Electronic products
While lithium-ion chemistry offers many advantages over other battery chemistry methods, its biggest drawback is its safety. Lithium-ion batteries can experience thermal events if used improperly. Robust electronics and fuses need to be incorporated into the battery design to ensure safe operation.
A typical battery management system (BMS) is commonly used in lithium-ion batteries. The most important element of a battery management system is ensuring the proper use of the lithium-ion battery. This includes monitoring the following to ensure it operates under normal conditions:
• Discharge current: This is the current that leaves the battery to supply power to a load. All batteries are rated for a specific discharge current that they can handle. Some batteries are rated to operate at temperatures up to 100°C (100 times their rated capacity), while others can only operate at temperatures below 1°C.
• Charging current: This is the current that enters the battery to charge it. All batteries are rated for a specific charging current that they can handle. Some batteries have a higher rated charging current capability to enable fast charging.
• Voltage: This is the voltage on a single cell. Lithium-ion batteries need to be maintained within a certain voltage range. Exceeding this range, especially at the high end, can cause thermal problems. Overvoltage is one of the most dangerous things you can do to a battery.
• Temperature: This is the temperature on the battery. Cells need to operate within a given range. Typically, it should be between -20°C and 60°C during discharge and between 0°C and 40°C during charging.
If the electronic device detects that any of the above factors exceed the battery's specified range, it will shut down the battery. There are several ways to shut down the battery. Most BMS systems use a combination of these methods.
1. PTC – Positive Temperature Coefficient Thermistor connected in series with the battery. As current increases, the resistance of the PTC increases with temperature, creating an open circuit and disconnecting the battery from the application. This is a very common way to add protection to the battery. Some batteries have a built-in PTC for added protection.
2. Current Fuse – Another method of current protection is to connect a current fuse in series with the battery. Once the current exceeds the fuse's rating, it will trip. Most systems use this as a secondary, last resort option because it will permanently shut down the battery.
3. Thermal fuse – This fuse will open when the temperature exceeds its rated value. Like an electrical fuse, it is permanently closed.
4. MOSFET – One of the most common ways to turn off a battery is by using a MOSFET. MOSFETs are typically connected to a protection IC. The protection IC measures the voltage on the battery, the current flowing into and out of the battery, and in some cases, the temperature of the battery. If the IC detects any of these values are out of range, it sends a signal to the MOSFET to turn it off. Because the current can be bidirectional (charging and discharging), back-to-back MOSFETs are used.
If an N-channel MOSFET is used on the positive side of the battery pack, the IC needs a charge pump to provide a gate voltage higher than the battery voltage. Most simple protection ICs do not have a charge pump, so there are two options:
• Disconnect the battery using an N-channel MOSFET on the battery pack side. The drawback here is the need to understand and mitigate the impact on the system when the battery is disconnected from ground.
• Disconnect the battery using a P-channel MOSFET on the + side of the battery pack. P-channel MOSFETs are less common than N-channel MOSFETs, and the selection may not be as good. They are generally more expensive than N-channel MOSFETs.
Several manufacturers offer protection ICs, such as Texas Instruments (TI), Seiko, and Sammi.
5. Chemical Fuses – Chemical fuses are also used in battery packs for secondary overvoltage protection. A chemical fuse works similarly to an current fuse, but it also has the ability to self-blow using a built-in heater. Terminal T3 is grounded, activating the heater and blowing the fuse. The most common use of chemical fuses in batteries is for secondary overvoltage protection. Terminal T3 is connected to the secondary overvoltage protection IC, which activates when it detects one or more batteries exceeding a specific voltage range. For secondary protection to activate, this must mean that the primary protection circuit is not working and there is a serious problem with the battery pack. In this case, permanently disabling the battery pack is advisable. Many battery packs that require UL2054 testing have a chemical fuse for secondary overvoltage protection.
6. Relays/Contactors – Another option is to use relays or contactors to switch battery loads. This is a very popular method for larger battery packs (forklift batteries, vehicle batteries, etc.). The main advantage is that relays/contactors can carry large currents. They are also independently controlled (unlike MOSFETs), so a low voltage signal can switch the input/output to a high voltage output.
Although lithium-ion batteries frequently make headlines, there are many ways to ensure their safety in electronic devices. It is strongly recommended that you always incorporate redundant protection in your battery pack design. At least two independent overcurrent and overvoltage shutdown mechanisms should be included to mitigate any failure or damage to the main protection circuitry.
