Lithium battery power monitoring principle

1. Battery level monitoring
2. Traditional battery power monitoring methods
   1. Traditional battery pack side fuel gauge
   2. What are the functions of the electricity meter?
   3. How to realize power monitoring?
   4. SOC error of voltage-based electricity monitoring
3. Voltage-based fuel gauge
4. The benefits of electricity monitoring
5. Summary

Battery fuel gauge is a technique used to predict battery power under all system operating and idle conditions. Battery capacity includes percentage, runtime, time to drain/full, milliamp hours (mAh), watt hours (Wh), talk time, limited time, and more.

Battery level monitoring

The high-current discharge capability of lithium-ion batteries can be tuned over a wide range by using thicker or thinner layers of active material. A thinner active material layer means that battery has a higher high-current discharge capability, but a lower energy density.

Standard 18650 cylindrical cells used in laptops are designed for maximum C-rate discharge. However, some batteries are rated for 10C discharge, and even some batteries can discharge at a rate of 60C, mainly for backup power or regenerative braking in gas-electric hybrid vehicles.

The high-current discharge capacity will be severely reduced at low temperature below 0 °C, which is due to the low conductivity of the organic electrolyte. Electrolytes vary in conductivity, so it is important to consult the manufacturer’s data on low temperature discharges.

Traditional battery power monitoring methods

The main purpose of battery power monitoring is to maximize the use of the capacity in the battery. Generally speaking, it is difficult for us to use 100% of the capacity in battery power. The reasons are as follows:

There are 2 factors here. First of all, when charging, it is difficult for the charging voltage to be exactly the full charging voltage of the battery. Usually, in order to prevent battery power from overshooting, the charging voltage error is biased downward. That is to say, the charging voltage of the battery may be or so, if charging at this low charging voltage, the capacity to be charged may be less.

In addition, due to the inaccuracy of battery power monitoring, users may estimate the power conservatively for safety and to prevent data loss due to sudden critical issues. Of course, the user experience feels that the capacity of battery power has become smaller, which is a disadvantage.

And the consequence of this is that the capacity of battery cannot be fully utilized. The battery power monitoring technology is to maximize the monitoring of battery power, allowing users to maximize the use of the current battery capacity.

Traditional battery pack side fuel gauge

The traditional battery pack power monitoring technology is a frame structure, and the power monitoring chip is generally placed in the battery pack. There is also a protector that controls the MOS tube. This protector switches the MOS to protect the battery cell when battery is overcharged or overdischarged.

Generally, a thermistor is placed in battery pack to monitor the battery pack. Temperature, except that the left side refers to the system board of the mobile phone or tablet, the power management chip and the host’s processor are mainly related to the fuel gauge on this system board. The host’s processor is usually through I2C or single wire. HDQ bus to read the power information in the fuel gauge.

What are the functions of the electricity meter?

It can measure battery voltage, charge or discharge current, temperature. As well as it can provide battery runtime and remaining capacity, battery power health information, overall battery power management.

The main function of the fuel gauge is that the fuel gauge must first complete the communication between the system and battery power. As a fuel gauge, these basic analog information is more important to provide the capacity information of the battery, that is, battery remaining capacity, battery operating time, and battery information on the health.

Another is that the chip itself must be able to complete the transition of the working state, that is to say, it must switch from the normal working mode to the low-power mode to realize this transition and achieve the purpose of saving power.

How to realize power monitoring?

The first method is voltage-based gas monitoring. Another method is coulomb counting, which is an energy obtained by integrating the current charged into the battery or discharged from battery power. The latest algorithm is the impedance tracking algorithm. In fact, this algorithm is based on the real-time internal resistance measurement of battery to obtain the battery capacity.

That is, the terminal voltage V of the battery is equal to the open-circuit voltage of the battery minus the current multiplied by the internal resistance of a battery. This current refers to the total current flowing into or out of battery power.

SOC error of voltage-based electricity monitoring

After 100 cycles, the internal resistance of battery doubles. According to this rule, the error will become larger and larger, and the error caused by the impedance error to the capacity will also increase accordingly. So basically after 300 cycles, the error caused by the lower capacity will be very large, 75% or more.

Therefore, the electricity calculation technology based on voltage measurement can only be used in those occasions with low requirements, and its error is relatively large. Then the biggest impact on this capacity calculation is the internal resistance of a battery.

The reason for the relatively large change in the internal resistance of the battery is the deviation of the internal resistance of the battery caused by the manufacturing process of battery power. The other is that the internal resistance of  battery will change greatly due to the delay of battery power usage time, so the estimated result will have a relatively large deviation from the actual result.

Voltage-based fuel gauge

The disadvantage of voltage-based fuel gauges is that they are not accurate due to the internal impedance of battery. In addition, batteries have self-discharge characteristics. The battery power is placed there. Even if the load does not work, battery power will discharge itself.

After a long time, the power will become less and less, and the voltage will become lower and lower. Then the fuel gauge of the voltage monitoring only needs to judge the capacity according to the voltage.

So now the capacity is reported as much as the voltage is, so there is no need to care too much about the self-discharge. So this kind of voltage-based fuel gauge can still achieve a certain accuracy if the current is very small. But at present, various applications are becoming more and more complex, and the current changes are becoming larger and larger.

The benefits of electricity monitoring

In fact, in the impedance tracking technology, with the continuous improvement of the algorithm, the impedance change caused by the continuous rise and fall of the battery temperature during use is added to the estimation of the temperature model, and thermal simulation is introduced to adjust the heating of the battery power. Master the voltage drops due to load changes, these voltage drops are actually also considered in the battery power calculation.

Of course, for the aging battery, because the impedance tracking calculates the impedance of the battery power in real time, it does not need to use a model to estimate the aging battery. It is the measured impedance, so the effect of aging is relatively small for it. Due to its precise computing capacity, it can maximize the use time, as you can clearly see from the pictures just now.

With the impedance tracking chip, our host system does not need any algorithm to calculate the capacity of the battery power, and only needs to simply read the specified register to get the capacity. With impedance tracking technology, you can also conduct some thorough analysis of the battery power, such as battery aging, battery health, and so on.

Summary

So as far as battery products are concerned, an accurate gauge is very important for the battery power to get a long run. Then, if you use an accurate fuel gauge to maximize the available capacity of the system, you can use a relatively low-capacity battery, which can bring cost savings.

There are many battery power schemes available, and they are generally based on the combination of voltage monitoring or coulomb counting, and rarely use pure voltage monitoring or pure coulomb counting.

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