News
What Is Internal Resistance in Batteries? How It Impacts Efficiency, Performance, and Lifetime
- November 17, 2025
Battery performance is determined by many factors, but Internal Resistance (IR) is one of the most critical and least understood.
For Energy Storage Systems (ESS), IR directly affects efficiency, available power, heat generation, cycle life, and the system’s long-term economic value.
Understanding how IR works—and how it changes over time—is essential for designing, operating, and maintaining a high-performance and safe ESS.
What Is Internal Resistance?
Internal Resistance is the inherent opposition inside a battery that resists the flow of current.
It has two major components:
(1) Ohmic Resistance
From current collectors, electrolyte, separators, and connectors
Causes instant voltage drop (IR drop) during charge/discharge
(2) Polarization Resistance
From electrochemical reactions
Includes:
Charge-transfer resistance
Diffusion resistance
SEI (Solid Electrolyte Interphase) resistance
Total IR = Ohmic Resistance + Polarization Resistance
When current flows through the battery, internal resistance converts some electrical energy into heat:
Heat = I² × R
This is why high IR leads to more heat, lower efficiency, and faster aging.
How Internal Resistance Affects ESS Performance
(1) Lower Efficiency (More Losses)
Higher IR means more electrical energy is lost as heat.
For ESS operation, this results in:
Lower Round-Trip Efficiency (RTE)
More energy wasted during charge/discharge
Higher long-term operational cost
(2) Reduced Power Output
High IR leads to:
Increased voltage drop
Reduced usable voltage
Restricted charge/discharge current
This forces PCS and BMS to limit power to prevent overheating.
The result: the ESS cannot deliver its rated power when IR becomes too high.
(3) Increased Heat and Safety Risks
High IR → Higher heat → Faster aging → Even higher IR
This positive feedback loop can lead to:
Accelerated SEI growth
Lithium plating at low temperatures
Increased risk of thermal incidents
This is why IR is a key parameter monitored by modern BMS.
(4) Shorter Cycle Life
As IR increases:
Capacity decreases
Power capability drops
Cell inconsistency worsens
The system ages faster and becomes less dependable.
What Causes Internal Resistance to Increase?
(1) Natural Aging
Electrode changes and SEI growth are unavoidable.
(2) High Temperature
Chemical aging nearly doubles for every 10°C rise.
(3) High C-rate Charging/Discharging
Accelerates polarization resistance and heat generation.
(4) Low Temperature Operation
Causes lithium plating, which sharply increases IR.
(5) Cell Inconsistency
Weak cells force pack-level derating.
(6) Poor Thermal Management
Hot spots create uneven IR growth and imbalance.
How BMS and AI Monitor Internal Resistance
Modern ESS platforms continuously track IR to ensure safety and performance.
Common measurement methods:
DCIR (Direct Current Internal Resistance)
ACIR (Alternating Current Internal Resistance)
Pulse current testing
Machine learning models for SOH estimation
AI-enabled cloud EMS can:
Detect abnormal IR increase early
Predict future degradation
Trigger maintenance warnings
Extend system lifetime and protect ROI
How to Maintain Low Internal Resistance
For ESS Operators
Maintain 20–35°C operating temperature
Avoid unnecessary high C-rate cycles
Avoid charging below 0°C
Use high-quality LFP cells
Keep SOC in a moderate range (10–90%)
Ensure proper EMS/PCS tuning
For System Designers
Liquid cooling for temperature uniformity
High-quality busbars and connectors
Advanced BMS for precise balancing
AI-based strategies to optimize cycling
Conclusion
Internal resistance is a fundamental metric that determines the efficiency, safety, and lifetime of any energy storage system.
Low IR provides:
Higher power output
Higher energy efficiency
Lower heat generation
Longer cycle life
Improved economic returns
Monitoring and controlling IR is essential for achieving safe, reliable, and high-value ESS operation—especially as global energy storage deployment accelerates.