In part 1 of Dissecting A Smart Battery I mentioned that smart batteries have contained within them specialized hardware that when working in concert provides the power necessary to run a device such as a PDA, digital camera, or ipod player. Continuing the dissection of a smart battery this article of the series will look at the smart battery’s fuse, charge and discharge FETs , the cell pack, and the sense resistor (RSENSE) to discover what role they each play within the smart battery.
Before we begin let’s just recap some of the specialized hardware within the smart battery:
1. the connector
2. the fuse
3. the charge and discharge FETs
4. the cell pack
5. the sense resistor (RSENSE)
6. the primary and secondary protection ICs
7. the fuel-gauge IC
8. the thermistor
9. the pc board
10. the EEPROM or firmware for the fuel-gauge IC.
11. and the SMBus
The Smart Battery Fuse
When we discuss fuses in relation to electronics we are speaking directly of a fusible link that is responsible for protecting the device from over current. Fusible links have a metal wire that melts when heated to a predetermined electric current rating. When melted the electrical circuit is opened and thereby protecting the circuit from an over-current condition. The obvious concern here is the selection of the fuse – an improperly selected fuse will not protect from over-current conditions and the result will be a fire or damage due to a short circuits.
In a smart battery a typical fuse has three-terminal components that limit current flow based on the temperature, current, and or power across the heating wire. Besides temperature ratings other important factors when selecting the proper fuse to work with each smart battery is hold current, trip current, maximum battery voltage, and fuse size.
The Smart Battery’s FET (field effect transistor)
Smart batteries must have a series FET (field effect transistor) switch to open and protect the battery’s cells. A FET is a transistor that uses an electric field to control the conductivity of a particular 'channel' in a semiconductor material. FETs at times are used as voltage-controlled resistors. As such field effect transistors are chosen based upon their designed ability to dissipate on demand power.
The Smart Battery’s Cell Pack
The battery cell can be thought of as the holding area of the battery’s chemical. The battery cell pack is critical to the overall capability of the smart battery. Cell packs have to be designed and integrated based upon the vitals of the battery including chemistry type (Li-ion, Li-po, NICD, NIMH, etc.) cycle life, storage-capacity loss, shelf life, impedance, capacity at different rates of discharge and temperature, and mechanical and environmental requirements. It is critical to say the least.
The Smart Battery’s Sense Resistor
The final specialized hardware I want to review in this article is the sense resistor (RSENSE). In electronics, sense, is generally referred to the task of producing the correct voltage. Current not temperered will cause damage so sense resistors need to be integrated in order to control power and temperature.
In my next article on the dissection of a smart battery I will cover secondary protection ICs, the fuel-gauge IC, the thermistor, the pc board, and the EEPROM.
Until next time, Dan Hagopian – www.batteryship.com
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