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Protecting Measures Taken by LiPo Battery II

Intro

In last blog, we have introduced short circuit protection, over-current protection, temperature protection and voltage protection, now, we will continue to talk about protection measures taken by lipo battery. They are MOS protection, how to decrease self discharge, how to balance lipo battery. If you are interested about them, just keep reading!

MOS Production of Lipo Battery

The main focus of MOS protection in polymer lithium (LiPo) batteries revolves around voltage, current, and temperature, which also involves the selection of MOS transistors. It is essential for the withstand voltage of the MOS transistor to exceed the battery pack voltage. The current refers to the temperature rise on the MOS transistor body, which generally should not exceed 25 degrees Celsius when operating under rated current. Please note that this is based on personal experience and should be used as a reference only.

When driving the MOS transistor, it is recommended to use a MOS transistor with low internal resistance and high current capacity. However, why does it still experience relatively high temperatures? This is because the MOS transistor’s driving part may not be well designed. Driving the MOS transistor requires a sufficiently large current, which needs to be determined based on the input capacitance of the power MOS transistor. Therefore, direct driving of the MOS transistor by the chip is not recommended for overcurrent and short circuit conditions.

Extra driving measures are necessary. When operating at high currents (exceeding 50A), multiple stages and multiple paths of driving are required to ensure proper opening and closing of the MOS transistor at the same time and under the same current. This is because the MOS transistor has an input capacitance, and as the power and current increase, the input capacitance also increases. Without sufficient current, complete control cannot be achieved within a short period. Especially when the current exceeds 50A, more careful current design is required, and multi-stage and multi-path driving control must be implemented to ensure proper overcurrent and short circuit protection of the MOS transistor.

MOS current balance mainly refers to the situation where multiple MOS transistors are used together. It is crucial to ensure that the current passing through each MOS transistor has consistent opening and closing times. This requires careful layout design, with symmetric input and output configurations, to ensure that the current passing through each transistor is consistent. This is the ultimate goal.

How to Decrease Self Discharge of Lipo Battery

For polymer lithium (LiPo) batteries, the self-discharge rate should ideally be as low as possible, preferably zero. However, achieving zero self-discharge is not feasible. Many people aim to minimize this parameter, and some have even unrealistic expectations. The protection board contains chips that need to work, and while it is possible to achieve very low self-discharge rates, reliability should also be considered. It is important to assess self-discharge only when the performance is reliable and stable.

Self-discharge can be categorized into overall self-discharge and self-discharge for each cell group. For Lipo batteries, an overall self-discharge rate of 100 to 500uA is generally acceptable because these batteries have a large capacity. For example, for a 5AH battery, a discharge rate of 500uA would take a long time to significantly affect the entire battery pack.

The self-discharge of each lipo cell group is more critical, and it cannot be zero either. However, it is crucial to ensure that the self-discharge rate is consistent for each cell group. Typically, the difference in self-discharge between each cell group should not exceed 5uA. If there are differences in self-discharge between cell groups, the battery’s capacity will inevitably vary over time, especially during prolonged periods of inactivity.

How to Balance Lipo Battery

Balance is the focus of this article. Currently, there are two main types of balance methods: energy-consuming balance and energy transfer balance. Energy-consuming balance primarily dissipates excess electrical energy from a high-voltage or high-capacity cell in a multi-cell battery through resistance. There are three sub-types:

  1. Charge-based balance: This balance is activated when any one cell’s voltage exceeds the average voltage during charging. It is mainly applied in intelligent software solutions. The advantage is that it allows more time for voltage balance in Lipo batteries.
  2. Voltage-based balance: This balance is set at a specific voltage point, such as 4.2V for manganese lithium batteries. It only occurs at the end of the battery charging process, so the balance time is short.
  3. Static automatic balance: This balance can occur during the charging or discharging process of Lipo batteries. It also balances the voltage when the batteries are statically stored until their voltages become consistent. However, some people question why the protection board still generates heat when the batteries are not in use.

All three methods achieve balance based on reference voltage. However, high voltage does not necessarily indicate high capacity, and vice versa. The details are explained below.

The advantages of energy-consuming balance are low cost and simple design. It can effectively address the voltage inconsistency caused by long-term self-discharge of Lipo batteries, but its feasibility is weak.

The disadvantages include the complexity of the circuit, a high number of components, high temperature, poor electrostatic protection, and high failure rate of Lipo batteries.

In a PACK composed of individual batteries after capacity, voltage, and internal resistance partitioning, there will always be individual cells with lower capacity. Usually, the cell with the lowest capacity has the fastest voltage rise during charging and reaches the balance activation voltage first. At this point, the higher-capacity cells have not reached the voltage point for balance activation yet. As a result, the smaller capacity cell undergoes continuous charge and discharge cycles while the larger capacity cells have not started balancing. This leads to accelerated aging and increased internal resistance of the smaller capacity cell, creating a detrimental cycle. This is a significant drawback.

The more components in the Lipo battery, the higher the failure rate.

As for temperature, it is evident that energy-consuming balance generates heat as it dissipates excess electrical energy through resistance. This makes it a significant heat source. High temperatures are extremely harmful to the battery itself; it can cause battery combustion or even explosion. While efforts are made to reduce the overall temperature of the battery pack, energy-consuming balance, on the other hand, generates alarming heat. One can conduct tests in a completely enclosed environment to verify this. Overall, it acts as a heat source, and heat is the battery’s mortal enemy.

Regarding electrostatics, when designing a protection board, personally, I never use low-power MOS transistors, not even one. I have suffered too many losses in this regard, specifically due to the static electricity issues of MOS transistors. Let alone the working environment of small MOS transistors, during the production and assembly of PCBA, if the humidity in the workshop is below 60%, the defect rate of small MOS production will exceed 10%. However, if the humidity is adjusted to 80%, the defect rate drops to zero. You can try it out. What does this imply? Whether small MOS transistors can pass through in winter in northern regions where Lipo batteries are used requires time to verify.

Furthermore, MOS transistors are vulnerable to short circuits, and if a short circuit occurs, it implies that the battery pack is about to be damaged. Considering the extensive use of small MOS transistors in balance systems, this poses additional risks.

Energy transfer balance allows large-capacity batteries to transfer energy to smaller-capacity batteries in the form of energy storage. It sounds intelligent and practical. It also includes real-time capacity balance and fixed-point capacity balance. It achieves balance based on battery capacity, but it seems to overlook battery voltage. For example, using a 10AH battery pack as an example, if there is one cell with a capacity of 10.1AH and another with a slightly smaller capacity of 9.8AH, and the charging current is 2A while the energy balance current is 0.5A, the 10.1AH cell will transfer energy to the 9.8AH cell during charging. As a result, the charging current for the 9.8AH cell becomes 2A + 0.5A = 2.5A. However, what will be the voltage of the 9.8AH cell? It will obviously rise faster than the other cells. If it reaches overcharge protection prematurely at the end of charging, the smaller capacity cell will be subjected to deep charging and discharging in every cycle. Meanwhile, it remains uncertain whether other cells have reached full charge.

Conclusion

Above all are all our protection measures taken by lipo battery. In conclusion, lipo battery is a kind of rechargeable battery  with sooo many advantages. The most important thing is lipo battery is very safe and affordable for a long time. By the way, lipo batteries in FPpower are still excellent. If you have some some design plan, our lipo batteries may be your good choice.

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