How to Measure an Ultracapacitor’s State of Charge & State of Health

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.

13 comments on “How to Measure an Ultracapacitor’s State of Charge & State of Health”

1. SunitaT
June 26, 2014

@Mike, please elaborate more on state of charge and state of health of the battery.

How different are ultracapacitors compared to conevntional capacitors?

2. RedDerek
June 26, 2014

@SunitaT0 – Ultracaps are typically low voltage, but with capacitances in the 10's to 100's times higher than the conventional caps. Ultracaps also tend to have higher charge and discharge current capabilities.

For some electric vehicles, they use ultracaps to provide that fast quick boost in acceleration. I believe Tesla uses them to supplement the battery power.

3. Netcrawl
June 26, 2014

@redDerek I agree with you about ultracapcitors, they have these unique characteristics when compared to other energy storage devices, they are designed with a very low equivalent series resistance, which allow them to deliver and absorb very high current. The low ESR capability of ultracapacitors allows them to charged quickly, which make them well-suited for other quick-charge scenarios and regenrative braking applications.

4. Netcrawl
June 26, 2014

@SunitaT0 ultarcapacitors are electrochemicals capacitors that have a very high energy density when compared to conventional capacitors, they are able to hold hundred of times the amount of electrical charge as conventional capacitors, making them a suitable replacement for electrochemical batteries in most industrial applications. While they're able to store much more energy than conventional capacitors, they're limited in their ability to withstand high voltage- around 5 volts.

5. samicksha
June 27, 2014

Considering a situation where energy supply vary, measurement of buffered energy by using the supercapacitor terminal voltage is not up to the mark or accurate as this does not fully comprehend the physical state of charge.

6. SunitaT
June 27, 2014

While they're able to store much more energy than conventional capacitors, they're limited in their ability to withstand high voltage- around 5 volts.

In order to use ultracapacitors at higher voltages, they must be connected in series to add up their voltage ratings. When multiple capacitors of any type are connected in series, special precautions are required to equalize the voltage across the individual components. Otherwise, one of them might contribute more than its share of the voltage and burn out as a result.

7. SunitaT
June 27, 2014

Ultracapacitors being electrochemial capacitors will there be any dielectric and conducting plates concepts involved?

Capacitance of conventional capacitors depend on dielectric constant, distance between plates and surface area of conducting plates. How are these features of conventional capacitors designed in ultracapacitors or they are designed totally with different concepts?

8. geek
June 29, 2014

“The low ESR capability of ultracapacitors allows them to charged quickly, which make them well-suited for other quick-charge scenarios and regenrative braking applications. “

@Netcrawl: Any particular specific applications that ultracapacitors are most suitable for? Also, what about their life span? Is it the same as a normal capacitor? Essentially, the maintenance costs are also important to consider.

9. Netcrawl
June 29, 2014

@tzubair they're typically used in applications where batteries have a short fall when it comes to high power and life span, ultracapcitors simply do not have the hard end life failure, they're end of life is defined as when ESR has degraded beyond application needs.

10. samicksha
June 30, 2014

Yes i agree on this, compared to batteries, they have a low internal resistance, providing high power density capability and high current charging and discharging is achievable without any damage to the parts.

11. SunitaT
June 30, 2014

Cold engine start in automative, pulse power generation in Radar, transmission lines to absorb power surges are some of the applications of ultracapacitors.

12. yalanand
August 31, 2014

How different are ultracapacitors compared to conevntional capacitors?

@SunitaT0,The battery is for energy and long term voltage stability whereas the ultracapacitor is for short term, high power bursts.

13. yalanand
August 31, 2014

Any particular specific applications that ultracapacitors are most suitable for?

@tzubair, Ultracapacitors can be used in all the application requiring a short duration power boost. They are largely used in militarly applications.

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