A modern car is a complex electromechanical complex, where the health of the power source plays a decisive role. Many vehicle owners are accustomed to assessing the condition of the battery solely by the voltage at the terminals or the date of manufacture, ignoring deeper parameters. However, it is internal resistance is the most accurate indicator of the battery's ability to deliver powerful starting current, especially in winter.
A drop in this indicator below acceptable limits often precedes the complete failure of the device. Understanding the physical processes occurring inside a lead-acid battery allows you to anticipate engine starting problems in advance. In this article we will analyze in detail which values are considered reference values for different types of batteries and how to correctly interpret measurement data.
The physical nature of the internal resistance of the battery
Internal resistance is the sum of the resistances of all battery components, including the electrolyte, plates, separators and leads. Unlike voltage, which can remain high even with a “tired” battery without load, impedance (impedance) increases sharply as the device ages. This occurs due to sulfation of the plates, drying out of the electrolyte or detachment of the active mass.
The process of charge and discharge in a lead-acid system is inextricably linked with chemical reactions, the speed of which depends on the contact area and conductivity of the electrolyte. The critical threshold is considered to be an increase in resistance by 100% relative to the factory value, which indicates the need for urgent replacement of the power source. At the same time, the idle voltage can remain in the range of 12.6–12.8 Volts, creating the illusion of serviceability.
⚠️ Attention: Measuring voltage without load often gives a false positive result. Only a load test or impedance measurement shows the actual ability of the battery to turn the starter.
Ambient temperature has a tremendous impact on the electrical conductivity of the electrolyte. As the temperature decreases, the viscosity of the liquid increases, the ions move more slowly, which naturally increases the internal resistance. That is why in summer a battery with increased impedance can still start the engine, but in winter the car will not start.
Standard values for various types of batteries
There is no single standard for all batteries, since the values depend on capacity, production technology (Ca/Ca, AGM, EFB, GEL) and design features. For standard 55-60Ah lead-acid batteries, the range of 4 to 6 milliohms (mOhms) is considered normal. More powerful batteries with a capacity of 90–100 Ah have lower resistance, usually in the range of 2.5–4 mOhm.
Technology AGM (Absorbent Glass Mat), widely used in Start-Stop systems, has its own characteristics. Thanks to the dense assembly of the blocks and the use of fiberglass, such batteries initially have lower internal resistance compared to their liquid counterparts. This allows them to deliver enormous currents in fractions of a second, which is critical for the operation of modern electronic systems.
Below is a table of approximate resistance values for serviceable batteries of different capacities at a temperature of +25°C:
| Battery capacity (Ah) | Battery type | Normal (mOhm) | Critical value (mOhm) |
|---|---|---|---|
| 40–50 | Liquid (WET) | 6.0 – 8.0 | > 12.0 |
| 55–65 | Liquid/Ca | 4.0 – 6.0 | > 10.0 |
| 70–80 | AGM / EFB | 3.0 – 4.5 | > 7.0 |
| 90–100 | AGM / WET | 2.5 – 3.5 | > 6.0 |
It is important to understand that the range of values may vary depending on the manufacturer. For example, premium batteries from brands like Varta or Bosch often have lower initial ratings due to lead purity and improved lattice structure.
If your measurements show values close to critical, but the engine starts confidently, this may be a sign of incipient sulfation. At this point, you can still try to carry out desulfation with low currents to extend the life of the device.
Methods and tools for accurate diagnosis
To measure internal resistance, conventional multimeters in voltmeter mode are not suitable, since they operate on direct current and do not take into account the reactive component. Professionals use special battery testers (for example, Midtronics, Konnwei), which supply a short-term alternating signal with a frequency of about 1 kHz. This method allows you to separate active resistance from capacitive resistance.
There is also a load fork method that simulates the inrush current. Although it does not directly indicate resistance in milliohms, the voltage drop under load indirectly indicates its magnitude. If, when connecting the plug, the voltage drops below 9.6–10 Volts, the internal resistance is clearly higher than normal.
The operating algorithm of modern analyzers includes the following stages:
- 🔋 Read open circuit voltage for calibration.
- ⚡ Supply a test signal (AC) to the terminals.
- 📊 Impedance calculation and comparison with declared capacitance (CCA).
- 📉 Output of the result as a percentage of health status (SoH).
