Capacity and voltage of a car battery: how to check its charge with a multimeter

Every car enthusiast will benefit from knowledge of methods for testing battery health. The days when the engine could be started by turning the recoil starter are long gone. The battery (storage battery) in a modern car, in addition to starting the engine, is responsible for powering security systems, alarms, and electric locks. The good condition of the battery is the most important factor in the correct operation of the vehicle.

There can be many cases when a battery health check is required. For example, the car has been idle for a long time, or severe frosts have set in the street, which increases the load on the battery, or a long vacation trip is planned. You can check the reliability of this device by measuring three main parameters: voltage, capacity and density of the electrolyte.

Measuring instruments

Several devices are used to test the battery's performance and perform a number of simple manipulations:

1. A multimeter is an affordable and convenient device that allows you to determine the state of the battery with acceptable accuracy.
2. Load plug - a device that allows you to simulate the starting load on the battery when starting the engine and at the same time measure the voltage drop at the battery terminals.
3. The hydrometer is used to check the density of the electrolyte in the battery banks.
4. A specialized window on the battery case is a color indicator of the charging status of maintenance-free and low-maintenance batteries. Green indicates 100% charge, white indicates a low electrolyte level, and black indicates the need to charge.

Checking the battery with a multimeter

The technique of how to check a car battery with a multimeter is of interest to many motorists. A multimeter (also called a tester) is the most readily available and versatile instrument for all electrical measurements. Used in electrics and electronics. Perfect for testing automotive electrical circuits. They can measure basic parameters such as resistance, voltage, current. As a result, it is possible to measure the battery capacity. Anyone familiar with the basic fundamentals of electricity can do this. Simply put, a high school student can easily cope with the task.

Checking the car battery with a multimeter can be divided conditionally in two stages:

1. Checking the charging by voltage in the battery.
2. Checking the battery capacity.

These two most important parameters are measured quite simply, but they require care when connecting a measuring device, since The battery has a powerful current despite the low voltage, which, if connected incorrectly, can damage the tester.

There are many articles on the Internet about how to check battery charging with a multimeter. But they all boil down to one simple method - measuring the voltage at the terminals. The method allows you to determine the battery charge level, as well as determine if the battery needs service. This is done as follows:

• Disconnect the vehicle power terminals from the battery.
• Set the measuring range of the multimeter to a value from 0 to 20 V DC. It is important not to be confused: the multimeter should be switched on to the voltage measurement mode, measured in volts, and not in the position of the current measurement, measured in amperes.
• Connect the multimeter to the battery terminals, observing the polarity.
• Remember the readings.

From the readings obtained, one can easily understand what is the state of the battery:

• A 100% charged battery will deliver a voltage of 12.7 V and higher, on some battery models the voltage may even be 13 V.
• A voltage lower than 12.7 V indicates a discharge of the device. The lower it is, the more the battery is discharged. For example, if the meter reads 12.2 V, the battery is about 50% discharged.
• A reading of 11.8 V indicates a critical discharge and the battery needs to be charged immediately. He most likely will not be able to start the engine.
• If the readings are below 11 V, it is better to replace the battery, otherwise it can damage the electrical circuits of the car, damage the generator or the charging controller.

To obtain more accurate information about the state of charge of the device, measurements should be made at least four hours after disconnecting the vehicle power terminals.

When checking the condition of the battery, it will be useful to test for possible excessive leakage currents.

Voltage leakage potential

Tips on how to test your car's battery for excess leakage current with a tester will help conserve precious power for starting the engine after long periods of parking or during freezing weather.

It should be noted right away that there is a regular leak in modern cars. It arises from the constant consumption of power by car security systems, lock controllers, etc. This leak should not exceed 75 milliamps per hour, otherwise the battery will drain quickly. In reality, the electrical consumption with constant power supply of the devices is much less.

But there are also non-standard ones, excess leaks. They can occur both through the body of the battery itself, and in the electrical circuits and circuits of the car, leading to the premature discharge of the battery.

Leakage through the battery case

Microcurrents through the battery case can pass through various drips (for example, electrolyte drips) and contaminants. To determine the places of passage of microcurrents, it is necessary:

• Disconnect the battery from the vehicle systems.
• Set the multimeter to measure DC voltage in the range from 0 to 20 V.
• Connect the positive test lead of the multimeter to the positive of the battery.
• Drive slowly with a minus probe in places where there are smudges and dirt.

