
State of charge (SOC) quantifies the remaining capacity available in a battery at a given time and in relation to a given state of ageing.
It is usually expressed as percentage (0% = empty; 100% = full). An alternative form of the same measure is the
depth of discharge (DOD), calculated as 1 − SOC (100% = empty; 0% = full). It refers to the amount of charge that may be used up if the cell is fully discharged. State of charge is normally used when discussing the present state of a battery in use, while depth of discharge is most often used to discuss a constant variation of state of charge during repeated cycles.
In electric vehicles
In a
battery electric vehicle
A battery electric vehicle (BEV), pure electric vehicle, only-electric vehicle, fully electric vehicle or all-electric vehicle is a type of electric vehicle (EV) that uses electrical energy exclusively from an electric vehicle battery, on-boa ...
(BEV), the state of charge indicates the remaining energy in the
battery pack
A battery pack is a set of any number of (preferably) identical Battery (electricity), batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage and current. The term ' ...
. It is the equivalent of a
fuel gauge.
The state of charge can help to reduce electrical car owners' anxiety when they are waiting in the line or stay at home since it will reflect the progress of charging and let owners know when it will be ready. However on any vehicle dashboard, especially in
plug-in hybrid vehicles, the state of charge presented as a gauge or percentage value may not be representative of a real level of charge. A noticeable amount of energy may be reserved for
hybrid-work operations. Examples of such cars are
Mitsubishi Outlander PHEV (all versions/years of production), where a charge level of zero is indicated to the driver when the real charge level is 20–22%. Another one is the BMW i3 REX (Range Extender version), where about 6% of SoC is reserved for PHEV-alike operations.
State of charge is also known to impact battery aging.
To extend battery lifetime, extremes of state of charge should be avoided and reduced variations windows are also preferable.
Determining SoC
Usually, SoC cannot be measured directly but it can be estimated from direct measurement variables in two ways: offline and online. In offline techniques, the battery desires to be charged and discharged in constant rate such as Coulomb-counting. This method gives precise estimation of battery SoC, but they are protracted, costly, and interrupt main battery performance. Therefore, researchers are looking for some online techniques. In general there are five methods to determine SoC indirectly:
* chemical
* voltage
* current integration
* Kalman filtering
* pressure
Chemical method
This method works only with batteries that offer access to their liquid
electrolyte
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
, such as non-sealed
lead acid batteries. The
specific gravity
Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...
of the electrolyte can be used to indicate the SoC of the battery.
Hydrometer
A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically Calibration, calibrated and Graduation (instrument), graduated with one or more scales suc ...
s are used to calculate the specific gravity of a battery. To find specific gravity, it is necessary to measure out volume of the electrolyte and to weigh it. Then specific gravity is given by (mass of electrolyte
volume of electrolyte
l/ (Density of Water, i.e. 1g/1ml). To find SoC from specific gravity, a look-up table of SG vs SoC is needed.
Refractometry has been shown to be a viable method for continuous monitoring of the state of charge. The refractive index of the battery electrolyte is directly relatable to the specific gravity or density of the electrolyte of the cell.
Notably, analysis of electrolyte does not provide information about the state-of-charge in the case of
lithium-ion batteries and other batteries, that do not produce or consume solvent or dissolved species during their operation. The method works for
lead-acid batteries, because the concentration of
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
changes with the battery's state-of-charge according to the following reaction:
:(s) + (s) + 2(aq) → 2(s) + 2(l)
Voltage method
This method converts a reading of the battery
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
to SoC, using the known discharge curve (voltage vs. SoC) of the battery. However, the voltage is more significantly affected by the battery current (due to the battery's
electrochemical
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...
kinetics) and temperature. This method can be made more accurate by compensating the voltage reading by a correction term proportional to the battery current, and by using a look-up table of battery's open circuit voltage vs. temperature.
In fact, it is a stated goal of battery design to provide a voltage as constant as possible no matter the SoC, which makes this method difficult to apply. For batteries, that have voltage independent on their state-of-charge (such as
lithium iron phosphate battery
The lithium iron phosphate battery ( battery) or LFP battery (''lithium ferrophosphate'') is a type of lithium-ion battery using lithium iron phosphate () as the cathode material, and a graphitic carbon electrode with a metallic backing as t ...
), open-circuit voltage measurements cannot provide a reliable estimate of the SoC. On the other hand, batteries with a sloping voltage-charge curves (such a
nickel-cobalt-manganese battery, are more amenable to SoC estimation from the
open-circuit voltage
Open-circuit voltage (abbreviated as OCV or VOC) is the voltage, difference of electrical potential between two Terminal (electronics), terminals of an electronic device when disconnected from any Electric Circuit, circuit. There is no External ...
measurements.
Current integration method
This method, also known as ''
coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).
It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...
counting'', calculates the SoC by measuring the battery current and
integrating it in time.
Since no measurement can be perfect, this method suffers from long-term drift and lack of a reference point: therefore, the SoC must be re-calibrated on a regular basis, such as by resetting the SoC to 100% when a charger determines that the battery is fully charged (using one of the other methods described here).
Combined approaches
Maxim Integrated touts a combined voltage and charge approach that is claimed superior to either method alone; it is implemented in their ModelGauge m3 series of chips, such as MAX17050, which is used in the
Nexus 6 and
Nexus 9
The Nexus 9 (codenamed Volantis or Flounder) is a tablet computer co-developed by Google and HTC that runs the Android operating system. It is the fourth tablet in the Google Nexus series, a family of Android consumer devices marketed by Googl ...
Android devices, for example.
Kalman filtering
To overcome the shortcomings of the voltage method and the current integration method, a
Kalman filter
In statistics and control theory, Kalman filtering (also known as linear quadratic estimation) is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, to produce estimates of unk ...
can be used. The battery can be described with an electrical model which the Kalman filter will use to predict the over-voltage given the observed current. In combination with coulomb counting, it can make an accurate estimation of the state of charge. The strength of this technique is that a Kalman filter adjusts its relative trust of the battery voltage and coulomb counting in real time.
Pressure method
This method can be used with certain
NiMH batteries, whose internal pressure increases rapidly when the battery is charged. More commonly, a pressure switch indicates if the battery is fully charged. This method may be improved by taking into account
Peukert's law which is a function of charge/discharge current.
See also
*
Battery balancing
Battery balancing and battery redistribution refer to techniques that improve the available capacity of a battery pack with multiple cells (usually in series) and increase each cell's longevity. A ''battery balancer'' or ''regulator'' is a ...
*
Battery charger
A battery charger, recharger, or simply charger, is a device that stores energy in an electric battery by running current through it. The charging protocol—how much voltage and current, for how long and what to do when charging is complete� ...
*
Battery management system (BMS)
*
Battery monitoring
*
Depth of discharge (DoD)
*
State of Health (SoH)
*
Smart battery
References
{{reflist
Battery charging
Electric vehicle technologies