The secret to long life: How to tell whether the battery is excellent, good or damaged by measuring internal resistance
Internal resistance is one of the best indicators of the condition of a battery. It is like blood pressure for a person – it gives a lot of information about its “health”. Let’s take a closer look at how we can use internal resistance to assess the health of different types of batteries.
What is internal resistance?
The internal resistance (Rint) of a battery is a measure of the resistance of all the components inside it that impede the flow of electric current. This includes the resistance of the electrolyte, the electrodes, the separator, and the connections between them. The lower the internal resistance, the more efficiently the battery can deliver and receive energy.
Why is it important?
Over time and use, the internal resistance of a battery increases. This is due to various processes such as:
- Electrode wear: Formation of layers with higher resistance, change in crystal structure.
- Electrolyte degradation: Increase in viscosity, decrease in ionic conductivity.
- Loss of active material: Contraction and expansion leading to loss of contact.
Increased internal resistance leads to:
- Reduced capacity: The battery cannot store as much energy.
- Reduced power: It cannot deliver high current, especially under continuous load.
- Increased heating: More energy is lost as heat during charging and discharging.
- Faster self-discharge: Not directly related, but often goes hand in hand with other degradation processes.
How is internal resistance measured?
There are two main measurement methods:
- AC (alternating current) method: This is the most common and accurate method. The device applies a small alternating current of a certain frequency (for example, 1 kHz) through the battery and measures the voltage drop. Internal resistance is calculated using Ohm’s law (R=V/I). Most specialized battery testers use this method.
- DC (direct current) method: The voltage drop when a known load is applied is measured. This method is simpler, but less accurate, as it depends on the state of charge and temperature. However, it is more accessible in practice, because it does not require specialized devices – two multimeters and a sufficiently powerful resistor with accurate resistance are sufficient (for example). There are very good descriptions of this method on Youtube.
Classification of batteries by internal resistance
It should be borne in mind that there are no universal, strictly defined values of internal resistance that are applicable to absolutely all batteries, as they depend on:
- Chemical composition: Li-ion, LiFePO4, NiMH, lead-acid have different typical values.
- Capacity: A battery with a higher capacity usually has a lower internal resistance.
- Size/Form factor: 18650, 21700, 26650 have different designs and sizes.
- Manufacturer: Different manufacturers and series have differences.
- Purpose: Batteries for high currents (power cells) have a much lower internal resistance than those for high capacity (capacity cells).
- Temperature: At lower temperatures, the internal resistance increases.
- State of charge (SoC): At very low or very high charge the internal resistance may increase slightly. For the most accurate measurements, it is recommended that the battery be about 50-80% charged.
Guidelines
The following are guidelines for determining the quality of different types of batteries.
Lithium-ion batteries (18650, 21700, 26650, etc.)
These batteries are very sensitive to internal resistance. Values are often measured in milliohms (mΩ).
| Battery Condition | 18650/21700 (Power Cells – for high current, e.g. for power tools, vapes) | 18650/21700 (Capacity Cells – for high capacity, e.g. for laptops, flashlights) | 26650 (and similar larger Li-ion) |
| Excellent | <15 mΩ (often 8−12 mΩ for new) | <30 mΩ (often 15−25 mΩ for new) | <20 mΩ (often 10−15 mΩ for new) |
| Good | 15−25 mΩ | 30−50 mΩ | 20−35 mΩ |
| Acceptable / Starting to degrade | 25−40 mΩ | 50−80 mΩ | 35−50 mΩ |
| Damaged / Bad | >40 mΩ | >80 mΩ | >50 mΩ |
Table with updated indicative values for 18650/21700 according to purpose
| Battery condition | 18650/21700 (Power Cells – for high current) | 18650/21700 (Capacity Cells – for high capacity) | 18650/21700 (Low Drain – for low current/long life) |
| Excellent (New) | <15 mΩ (8−12 mΩ) | 15−30 mΩ (20−25 mΩ) | 30−60 mΩ (40−50 mΩ) |
| Good | 15−25 mΩ | 30−50 mΩ | 60−90 mΩ |
| Acceptable / Starting to degrade | 25−40 mΩ | 50−80 mΩ | 90−120 mΩ |
| Damaged / Bad | >40 mΩ | >80 mΩ | >120mΩ |
Internal resistance of lithium “coin” batteries (CR2032, CR2016, CR2025, etc.)
These batteries are of the type primary lithium batteries (non-rechargeable). Their chemical composition is different (lithium-manganese dioxide usually) and they are designed for very low discharge current for an extremely long period (years).
- Intended use: Watches, computer motherboards (CMOS batteries), small remotes, toys, low-power medical devices.
- Typical internal resistance: The internal resistance of these batteries is significantly higher than that of rechargeable lithium-ion batteries, but this is normal for their purpose.
| Battery condition | CR2032/2016/2025 (Primary lithium) |
| Excellent (new) | 100−500 mΩ (may vary greatly depending on manufacturer) |
| Good | 500 mΩ−1Ω (1000 mΩ) |
| Worn / End of Life | >1Ω (1000 mΩ) |
Important notes about “coin” batteries:
- High sensitivity of the meter: Some internal resistance testers, especially those designed for Li-ion batteries, may not give accurate or reliable readings for coin batteries due to their very high impedance and different frequency characteristics.
