A comprehensive test of the battery allows you to evaluate its main characteristics, general condition, potential resource and suitability for further use. This material is exclusively about automotive lead-acid starter batteries with liquid electrolyte. Although some of the methods described here are also suitable for testing AGM, GEL, LifePO4 and Li-ion batteries.
Purposes of battery testing
Knowledge of available methods for testing car batteries and the ability to apply them in practice allows you to:
- choose the best new battery from those presented in the store window;
- assess the condition of the battery when buying a used car;
- promptly determine the malfunction of the generator and voltage regulator relay;
- quickly determine whether the battery is discharged while parked;
- determine when the battery needs servicing;
- estimate the remaining resource;
- avoid a situation where the car does not start due to a dead battery;
- evaluate the dynamics after desulfation, correction of the level and density of the electrolyte.
Note. Checking a battery and getting all the information about its condition is a very difficult task. In nature, there is no universal device for checking batteries. Those tools that are used by car enthusiasts and professional battery technicians individually do not allow us to see the full picture. In addition, the reliability of some methods does not exceed 50%, and their use without comprehensive testing is often misleading. Therefore, when a battery test is required, do it in several ways to get the most objective result.
Methods for checking a car battery
For a comprehensive test of a lead-acid battery with liquid electrolyte, we have the following methods at our disposal:
- Open circuit voltage.
- Electrolyte density.
- Internal resistance.
- Voltage drop under load.
- Real capacity.
- Charging time.
- Battery life tester.
- Self-discharge.
- Weight.
- Visual check.
Let's consider each of these methods according to the principle: what it is, how it is measured, in what units, what allows you to find out and how reliable the result can be obtained.
Open circuit voltage
Open circuit voltage (OCV) is the voltage across the battery terminals without a load. Measured with a voltmeter or multimeter in the appropriate mode. The unit of measurement is volt (V). Allows you to find out approximately how much % the battery is charged. The reliability of the results largely depends on the measurement conditions.
In order for testing a battery with a multimeter to give a reliable result, the open circuit voltage must be measured under the following conditions:
- the battery is disconnected from the vehicle's on-board network;
- no load is connected to the terminals;
- the battery has stood for at least 12 hours after charging and other manipulations.
For example, if your car sat overnight in a garage or in a parking lot, then in the morning you can easily assess the charge level by measuring the voltage at the terminals. To do this, use the following table.
|
Quiescent voltage (V) |
Charge (%) |
|
< 11.90 |
0 |
|
11.95 |
10 |
|
12.00 |
20 |
|
12.05 |
30 |
|
12.15 |
40 |
|
12.20 |
50 |
|
12.30 |
60 |
|
12.40 |
70 |
|
12.50 |
80 |
|
12.60 |
90 |
|
12.70 |
100 |
|
>12.71 |
Incorrect measurement |
It is useless to measure open circuit voltage immediately after charging and discharging the battery. The same applies to cases when the car was in operation and the battery was charged from the generator. Reliable data can only be obtained after the battery has been completely at rest for at least 12 hours.
Note. Sometimes OCV is used to determine the stratification (stratification) of an electrolyte. This is when the density in the lower layers of the battery is higher than on the top. Measurements are carried out after the battery is fully charged and left to stand for the time specified above. If, under the described conditions, checking the battery voltage gives significantly more than 13 volts, then this usually indicates electrolyte separation.
Electrolyte density
Electrolyte density is a measure of the concentration of sulfuric acid dissolved in distilled water. Measured using a hydrometer or refractometer. The unit of measurement is g/cm3. Electrolyte density is the most accurate and reliable way to assess the battery charge level. Moreover, unlike voltage, density can be measured at any time. The method also allows you to detect sulfation (if, after proper charging, the density does not return to normal, then this is a clear sign of sulfation). Reliability is extremely high. The only drawback is that it is only suitable for batteries with liquid electrolyte, to which you have access.
To find out the charge level, you need to measure the density of the electrolyte using a hydrometer or refractometer. It is advisable to check all compartments, which will additionally allow you to find out if there is a big difference between them. See this table for the relationship between density and charge level.
|
Density (g/cm3) |
Charge (%) |
|
< 1.12 |
0 |
|
1.14 |
10 |
|
1.15 |
20 |
|
1.16 |
30 |
|
1.18 |
40 |
|
1.19 |
50 |
|
1.20 |
60 |
|
1.22 |
70 |
|
1.23 |
80 |
|
1.25 |
90 |
|
1.26 |
100 |
|
> 1.26 |
Density above normal |
Note. A refractometer is a more modern and accurate instrument than a hydrometer. However, its accuracy greatly depends on the lighting of the room and the transparency of the electrolyte. If the garage is dark and the electrolyte is cloudy, it’s better to use a good old hydrometer. The readings of both devices are insignificantly affected by the temperature of the electrolyte. For example, if after intensive charging of the battery the electrolyte temperature exceeds +35 degrees Celsius, then you can safely add 0.01 g/cm3 to the obtained value.
Internal resistance
Internal resistance is the resistance that a battery offers to electrical current flowing through it. It is measured with special devices. The unit of measurement is mOhm (milliohm). Allows you to evaluate the “health” and state of charge of a “healthy” battery. The lower the internal resistance, the healthier and more charged the battery. The normal internal resistance of a fully charged and “healthy” battery with a capacity of 60 Ah should be 4-6 mOhm. The reliability of the method is high.
Many car battery testing instruments calculate the cold cranking current and overall condition as a percentage based on the measured internal resistance. It should be emphasized that such devices do not measure the maximum current directly, but only calculate it based on voltage and internal resistance. If the reliability of the internal resistance readings is close to 100%, then the “life” and maximum current indicators should not be trusted unconditionally.
