A constant current discharge test can be useful for customer evaluation of an ultracapacitor module prior to application testing. Please note that, for safety reasons, Tecate stores all of its ultracapacitor cells and modules in a discharged state and recommends that customers completely discharge any ultracapacitors that will not be immediately installed into equipment.
- Ultracapacitor module
- Bidirectional power supply (supply/load) or separate power supply and programmable load (capable of providing a constant current)
- Voltage vs. time measurement/recording device (digital scope or other data acquisition)
- Current vs. time measurement/recording device (optional if the power supply and load settings are trustworthy)
Before testing, connect the data-acquisition equipment to the device terminals and set recording speeds as fast as reasonably possible (<<100msec preferred). The faster the recording speeds, the more accurate the calculations will be.
- Set the power supply to the appropriate voltage and current limits, and turn the supply output off. The current limit can be anything at or less than the maximum rated current for the lowest-rated cell in the series. The voltage limit is the maximum cell voltage times the number of cells in the series. Each cell's voltage should be limited to the maximum rated voltage for the lowest-rated cell in the series.
- Having preset the current and voltage limits, connect the ultracapacitor module to the power supply. Cooling air may be required to keep the module within operating temperature limits, depending on the test current and duration.
- Connect the voltage and current measuring/recording devices.
- With the power supply preset and the ultracapacitor module connected, turn the supply output on.
- Charge the module at the appropriate current to the appropriate voltage.
Note: If you are using a separate programmable load instead of an integrated bidirectional power supply, disconnect the charging power supply prior to discharging. Do not simply turn it off or change its set points, as many supplies will sink current when not regulating.
- Set the load to the appropriate constant current, and discharge the module to 0.1V, or as low as the load can be controlled. Depending on the equipment, some modules can be discharged to 0.1V and others to half of their initial voltage. Capacitance values will be slightly higher when the modules are discharged to half of their initial voltage rather than 0.1V.
- Immediately remove the load once the minimum voltage is reached, allowing the module's voltage to bounce back.
- Measure the following parameters:
- Vw (inital working voltage)
- Vmin (minimum voltage under load)
- Id (discharge current)
- Vf (voltage 5 seconds after removal of load)
- td (time to discharge from initial voltage to minimum voltage)
- Calculate the capacitance according to the following formula: Capacitance = (Id * td)/(Vw – Vf) (This change in voltage (Vw – Vf) is used because it eliminates the voltage drop due to the equivalent series resistance (ESR).)
- Calculate the ESR according to the following formula: ESR = (Vf – Vmin)/Id (An LCR meter or bridge can be used to measure ESR at higher frequencies. The ESR at frequencies up to 100Hz will typically be 50-60% of the DC ESR. The capacitance will be much lower, due to the structure of the electrode.)
Representative Measurement Points for Constant Current Test
As with all electrical testing, the investigator should take appropriate precautions in the design and execution of the test. Proper precautions for the voltage should be observed. Any interconnections should be sized for the maximum anticipated current and insulated for the appropriate voltage. If repeated testing will be performed, cooling air may be required to keep the test module within its operating temperature range.