Suggested URL: /lithium-ion-capacitor-high-power-performance/
Meta Title: LIC High-Power Performance: ESR, Pulse Current and Thermal Design
Meta Description: Explore how ESR, pulse current, voltage drop and heat generation influence lithium-ion capacitor performance in high-power applications.
1. Introduction: Why Power Performance Matters
In many industrial systems, energy demand is not constant.
Motors, actuators, communication equipment, power conversion systems and other electrical loads can generate sudden increases in power demand. These short-duration peaks may place considerable stress on the primary power source.
A Lithium-Ion Capacitor (LIC) can be used as an energy-storage and power-buffering component to help manage these dynamic loads.
However, high-power performance cannot be evaluated by capacitance alone.
A complete assessment must consider equivalent series resistance (ESR), operating voltage, current capability, pulse duration, temperature rise and the electrical characteristics of the overall system.
2. ESR: A Critical Parameter in High-Power Applications
Equivalent Series Resistance is one of the most important parameters affecting LIC power performance.
When current flows through an LIC, its internal resistance causes a voltage drop.
A simplified relationship is:
\[ \Delta V=I\times ESR \]
where:
- \(\Delta V\) is the resistive voltage drop.
- \(I\) is the current.
- \(ESR\) is the equivalent series resistance.
For example, when a high current is drawn over a short period, a device with lower ESR generally experiences less resistive voltage drop than one with higher ESR, assuming comparable test conditions.
ESR also influences internal heat generation.
The approximate resistive power loss is:
\[ P_{\mathrm{loss}}=I_{\mathrm{RMS}}^{2}\times ESR \]
This relationship explains why even a relatively small increase in ESR can become important under high-current operating conditions.
As current increases, resistive heating rises with the square of current.
For this reason, ESR is not merely a datasheet value. It is directly connected to voltage stability, thermal behavior and power-delivery capability.
3. Understanding Pulse Current and Continuous Current
One common mistake in capacitor selection is treating maximum pulse current and continuous current as interchangeable.
They are not the same.
Continuous current
Continuous current refers to the current that a device can support under specified operating conditions without exceeding its permitted temperature rise or other performance limits.
Pulse current
Pulse current refers to a higher current that can be supported for a limited duration.
The permitted pulse duration, initial temperature, voltage condition and recovery interval all affect the result.
A device may tolerate a short current pulse but be unable to sustain the same current continuously.
For engineering evaluation, the following parameters should be defined:
- Peak current.
- Continuous current.
- Pulse duration.
- Pulse repetition frequency.
- Duty cycle.
- Initial operating temperature.
- Maximum permitted temperature rise.
Without these conditions, a maximum-current figure can be misleading.
4. Voltage Drop During High-Current Discharge
LIC voltage changes during charge and discharge.
There are two important contributors:
- The voltage change associated with the amount of charge transferred.
- The instantaneous voltage drop associated with ESR.
For an ideal capacitor under constant current:
\[ \Delta V_C=\frac{I\times \Delta t}{C} \]
where:
- \(I\) is discharge current.
- \(\Delta t\) is discharge duration.
- \(C\) is capacitance.
The practical terminal voltage also includes the resistive voltage drop.
During discharge, a simplified approximation is:
\[ V_{\mathrm{terminal}}\approx V_{\mathrm{internal}}-I\times ESR \]
This distinction matters because the voltage measured at the terminals under load may be lower than the voltage associated with the stored charge alone.
System designers must therefore evaluate both the usable voltage window and the voltage drop under the expected load profile.
5. Thermal Management: The Connection Between Current and Temperature
Heat generation is a critical factor in high-power energy storage systems.
For an LIC, resistive heating is approximately proportional to the square of RMS current.
A simplified expression is:
\[ P_{\mathrm{heat}}\approx I_{\mathrm{RMS}}^{2}\times ESR \]
The actual temperature rise also depends on:
- Cell geometry and construction.
- Thermal conductivity of the materials.
- Surface area.
- Installation orientation.
- Airflow and cooling conditions.
- Ambient temperature.
- Pulse duration and repetition rate.
A cell that performs well during a short laboratory pulse may behave differently in an enclosed industrial system with limited heat dissipation.
Therefore, power capability should be validated under application-relevant operating conditions rather than inferred from a single peak-current test.
6. How to Evaluate an LIC for High-Power Applications
A useful evaluation process should include the following steps.
Step 1: Define the load profile
Identify the actual operating current, peak demand, pulse duration and repetition frequency.
Step 2: Confirm the voltage window
Determine the minimum and maximum permitted operating voltages and the minimum voltage required by the connected equipment.
Step 3: Evaluate ESR
Check both initial ESR and its change over the intended operating life.
Step 4: Calculate expected voltage drop
Estimate the resistive drop and capacitive voltage change under the expected load.
Step 5: Evaluate thermal behavior
Measure temperature rise under representative continuous and pulsed operating conditions.
Step 6: Validate cycle performance
Test the LIC under the actual voltage window, current profile and temperature conditions expected in the application.
7. Why High Power and Long Life Must Be Considered Together
A high-power application often involves frequent charge-discharge cycles.
Although LIC technology is designed for applications requiring repeated energy exchange, cycle life is not independent of operating conditions.
High current, elevated temperature, excessive voltage and unsuitable charging conditions can accelerate performance degradation.
A meaningful cycle-life specification should always identify:
- Charge and discharge voltage limits.
- Charge and discharge current.
- Test temperature.
- Cycle definition.
- Capacitance-retention threshold.
- ESR-increase threshold.
For example, a stated cycle life of 100,000 cycles cannot be compared directly with another product rated at 100,000 cycles unless the test conditions and end-of-life criteria are comparable.
8. Conclusion
High-power LIC performance is the result of several interconnected electrical and thermal characteristics.
ESR affects voltage drop and heat generation. Capacitance determines the rate of voltage change under a given current. Pulse duration and repetition rate influence thermal accumulation. The operating voltage window determines how much energy can actually be delivered.
For equipment manufacturers and system integrators, evaluating these factors together is essential when selecting an LIC for high-power applications.
High-power performance is not defined by peak current alone. It is defined by the ability to deliver the required power within acceptable electrical, thermal and lifetime limits.
Technical references
- Influences of electrode structure on the electrical performances of lithium-ion capacitor
- Electrochemical performances and capacity fading behaviors of activated carbon/hard carbon lithium ion capacitor
- LICAP 3.8V, 2200F Lithium-Ion Capacitor Datasheet — an example of manufacturer specifications showing capacitance, ESR, continuous current and usable energy as separate parameters.