Battery internal resistance is the opposition within a battery that restricts the flow of electric current, leading to energy loss and reduced performance.
Did you know that even a fully charged battery can fail to power your device properly if it has high internal resistance? While often overlooked, internal resistance is a key factor that influences a battery’s efficiency, lifespan, and safety. It determines how much energy is wasted as heat and how effectively the battery can deliver current when demanded.
Whether you’re a DIY electronics enthusiast, a professional engineer, or just trying to figure out why your car won’t start despite a “full” battery, understanding internal resistance is crucial.
Top 3 Battery Internal Resistance Testers for Accurate Readings
When it comes to measuring battery internal resistance, using a reliable and accurate tester is key. Here are three of the best on the market:
Hioki BT3554-01 Battery Tester
Why we picked it: Offers ultra-fast, accurate resistance and voltage readings with Bluetooth data transfer and built-in comparator functionality for on-site diagnostics.
Fluke BT521 Battery Analyzer
Why we picked it: Professional-grade tool with advanced diagnostics, data logging, and a guided user interface ideal for stationary battery systems and industrial applications.
Aitelong AT528 Battery Internal Resistance Tester
Why we picked it: Affordable yet dependable option for hobbyists and field technicians, offering real-time resistance and voltage readings with easy-to-read LCD display.
What Is Battery Internal Resistance?
Battery internal resistance refers to the inherent opposition to current flow within a battery. It’s caused by the battery’s materials, construction, and chemical composition, and it directly affects how efficiently the battery can deliver power.
When current flows through a battery, some energy is inevitably lost as heat due to this resistance. This means less energy reaches your device. The internal resistance is typically measured in milliohms (mΩ), and even small increases can have a big impact, especially in high-performance or sensitive applications like electric vehicles or medical devices.
There are two main types of internal resistance:
- Ohmic Resistance: Caused by the resistance of the battery’s electrodes, electrolyte, and connectors.
- Electrochemical Resistance: Arises from chemical reactions inside the battery and varies depending on the state of charge and temperature.
Key Factors That Influence Internal Resistance
- Battery Type (Li-ion, NiMH, Lead-acid, etc.)
- Age and Usage History
- Temperature (resistance typically increases in cold conditions)
- State of Charge (partially charged batteries often show higher resistance)
Why It Matters
- Heat Generation: Higher resistance causes more heat, which can degrade the battery faster.
- Voltage Drop: Results in reduced performance under load.
- Shortened Lifespan: Increased resistance is a key sign of battery aging or failure.
Pro tip: A brand-new battery should have very low internal resistance. Regularly checking it helps in predicting battery health over time.
How Internal Resistance Affects Battery Performance
Internal resistance may be invisible, but its impact on battery performance is undeniable. From power delivery to lifespan, it plays a central role in how well a battery can do its job.
Power Output and Efficiency
The higher the internal resistance, the more voltage is lost internally when the battery is under load. This means less voltage reaches the device, leading to poor performance or even malfunction. For example, a battery with just a few extra milliohms of resistance might cause LED lights to dim or power tools to underperform.
- Formula in action:
Voltage drop = Current × Internal Resistance
If current demand is high, even a small resistance leads to significant energy loss.
Battery Heating
High resistance causes heat build-up, especially under load. This not only wastes energy but can also damage the internal structure of the battery over time.
- Thermal runaway risk: In lithium-ion batteries, excessive heat may trigger dangerous chain reactions, leading to swelling, leaks, or even fire.
Capacity and Runtime Reduction
As internal resistance increases with age, the effective capacity of the battery drops. You might notice your phone dies quicker or your cordless drill loses torque faster, even if the battery is fully charged.
Impact on Charging
Chargers compensate for internal resistance by adjusting voltage and current. High resistance can confuse charging systems, causing undercharging, overheating, or failed charging cycles.
Bottom line: Internal resistance is a silent performance killer—monitoring and minimizing it is essential for getting the most out of your battery.
What Causes Internal Resistance in Batteries?
Internal resistance isn’t the result of just one component—it’s a combination of chemical, physical, and environmental factors inside the battery. Understanding these root causes helps diagnose performance issues and choose the right battery for specific applications.
Material Composition
The type of materials used in a battery’s electrodes and electrolyte largely determines its resistance.
- High-purity metals like copper and aluminum offer low resistance, while cheaper alloys may increase it.
- Electrolyte viscosity also matters—thicker electrolytes slow ion movement, raising resistance.
For example, lithium-ion batteries generally have lower internal resistance compared to nickel-cadmium or lead-acid batteries due to more efficient materials and compact design.
Battery Design and Construction
How a battery is physically built impacts how well electrons and ions move.
- Thin separators and tightly packed cells reduce resistance by shortening ion travel paths.
- In contrast, poor welding or loose connections within the battery can introduce micro-resistances that build up over time.
Think of it like plumbing: a wider, smoother pipe allows water to flow freely, while a narrow, clogged one restricts it—battery internals work the same way with electricity.
Age and Usage Patterns
As batteries age, their internal chemistry begins to degrade.
- Electrolyte breakdown and crystal formation on electrodes (like sulfation in lead-acid batteries) increase resistance.
- Repeated deep discharges, high-load usage, or improper charging cycles accelerate this process.