Note. Never try to measure the internal resistance of a battery with a multimeter in ohmmeter mode. This mode is designed to measure the resistance of simple conductors, while the battery is not a simple conductor of current, but its source. How to measure the internal resistance of a battery correctly? There are special devices for this. An example is shown below.
In general, the internal resistance of lead-acid batteries increases due to sulfation. The plates overgrown with lead sulfate pass electric current less well, which is why the battery loses capacity and starting characteristics. Desulfation in some cases helps to reduce internal resistance and partially restore battery parameters. Therefore, during mass battery maintenance, such a device will be very useful for assessing dynamics.
Detailed material about the internal resistance of the battery
Voltage drop under load
Load sag is the difference between the open circuit voltage and the loaded battery voltage. A load fork is used for measurement. The unit of measurement is volts. Allows you to fairly accurately assess the “health” of a car battery in conditions close to real ones. The load fork simulates the operation of a car starter, displaying the current voltage on the display. The battery must be charged. If the voltage of a 12-volt battery under load is not lower than 10 V, it can be used further.
Note. Unlike a multimeter and a device for measuring internal resistance, the load plug actually loads the battery. If you need the battery to start your car engine now on a frosty morning, stop using the load fork. Use this tool only in cases where it is possible to recharge the battery.
Real capacity
Real capacity is the amount of electricity that a battery can actually store and deliver. Measured using an electronic load. The unit of measurement is ampere-hour. Allows you to evaluate the wear of a used battery. What the measurement of the real capacity of a new battery gives, I explained and clearly showed in the material at the link. When using an electronic load and following the discharge rules, the accuracy of the method is extremely high.
Note. The essence of measuring capacity is to discharge a charged battery with a current of 10% of the rated capacity (indicated on the label). The discharge is carried out to a voltage of 10.5 volts. If the measurement is performed by an electronic load, it calculates the actual capacity in ampere-hours based on the voltage, current and discharge time. A similar test can be performed using a car light bulb of known power and a chronometer.
Charging time
Charge time is the time required to fully charge the battery in accordance with the charging rules. Measured using a watch or the charger itself, if such a function is provided. The unit of measurement is hour. Allows you to assess the general condition of the battery, identify sulfation and other problems. The reliability of the method is relatively high.
Normal charging time for a 12-volt lead-acid starter battery is about 14-16 hours. This is provided that the battery was completely discharged and charging was carried out without exceeding the voltage and current. If the standard charge ends much earlier than after 14-16 hours, this indicates battery wear. The most common cause is sulfation.
Note. To obtain a correct estimate using this method, you need to know exactly the actual capacity of the battery, and not what is indicated on the case. If the battery is worn out, say, by 50%, and you charge it with a current of 10% of the rated capacity, then it will charge very quickly. Faster than if you set the charge current to 10% of the actual capacity.
Battery life tester
A battery tester is a device that, based on the voltage at the terminals and internal resistance, calculates the maximum starting current and the overall “health” of the battery. Typically, all such devices display voltage in volts, internal resistance in milliohms, maximum starting current in amperes and the general condition of the battery as a percentage.
One well-known American company, which has been developing and producing such devices for many decades, tested this method on hundreds of batteries. Tests have shown that its reliability is approximately 50%. That is, in half the cases such a device does not reveal the actual wear of the battery.
Note. Despite the above, battery testers have not lost popularity. Moreover, thanks to some of the patents revoked by this American company, the Chinese began making such devices en masse. Since they work on the same principle, you should also only trust them 50%. Like many of my colleagues, I personally believe that it makes sense to use such testers. But only in combination with other methods discussed in this material.
Self-discharge
Self-discharge is the tendency of a battery to lose its accumulated charge without load. To evaluate this parameter, measurements of open circuit voltage and battery idle time are used. Additionally, you can measure the density of the electrolyte (hydrometer vs refractometer). The self-discharge of a “healthy” battery that has been in storage mode for two weeks does not exceed 10%. Modern batteries have low self-discharge, and under certain conditions can be stored without consequences for their “health” for 1 year.
Note. When the battery is used in a car, self-discharge and systematic discharge due to current leakage should not be confused. Especially if it is a current leak through the generator. There are good materials on these topics on the Auto without a service station website. Follow the links and study.
Weight
Weight is the mass of the battery. Measured by weighing on a scale. The unit of measurement is kilogram. Allows you to find out how much lead the manufacturer used to make a particular battery. If you compare batteries of the same type, manufactured using the same technology, then weighing is a fairly accurate indicator of the quality of the battery.
The normal weight of a 12-volt lead-acid battery with liquid electrolyte for 60 ampere hours is 15 kg. If, when choosing a new battery, you took scales with you, refuse to buy those that weigh 11-13 kg. Such a small weight for a “sixty” means that the manufacturer saved on lead. This means that the actual characteristics will most likely not correspond to those stated on the label.
Visual inspection
Visual inspection - allows you to assess the general physical condition of the battery, identify damage to the case, depressurization and sulfation. If there is deformation on the battery case, this often indicates an internal short circuit or severe uncontrolled overcharging with critical overheating. Sulfation is visible on negative plates as a light gray coating. This is sometimes used to assess progress after desulfation.
You can also visually assess the condition of the electrolyte. If it is cloudy, brown, with suspensions, most likely your battery will not last long.
Conclusion
Checking your car battery is a very useful activity. Not only professional battery technicians, but also ordinary car enthusiasts should know how to test batteries. At the stage of choosing and purchasing a new battery, this skill will help you avoid buying an outright defect. During operation, you will be able to independently assess the condition of the battery, take care of it in a timely manner and replace it before it fails you.
