Supercapacitor has a longer working life than the secondary battery, but its working life is not infinite, supercapacitor in the form of basic failure was an increase in capacitance resistance (ESR) and (or) lower capacitance, the actual failure forms often associated with the application of the user, the long-term thermal (temperature) overvoltage (voltage), or frequent large current discharge will lead to increase or decrease the capacity of capacitance resistance. Using supercapacitors within the specified parameters can effectively extend the working life of supercapacitors. In general, supercapacitors have a similar structure to ordinary capacitors. They are sealed with liquid electrolyte in an aluminum shell. After a few years, the electrolyte will gradually dry up.
Ultracapacitors have a recommended operating voltage or optimal operating voltage, which is determined by the maximum operating time of the capacitor at the maximum setting temperature. If the applied voltage is higher than the recommended voltage, the life of the capacitor will be shortened. If the voltage is too high for a long time, the electrolyte inside the capacitor will decompose into gas, and when the gas pressure gradually increases, the safety hole of the capacitor will break or break through. Short periods of over voltage are tolerable for capacitors.
Supercapacitors are designed with symmetrical electrodes, which means they have a similar structure. When the capacitor is assembled for the first time, each electrode can be treated as a positive or negative electrode. Once the capacitor is 100% charged for the first time, the capacitor will become polar. On the shell of each super capacitor, there is a negative electrode mark or logo. Although they can be short-circuited to reduce the voltage to zero volts, the electrodes retain very little charge and changing polarity is not recommended. The longer it takes for capacitors to be charged in one direction, the stronger their polarity becomes. If a capacitor changes polarity after being charged in one direction for a long time, the life of the capacitors will be shortened.
Super capacitor , is the normal operating temperature -40 ℃ ~ +70 ℃, the combination of temperature and voltage is one of the important factors affecting the working life of the super capacitor. Under normal circumstances, the super capacitor each elevated temperature is 10 ℃, the life of the capacitor will be reduced by 30% ~ 50%, also said that, if possible, try to reduce the use of super capacitor temperature, in order to lower the attenuation of the capacitance and resistance increases, if impossible to reduce the temperature, so can reduce the voltage to against high temperature and the negative effect on the capacitor. For example, reduced to 1.8 V, if the working voltage of capacitor capacitance can work under 65 ℃ high temperature. If the ultracapacitor is used under room temperature, the ultracapacitor can work higher than the specified voltage without accelerating the degradation inside the ultracapacitor and affecting the service life of the ultracapacitor. The increase of the working voltage of the ultracapacitor at low temperature can effectively offset the increase of the internal resistance of the ultracapacitor at low temperature. Under the condition of high temperature, capacitance resistance increases, the change is permanent and irreversible (electrolyte decomposition), under low temperature, the capacitance is a temporary phenomenon, the rise of resistance because of low temperature, the electrolyte is sticky increases, reduces the movement of ions.
When a supercapacitor discharges, it discharges along a slope curve. When an application has defined the capacitance and internal resistance requirements, it is important to understand the effect of resistance and capacitance on discharge characteristics. Resistance is the most important factor in pulse applications and capacity is the most important factor in low current applications. The calculation formula is as follows:
Vdrop = I (R + t/C)
Where Vdrop is the difference between the initial working voltage and the cut-off working voltage, I is the discharge current, R is the capacitance and the dc internal resistance, t is the discharge time, and C is the capacitance capacity
In pulse applications, due to the large instantaneous current, in order to reduce voltage sag, low internal resistance (ESR) ultracapacitor (R value) is selected. In low-current applications, in order to reduce voltage sag, high-capacity ultracapacitor (C value) is required.
Supercapacitors can be charged in a variety of ways, such as constant current, constant power and constant voltage. Or in conjunction with a power source, such as a battery, fuel cell, DC converter, etc. If a capacitor is connected in parallel with a battery, a resistor in series in the capacitor loop will reduce the capacitor's charging current and improve the battery's life. If the resistance is in series, make sure that the voltage output of the capacitor is directly connected to the load without passing through the resistance, otherwise the capacitor's low resistance characteristic will be invalid. Many battery systems do not allow for instantaneous large current discharge, which would affect the battery life. The recommended charging current for a maximum capacitor is calculated as follows: I=Vw/5R
Where I is the recommended maximum charging current, Vw is the charging voltage, and R is the internal dc resistance of the capacitor.
The capacitor is continuously charged with high current or overvoltage. Will cause the capacitor to heat up, overheating will lead to an increase in the internal resistance of the capacitor, electrolyte decomposition to produce gas, shortened life, increased leakage current or capacitor rupture.
