How to implement a battery charger using lithium-ion technology?
In this article, we will provide an example of how to implement a battery
charger using lithium-ion technology. Lithium-ion battery chargers usually use a
constant current (CC) - constant voltage (CV) charging curve. The charging
process goes through several different stages, ensuring the battery is fully
charged while complying with specific safety rules. The CC-CV curve includes the
following stages:
1.Precharge
2.Activate
3.Constant current
4.Constant voltage
Charging begins with a pre-charge phase to check whether the battery is in
good condition. In this stage, a small amount of current of 5% to 15% of the
battery capacity is usually provided to the battery. If the battery voltage
rises above 2.8V, the battery is considered to be in good condition and can
enter the activation stage. During this phase, the same current is supplied to
the battery, but for a longer period of time. When the battery voltage rises
above 3V, fast charging is started and a constant current equal to or lower than
the battery capacity is provided. The constant current phase ends when the
battery voltage rises to the full charge voltage (4.2V) or when a timeout occurs
(whichever occurs first). When the battery voltage reaches the full charging
voltage, charging enters the constant voltage stage and the battery voltage
remains constant. To do this, the charging current must decrease over time. The
charging process at this stage takes the longest time compared to other charging
stages. During this process, when the charging current drops below the end
current limit, usually 2% of the battery capacity, the battery is fully charged
and the charging process ends. Please note that each stage of the charging
process has a time limit, which is an important safety feature.
In order to implement this charging profile, the battery voltage and
charging current must be known at all times. Also, check the temperature of the
battery. Because batteries tend to get hot when charging. If the temperature
exceeds the specified limit of the battery, it may cause damage to the
battery.
In terms of battery charger implementation, users have two options. One is
to use a specialized battery charger IC, and the other is to use a more general
microcontroller. The first solution solves the problem quickly, but has limited
configurability and user interface options (LED indicators). The second option
uses a microcontroller, which takes slightly longer to design, but provides
configurability options and can integrate other functions, such as battery state
of charge (SOC) calculation and communication interface to the host in the
system. The device sends information, etc. In addition, the microcontroller
cannot provide the power circuitry necessary for the charger, and an external
BJT or MOSFET is required. However, the cost of these power components is much
lower than that of microcontrollers or specialized charger ICs.
Charger architecture
We can see from the charging curve that a single-cell lithium-ion battery
charger requires a controllable current source. The current source output should
change based on battery status. Considering the above requirements, a
microcontroller-based implementation requires the following functional
modules:
1. Current control circuit
2. Battery parameter (voltage, current, temperature) measurement
circuit
3. Charging algorithm (used to implement CC-CV charging curve)
Current control circuits can be constructed using voltage source and
current feedback techniques. Its working principle is similar to a typical
negative feedback control system. The charging current is allowed to pass
through a small resistor to obtain feedback, thereby producing a certain
voltage.
Voltage sources can be created in two ways:
1.Linear topology
2. Switch: buck or boost topology
Linear topologies employ series pass elements (BJTs or MOSFETs) in linear
mode.
The charging current is controlled by controlling the bias of the series
pass transistor Q1. Bias can be controlled using an analog-to-digital converter
(ADC) or a pulse width modulator (PWM) with an external RC low-pass filter. The
linear method is suitable for low charging currents (<1A) because series pass
components face power dissipation issues.
The switching topology itself has the advantage of low power consumption
and can achieve higher charging current.
The charging current is set by the PWM duty cycle driving the MOSFET.
Battery parameter measurement circuit: The feedback signal needs to be
measured using an ADC. Currently, most microcontrollers can provide ADC
peripherals.
The negative battery terminal serves as the microcontroller ground. This
allows voltage, temperature, and current feedback to be referenced to the
microcontroller ground and enables single-ended ADC measurements. For current
feedback, a positive offset voltage needs to be introduced, while the feedback
voltage will be negative when the battery is charging. As shown in Figure 5,
resistors R3 and R4 provide the required offset voltage.
Charging Algorithm: This action will end the loop. The CPU reads the ADC to
obtain voltage, charging current and temperature readings, and controls the PWM
duty cycle based on the charging curve. The speed at which the CPU monitors the
ADC results and controls the PWM depends on the balance between loop response
time and CPU bandwidth consumption.
ADC Parameters and PWM Resolution: ADC resolution and accuracy along with
PWM resolution are important parameters that should be taken into consideration
while designing a battery charger. ADC resolution defines the accuracy of the
input voltage measurement (in this case, the feedback voltage). The PWM
resolution defines the accuracy of changing the duty cycle of the output signal,
which in turn determines the output voltage of the current control circuit. When
charging lithium-ion batteries, the battery voltage needs to be controlled
accurately and with high precision. This is especially important when the
battery voltage is close to full. Controllability depends on the ADC resolution,
the accuracy of the measurements, and the granularity of the duty cycle
changes.
Example of charger architecture implemented using Cypress CY8C24x23 PSoC
device. Microcontrollers work with general-purpose digital and analog modules
that can be configured for specific circuit functions. For example, duration
analog blocks can be used to implement programmable gain amplifiers and
comparators. Switched capacitor analog modules are used in many different
applications, including filters, digital-to-analog converters (DAC), and
analog-to-digital converters (ADC). Digital basic modules can be used to
implement PWM, counters, timers and buffers, while digital communication modules
can be used to implement communication interfaces such as SPI, UART, IrDA RX and
TX. In addition, the device is available with an I2C module that can be used as
a master or slave device.
Looking at the device resource consumption of a single-cell battery charger
application, we see that there are still enough digital and analog blocks to
implement other useful functions, which provides the system with more
integration options, thereby helping to reduce system cost and size.
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