What is a Li-Po Battery?
A Li-Po battery is a rechargeable battery that uses a polymer electrolyte instead of a liquid electrolyte like in Li-ion batteries. The polymer electrolyte allows the battery to be made in a pouch format that can be custom-shaped to fit the device. Li-Po batteries have a nominal voltage of 3.7V per cell and can be connected in series and parallel to increase voltage and capacity.
Some key characteristics of Li-Po batteries include:
- High energy density (up to 200 Wh/kg)
- Lightweight
- Can be made very thin (<1mm)
- No memory effect
- Relatively low self-discharge rate
- Require careful charging to prevent overcharge, over-discharge, and overheating
Li-Po Battery Charging Basics
To charge a Li-Po battery safely and effectively, a charger circuit needs to follow a specific charging profile. The standard charging profile for Li-Po batteries consists of two stages:
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Constant Current (CC) Stage: In this stage, the charger supplies a constant current to the battery while the battery voltage increases. The current is typically set to a value between 0.5C to 1C, where C is the capacity of the battery in Amp-hours (Ah). For example, for a 1000mAh battery, 1C would be 1A. This stage continues until the battery voltage reaches 4.2V per cell.
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Constant Voltage (CV) Stage: Once the battery voltage reaches 4.2V per cell, the charger switches to constant voltage mode and gradually reduces the charging current. This stage continues until the current drops to around 0.1C (or 10% of the initial charging current), at which point the battery is considered fully charged.
Here is a table summarizing the standard Li-Po charging profile:
| Stage | Voltage | Current |
|---|---|---|
| Constant Current (CC) | Increases to 4.2V/cell | Constant (0.5C to 1C) |
| Constant Voltage (CV) | Constant at 4.2V/cell | Decreases to 0.1C |
It’s important to note that overcharging a Li-Po battery beyond 4.2V per cell can cause permanent damage and even lead to fire or explosion. Similarly, over-discharging below 3V per cell can also degrade the battery. Therefore, a Li-Po charger circuit must include voltage and current monitoring and protection features.
Li-Po Battery Charger Circuit Design
A basic Li-Po battery charger circuit consists of the following components:
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Voltage Regulator: A voltage regulator is used to step down the input voltage (e.g. from a 5V USB port) to the desired charging voltage (4.2V per cell). A common choice is a switching regulator like the MP2307 or a linear regulator like the LM317.
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Current Regulator: A current regulator is used to limit the charging current to the desired value (e.g. 1A). This can be implemented using a current sense resistor and a op-amp comparator circuit that adjusts the voltage regulator output based on the measured current.
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Voltage Monitor: A voltage monitor circuit is used to measure the battery voltage and switch from CC mode to CV mode when the voltage reaches 4.2V per cell. This can be implemented using a voltage divider and comparator.
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Current Monitor: A current monitor circuit is used to measure the charging current and terminate charging when the current drops below a threshold (e.g. 0.1C). This can be implemented using a current sense resistor and comparator.
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Protection Circuits: Additional protection circuits are needed to prevent overcharge, over-discharge, short circuit, and reverse polarity conditions. These typically include a MOSFET switch to disconnect the battery, and a fuse or polyfuse for overcurrent protection.
Here is a simplified schematic of a single-cell Li-Po charger circuit:
+-----------------+
| |
| Voltage Reg |
| (e.g. MP2307) |
| |
+-----+ | |
| | | |
| USB |---+----------+ |
| | | | |
+-----+ | | |
| +------+----------+
| |
| |
| |
| +------+----------+
| | |
| | Current Reg |
| | (e.g. op-amp) |
| | |
| +-----------------+
|
+---------+-------------------------+
| |
| Voltage Monitor |
| |
+----------------------------------+
|
+---------+-------------------------+
| |
| Current Monitor |
| |
+----------------------------------+
|
+---------+-------------------------+
| |
| Protection Circuits |
| |
+----------------------------------+
|
|
|
+--+--+
| |
| Bat |
| |
+-----+
The voltage regulator steps down the USB voltage to 4.2V and supplies current to the battery through the current regulator. The voltage and current monitors measure the battery voltage and current and control the regulators and protection circuits accordingly.
For multi-cell batteries, a separate monitoring circuit is needed for each cell to ensure balanced charging. This is typically done using a dedicated battery management system (BMS) IC that includes cell balancing features.
Choosing Charging Current
The charging current is an important parameter in Li-Po charger design as it affects the charging time and the heat generated in the battery. A higher charging current will charge the battery faster but also generate more heat, which can degrade the battery life if not properly managed.
The maximum safe charging current for a Li-Po battery is usually specified by the manufacturer in terms of the C-rate. For example, a 1C rate means the current equals the capacity of the battery in Amp-hours. So for a 1000mAh battery, 1C would be 1A.
Most Li-Po batteries can be safely charged at rates up to 1C. However, many manufacturers recommend lower rates like 0.5C to prolong battery life. The table below shows some common C-rates and the corresponding charging times:
| C-rate | Current (for 1Ah battery) | Charging Time |
|---|---|---|
| 0.2C | 0.2A | 5 hours |
| 0.5C | 0.5A | 2 hours |
| 1C | 1A | 1 hour |
| 2C | 2A | 30 minutes |
In practice, the charging current is often limited by the charger circuit power supply rather than the battery. For example, most USB ports are limited to 0.5A or 1A, so a USB-powered Li-Po charger would have to limit the current accordingly even if the battery supports higher rates.
