James J. D’Amato
Section L01/Dr. Milor
Tesla Tech
Li-Ion Battery and Power-Efficient Li-Ion Battery Charger Circuits
Introduction
The demand for greater battery life in low-power consumer electronics, deep sea exploration, and
implantable medical devices presents a need for improved energy efficiency in the management of small
rechargeable cells. The advent of long-lasting rechargeable battery cells and power-efficient charging
circuits has permitted engineers, scientists, and industry to excel in self-sufficient products. A variety of
rechargeable battery cells exist in the present market such as Lead-acid, Nickel-Cadmium, and Lithiumion. This paper focuses on Lithium-ion batteries and their respective chargers, discusses some of the
technologies that are commercially available, and identifies some of the techniques used to charge
Lithium-ion batteries in the most accurate, power-efficient, and sustainable manner.
Commercial Applications
Rechargeable Lithium-ion battery cells are presently the most self-sustained battery on the market.
Although, the high energy density of Li-ion batteries is high, the costs are much greater than alternatives
(NiCad). For a Lithium-ion battery producing 4.2 V at 1100 mAh, the costs range from $15 to $80 from
manufacturers such as Energizer, Ultrafire, Sony, and Tenergy. Li-ion chargers are manufactured by an
abundance of electronic companies such as Texas Instruments, Allied Electronics, and Tenergy as well,
ranging in costs from $25 to $80. Lithium-ion batteries and the respective chargers are used in today’s
most sophisticated products such as Apple’s iPhone4, Lenovo’s ThinkPad, and Samsung’s Galaxy tablet
[1].
Lithium-Ion Battery
If environmental pressure is high, if the temperature is 5° C or less, and if conditions are unsuitable for
many chemical reactions, electrical storage can be an extremely difficult task. The Li-ion battery is
chosen due to its high energy density and versatility. During discharge, lithium ions, Li+, carry the current
from the negative to the positive electrode through the non-aqueous electrolyte and separator diaphragm.
During charging, an external electrical power source (the charging circuit) applies a higher voltage (but of
the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction.
The lithium ions then migrate from the positive to the negative electrode, where they become embedded
in the porous electrode material in a process known as intercalation [2]. The battery’s voltage must not
fall below a certain voltage and must not be above a certain voltage. Thus, care must be taken to ensure
that the circuit adheres to close voltage tolerances.
Lithium-Ion Battery Charging Circuitry
The charging profile of a Li-Ion battery can be divided into four distinct regions, based on the battery’s
constraints. The trickle charge, constant current, constant voltage, and end-of-charge distinguish the four
charging profile sections when charging a Li-ion battery. During trickle charge, the battery is charged
with a small amount of current, typically no more than 0.1 times the rated capacity expressed in terms of
Ah, or C. Charging currents greater than 0.1 C may be hazardous since the battery has high internal
impedance at these low voltages and is susceptible to thermal runaway [3]. Above the rated threshold of
the battery, the battery may be charged at higher currents, typically less than 1 C; this charging profile
section represents the constant-current region. As the battery voltage approaches its maximum respective
capacity, the charging profile enters the constant-voltage region. In this region, the charging current
should be progressively decreased as the battery voltage approaches its respective maximum. Charging
current should be decreased until a certain threshold is met, typically about 2% of the rated battery
capacity [4]. Once this charging current is reached, the charger enters the end-of-charge region. The
charging circuit can either be designed using digital logic or in the analog domain using the hyperbolic
tangent (tanh) basis. In the analog domain, the efficiency is much greater (89.7%) due to CMOS
technology [3].
Components for Implementation
Lithium-ion batteries are essential for demanding applications because of the high energy density, low
discharge rate, and low maintenance. The battery that is used must have a simple structure that is the
cathode, anode, and electrolyte. In addition, the Li-ion battery must also have low running cost, high
reliability, and the capability to recharge [5]. A sub-threshold operational transconductance amplifier
(OTA), a reference circuit, a current-gain circuit, and an end-of-charge detector circuit comprise the
Lithium-ion charging circuitry. The OTA consists of CMOS transistors used in configurations as current
mirrors and differential amplifiers. The reference circuit is designed using standard operational amplifiers
along with resistors and standard switching diodes. The current gain stage is interconnected to the OTA
with a single P-channel MOSFET. The end-of-charge detection circuit uses CMOS transistors in a
differential amplifier configuration [6]. The complete circuit can be mounted on a printed circuit board
(PCB) for durability and safety using surface mounted parts.
References
[1]
Institute of Information Technology (2009, Aug.). Comparison of Battery Technology.
Advanced Technology Program. [Online]. Available:
http://www.atp.nist.gov/eao/wp05-01/append-4.htm
[2]
Hyakudome, T.; Yoshida, H.; Ishibashi, S.; Sawa, T.; Nakamura, M.; , "Development of
advanced Lithium-ion battery for underwater vehicle,"Underwater Technology (UT), 2011 IEEE
Symposium on and 2011 Workshop on Scientific Use of Submarine Cables and Related
Technologies (SSC), vol., no., pp.1-4, 5-8 April 2011
doi: 10.1109/UT.2011.5774116
URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5774116&isnumber=5774075
[3]
Do Valle, B.; Wentz, C.T.; Sarpeshkar, R.; , "An Area and Power-Efficient Analog Li-Ion Battery
Charger Circuit,"Biomedical Circuits and Systems, IEEE Transactions on, vol.5, no.2, pp.131137, April 2011
doi: 10.1109/TBCAS.2011.2106125
URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5713828&isnumber=5765036
[4]
Min Chen; Rincon-Mora, G.A.; , "Accurate, Compact, and Power-Efficient Li-Ion Battery
Charger Circuit,"Circuits and Systems II: Express Briefs, IEEE Transactions on, vol.53, no.11,
pp.1180-1184, Nov. 2006
doi: 10.1109/TCSII.2006.883220
URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4012370&isnumber=4012365
[5]
W. Howard, C. Schmidt, E. Scott; “Lithium Ion Battery,” U.S. Patent 7 803 481 B2, Sep. 28,
2010.
[6]
Freescale Semiconductor, (2008, Apr.) 28V-Input-Voltage Single-CellLi-Ion Battery Charger
with 10mA Regulator. Tempe, AZ. [Datasheet]. Document Number: MC34675.