power consumption. Higher electric power is used in digital cameras than in comparison with photographic cameras. In particular when the display device is used as a finder, the power consumption of the battery comprising the power source of the digital camera is accelerated due to the large power consumption of the display device, thereby reducing the time photography is possible. During the operation of the digital camera, if the functions of these power consumptive elements are not necessary, the related switch will be turned off to stop supplying power. As the digital camera is usually equipped as a portable device for photographing various events, battery power is used as a main operation power. When the battery gets weak, the digital camera loses its ability to function and cannot continue to photograph without the assistance of a spare battery. In order to avoid the camera from being out of function or shut down due to the insufficient output voltage of the battery, a digital camera is generally installed with a battery low detection circuit. When the output voltage of the battery is lower than a predetermined threshold limit value, the digital camera will generate a warning signal or sound to inform the user to replace the battery. A digital camera is generally provided with a battery fuel gauge to keep the user informed regarding the reliability of the battery power. A battery fuel gauge is an indicator that shows remaining energy capacity or charge level of the battery. The fuel gauge keeps a user informed of a current or remaining battery charge level and, by extension, a probable remaining operating time of the electronic device.