- Focus on voltage: With these batteries, it is often more practical to relies on measuring the voltage under minimal load, as it drops significantly as the battery nears the end of its life. A new CR2032 is around 3V, while below 2.8V is already considered worn out for most applications. However, internal resistance is a better indicator of remaining life than open circuit voltage alone.
- They are not for high current: Trying to draw high current from such a battery would result in a huge voltage drop due to the high internal resistance.
Differences in internal resistance according to the purpose of lithium-ion batteries
Lithium-ion batteries of the same form factor (e.g. 18650) are manufactured for different purposes, which directly affects their internal resistance:
- Power Cells:
- Purpose: Power tools, vaping devices, drones, electric bicycles/scooters – applications where it is necessary to deliver high currents for a short or long period.
- Design: Optimized for minimal internal resistance through thinner separators, larger electrode area, and higher conductivity materials. This is usually at the expense of capacitance.
- Typical internal resistance (new): Very low, typically below 15 mΩ, often in the 8−12 mΩ range.
- High Capacity Batteries (Capacity Cells):
- Intended Use: Laptops, flashlights, power banks, medical devices – applications where maximum capacity is important, not so much the ability to deliver very high current.
- Design: Optimized for maximum capacity through a larger amount of active material. This can lead to slightly higher internal resistance compared to “power” cells.
- Typical internal resistance (new): Moderately low, usually in the 20−30 mΩ range, although some newer and more efficient models can be below 20 mΩ.
- Low Drain / Long Life Cells:
- Use: Applications where very low, constant current is required for a long period of time, such as watches, remote controls, sensors, backup batteries.
- Design: The focus is on voltage stability, low self-discharge and long life at low loads. Internal resistance is not as critical a factor as with “power” cells, but should still be within acceptable limits.
- Typical internal resistance (new): Higher than “power” and “capacity” cells, but still within reasonable limits for lithium-ion technology. For 18650 or similar cells, values of 40−60 mΩ for new cells designed for very low current only can be considered normal. Important: Values above 100 mΩ for 18650 even for this type will indicate degradation.
Additional notes for Li-ion batteries:
- New batteries: Will always have the lowest internal resistance.
- Parallel connected: With parallel connected cells it is critical that they all have similar internal resistance to balance the load and prevent overloading of the weaker cells. With such a connection, the total resistance is lower (calculated as with parallel resistors) and, accordingly, they can deliver more current at the same voltage.
- LiFePO4 (Lithium Iron Phosphate): These batteries usually have a lower internal resistance than standard Li-ion, especially when they are designed for high currents. For example, a new LiFePO4 32650 battery may have an internal resistance of less than 5 mΩ.
- Get batteries from manufacturers that guarantee a certain internal resistance in their specifications. Avoid battery “repackers”, no matter how much they praise them in forums and reviews. The claim that 200 mΩ can be a good internal resistance for an 18650 is false – this is too high a value even for low-current batteries. At 200 mΩ, an 18650 battery, regardless of its purpose, would be severely degraded and with very limited capacity and power, practically unusable.
Nickel-Metal Hydride (NiMH) and Nickel-Cadmium (NiCd) batteries
These batteries usually have a higher internal resistance than lithium-ion, especially in smaller sizes (AA, AAA).
| Battery condition | AA/AAA NiMH/NiCd (small sizes) | C/D Cell NiMH/NiCd (larger sizes) |
| Excellent | <50 mΩ (often 20−40 mΩ for new) | <20 mΩ (often 10−15 mΩ for new) |
| Good | 50−100 mΩ | 20−40 mΩ |
| Acceptable / Starting to degrade | 100−200 mΩ | 40−70 mΩ |
| Damaged / Bad | >200 mΩ | >70 mΩ |
Additional notes for NiMH/NiCd:
- “Memory effect” and crystallization: These effects can increase internal resistance.
- The discharge-charge limits during initial operation are important due to the memory effect.
Lead-acid batteries (car, some stationary, etc.)
In car batteries, the internal resistance is extremely low and is measured in microohms (μΩ) or tenths of a milliohm (mΩ). It is a key indicator of the battery’s ability to deliver high cranking current (CCA – Cold Cranking Amps).
| Battery Condition | Car Battery (12V, 60-80 Ah) |
| Excellent | <4 mΩ (often 2−3 mΩ for new) |
| Good | 4−6 mΩ |
| Acceptable / Starting to degrade | 6−10 mΩ |
| Damaged / Bad | >10 mΩ |
Additional notes for lead-acid batteries:
- State of charge: In these batteries, the state of charge has a significant impact on the internal resistance. Measurements should be made with a fully charged battery.
- Temperature: At low temperatures, the internal resistance increases significantly, which is the cause of difficulties when starting a car in winter.
- Sulfation: This is the main cause of increased internal resistance in lead-acid batteries.
General advice and conclusions
- Measure with the same device: To obtain the most reliable results, always use the same internal resistance tester. Different devices may give slightly different readings.
