Unlike the OCV method, this method is able to determine the evolution of the state of charge during battery use. This integration directly gives the number of electrical charges injected or withdrawn from the battery, thus making it possible to precisely quantify the SoC of the battery. To track the state of charge when using the battery, the most intuitive method is to follow the current by integrating it during cell use. A precise measurement, coupled with a modeled load curve, allows SoC measurements to be obtained with an accuracy of 10 to 15%.Ģ/ SoC estimation using the Coulomb Counting method However, calibrated charge indicators can be used specifically for lithium-ion batteries in general and lithium iron phosphate batteries in particular. The diagram below shows that the voltage measurement difference between a DoD value of 40% and 80% is about 6.0V for a 48V battery in lead-acid technology, while it is only 0.5V for lithium-iron-phosphate! Indeed, the voltage difference between two SoC values may be so small that it is not possible to estimate the state of charge with good precision. Lithium Iron Phosphate technology has the flattest discharge curve, which makes it very difficult to estimate SoC on a simple voltage measurement. However, Lithium-ion batteries have a much flatter discharge curve, which means that over a wide operating range, the voltage at the battery terminals changes very slightly. It can be seen that lead-acid batteries have a relatively linear curve, which allows a good estimation of the state of charge: for a measured voltage, it is possible to estimate fairly precisely the value of the associated SoC. As an example, the diagram below compares the discharge curves between a lead battery and a Lithium-Ion battery. This relationship between voltage and SOC depends directly on the battery technology used. The voltage will be highest when the battery is fully charged and lowest when it is empty. 1/ SoC estimation using Open Circuit Voltage Method (OCV)Īll types of batteries have one thing in common: the voltage at their terminals decreases or increases depending on their charge level. We will detail here the two most common and simplest methods to estimate the state of charge of a battery: voltage method or Open Circuit Voltage (OCV) and coulomb counting method. Some methods are quite complicated to implement and require complex equipment (impedance spectroscopy or hydrometer gauge for lead-acid batteries). There are several ways to get Lithium-Ion State of Charge (SoC) measurement or Depth of Discharge (DoD) for a lithium battery. An alternative form of the same measure is the depth of discharge (DoD), the inverse of SoC (100% = empty 0% = full). The units of SoC are percentage points (0% = empty 100% = full). State of charge (SoC) is the level of charge of an electric battery relative to its capacity. The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. In general, the SOC of a battery is defined as the ratio of its current capacity () to the nominal capacity (). SOC is one of the most important parameters for batteries, but its definition presents many different issues. Results demonstrate a more than third-order of magnitude outperformance of the proposed system in terms of compression gain and computational efficiency while assuring an analogous SoC estimation precision.ĭefinition and Classification of SOC Estimation The devised system comparison is made with the traditional counterparts. The estimated SoC is calibrated by using an original event-driven Open Circuit Voltage (OCV) to SoC curve relation. Recent BMSs are becoming sophisticated and causes a higher consumption overhead on the battery. To assure an effective battery utilization and longer life, the battery management systems (BMS) are employed. The lithium-ion batteries are recurrently used in a variety of applications. Lithium-Ion State of Charge (SoC) measurement
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