Factors and metering methods of battery capacity

　　Author ：Iflowpower – Portable Power Station Supplier

After the mobile phone came out, the rechargeable battery and its related power indication have become an integral part of our information society. For us, they are as important as the automotive fuel indicator that has played an important use in the past 100 years. The only difference is that the driver can not tolerate an inaccurate fuel instruction, while mobile phone users want to get high precision.

High resolution power indicator. After the many technical issues have been solved until the 1997 lithium-ion battery began mass production. Because the highest energy density (volume density and weight density) can be supplied, they are widely used in various systems from mobile phones to electric vehicles.

Lithium-ion batteries have some key features that affect power, the battery pack must contain various safety mechanisms to prevent battery overcharge, depth discharge or reverse connection. Since the lithium element is very active, there is a potential explosion risk, so the lithium ion battery is not exposed to a high temperature environment. The anode of the lithium ion battery is composed of carbide, and the cathode is composed of metal oxide, and lithium is added to the smallest method of deterioration of the lattice.

This process is called implantation. Metal lithium will have a strong reaction with water, so the lithium-ion battery uses a non-liquid organic lithium salt as an electrolyte. When charging the lithium ion battery, the lithium atom is transmitted to the anode by electrolyte by electrolyte.

Battery capacity battery is the most important parameter (except for voltage) is capacity, the unit is mAh (MAH), its meaning is the maximum amount of power supplied by the battery. The capacity of the manufacturer's meaning is the value of the battery under specific discharge conditions, but the battery will change after the battery is exported. The battery capacity is related to the battery temperature (Fig.

1), and the uppermost curve shows the process of constant current constant pressure at different temperatures at different temperatures. As can be seen from the curve, the battery can be charged with 20% by the battery over higher temperatures compared to charging data at -20 ° C. The two curves below Figure 1 indicate that the temperature is more affected by the temperature, and these curves show a complete battery to discharge the remaining electricity at two different discharge currents, from these two The curve can be seen that the residual capacity of the battery is related to the discharge current.

At a given temperature and discharge rate, the lithium ion battery capacity that can be obtained is the difference between the uppermost curve and the lower corresponding curve. Therefore, when low temperature or large current discharges, the capacity of the lithium ion battery will be greatly reduced. At low temperatures or large current discharge, the battery remaining is large, and can be discharged at a smaller current at the same temperature.

Due to the mixed impurities in the electrolyte, there is an undesired chemical reaction inside the battery, resulting in electricity loss. Typical self-discharge ratio of common battery type at room temperature is shown in Table 1. The speed of chemical reaction is affected by temperature, so self-discharge is related to temperature.

With regard to different types of batteries, self-discharge can be modeled with a parallel resistance consuming leakage current. Battery capacity decreases with the new number of charge and discharge, which is quantified to work life, that is, a battery before its capacity falls to 80% of the initial capacity of 80%, discharge. Typical lithium-ion batteries have a working life of 300 ~ 500 charge / discharge.

Lithium-ion battery life is also affected by time, regardless of whether or not, its capacity begins to gradually decline after the factory. At 25 ° C, this effect can cause a completely filled battery to lose 20% per year; loss of 35% at 40 ° C. About the battery that is not fully charged, this aging process is slower: 25 ° C, the residual 40% battery is lost annually by about 4% of the amount of electricity.

The battery's data manual specifies the discharge character curve under certain conditions, one of which affects the battery voltage is load current. However, the load current cannot be modeled through a simple source resistance, because the resistance depends on other parameters, such as battery aging and electricity levels. Compared with the original battery, the rechargeable lithium ion battery exhibits very flat discharge curve.

System developers prefer this feature because the voltage supplied by the battery is substantially unchanged. However, with the discharge of the battery, the battery voltage is almost associated with the remaining power. Simple is not equal to "shortcut" to determine the available power of the battery, first requiring a simple detection method, the detection circuit consumes only trace power consumption, allowing the user to calculate the electricity level level from the battery voltage (ideal).

However, since there is no clear relationship between voltage and electricity, the results of detecting the supply of battery voltages can be unreliable. In addition, the battery voltage is also dependent on temperature and dynamic release effect (which makes the end voltage slightly slow) when lowering the load current. Therefore, the simple voltage detection method is difficult to ensure that the power monitoring accuracy is higher than 25%.

The relative level of the power is often referred to as a charging state (SOC), refers to the ratio of the remaining power and battery capacity. The determination of this parameter is to monitor inflow, flow out of the amount of charge - a so-called "coulombometer" method. The actual coulombium is achieved by accumulating the current flowing out and out of the battery.

When the current is measured with a high resolution ADC, a small resistance is usually used in series with a small resistance and a battery anode. Because the function relationship between the battery SOC and some of the parameters mentioned above, the battery capacity must be determined by relevant experience. There is currently no detailed analysis model (with sufficient accuracy) for calculating capacity of specific operating conditions (such as temperature, charge, current, etc.

). The theoretical model is only suitable for determining conditions, in order to obtain relative charging levels, these models are used for specific conditions and overall calibration. In order to achieve a sufficiently high power metering accuracy, the model parameters must be continuously calibrated - using the so-called power "learning" mode, with coulombi, this method can make the power metering accuracy to several percentage points.

Electrical measuring method About rechargeable batteries in different types, configuration, and applications, modern integrated circuits can determine their SOC. Despite consuming a small amount of power supply (60mA consumption mode, sleep mode consumption is 1mA), these chips can still achieve higher precision. The electric quantimeter chip is divided into three types (Table 2) because the lithium ion battery is the preferred choice for most applications, which exemplifies the power metering circuit of a lithium ion and lithium polymer battery.

Coulombau, also known as battery monitor, conversion for measurement, counting, and battery parameters, including electricity, temperature, voltage, charging number, etc. Coulombia cannot measure variables, no intelligence. The DS2762 in this type of chip contains high-precision 25MΩ swelled resistors, can also monitor temperature, battery voltage and current, communicating through the 1-Wire bus, allowing the battery pack or the microcontroller in the battery pack or host system to read all data.

You can form a flexible low-cost system, but you have to understand a significant background knowledge and make certain development work, software, models, and support supplied by IC vendors, can reduce development costs. Another method is to use the power meter to calculate the quaguity, which can run electricity measurements with learning algorithms and all necessary measurements. Smart batteries typically use power meters to perform automatic monitoring, using less development work required to use integrated power meters, help to shorten product listing time.

The DS2780 is a fully integrated power meter that allows the host to read the SOC through the 1-Wire bus, and supplies the necessary safety protection circuit for lithium ion battery. Another option is to use a programmable power meter, which includes a microcontroller, which can supply considerable flexibility. For example, MAX1781, internal integration of RISC core, E2PROM and RAM.

Developers can achieve battery modeling, power meter programming and necessary measurements. Simple, accurate SOC indication can be implemented by internal LED drive. Conclusion Affected by multiple interrelated parameters, the amount of electricity measurement of rechargeable batteries becomes a complex task.

Simple measurement cannot supply accurate results, only applicable to some unimportant applications. By using the ready-made electric meter, high-precision, reliable electricity metering can be achieved.