State of charge
Encyclopedia
State of charge is the equivalent of a fuel gauge
for the battery pack
in a battery electric vehicle
(BEV), hybrid vehicle
(HEV), or plug-in hybrid electric vehicle (PHEV). The units of SOC are percentage points (0% = empty; 100% = full).
In general there are four methods to determine SOC indirectly:
, such as non-sealed lead acid batteries. The specific gravity
or pH
of the electrolyte can be used to indicate the SOC of the battery.
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
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.
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 3 methods described here).
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 rate or ampere
.
Fuel gauge
A fuel gauge is an instrument used to indicate the level of fuel contained in a tank. Commonly used in cars, these may also be used for any tank including underground storage tanks.As used in cars, the gauge consists of two parts:...
for the battery pack
Battery pack
A battery pack is a set of any number of identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density...
in a battery electric vehicle
Battery electric vehicle
A battery electric vehicle, or BEV, is a type of electric vehicle that uses chemical energy stored in rechargeable battery packs. BEVs use electric motors and motor controllers instead of, or in addition to, internal combustion engines for propulsion.A battery-only electric vehicle or...
(BEV), hybrid vehicle
Hybrid vehicle
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles , which combine an internal combustion engine and one or more electric motors.-Power:...
(HEV), or plug-in hybrid electric vehicle (PHEV). The units of SOC are percentage points (0% = empty; 100% = full).
Determining SOC
SOC cannot usually be determined directly.In general there are four methods to determine SOC indirectly:
- chemical
- voltage
- current integration
- pressure
Chemical method
This method works only with batteries that offer access to their liquid electrolyteElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
, such as non-sealed lead acid batteries. The specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
or pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the electrolyte can be used to indicate the SOC of the battery.
Voltage method
This method converts a reading of the battery voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
) 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.
Current integration method
This method, also known as "coulomb counting", calculates the SOC by measuring the battery current and integratingIntegral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
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 3 methods described here).
Pressure method
This method can be used with certain NiMHNIMH
NIMH or NiMH may refer to:*Nickel-metal hydride battery, a type of rechargeable battery*National Institute of Mental Health, a part of the United States National Institutes of Health...
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
Peukert's law
Peukert's law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a lead–acid battery in terms of the rate at which it is discharged. As the rate increases, the battery's available capacity decreases....
which is a function of charge/discharge rate or ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
.
See also
- Battery balancingBattery balancingBattery balancing and battery redistribution refer to techniques that maximize a battery's capacity to make all of its energy available for use and increase the battery's lifetime. A battery balancer or battery regulator is a device in a battery pack that performs battery balancing...
- Battery balancer
- Battery chargerBattery chargerA battery charger is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it.The charge current depends upon the technology and capacity of the battery being charged...
- Battery management systemBattery Management SystemA battery management system is any electronic system that manages a rechargeable battery , such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing it.-Monitor:A BMS may monitor the state of the...
(BMS) - Battery monitoring
- Depth of dischargeDepth of dischargeDepth of discharge is an alternate method to indicate a battery's state of charge . The DOD is the inverse of SOC: as one increases, the other decreases. While the SOC units are percent points , the units for DOD can be Ah or percent points...
(DOD) - State Of HealthState Of HealthState of health is a figure of merit of the condition of a battery , compared to its ideal conditions. The units of SOH are percent points ....
(SOH)