MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 12

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Introduction
Powering today’s portable world poses many challenges for
system designers. The use of batteries as a prime power source
is on the rise. As a result, a burden has been placed on the
system designer to create sophisticated systems utilizing the
battery’s full potential.
Each application is unique, but one common theme rings
through: maximize battery capacity usage. This theme directly
relates to how energy is properly restored to rechargeable
batteries. No single method is ideal for all applications. An
understanding of the charging characteristics of the battery and
the application’s requirements is essential in order to design
an appropriate and reliable battery charging system. Each
method has its associated advantages and disadvantages. It is
the particular application with its individual requirements that
determines which method will be the best to use.
Far too often, the charging system is given low priority, especially
in cost-sensitive applications. The quality of the charging system,
however, plays a key role in the life and reliability of the battery.
In this article, the fundamentals of charging Lithium-Ion (Li-Ion)
batteries are explored. In particular, linear charging solutions
and a microcontroller-based, switch-mode solution shall be
explored. Microchip’s MCP73843 and MCP73861 linear charge
management controllers and PIC16F684 microcontroller along
with a MCP1630 pulse width modulator (PWM), shall be used as
examples.
Li-Ion Charging
The rate of charge or discharge is often expressed in relation to
the capacity of the battery. This rate is known as the C-Rate. The
C-Rate equates to a charge or discharge current and is defined as:
where:
I = charge or discharge current, A
M = multiple or fraction of C
C = numerical value of rated capacity, Ah
n = time in hours at which C is declared.
A battery discharging at a C-rate of 1 will deliver its nominal
rated capacity in one hour. For example, if the rated capacity is
1000 mAhr, a discharge rate of 1C corresponds to a discharge
current of 1000 mA. A rate of C/10 corresponds to a discharge
current of 100 mA.
Typically, manufacturers specify the capacity of a battery at a 5
hour rate, n = 5. For example, the above-mentioned battery would
provide 5 hours of operating time when discharged at a constant
current of 200 mA. In theory, the battery would provide 1 hour
of operating time when discharged at a constant current of
1000 mA. In practice, however, the operating time will be less
than 1 hour due to inefficiencies in the discharge cycle.
So how is energy properly restored to a Li-Ion battery? The
preferred charge algorithm for Li-Ion battery chemistries is a
constant, or controlled, current – constant voltage algorithm
that can be broken up into four stages: trickle charge, constant
current charge, constant voltage charge and charge termination.
Refer to Figure 1.
10
Charging Lithium-Ion Batteries:
Not All Charging Systems Are Created Equal
By Scott Dearborn, Microchip Technology Inc.
Analog and Interface Guide – Volume 2
Charging Lithium Batteries
I = M x Cn
Figure 1: Li-Ion Charge Profile.
Stage 1: Trickle Charge – Trickle charge is employed to restore
charge to deeply depleted cells. When the cell voltage is below
approximately 3V, the cell is charged with a constant current of
0.1C maximum.
Stage 2: Constant Current Charge – After the cell voltage has
risen above the trickle charge threshold, the charge current
is raised to perform constant current charging. The constant
current charge should be in the 0.2C to 1.0C range. The constant
current does not need to be precise and semi-constant current
is allowed. Often, in linear chargers, the current is ramped-up as
the cell voltage rises in order to minimize heat dissipation in the
pass transistor.
Charging at constant current rates above 1C does not reduce the
overall charge cycle time and should be avoided. When charging
at higher currents, the cell voltage rises more rapidly due to
over-voltage in the electrode reactions and the increased voltage
across the internal resistance of the cell. The constant current
stage becomes shorter, but the overall charge cycle time is not
reduced because the percentage of time in the constant voltage
stage increases proportionately.
Stage 3: Constant Voltage – Constant current charge ends and the
constant voltage stage is invoked when the cell voltage reaches
4.2V. In order to maximize performance, the voltage regulation
tolerance should be better than +1%.
Stage 4: Charge Termination – Unlike nickel-based batteries, it is
not recommended to continue to trickle charge Li-Ion batteries.
Continuing to trickle charge can cause plating of metallic lithium,
a condition that makes the battery unstable. The result can be
sudden, automatic and rapid disassembly.
Charging is typically terminated by one of two methods: minimum
charge current or a timer (or a combination of the two). The
minimum current approach monitors the charge current during
the constant voltage stage and terminates the charge when the
charge current diminishes in the range of 0.02C to 0.07C. The
second method determines when the constant voltage stage is
invoked. Charging continues for an additional two hours, and then
the charge is terminated.
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0.0
0.5
1.0 1.5
Time (Hours)
2.0
Capacity
Charge Current
Cell Voltage
2.5
3.0
3.5
4.0
4.5
1200
1000
800
600
400
200