e>
An Interleaved Dual-Battery Power Supply for Battery- Operated Electronics

Qing Wu, Qinru Qiu and Massoud Pedram
Department of Electrical Engineering-Systems
University of Southern California
Los Angeles, CA 90089, USA
Tel: +1-213-740-4480
Fax: +1-213-740-7290
E-mail: {qingwu, qinru, massoud}@zugros.usc.edu


Abstract
After a detailed analysis and discussion of two
important characteristics of todays battery cells (i.e., their
current-capacity and current-voltage curves), this paper describes
the design principles and architecture of a dual-battery power
supply system for portable electronics. The key idea is to integrate
two battery types with different energy capacity and current rate
curves into the power supply system, and then use them in an
interleaved manner in response to varying current requirement of
the VLSI circuit that is powered by this dual-battery system.
Analytical and empirical results demonstrate the effectiveness of
the new battery architecture in maximizing the service life of a
battery system with fixed volume (or weight).
I. I
NTRODUCTION
With the Moore's law still in effect, integrated circuit densities and
operating speeds continue to rise. Chips however cannot get larger
and faster without a sharp increase in power consumption beyond
the current levels. Minimization of power consumption in VLSI
chips has thus become an important design objective. In fact, with
exponential growth in demand for portable, battery-powered
electronics and the usual push toward more complex and higher
performance, power consumption has in many cases become the
limiting factor in satisfying the market demand.
This challenge has been met by an active research and
development community both in industry and academia. Rapid
advances are taking place in low-power process technologies,
architecture and circuit optimization techniques, power-aware
simple and complex cell design, use of variable and/or multiple
supply voltages and dynamic power management schemes, and
low power computer aided design (CAD) tools from system and
software levels to layout and transistor levels.
A battery-powered digital system consists of the VLSI circuit, the
battery, and the DC/DC converter. In spite of the fact that the goal
of low-power design for battery-powered electronics is to extend
the battery service time, most research works on low-power design
metrics and methodologies have focused on the VLSI circuit itself.
People usually assume that the battery sub-system (battery and the
DC/DC converter) is an ideal source that outputs a constant
voltage and stores/delivers a fixed amount of energy [4].
However, in reality, the battery sub-system has complicated
characteristics rather than ideal. Research has been done [7][8] to
study the influence of the current-capacity characteristics of the
battery on CMOS digital design. It has been found that, by
selecting the optimal supply voltage that minimizes the Battery
Discharge-Delay product, we can achieve the best trade-off
between battery service life and circuit delay. It is also shown in
[8] that, even with the same mean value, different current
discharge (energy dissipation) profiles will lead to different
battery service life. However, these research works have not
considered other important characteristics of the battery sub-
system.
In this paper, we extend the work of [8] by doing analysis of
optimal supply voltage for a VLSI circuit by considering not only
the current-capacity characteristics, but also the current-voltage
characteristics of the battery. In a significant departure from the
work reported in [7][8], in this paper, we also present design
principles and a design procedure for constructing a dual-battery
power supply system which would interleave two different batter
types (i.e., with different energy capacity and current rate) in order
to match the current requirements of the VLSI circuit which is
powered by this dual-battery power supply. The goal is to
maximize the total battery service life for given battery volume (or
weight). Analytical and empirical results demonstrate that battery
life increases of up to 60% can be achieved if the dual-battery
power supply is designed and used properly.
This paper is organized as follows. Section II provides some
background on battery characteristics. Section III considers the
problem of optimal supply voltage selection. Section 0 gives the
design and analysis of dual-battery interleaving power supply
system. Sections V presents conclusions.
II. B
ACKGROUND
A. Notation
We first give some useful notations:
T: Clock cycle time for one operation
V
0
: Output voltage of the battery
I
0
: Average output current of the battery over time N T
V
dd
: Supply voltage of the circuit
I
dd
: Average supply current of the circuit over time N T
µ
: Efficiency factor of the battery : Efficiency of the DC/DC converter
E
ide
: Ideal energy needed to complete an operation
E
act
: Actual battery energy needed to complete an operation
CAP
0
: Total energy stored in a new battery
BD: Battery discharge
B.
Battery Overview and Characteristics
Many different types of batteries are being used in a wide range of
applications [10]-[12]. Among these, the Nickel-Metal Hydride
battery and the Lithium-Ion battery are currently the most popular
secondary batteries for portable electronic devices, ranging from
cellular phones to notebook computers.











Figure 1 The relations between battery capacity, output
voltage and discharge current.
A typical lithium rechargeable battery consists of the lithium foil
anode, the composite cathode, and the electrolyte that serves as an
ionic path between electrodes and separates the two materials.
During discharge, the electrochemical process involves the
dissolution of lithium ions at the anode, their migration across the
electrolyte and their insertion within the crystal structure of the
An Interleaved Dual-Battery Power Supply for Battery-
Operated Electronics
cathode. Positive current flows in the opposite direction in the
external circuitry. Applying electrical recharging can reverse the
reaction; hence the battery can be used for multiple times
(normally several hundred times).
The principles of electrochemical reaction make the battery not an
ideal energy source. Many factors define the complicated relations
between battery capacity, output voltage, and discharge current.
Some major relations for typical lithium batteries [10][11] are
shown in Figure 1. We can conclude from Figure 1 that:
1. The
deliverable capacity (in brief, capacity) of the battery
decreases when the discharge current increases.
2. The output voltage of the battery decreases when the
discharge current increases.
For our analysis, we need to model and approximate these
relations using analytical equations. To simplify the analysis and
presentation, in this paper, we use linear functions for
approximation; high-order approximations can be used to obtain
higher accuracy.
A decrease of actual battery capacity is equivalent to an increase
of the actual energy drawn from the battery [8]. Therefore the
actual energy that is taken out of the battery is:

1
0

,
0
0
=
µ
µ
T
I
V
E
act
(2.1)
where
µ
is called the battery efficiency (or utilization) factor. The
efficiency factor
µ
is a function of discharge current I
0
, we
approximate it as:

0
1
I
= µ
(2.2)
where is a positive constant number.
Similarly we approximate the relationship between the battery
output voltage and the discharge current as a linear function:

0
0
I
V
V
OC
= (2.3)
where V
OC
is the open-circuit output voltage of the battery.
C. DC/DC
Converters
The role of a DC/DC converter is to convert the battery output
voltage to and stabilize at the operation voltage of the CMOS
circuit. If we define as the conversion efficiency of the DC/DC
converter, we have:

dd
dd
I
V
I
V =
0
0 (2.4)
where I
0
and I
dd
are average input and output current of the
DC/DC converter over some period of ti