If you're reading this article on your laptop or smartphone, you can likely easily spot the battery icon with a quick glance at the top or bottom of your screen. For devices so central to our lives—almost extensions of our hands—it seems natural to us to expect that the device "knows" its battery level and can inform us how much uninterrupted usage time remains.
However, the journey to the familiar battery icon and the charge percentage beside it involves fascinating engineering processes. Let's take a brief look at how they work.
Most smartphones and laptops are powered by lithium-ion batteries, and the charge level of these batteries is professionally referred to as SOC (State of Charge). SOC estimates the available electric charge, measured in ampere-hours.
Electric charge is a fundamental property of particles that determines their ability to interact with an electric field and is related to the amount of usable energy, which is calculated by multiplying the charge by the electric voltage. Alongside SOC, another metric is SOH (State of Health). SOH reflects the battery's capacity to store and deliver electrical energy to a device, compared to a brand-new, unused battery.
For us as users, SOC and SOH calculations are key to understanding our devices' performance. But these metrics are even more critical for battery manufacturers who (we hope) strive to produce durable batteries with long lifespans. Currently, there are three primary methods for calculating SOC.
The Coulomb Counting method
This is the most practical and widely used method for lithium-ion batteries. Measuring the electric current - which, in simple terms, represents the flow rate of electrical charges, or more precisely, the change in charge over time - is relatively straightforward. By continuously measuring the current and accumulating the data over time, the amount of available charge can be calculated. Charge is typically measured in units of Coulombs (C) or ampere-hours (Ah), which is why this method is also referred to as Coulomb Counting or the Ah method.
3 View gallery
Electrical voltage, or potential difference, represents the ability to move charges between two points. Measuring voltage in a car battery
(Photo: Shutterstock, Peter Gudella)
However, some electric charge is inevitably lost during battery operation, leading to errors in the calculation. These losses are primarily due to self-discharge, a phenomenon caused by chemical reactions within the battery, even when it is not connected to an external device. While SOC calculations using this method are designed to account for these charge losses, regular recalibration of the SOC is required to ensure accurate readings.
The voltage method
Electrical voltage, or potential difference, represents the capacity to move charges from one place to another. In the voltage method for calculating SOC, the battery voltage is measured, and the result is converted into SOC using pre-established curves. The relationship between electrical voltage and SOC is also reflected in Ohm's law, which defines the relationship between electric current and voltage through resistance that connects them. According to Ohm's law, voltage is the product of current and resistance.
A significant drawback of this method is that voltage readings must be taken when the device is inactive, unlike the Coulomb Counting method, which allows measurements during operation. In practice, voltage measurements require an open-circuit state—meaning no device activity—for at least four hours or even longer. This delay is necessary because the measurement is affected by the movement of charges within the battery and by its temperature, which increases during use.
Additionally, the method demands a relatively long measurement period. These factors make the voltage method less practical for real-world applications.
The Kalman filter method
The Kalman filter is an algorithm designed to bridge the gap between predictions and observations. In theory, the operational behavior of a system—in this case, a battery—is predefined as part of a model previously studied or designed, enabling predictions about its future state.
However, real-world outcomes often deviate slightly from predictions due to various internal and external factors, collectively referred to as "noise."
To address this noise and provide a real-time picture of the system's state, Rudolf Kalman, a Hungarian-American electrical engineer of Jewish descent, proposed continuously updating predictions based on real-time data from the system.
Get the Ynetnews app on your smartphone: Google Play: https://bit.ly/4eJ37pE | Apple App Store: https://bit.ly/3ZL7iNv
Since each sensor in the system typically provides only partial information, and each introduces its own noise, data integration is essential. The algorithm itself is complex and merits a separate discussion to explain fully. However, just as it was instrumental in enabling Apollo 11 spacecraft launch, it is also used for measuring SOC.
While manufacturers can predict battery current behavior over time using prior knowledge, real-time measurements enhance accuracy. The primary drawback of this method is that it requires a tailored model for each battery. making it complex and computationally demanding. As a result, it has largely fallen out of favor. Surprisingly, despite advancements in artificial intelligence and electrochemistry, modern adaptations of this algorithm remain unsuitable for lithium-ion batteries.
Not starting from scratch
Thus, the Coulomb Counting method remains the dominant “player” in the "field" of SOC measurement. While alternative methods, such as chemical or pressure-based measurements, have been proposed over the years, Coulomb Counting continues to be the most widely used technique.
A critical factor to consider is the battery’s initial charge. In fact, for any problem involving the change of a quantity over time, knowing the initial conditions is essential for accurately calculating the full solution. Otherwise, it's like trying to map out a running track without specifying whether the run takes place in a stadium (on a circular track) or in a park (on a straight path). In this context, the focus is on determining the battery’s initial SOC. But how is it determined?
Batteries can exist in only three states: charging, discharging, or an open circuit (with no electrical activity). To determine the initial SOC, the open-circuit voltage of the battery—after it has sufficiently stabilized—is measured and compared to manufacturer-provided curves that correlate voltage with SOC. In essence, this process is a specific application of the voltage method.
So, the next time you check your battery's status, don’t take it for granted—it’s much more complex than it appears.