Internal Resistance in Lithium Batteries Explained
Internal resistance is one of the most important but often misunderstood characteristics of a lithium battery. It directly affects how the battery performs under load, how efficiently it operates, and how much usable power it can deliver.
Understanding internal resistance helps explain common issues such as voltage drop, reduced performance under load, and differences between battery quality levels.
This guide explains what internal resistance is, how it works, and why it matters in real-world applications.
What is Internal Resistance?
Internal resistance is the natural resistance to electrical flow within a battery.
All batteries have some level of internal resistance due to the materials and chemical processes involved in storing and releasing energy.
When current flows through the battery, this resistance causes a loss of energy in the form of heat and results in a drop in voltage.
How Internal Resistance Affects Performance
Internal resistance becomes most noticeable when the battery is under load.
As current increases, the voltage drop caused by internal resistance also increases.
This can lead to:
- Reduced voltage at the output
- Lower available power
- Increased heat generation
- Reduced efficiency
In high-demand applications, this effect can significantly impact system performance.
Voltage Sag Explained
Voltage sag refers to the drop in voltage that occurs when a battery is placed under load.
This is largely caused by internal resistance.
The higher the internal resistance, the greater the voltage drop under the same load conditions.
This is why some batteries appear to perform well at low loads but struggle under higher demand.
Why Low Internal Resistance is Important
Lower internal resistance allows a battery to deliver higher current more efficiently.
This results in:
- Better performance under load
- More stable voltage output
- Reduced heat generation
- Improved overall efficiency
This is particularly important for applications such as inverters, mobility systems, and high-power equipment.
What Causes Internal Resistance to Increase?
Internal resistance is not constant and can change over time.
Factors that increase internal resistance include:
- Ageing of the battery
- High operating temperatures
- Deep discharge cycles
- Poor cell quality or manufacturing
As internal resistance increases, performance gradually declines.
Internal Resistance and Battery Ageing
As a battery ages, internal resistance naturally increases.
This is one of the key reasons why older batteries struggle to deliver the same performance as new ones.
Even if the battery still holds charge, increased resistance can limit its ability to deliver power effectively.
Real-World Example
In a system running an inverter, a battery with high internal resistance may experience significant voltage sag under load.
This can cause the inverter to shut down, often due to BMS protection under load, even if the battery still has available capacity.
This is often mistaken for a capacity issue, when in reality it is a performance limitation caused by resistance.
How Internal Resistance Relates to Cable Losses
Internal resistance within the battery is only one part of the overall system resistance.
Cables, connectors, and terminations also contribute to total resistance and voltage drop.
This is why proper cable sizing and system design are critical for maintaining performance.
What to Look For in a High-Quality Battery
A well-designed lithium battery will have low and stable internal resistance.
This depends on:
- Cell quality and chemistry
- BMS design and control
- Manufacturing standards
Lower resistance is typically associated with higher performance and better reliability.
Conclusion
Internal resistance plays a critical role in how a lithium battery performs in real-world applications.
It affects voltage stability, power delivery, efficiency, and long-term performance.
Understanding this concept helps explain why two batteries with similar specifications can behave very differently under load.