🔋 Internal Resistance Calculator
Calculate internal resistance of batteries and voltage sources
Open-circuit voltage (no-load voltage)
Voltage across load when current is flowing
Current flowing through the circuit
How to Use This Calculator
Enter EMF (Electromotive Force)
Input the open-circuit voltage (EMF) of the battery or voltage source in Volts. This is measured when no current flows (no load connected).
Enter Load Voltage
Input the voltage across the load when current is flowing, in Volts. This is measured when the battery is connected to a load and current is flowing.
Enter Current
Input the current flowing through the circuit in Amperes. This current causes the voltage drop due to internal resistance.
Calculate
Click the "Calculate Internal Resistance" button to get the internal resistance in Ohms. The calculator also shows the voltage drop across the internal resistance.
Formula
r = (EMF - V) / I
Where:
- r = Internal Resistance (Ohms, Ω)
- EMF = Electromotive Force = Open-circuit voltage (Volts, V)
- V = Load Voltage = Terminal voltage under load (Volts, V)
- I = Current (Amperes, A)
Equivalent Circuit:
V = EMF - I×r
The terminal voltage equals EMF minus the voltage drop across internal resistance.
Example Calculation:
For a battery with EMF = 12 V, load voltage = 11.5 V, current = 5 A:
r = (12 - 11.5) / 5
r = 0.5 / 5 = 0.1 Ω
Note: Internal resistance causes voltage to drop as current increases. Higher current means more voltage drop, resulting in lower terminal voltage.
About Internal Resistance Calculator
The Internal Resistance Calculator determines the internal resistance of a battery or voltage source by measuring the voltage drop that occurs when current flows. All real voltage sources have internal resistance, which causes the terminal voltage to drop below the EMF when delivering current. Understanding internal resistance is crucial for battery performance analysis and circuit design.
When to Use This Calculator
- Battery Testing: Determine internal resistance to assess battery health and performance
- Circuit Design: Account for internal resistance in voltage source modeling
- Power Systems: Calculate voltage regulation and efficiency losses
- Electronics: Analyze voltage source behavior under load
- Battery Diagnostics: Identify degraded batteries with high internal resistance
Why Use Our Calculator?
- ✅ Quick Calculation: Instantly determine internal resistance from voltage and current measurements
- ✅ Battery Health: Essential for evaluating battery condition and remaining capacity
- ✅ Circuit Analysis: Important for accurate circuit modeling and design
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Learn about voltage sources and their limitations
Common Applications
Battery Health Assessment: Measure internal resistance to evaluate battery condition. As batteries age, internal resistance increases, causing greater voltage drop under load. A battery with significantly increased internal resistance may need replacement, even if its open-circuit voltage appears normal.
Voltage Regulation Analysis: Understand why terminal voltage drops when load current increases. Internal resistance limits the maximum current a battery can deliver and causes voltage regulation issues in power supply systems. Lower internal resistance means better voltage regulation and higher efficiency.
Circuit Design: Model voltage sources accurately in circuit analysis. Accounting for internal resistance helps predict actual terminal voltage under load, ensuring circuits operate correctly. This is especially important for low-voltage circuits where voltage drops matter significantly.
Tips for Best Results
- Internal resistance typically increases as batteries age or discharge
- Lower internal resistance means better performance (less voltage drop)
- Measure EMF when no current flows (open circuit)
- For accurate measurements, use consistent measurement conditions
- Temperature affects internal resistance - measure at consistent temperature
Frequently Asked Questions
What is internal resistance?
Internal resistance is the resistance inside a battery or voltage source that causes voltage drop when current flows. It represents resistance from electrodes, electrolyte, and connections. When current flows, some voltage is "lost" across this internal resistance, reducing terminal voltage below EMF.
Why does voltage drop when current increases?
According to Ohm's law (V = I×r), voltage drop across internal resistance increases with current. Since terminal voltage V = EMF - I×r, higher current means more voltage drop, resulting in lower terminal voltage. This is why batteries can't maintain their open-circuit voltage under heavy loads.
What is a typical internal resistance value?
Values vary widely: small batteries (AA, AAA) might have 0.1-0.3 Ω, car batteries 0.01-0.05 Ω, lithium-ion cells 0.02-0.1 Ω. Internal resistance increases as batteries age, discharge, or operate at extreme temperatures. Lower is better - it means less voltage drop and better performance.
How does internal resistance affect battery performance?
Higher internal resistance causes more voltage drop under load, reducing available power and efficiency. It limits maximum current capability and causes batteries to heat up more under load. Batteries with high internal resistance may appear fully charged but fail to deliver power effectively.
Can internal resistance be measured with a multimeter?
Not directly. You need to measure both open-circuit voltage (EMF) and load voltage under known current. Some advanced multimeters have internal resistance measurement modes. The method used here (measuring voltage drop with known current) is the standard approach.
Does internal resistance change with battery state?
Yes, internal resistance increases as batteries discharge, age, or operate at extreme temperatures. It's lowest when fully charged and at moderate temperature. Measuring internal resistance is a way to assess battery health - significant increases indicate degradation or aging.