- Consistency: Measure under similar conditions – temperature and state of charge (for Li-ion around 50-80%, for lead-acid fully charged).
- Combine with other indicators: Internal resistance is an excellent indicator, but it is not the only one. Combine it with capacity measurement (especially for Li-ion), voltage under load and visual inspection (for swelling, leaks, etc.).
- Track the development: The best assessment of the condition of the battery is obtained when you track its internal resistance over time. If it starts to increase rapidly, this is a sure sign of degradation.
- Unit of measurement: Be careful with the units of measurement – milliohms (mΩ) and microohms (μΩ). 1 mΩ=1000μΩ.
Using the FNIRSI HRM-10: Practical Aspects
FNIRSI HRM-10 is a popular and relatively affordable battery internal resistance tester, and with it we can easily practically understand whether the battery is excellent, good or damaged.
It uses the AC (alternating current) method at 1 kHz, which is a standard and reliable approach to measuring the internal resistance of batteries. Importantly, it is a four-wire (Kelvin) tester, which is key to accuracy. Four-wire measurement eliminates the influence of test lead resistance and battery terminal contact, which is a major source of error in simpler two-wire methods.
Overall, you can expect the FNIRSI HRM-10 to provide fairly accurate and reliable results for its price, especially for 18650, 21700, 26650 lithium-ion batteries, as well as automotive batteries.
It is recommended to consider the following features to be sure whether the battery is excellent, good or damaged:
- Accuracy at extreme values:
- Very low resistances (below 1 mΩ): Although the HRM-10 is quite good at measuring the internal resistance of new, high-quality car batteries (which can drop below 2-3 mΩ), there may be slight deviations from ideal laboratory instruments. However, for everyday use and condition determination, a difference of 0.1−0.2 mΩ will not be fatal.
- Very high resistances (above 1-2 ohms): Although it can measure up to 200 ohms, with coin batteries (CR2032 and similar), whose resistance can be hundreds of milliohms to a few ohms, the results may be less stable or vary more. This is not a problem with the device specifically, but with the specificity of these batteries and the fact that testers are optimized for lower values.
- Calibration: Although the FNIRSI HRM-10 comes factory calibrated, over time or under extreme conditions of use, the accuracy may shift slightly. The device has the ability to be calibrated, which is a plus. For maximum accuracy, periodic checking with a known precision resistor (or other calibrated device) would be very useful.
- Temperature: As mentioned, temperature has a strong effect on internal resistance. Make sure the batteries you are testing are at room temperature (around 20−25∘C) for the most consistent and comparable results. If you are testing a battery that has just been used heavily (and is warmed up) or has been in the cold, the results will be distorted.
- State of Charge (SoC): Although the AC method is less sensitive to SoC than the DC method, it is still advisable to test Li-ion batteries at a similar state of charge (e.g. 50−80%). Lead-acid batteries must be fully charged for an adequate assessment.
- Terminal Contact: Although it is a four-wire, poor contact between the probes/clamps and the battery terminals can lead to inaccurate and/or variable readings. Make sure the terminals are clean and you have a solid, tight contact.
- “Noise” in the measurement: In some cases, especially with lower internal resistance values or in the presence of interference, slight fluctuations in the readings may be observed. This is normal for most handheld devices. The approach in this case is to take several measurements and average the results if necessary.
Overall impressions of FNIRSI HRM-10:
- Advantages:
- Four-wire method (Kelvin connection): Excellent basis for precise measurements.
- 1 kHz AC measurement: Industry standard.
- Wide range: Allows testing of various types of batteries – from small lithium-ion to automotive.
- Sorting function: Allows you to set thresholds for “good” and “bad” batteries, which is very useful for sorting large numbers of cells.
- Price/performance: Offers very good functionality for its price range.
- Disadvantages (usually minor):
- May require a little time to get used to the menus.
- As with any device, there may be slight variations between individual units.
Conclusion
FNIRSI HRM-10 is a very good choice for practical use. The theory we discussed is fully applicable to its measurements. Do not expect significant differences that would make the theory meaningless, but you should also be aware of the factors that can affect the accuracy of the measurement (temperature, SoC, contact) and minimize them according to your capabilities and circumstances to get the most reliable results.
If you follow these basic rules, the FNIRSI HRM-10 will be an extremely useful tool for assessing the condition of batteries and will help you make informed decisions about which ones are still usable and which ones should be discarded.
Last words
All data from this article is only a starting point. It is recommended to collect data from one’s own practice and real experience of trusted people from the amateur radio community about the internal resistance and capacity of batteries, from which to draw conclusions about their quality. User reviews, unless supported by real measurements, are not the best source for an informed decision.
According to my experience so far, batteries should only be purchased from manufacturers who are not afraid to publish real data about their batteries. The so-called “repackers”, if they publish any specifications at all, they are unrealistic and often obviously fake, but with a tempting price. But there is no guarantee that if their batteries are more expensive, they are of better quality.
Whether the battery is excellent, good or damaged?
We hope that this research of mine was useful to you and I will be glad to share your opinion in the comments, no matter what it is.
