(January 2007) Battery Fuel Gauges: Accuracy is the Game
From Battery Power Products & Technology, September/October 2006, pp.19-21
Determining the amount of charge remaining in any secondary battery is complicated by the interdependent factors controlling the remaining charge. Using Lithium-ion chemistry as a model because of its popularity, the complete requirements for determining State Of Charge (SOC) are considered in producing a microchip which will be accurate. Factors include:
1. Capacity as manufactured: Voltage at full charge, discharge rate (C), Ampere-hour rating and discharge characteristic.
2. Temperature: Battery capacity increases by 20% at 80 0C compared to -20 0C.
3. Self discharge: Lithium-ion discharges at 2-3%/month whereas Nickel-metal hydride discharges at 30%/month.
4. Cycle life: Lithium-ion can withstand up to 500 complete discharges before falling to 80% of original capacity.
5. Calendar life: Fully charged Lithum-ion loses 20% of capacity at 25 0C and 35% at 40 0C. A battery charged to 40% of capacity looses 4%/year at 25 0C.
Since a battery is defined by its above dependencies, simple present terminal Voltage measurements are not more than 25% accurate. Current SOC determining ICs employ Coulomb counting (measuring the charge/time in vs. the charge/time out with Voltage and temperature) and Fuel gauging (smart batteries which have either on-battery or in-host software to perform measurements which provide inputs to algorithms which integrate dependencies to estimate current SOC). An example of a connection is the 1-wire bus.) A third expanded fuel gauge is the Smart Battery Pack which includes a full microcomputer processor on board the battery pack to perform measurements and utilize flexible software algorithms to evaulate SOC which can be indicated by integral LEDs. The full story is very worthwhile for battery implementers and users, written at an easy-to-understand level. BD
Dual comparator thermally protects Lithium-ion battery
EDN Magazine, Sept. 18, 2003
Using a high temperature (< 500 C) and low temperature reference thermistor (> 00 C) eliminates a precision reference. The comparators enable a single chip Lithium-ion battery charger.
Low-battery indicator uses fleapower
EDN Magazine, Sept. 4, 2003, pp.90,92
To increase the life of a 9- Volt Alkaline battery, the proposed circuit uses only 1.2 microAmps to indicate a low battery with a blinking LED. The circuit uses a single popular IC and ten passive components.
New ICs Simplify Battery Charging from the USB
Linear Technology offers a single linear IC which provides constant current/constant Voltage charging to a single cell Lithium-ion battery via the USB. The device supports the suspend, low power and high power modes. It can be configured to function with the USB alone or with an associated wall adapter. By adding a USB power manager IC, the circuit will provide a full 2.5 Watts to the load.
Linear Technology Design Note 320
(July 2004) Proper charging for LiIon batteries
From the IC charger supplier’s viewpoint, many Lithium-ion charge features can be provided to the charger designer from Voltage ranges, measurement accuracies, high duty cycles and ability to suppress noise form ceramic capacitors with popular inductor sizes.
Portable Design, June 2004, p 22
(July 2004) How much time before recharging your battery pack
Semiconductor vendors offer sophisticated gas-gauge IC algorithms to maximize recharging and also compensate for aging effects. Difficulty in determining charge content is amplified by varying rates of discharge. Updating capacity algorithms use methods such as cycle counts, coulomb counting, and ever improving impedance measuring techniques.
Portable Design, June 2004, p. 26
(July 2004) Avoid pitfalls when recharging batteries to ensure long life.
Portable Design, June 2004, pp. 18-28
An excellent overview of charging for Nickel and Lithium-ion based chemistries is presented. This article should be in every battery designer’s notebook. The considerations for delta V and delta T in nickel charging are presented. False peaks, temperature conditions, charge times and even some consideration of aging are presented.
(Febuary 2004) Chips Aid Battery Backup In Industrial Applications
Linear Technology is building a singel cell, stand alone Lithium-ion battery charger for long life industrial backup batteryu applications. The chip only charges to 4.0 Volts instead of the popular 4.2 Volts to slow the aging process and allow operation to 3 years or more.
AMI offers achip which measures power failure durations for up to 5 hours with one second resolution. Current consumption is less than one microAmp.
Electronic Design, November 24, 2003, p 34
Advancements in battery rapid-testing : April 2003 Not just a concept to provide greater information about battery charge conditions,Electrochemical Impedance Spectroscopy (EIS) now provides data to show superior perofrmance. The electrochemical characteristics of a battery are determined by applying an AC potential at varying frequencies and measuring the current response of the lectrochemical cell.