Any deviation from zero in the meter readings says about the presence of microcurrents on the battery case.

Such leaks are easily eliminated. The battery case must be treated with a baking soda solution (one tablespoon of baking soda per glass of water). The battery is carefully wiped from smudges and dirt, and then dried. If the testing did not reveal any leaks, but there are leaks and contamination on the battery, they must still be removed, since sooner or later contamination will lead to leaks and self-discharge of the device.

Lack of charge in the circuit

Battery charge leaks can occur in the electrical circuits of the vehicle. To determine in which particular circuit the increased power consumption occurs, it is necessary:

• Disconnect all sources of electricity consumption: headlights, radio, interior light
• Set the multimeter to the position for measuring amperage up to 10 amperes.
• Disconnect positive terminal from battery.
• Connect the negative terminal of the tester to the contact terminal of the battery, connect the positive terminal of the multimeter to the positive terminal of the vehicle.
• After about a minute, take the readings (the car's security systems are activated, as a rule, after a minute).

If the device's leakage current readings exceed 75 milliamps, the following steps must be taken:

• Remove the fuses and relays one by one for subsequent monitoring of the instrument readings.
• After the leak reading returns to normal, determine which circuit is leaking.

Knowing in which of the car's circuits the fault is located, it is much easier to find and fix it.

Further elimination of such leaks is a reason to contact a professional auto electrician. The check makes it possible to understand that the battery is fully operational, and self-discharge occurs due to the fault of the car's internal electrical system and has nothing to do with the battery.

Capacity check

There are many ways to check the battery capacity of a car, but the simplest are two methods of measuring with a multimeter, namely:

• checking the capacity of the battery under load;
• checking the capacity of the battery by the discharge control method.

Load testing

The simplest way to check the capacity of a battery under load is to connect an ordinary car headlight with a power of 40 to 45 watts. The load is connected directly to the battery terminals. The headlamp should work for three minutes as a load. Then it is turned off and measured with a multimeter in the mode of measuring DC voltage in the range from 0 to 20 V.

If the battery capacity is normal, then the voltage at the terminals will exceed 12.4 V. Any problems with starting the engine are not rooted in the battery, but in the car's electrical system.

If the readings of the device are below 12.4 V, this indicates a loss of capacity by the battery, which means that soon you need to prepare for purchasing a new battery.

It is worth noting that if, when the headlight is connected to the battery, the light starts to fade after a few tens of seconds, it makes no sense to make further measurements, and the battery must be replaced.

Controlled discharge test

Before testing, you must fully charge the battery and have a passport of the tested battery.

A load is connected to the terminals of the device, corresponding to the nominal load specified in the technical passport of the battery. The load can be assembled from automotive lamps of various wattage. A tester is included in the circuit in the current measurement mode up to 10 amperes.

Next, you need to fix the discharge time of the battery in terms of current below 50%. The data obtained should be compared with the data specified in the technical passport of the battery. If the time indicator approaches the nominal value indicated in the passport, the battery is in order. If there is a significant difference in readings, the battery must be replaced.

Load fork measurements

The load plug is a device that allows you to measure the voltage while simulating the load on the battery while turning on the starter and starting the engine. Such measurements are the most accurate, but the device itself can only be found in a car service, therefore routine battery checks are usually performed with a multimeter.

Electrolyte density measurement

The battery charge test will be incomplete if the density and electrolyte level are not measured. To do this, you need a device - a hydrometer.

From the battery jar, electrolyte is pumped into the flask of the device, which also contains the measuring floats. Look at what point on the scale the hydrometer floats stop. The device is easy to use and in order to correctly navigate the indicators, you need to know the following:

• The maximum charged battery has an electrolyte density of 1.24-1.27 g / cm.
• 1.20 g / cm 3 - corresponds to 75% charge.
• 1.16 g / cm 3 - charge at 50%.
• 1.08-1.10 g / cm 3 - critical discharge.

This method of testing is unpopular, because modern batteries are mostly maintenance-free, and they do not have the ability to open cans and measure the density of the electrolyte.

It is more convenient and more accessible to measure battery performance using a multimeter. Timely battery checks will allow you to notice leaks and loss of capacity in time. This will make it possible to take timely measures to eliminate battery malfunctions, and also significantly extend the life of the device.