When using analyzers, it is important to press the probes tightly to the terminals, since oxidized contact can introduce an error of several milliOhms, which will distort the result. It should also be taken into account that immediately after charging or driving, the readings may be unstable due to surface charge.
Influence of temperature and operating conditions
The temperature coefficient of resistance for lead-acid batteries is approximately 0.8% per degree Celsius. This means that when the temperature decreases from +25°C to -15°C (a difference of 40 degrees), the resistance will increase by about 30-40%. It is this factor that often becomes fatal for old batteries in cold weather.
High temperatures under the hood in summer are also harmful, but have a different effect. Heat accelerates chemical reactions and corrosion of the gratings, which leads to an irreversible increase in resistance over time. Thermal runaway or simply constant overheating can reduce the battery life by half.
⚠️ Attention: Do not take measurements immediately after a long trip. Allow the battery to cool to ambient temperature, otherwise the readings will be incorrect.
Vibrations and shocks can cause the active mass to fall off the plates, which also changes the internal structure and increases resistance. If the car is driven on bad roads, mounting the battery becomes a mandatory procedure.
Install a thermal insulation cover on the battery if it is located close to hot engine components. This will extend the life of the electrolyte and plates.
Diagnostics of faults based on resistance indicators
A sharp increase in internal resistance often indicates specific defects. For example, if one of the cans (cells) inside the battery has a short circuit or break, the total resistance will change abruptly. If the circuit opens, the resistance will become infinite and the voltage will drop to zero.
Sulfation is the main enemy of battery longevity. Lead sulfate crystals do not conduct current, taking up space on the plates. This reduces the effective reaction area and increases resistance. In the early stages, this process is reversible, but if the battery has been left discharged for several months, the structure of the plates may be damaged irrevocably.
Signs of problems that can be identified through resistance analysis:
- 📉 Smooth growth of readings over the course of months - natural aging or sulfation.
- 🚀 A sharp jump after a deep discharge means possible damage to the structure of the plates.
- 🌡️ The dependence of the readings on the temperature above normal means problems with the composition of the electrolyte.
Is it possible to restore a battery with high resistance?
In some cases, washing with distilled water and long-term charging with low current (0.05C) helps. However, if the increase in resistance is caused by shedding of the mass or short circuit of the cans, restoration is impossible and dangerous.
Practical maintenance tips
To maintain low internal resistance, it is necessary to regularly check the electrolyte level (in serviced batteries) and density. The use of distilled water for topping up is mandatory, since impurities in ordinary water sharply reduce the battery life.
The terminals and terminals must be free of oxides and securely tightened. Poor terminal contact adds external resistance, which the tester may mistakenly interpret as internal resistance of the battery itself. Always clean the contacts before diagnosing.
☑️ Battery check plan
If the vehicle is rarely used, it is recommended to use smart charger with storage mode. It will keep the battery in good shape, preventing sulfation and resistance growth during idle periods.
When is a battery replacement necessary?
Replacing the power supply is required when the internal resistance exceeds the critical values listed in the table above and does not decrease after a full charge. Also a signal is the situation when a new battery with similar characteristics shows a resistance 2-3 times less than your current one.
You should not save on buying a new battery if diagnostics show it is worn out. High resistance leads to undercharging (the generator cannot “push” current through the high resistance) and overload of the starter, which can damage other electrical wiring elements.
If the resistance has increased by more than 50% of the factory value, the battery is considered exhausted, even if it still turns the starter.
When choosing a new battery, pay attention not only to the capacity and starting current, but also to the production date. Even without working, the battery loses its properties, and its internal resistance slowly increases during storage.
How often should you check the battery resistance?
It is recommended to carry out a full diagnosis, including measuring internal resistance, twice a year: before the onset of winter cold and after it. This will allow you to evaluate the effect of low temperatures on the condition of the battery.
Does engine type (diesel/gasoline) affect resistance requirements?
Yes, diesel engines require significantly more starting current due to their high compression ratio. Therefore, for diesel engines, the minimum internal resistance of the battery is critical, and the permissible wear threshold for them is lower than for gasoline cars.
Can a generator charge a high resistance battery?
Theoretically, it can, but the process will be very slow and ineffective. Most of the energy will be spent heating the battery. In such cases, an external stationary charger with desulfation capability is required.