Self Discharge and Leakage Current
Self-discharge and the leakage is essentially the same, in view of the super capacitor structure, equivalent to inside the capacitance between the positive and negative has a high resistance current path, it means at the time of capacitor charging, there will be an additional additional current at the same time, when is in charge, we can see this current as leakage current; When the charging voltage is removed and the capacitor is not connected to the load, this current makes the capacitor in the state of discharge. At this time, we consider this current as the self-discharge current.
In order to reliably measure leakage current or discharge current, the capacitance must be continuously charged for more than 72 hours, again depending on the structure of the capacitance. Super capacitor is a few models can be as different internal resistance of super capacitor in parallel, when charging, low internal resistance of the super capacitor charging speed, voltage up to and soon charging voltage are equal, when the charging voltage after removing, if high internal resistance of the super capacitor has not been filled with low internal resistance of super high internal resistance super capacitance shunting capacitor began to discharge, this will more quickly at the ends of the capacitor voltage drop, the impression is capacitance has larger self-discharge, it is important to note: when the capacitance, the greater the capacitance is filled with the longer the time required would be.
Series Configurations of Super capacitors
Monomer super capacitor voltage is 2.5 V or 2.7 V, commonly in many applications, the need to high voltage, so that can use a series of methods to improve the voltage of capacitor, must pay attention to, in the applications of series, every single capacitance cannot exceed the maximum pressure, once the over voltage for a long time, will cause the capacitor electrolyte decomposition, gases, life expectancy increased internal resistance and capacitance.
When discharging or charging, the difference in capacitance or the difference in leakage current under the stable state will lead to unbalanced partial voltage of the series capacitor. During charging, the capacitors in series will be divided into voltages, so that high-capacity capacitors will bear greater voltage pressure. For example, if two capacitors of 1F are connected in series, one is only +20% capacitance deviation and the other is only -20% capacitance deviation, the partial voltage of the capacitance is as follows:
Vcap1=Vsupply [Ccap1/(Ccap1+ Ccap2)] where Vcap1 is a capacitor with a capacitance deviation of +20% if the charging voltage is 5V
Vcap1 = 5 v x 1.2 / (1.2 + 0.8) = 3 v
As can be seen from the above equation, if the partial voltage is larger than the peak voltage of the capacitor, 3V, then the capacitance error must be within the same trend range, such as both +20% error or both -20% error. In addition, an active voltage balance circuit can be used to compensate for the voltage imbalance caused by capacitance mismatch.
Passive Voltage Balancing
A passive voltage balancing circuit USES a resistor in parallel with the capacitor to divide the voltage, which allows the current to flow from the higher voltage capacitor to the lower voltage capacitor, thus balancing the voltage in this way. It is very important to select the resistance value of the resistor so that the current allowed by the resistor is greater than the expected leakage current of the capacitor. It is important to remember that leakage current usually increases as the temperature increases.
Only in passive balance circuit is not frequently used in the application of capacitor charge, can tolerate balance resistance caused by the additional current at the same time, suggest choosing balance resistance tolerance, make balance resistance of more than 50 times greater than the capacitor leakage current, (balance resistance value is 3.3 K Ω - 22 K Ω, depends on the capacitance of the maximum operating temperature), although most balance balance circuit is relatively high resistance, but when the series capacitor is not matching, adequate protection is not enough.
Active Voltage Balancing
Active force balance circuit in series with the node voltage is consistent with the reference voltage, no matter how much voltage imbalance, in ensuring the accurate voltage balance at the same time, actively balancing circuit under steady state is only a very low current, only when the voltage is beyond the scope balance, will produce larger current, these features make the active balancing circuit is very suitable for need frequent charging and discharging.
Reverse Voltage Protection
When supercapacitors in series are charged quickly, the low capacitance voltage may turn into the reverse polarity, which is not allowed and will reduce the service life of the capacitor. A simple solution is to connect a diode at both ends of the capacitor. Replace standard diodes with a suitable zener regulated diode to protect against capacitor overvoltage. Note that the diode must be able to withstand the peak current of the power supply.
Although ultracapacitors have a relatively low internal resistance, compared with electrolytic capacitors, their internal resistance is relatively large, which is likely to cause internal heating when applied to pulsating current. As a result, the electrolyte inside the capacitor will decompose, the internal resistance will increase, and the capacitor life will be shortened. In order to guarantee the life of the capacitor, when applied to pulse occasions, best to ensure that the capacitor surface temperature rise of less than 5 ℃.
1.The above information is for reference only. If you have any questions, you can consult the HCCCap Technology Department.
2.Data parameters are for reference only. The actual product parameters produced in different batches and at different times may change. Subject to the product actually received.The exact parameters should be checked with the manufacturer in time.