Batteries are the only source of power for portable electronic devices. Whether you use smart phones, fitness trackers, sports cameras, outdoor navigation devices, cameras or handheld transceivers, you will encounter unexpected low power warning. In most cases, this kind of warning information will only bring inconvenience, but for safety and emergency equipment, it may cause serious consequences.

Determination of battery charging state by coulometer

Because the battery material, chemical composition and ambient temperature will change, it is not reliable to detect the battery voltage only by the coulometer. In addition, the battery impedance will change with the state of charge and battery aging, so it is more difficult to achieve accurate measurement. Each kind of battery chemical characteristics will produce a unique discharge characteristics, some are more suitable to represent the voltage based state of charge, while some voltage and load current discharge curve is almost a straight line, so that the coulometer can only display 100% or a flat curve.

So we need to introduce another method which can measure the battery charge and discharge current, that is the so-called coulometer method to determine the state of charge. This method takes into account the aging and self discharge characteristics of the battery.

The importance of state of charge to safety and consumer experience

With the increasing understanding of technology by consumers, whether the battery charging state can be accurately indicated has become a key indicator to measure the handheld consumer electronic devices. In order to accurately predict the battery state of charge, many manufacturers will customize the battery according to the specific application and use case, resulting in the delay of product launch, and in some cases, they need to ship to the third-party suppliers. However, the national regulations on the transportation of lithium-ion batteries are becoming more and more strict, and there are clear provisions on the mode of transportation and the remaining power.

In addition to the battery transportation regulations, in order to facilitate daily use and storage, other monitoring electronic devices need to be added to the design, so that the relatively volatile batteries (many batteries can provide hundreds of amperes of current) can operate in a safe range of parameters. In order to save costs, after-sales battery suppliers often ignore the battery security issues, so equipment manufacturers need to add battery encryption authentication technology in their battery and terminal product design to avoid security issues.

Challenges in measuring state of charge

As mentioned above, it is necessary to consider not only the power consumption during operation, but also the power consumption during standby to accurately measure the battery charging state and display it on the meter. In the process of transportation and storage, we also need to consider the quiescent current generated when the equipment is placed in the cabinet or “box”. The electricity meter itself consumes energy, so it needs to be taken into account when calculating how much power the battery can carry and whether the consumer has enough power to work after receiving the battery. As consumers, we like products to work without charging after they are received. In order to maintain the accuracy of the indicator, the relevant circuit must always be on. If the electricity meter is turned off before the consumer uses the product, the charging state indication will not be accurate enough. Please note that in order to ensure the safety during transportation and storage, the battery protection function (temperature, current and voltage monitoring) should be carefully activated in case of short circuit caused by excessive temperature or component failure.

Accurate solution of state of charge measurement

The maxim integrated max1730x low-power series is a good example. It integrates meter, protection and authentication functions in a 3mm × 3mm package. Max17301 is used to measure the charge state of lithium-ion battery or lithium polymer battery. When the output FET is activated, it has an ultra-low quiescent current of 24 μ a, especially as low as 18 μ a during sleep. When the FET is disabled, the current can be reduced to 0.1 μ a. This IC provides a comprehensive range of battery health and safety protection functions, including overvoltage (depending on temperature), overcharge current, battery under / over temperature, under voltage and over discharge / short circuit. The max1730x also has a one wire / I2C interface for communication with the main microcontroller to read the data and control registers of the max1730x (Fig. 1).

Solution of accurate measurement of state of charge based on electric quantity meter IC max1730x

Figure 1: Max integrated max1730x block diagram (source: Max integrated)

Battery health and protection requirements are determined by voltage, current and temperature. Maxim’s modelgauge M5 algorithm can be used to calculate the state of charge of battery. It has excellent short-term high precision and high linearity characteristics of coulometer, and excellent long-term stability of battery open circuit voltage measurement. The additional input of this temperature compensation algorithm produces accurate state of charge readings (Figure 2). The algorithm calculates the open circuit voltage of the battery, including the load condition.

Figure 2: calculating the corrected state of charge of Maxim max1730x using modelgauge M5 algorithm (source: Maxim integrated)

Max1730x compensates for the meter reading by battery aging characteristics and discharge rate. It provides percentage of state of charge or milliampere hour (MAH) readings under various operating conditions. The algorithm can also provide full charge time and aging prediction function to predict when the battery capacity will decline due to aging and use. The data recording function uses nonvolatile memory to record up to 13 parameters during the battery life cycle, including the time since the first power up. Figure 3 shows how the modelgauge M5 algorithm works.

Figure 3: schematic diagram detailing the maximum integrated modelgauge M5 algorithm (source: maximum integrated)

conclusion

It is the key to measure the success of a product that the charging state of the battery can be accurately displayed in the electric quantity meter without the time-consuming characteristic description of the battery. Maxim uses modelgauge M5 algorithm to realize this function, and saves valuable circuit board space and bill of materials (BOM) by combining protection and certification functions.

Editor in charge: GT

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