Internal resistance is the inherent electrical resistance inside a battery cell or battery pack that causes a voltage drop and heat generation when current flows. In EVs and charging systems, internal resistance directly affects charging speed, efficiency, and thermal stress, especially during high-power charging.
What Is Internal Resistance?
Every battery has internal resistance due to:
– Resistance of electrode materials and current collectors
– Ionic resistance in the electrolyte and separator
– Contact resistance at cell tabs, welds, and interconnects
– Interface effects inside the cell (electrochemical polarization)
It is often represented as a small value (e.g., milliohms) and can be described as:
– Ohmic resistance (instant voltage drop)
– Polarization resistance (voltage effects that change with time and current)
Why Internal Resistance Matters for EV Charging
Internal resistance limits how much current a battery can accept without overheating or exceeding voltage limits. Higher internal resistance leads to:
– More heat during charging and discharging (I²R losses)
– Faster voltage rise under charge current, triggering earlier power reduction
– Reduced peak charging power and a slower CC/CV charging profile transition
– Lower overall charging efficiency (more energy lost as heat)
This is one reason charging performance differs between EV models and changes over the life of the vehicle.
How Internal Resistance Affects Charging Power
During charging, the battery terminal voltage increases with current partly due to internal resistance:
– Higher current → larger voltage increase inside the battery
– If voltage approaches safety limits, the BMS reduces current
– Reduced current lowers charging power (kW), extending charging time
At low state-of-charge, many EVs can accept higher current; as the battery voltage rises and internal heating increases, charging power typically tapers.
What Influences Internal Resistance
Internal resistance is not fixed—it changes with operating conditions and battery age:
– Temperature: cold batteries typically have higher internal resistance
– State of charge (SoC): resistance can vary across SoC range
– Battery aging: resistance increases over time due to degradation mechanisms
– Cell chemistry: different chemistries have different resistance behavior
– Manufacturing and design: cell format, tabs, pack architecture, and cooling
This is why cold-weather charging often feels slower and why older batteries may charge less aggressively.
How Internal Resistance Is Measured or Estimated
Internal resistance can be assessed through:
– DC pulse tests (measuring voltage drop during a known current step)
– AC impedance methods (impedance spectroscopy)
– BMS-based estimation using operational data (current, voltage, temperature)
In EVs, the battery management system (BMS) continuously estimates internal behavior to manage safety and performance.
Practical Implications for Charging Infrastructure
While internal resistance is a battery property, it influences infrastructure expectations:
– Two vehicles on the same charger may charge at very different power levels
– Higher-power chargers do not guarantee faster charging if the battery cannot accept it
– Cold-climate sites may see more power tapering and longer dwell times
– Fleet planning should account for temperature and battery condition when scheduling charging
Related Glossary Terms
Battery Management System (BMS)
CC/CV Charging Profile
Charging Curve
Charge Tapering
Charging Efficiency
DC Fast Charging
Thermal Management
State of Charge (SoC)
Battery Degradation
Peak Charging Power