herunterladen
Introduction
Lithium Ion (Li-Ion) batteries are becoming more avail-
able in the marketplace, allowing system designers to
use both nickel metal-hydride (NiMH) and Li-Ion bat-
tery types to power their portable equipment. The bat-
teries, however, require different charge schemes. NiMH
batteries are usually fast-charged at a constant current
and terminated by either peak voltage detection, PVD, or
the increasing rise in temperature at the end of full
charge,
∆T/∆t. Li-Ion batteries are usually charged at a
constant voltage with a 1C current limit. Charge is usu-
ally terminated by time or when the charging current
drops to a very low rate, typically less than
C
⁄
30
, indicat-
ing that the battery is full.
In addition to battery charging, the designer has the task
of battery monitoring and capacity reporting. NiMH bat-
teries are typically monitored for end-of-discharge volt-
age, battery temperature, and charge and discharge
current. With a fairly flat discharge voltage over about
80% of its capacity, capacity gauging for NiMH is done by
determining the Amp-hour capacity removed during dis-
charge and replaced during charge. NiMH batteries
loose capacity due to self-discharge, which is determined
by the temperature of the battery and is about 1.5 to 2
percent at 25°C.
Li-Ion batteries also require monitoring for capacity and
state of charge. Li-Ion batteries using coke electrodes
have a sloping discharge as shown in Figure 1. In some
cases, the voltage during discharge can be used as an
indicator of state of charge, but the voltage must be cor-
rected for charge/discharge rate and ambient tempera-
ture. Voltage is acceptable for full or empty indication,
but the better approach would be to monitor the capacity
removed and the capacity replaced to determine the bat-
tery state of charge. This method would be more applica-
ble to the other type of Li-Ion battery, which uses
graphite electrodes. The graphite Li-Ion battery has a
much flatter discharge profile making voltage-based
gauging much less accurate than the coke Li-Ion batter-
ies. The self-discharge for both types of Li-Ion batteries is
about
1
⁄
10
th of that for NiMH batteries.
Benchmarq Microelectronics is developing charge con-
trollers, battery protectors, and capacity gauging ICs
specifically tailored for the Li-Ion battery. Today, how-
ever, Li-Ion batteries can be charged and monitored us-
ing existing Benchmarq products. The purpose of this
paper is to describe how a subsystem can be developed
that will support both NiMH and Li-Ion batteries using
existing Benchmarq ICs and easily transition to the new
IC developments.
Capacity Discharged, Amp-Hour
Cell Voltage, volts
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
4.1
4.3
0 0.2 0.4 0.6 0.8 1 1.2
1.0A
0.5A
0.33A
Figure 1. Li-Ion Battery Discharge Curve (Coke Electrodes)
Using NiMH and Li-Ion
Batteries in Portable Applications
Apr. 1995
1