Li-Po Battery Charger ICs
Designing a complete Li-Po battery charger circuit from scratch can be complex and time-consuming. Fortunately, there are many dedicated Li-Po charger ICs available that integrate most of the necessary functions into a single chip. These ICs typically require only a few external components like resistors and capacitors to set the desired parameters.
Some popular Li-Po charger ICs include:
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TP4056 – A complete constant-current/constant-voltage linear charger for single cell lithium-ion batteries. It can deliver up to 1A of charging current with a final float voltage accuracy of ±1%.
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MCP73831 – A highly advanced linear charge management controller for use in space-limited, cost-sensitive applications. It employs a constant-current/constant-voltage charge algorithm with selectable preconditioning and charge termination.
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BQ24074 – A switched-mode charger with integrated FETs that supports up to 4A charge current. It has a input voltage range of 5V to 28V and includes battery temperature monitoring and power path management.
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MAX1555 – A complete, single-cell Li-Po charger with thermal regulation, cell-voltage monitoring, and cell balancing. It can charge at up to 2A with an input voltage range of 3.7V to 7V.
Using a dedicated charger IC simplifies the design process and ensures reliable and safe operation. However, it’s still important to carefully read the datasheet and follow the recommended application circuit and layout guidelines.
Li-Po Battery Charger Safety Considerations
Li-Po batteries can be dangerous if not charged and handled properly. Some key safety considerations for Li-Po charger design include:
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Overcharge Protection: The charger must prevent the battery voltage from exceeding 4.2V per cell, which can cause the battery to swell, vent, or catch fire. This is typically done using a voltage monitoring circuit that shuts off the charger when the voltage reaches the threshold.
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Overcurrent Protection: The charger must limit the charging current to a safe value based on the battery specifications and the charger power supply. This is typically done using a current sensing resistor and a control loop that regulates the current.
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Short Circuit Protection: The charger must be able to detect and handle short circuit conditions without damaging the battery or the charger itself. This is typically done using a fast-acting fuse or a electronic switch that disconnects the battery.
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Reverse Polarity Protection: The charger must prevent damage if the battery is connected with the wrong polarity. This can be done using a series diode or a MOSFET switch.
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Temperature Monitoring: The charger should monitor the battery temperature and reduce or stop charging if the temperature exceeds a safe limit (typically 45°C). This can be done using a thermistor or a dedicated temperature monitoring IC.
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Timer Protection: The charger should include a timer that limits the total charging time to prevent overcharging if the voltage or current monitoring fails. The timer should be set based on the battery capacity and charging current.
By incorporating these safety features, a Li-Po charger can ensure reliable and safe operation even under fault conditions.
Li-Po Battery Charger Testing and Certification
Before using a Li-Po battery charger, it’s important to thoroughly test it to ensure it functions correctly and safely. Some key tests include:
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Functional Testing: Verify that the charger follows the correct charging profile and terminates charging when the battery is full. This can be done by monitoring the battery voltage and current during a complete charge cycle.
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Safety Testing: Verify that the charger includes all the necessary protection features and handles fault conditions safely. This includes testing overcharge, overcurrent, short circuit, reverse polarity, and temperature protection.
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Electromagnetic Compatibility (EMC) Testing: Verify that the charger does not generate excessive electromagnetic interference (EMI) that could affect nearby devices. This is typically done in an EMC lab using specialized equipment.
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Regulatory Certification: Depending on the intended market and application, the charger may need to be certified by regulatory bodies like UL, CE, or FCC to ensure it meets safety and EMC standards.
Testing and certification can be a complex and costly process, so it’s important to plan for it early in the design phase and work with experienced testing labs and certification bodies.
FAQs
1. Can I charge a Li-Po battery with a regular Li-ion charger?
No, Li-Po batteries require a charger specifically designed for the higher voltage (4.2V) and different charging profile of Li-Po cells. Using a Li-ion charger may overcharge and damage the Li-Po battery.
2. What happens if I overcharge a Li-Po battery?
Overcharging a Li-Po battery beyond 4.2V per cell can cause the battery to swell, vent flammable gases, and potentially catch fire or explode. Always use a charger with proper overcharge protection.
3. How long does it take to charge a Li-Po battery?
The charging time depends on the battery capacity and the charging current. At a 1C rate, a full charge takes approximately 1 hour. At lower rates like 0.5C, it can take 2 hours or more.
4. Can I charge a Li-Po battery with a solar panel?
Yes, but you need a charger circuit that can handle the variable voltage and current output of the solar panel and regulate it to the proper levels for the Li-Po battery. There are specialized solar Li-Po charger ICs available for this purpose.
5. How do I know when a Li-Po battery is fully charged?
A Li-Po battery is considered fully charged when the voltage reaches 4.2V per cell and the charging current drops below a certain threshold (typically 0.1C). Most Li-Po chargers have an indicator LED that turns on when the battery is full.
Conclusion
Designing a safe and reliable Li-Po battery charger circuit requires careful consideration of the battery specifications, charging profile, and protection features. Using dedicated Li-Po charger ICs can simplify the design process, but it’s still important to follow best practices for circuit design, layout, and testing. By understanding the key concepts and safety considerations involved in Li-Po battery charging, you can design chargers that maximize battery performance and longevity while minimizing the risk of damage or accidents.
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