Battery Internal Resistance Calculator
Calculate the internal resistance of a battery based on voltage measurements under different loads, and analyze its impact on performance and efficiency.
Battery equivalent circuit with internal resistance
Internal Resistance Results
What is Battery Internal Resistance?
Battery internal resistance is a measure of the opposition to current flow within the battery itself. It represents the combined effect of various factors that impede current flow, such as electrode resistance, electrolyte resistance, and polarization effects.
Internal resistance is a critical parameter in battery performance analysis, as it affects voltage drop under load, power loss, efficiency, and the battery's ability to deliver high currents.
Internal Resistance Formula
Internal Resistance (R) = (Open-Circuit Voltage - Load Voltage) / Load Current
R = (Voc - Vload) / I
Power Loss = I² × R
Efficiency (%) = (Vload / Voc) × 100
Measurement Method
To accurately measure internal resistance:
- Measure the battery voltage at rest (no load connected) to get the open-circuit voltage
- Connect a known load to the battery and allow the voltage to stabilize
- Measure the battery voltage under load and the current being drawn
- Calculate the internal resistance using the formula above
For more accurate results, use a load that draws a significant current (at least 20% of the battery's rated capacity) to ensure a measurable voltage drop.
- Internal resistance is not a constant value and varies with state of charge, temperature, age, and discharge rate
- For accurate measurements, the battery should be at a stable temperature and in a steady state
- The load should be applied long enough for the voltage to stabilize before taking measurements
- This calculation assumes a simple linear model of internal resistance, which is an approximation
- For very precise measurements, specialized equipment like battery analyzers or impedance spectroscopy may be required
Typical Internal Resistance Values
Internal resistance varies widely depending on battery chemistry, capacity, design, and condition
| Battery Type | Typical Internal Resistance | Notes |
|---|---|---|
| Lithium-ion 18650 cell | 20-60 mΩ | Higher capacity cells typically have lower resistance |
| LiFePO4 (lithium iron phosphate) | 5-20 mΩ | Excellent for high-current applications |
| Lead-acid (12V car battery) | 3-10 mΩ | Designed for high starting currents |
| Lead-acid (12V deep cycle) | 15-30 mΩ | Optimized for energy density over power density |
| NiMH AA cell | 20-150 mΩ | Varies widely by capacity and design |
| Alkaline AA cell | 150-300 mΩ | Not suitable for high-current applications |
Internal resistance typically increases by 25-100% before a battery is considered end-of-life, depending on the application requirements.
Factors Affecting Internal Resistance
State of Charge (SoC)
Internal resistance typically increases at very low and very high states of charge. The lowest resistance is usually found in the middle SoC range (30-70%).
Temperature
Temperature has a significant impact on internal resistance:
- Cold temperatures increase internal resistance substantially
- At 0°C (32°F), internal resistance can be 2-3 times higher than at room temperature
- Warm temperatures (20-30°C) generally provide the lowest internal resistance
- Very high temperatures temporarily reduce resistance but accelerate degradation
Age and Cycle Life
As batteries age and undergo charge-discharge cycles, internal resistance gradually increases due to:
- Growth of the solid electrolyte interphase (SEI) layer in lithium-ion batteries
- Loss of active material
- Electrode degradation
- Electrolyte deterioration
- Physical changes like corrosion and sulfation in lead-acid batteries
Frequently Asked Questions
What is battery internal resistance?
Battery internal resistance is the opposition to current flow within the battery itself, caused by electrochemical factors and physical limitations. It's represented as an electrical resistance in ohms (Ω). Internal resistance causes voltage drop under load, power loss as heat, and reduced efficiency. It's not a fixed value but varies with state of charge, temperature, age, and discharge rate.
How is battery internal resistance calculated?
Battery internal resistance is calculated using Ohm's Law based on the voltage drop under load: Internal Resistance (Ω) = (Open-Circuit Voltage - Load Voltage) / Load Current. For example, if a battery has an open-circuit voltage of 12.6V, drops to 12.0V when supplying 20A, the internal resistance is: (12.6V - 12.0V) / 20A = 0.03Ω.
How does internal resistance affect battery performance?
Internal resistance significantly impacts battery performance: 1) Voltage drop - Higher internal resistance causes greater voltage sag under load, 2) Power loss - Energy is wasted as heat (P = I²R), reducing runtime, 3) Efficiency reduction - Less of the stored energy is delivered to the load, 4) Capacity reduction - Effective capacity decreases, especially at high discharge rates, and 5) Peak current limitation - Maximum deliverable current is restricted.
What is considered a good internal resistance value?
Good internal resistance values vary by battery chemistry and size: Lithium-ion cells (18650): 20-60 mΩ when new, LiFePO4 cells: 5-20 mΩ for high-performance cells, Lead-acid batteries (12V): 3-10 mΩ for automotive batteries, 15-30 mΩ for deep cycle, NiMH AA cells: 20-150 mΩ depending on capacity and design. Generally, lower values indicate better performance.
How does temperature affect battery internal resistance?
Temperature significantly affects battery internal resistance: Cold temperatures increase internal resistance - At 0°C (32°F), resistance can be 2-3 times higher than at room temperature, severely limiting performance. Moderate warmth decreases resistance - Batteries often perform best around 20-30°C (68-86°F). High temperatures temporarily lower resistance but accelerate degradation - Operation above 40°C (104°F) can cause permanent damage.
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