Temperature Effects
Internal resistance rises in cold conditions as chemical reactions slow down. Conversely, extreme heat can damage internal components, also increasing resistance over time.
In short: Everything from chemistry to environment plays a part—knowing what causes resistance gives you a clearer picture of how to manage battery health.
How to Measure Battery Internal Resistance
Measuring internal resistance isn’t just for labs—it’s a practical step anyone can take to assess battery health. Whether you’re maintaining a battery bank, testing EV batteries, or diagnosing a weak power tool, there are several ways to measure internal resistance accurately.
Using a Dedicated Battery Internal Resistance Tester
This is the most accurate and straightforward method. These devices apply a known current load and calculate resistance using the voltage drop.
Steps:
- Fully charge the battery.
- Connect the tester probes to the battery terminals.
- The device will display resistance in milliohms (mΩ).
Best for: Technicians, professionals, or serious hobbyists who need precise readings.
Multimeter Method (Less Accurate)
While a basic multimeter won’t directly measure internal resistance, you can estimate it using Ohm’s Law.
Steps:
- Measure battery voltage under no load.
- Apply a known load and measure voltage again.
- Use the formula:
R=Vno load−VloadIR = \frac{V_{\text{no load}} – V_{\text{load}}}{I}R=IVno load−Vload
(where III is the current drawn by the load)
Note: This method has limitations and may not reflect real-time internal resistance under variable loads.
Electrochemical Impedance Spectroscopy (EIS)
Used mostly in research and industrial labs, this method applies alternating current at different frequencies to separate out ohmic and electrochemical resistance components.
Best for: Advanced users testing high-value or complex battery systems.
Pro Tips:
- Always test at room temperature for consistent readings.
- Fully charge the battery before testing.
- Avoid testing damaged or leaking batteries.
How to Reduce and Manage Battery Internal Resistance
While some level of internal resistance is inevitable, especially as a battery ages, the good news is there are several ways to reduce or manage it effectively. Proactive maintenance and mindful usage habits can significantly prolong battery life and preserve performance.
Choose High-Quality Batteries
Not all batteries are created equal. Low-quality or counterfeit batteries often use inferior materials and poor construction, leading to higher resistance right out of the box.
- Tip: Stick with trusted brands and certified battery sources to ensure consistent quality and lower initial resistance.
Avoid Deep Discharges
Frequent full discharges can accelerate internal resistance buildup, especially in lithium-ion and lead-acid batteries.
- Best practice: Recharge before dropping below 20–30% capacity to minimize chemical stress.
Store and Use Batteries at Optimal Temperatures
Extreme cold increases resistance temporarily, while excessive heat can cause permanent internal damage.
- Ideal range: Keep batteries between 20°C and 25°C (68°F–77°F) for optimal performance and longevity.
- Avoid: Leaving batteries in hot cars or unheated garages during winter.
Perform Routine Maintenance
For lead-acid batteries:
- Keep terminals clean and tight.
- Check electrolyte levels regularly.
- Equalize charging if needed (for flooded lead-acid types).
For lithium-based batteries:
- Use smart chargers with thermal and voltage regulation.
- Avoid overcharging or charging when the battery is hot.
Monitor Internal Resistance Regularly
Early detection is key. A gradual increase in resistance can be a warning sign before voltage or capacity issues become obvious.
- Schedule: Monthly checks for critical batteries, quarterly for casual use.
Conclusion
Battery internal resistance may be invisible, but its effects are felt in every device and system powered by batteries. From smartphones to solar power storage, internal resistance influences how efficiently a battery delivers energy, how long it lasts, and how safely it operates.
We’ve explored what internal resistance is, how it affects performance, what causes it, how to measure it, and how to manage it. Whether you’re a technician, engineer, or everyday user, understanding and monitoring internal resistance can help you maximize battery life and prevent unexpected failures.
Frequently Asked Questions About Battery Internal Resistance
Why is internal resistance important in a battery?
Internal resistance affects how efficiently a battery can deliver current. High resistance causes voltage drops, heat build-up, reduced capacity, and can lead to faster battery degradation. It’s especially critical in high-load applications like electric vehicles or power tools, where every milliohm counts.
How does internal resistance change over time?
As a battery ages, internal resistance naturally increases. This is due to chemical wear, electrode degradation, electrolyte breakdown, and crystal buildup (like sulfation in lead-acid batteries). Even with good care, resistance will rise gradually—but poor handling accelerates the process.
Can internal resistance be fixed or reversed?
Not entirely. While you can’t reverse the chemical wear inside a battery, you can slow it down. Proper charging, storage at moderate temperatures, and avoiding deep discharges can delay the rise of internal resistance. In some cases, desulfators may help recover lead-acid batteries temporarily.
What’s a “normal” internal resistance for a battery?
It varies by type and size:
- Li-ion (smartphone-sized): 50–150 milliohms
- Lead-acid (car battery): 3–10 milliohms
- 18650 Li-ion cells: 20–40 milliohms when new
Higher values typically indicate aging or damage.
Does internal resistance affect charging speed?
Yes. High internal resistance can interfere with the battery’s ability to accept charge efficiently. It may cause chargers to stop early or overcompensate, leading to overheating or incomplete charging cycles.