State Of Health
Encyclopedia
State of health is a figure of merit
of the condition of a battery
(or a cell
, or a battery pack
), compared to its ideal conditions. The units of SOH are percent points (100% = the battery's conditions match the battery's specifications).
Typically, a battery's SOH will be 100% at the time of manufacture and will decrease over time and use. However, a battery's performance at the time of manufacture may not meet its specifications, in which case its initial SOH will be less than 100%.
Knowing the SOH of a given battery and the SOH threshold of a given application:
The designer of a battery management system
may use any of the following parameters (singly or in combination) to derive an arbitrary value for the SOH.
In addition, the designer of the battery management system defines an arbitrary weight for each of the parameter's contribution to the SOH value. The definition of how SOH is evaluated can be a trade secret.
evaluates the SOH of a battery is arbitrary.
Similarly, the SOH threshold below which an application deems a particular battery unsuitable is also arbitrary; a given application may accept a battery with a SOH of 50% and above, while a more critical application may only accept batteries with an SOH of 90% and above.
Figure of merit
A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application...
of the condition of a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
(or a cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...
, or a 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...
), compared to its ideal conditions. The units of SOH are percent points (100% = the battery's conditions match the battery's specifications).
Typically, a battery's SOH will be 100% at the time of manufacture and will decrease over time and use. However, a battery's performance at the time of manufacture may not meet its specifications, in which case its initial SOH will be less than 100%.
SOH evaluation
- First, a 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...
evaluates the SOH of the battery under its management and reports it. - Then, the SOH is compared to a threshold (typically done by the application in which the battery is used), to determine the suitability of the battery to a given application.
Knowing the SOH of a given battery and the SOH threshold of a given application:
- a determination can be made whether the present battery conditions make it suitable for that application
- an estimate can be made of the battery's useful lifetime in that application
Parameters
As SOH does not correspond to a particular physical quality, there is no consensus in the industry on how SOH should be determined.The designer of a battery management system
Battery Management System
A 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...
may use any of the following parameters (singly or in combination) to derive an arbitrary value for the SOH.
- Internal resistance / impedance / conductance
- Capacity
- VoltageVoltageVoltage, 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...
- Self-dischargeSelf-dischargeSelf-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes...
- Ability to accept a charge
- Number of charge–discharge cycles
In addition, the designer of the battery management system defines an arbitrary weight for each of the parameter's contribution to the SOH value. The definition of how SOH is evaluated can be a trade secret.
SOH threshold
As stated before, the method by which the battery management systemBattery Management System
A 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...
evaluates the SOH of a battery is arbitrary.
Similarly, the SOH threshold below which an application deems a particular battery unsuitable is also arbitrary; a given application may accept a battery with a SOH of 50% and above, while a more critical application may only accept batteries with an SOH of 90% and above.
See also
- 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 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...
- State of chargeState of chargeState of charge is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle , hybrid vehicle , or plug-in hybrid electric vehicle...