The use of electrochemical energy storage in high-reliability applications, such as those supporting the critical societal needs for reduction in carbon dioxide emissions or stabilizing the aging power grid, generally requires either real-time direct knowledge or at the very least, predicated state of the health of the energy storage device. This is especially important for planning for replacement or service before a failure of the device can occur. These failures can be either due to natural causes or to some anomalous condition, such as manufacturing defects or operational stresses. State of charge is also important, as it determines if the energy storage system can be relied upon to execute its intended function.
To determine the state of health and state of charge of an ultracapacitor, one needs relatively little information, and that information is readily obtained during use of the device. To determine the state of charge of the device, one simply has to measure the voltage on the device. By comparing the voltage to the operating voltage range of the device, one can know how much discharge or charge voltage is left until the upper or lower limit is reached. Knowing the voltage can indicate how much energy is left in the device if one knows the capacitance. This holds true for single cells or a string of cells, as well. Once the capacitance and voltage are known, then the state of charge in terms of total energy in the device is determined by the straightforward equation:
E = ½*C*V2 , where:
E is the energy in the device in joules
C is the capacitance of the device in farads
V is the voltage of the device in volts
To determine the energy in the device between two specific operating voltages, the equation is modified to reflect the range of voltage:
E = ½*C*(V2 2 – V1 2 ), where:
V2 is the starting voltage of the discharge
V1 is the ending voltage of the discharge
The value of energy in joules in either case is the total energy contained in the device or the string of devices. However, it is not the usable energy by the application because one must account for the resistance losses of the devices and the system in order to understand how much energy is available for use.
To determine the capacitance of capacitive energy storage devices, one can use the following equation:
I = C*(dV/dt), where:
I is the constant current during a constant current discharge or charge
C is the capacitance to be determined
dV is the change in voltage during the charge or discharge
dt is the change in time of the charge or discharge
So it is simple to determine capacitance by measuring the current, time, and change in voltage of a constant current discharge.
The capacitance is a direct measure of the state of health of the device and one can quickly determine how much the cell has aged and how much lifetime is remaining in the device when it is used under conditions similar to those that have been used up to the point the measurements were made.
Stay tuned for my next blog post, in which I will further explore state of health and state of charge for battery devices. In the meantime, comment below with any questions.