How to properly charge your car battery

It’s worth starting with the fact that there are many different batteries for cars, but the most common type is a rechargeable acid battery (battery). The battery life is about 3-5 years, but it can be significantly extended if you get to know the device better and learn how to use it correctly.

Each car battery needs to be fully charged at least 2 times a year – before and after winter. But it’s better to do this every time the battery drops to 50-70%. If this is neglected, the battery will be constantly not fully charged, even with the generator running. This does not seem to be a problem, but it is worth understanding the topic a little more in detail. 

From the name it is clear that the battery runs on an acid solution. Active chemical reactions always take place inside the device. Therefore, the plates of an uncharged battery are covered with lead sulfate – this process is called sulfation. Because of it, the battery parameters drop significantly.

The longer the battery works in an uncharged state, the more “growths” on the plates and the lower the capacity of the battery. In the cold season, the starting current may simply not be enough to start the starter, and the car will not start. It is for this reason that demand for batteries grows in winter – people think that the device is broken, and they are in a hurry to buy a new one. 

But sulfation is reversible. When the battery is fully charged, lead sulfate breaks down back into its constituent parts of a chemical reaction. This prolongs its life and helps conserve battery capacity. Therefore, it is extremely important to know how to properly charge a car battery and how to measure its energy reserve. 

How to tell if a battery needs charging

If electrical appliances begin to work less when starting the engine or the car does not start at all, its battery may be dead. You can verify this in many ways. Almost all of them are related to the voltage of the battery or the density of the electrolyte – the solution that is contained in the battery.

The battery level is important to know not only in emergency cases. You need to monitor it regularly and charge the battery in time, if necessary. We will tell you how to identify this very need and protect the battery from rapid wear.

How to find out the battery charge by electrolyte density

It should be noted right away that this is a very accurate method of measurement. But it is not suitable for maintenance-free batteries, that is, for closed batteries without access to the solution. This method is based on a simple relationship: the higher the density of the electrolyte, the greater the battery charge. 

For measurement, a hydrometer or refractometer is useful. With their help, the exact density of the electrolyte is known. According to the result of measurements, we can conclude about the battery charge:

• 1,28 g/cm³ – 100%, the battery is fully charged, it can be used,
• 1,19 g / cm³ – 50%, the battery is half charged, it needs to be recharged,
• 1,10 g/cm³ – 0% battery is completely discharged and needs to be recharged or replaced.

Density also depends on temperature, this is also important to consider. The colder it is outside, the higher the density of the electrolyte. The above numbers are valid for measurement at 25 degrees. In warmer or colder climates, 100% charge will correspond to other values: from 1,24 g/cm³ to 1,31 g/cm³.

In addition, using a hydrometer, you can identify a defect in one of the battery cans. To do this, you need to measure the density in all containers – for a working battery, the value will be the same everywhere. But if a short circuit occurs in one of the jars, the density of the solution in it will be noticeably lower than in the others.

How to check battery charge voltage

The voltage can also determine how charged the battery is. But if the battery has just been removed from a charge or used in a car, its voltage will be higher. Therefore, for accurate measurement, it is better to let it “cool down” for several hours. 

There are two ways to measure battery voltage. The first is measurement without load. All you need is a multimeter or voltmeter. A multimeter is a versatile device for measuring various electrical characteristics. It includes the functions of a voltmeter, so that these devices can be taken as one – the sequence of actions will be the same in any case:

1. Turn off the ignition and all electrical appliances, as well as the light, otherwise the result will be inaccurate.
2. On the voltmeter, limit the measurement range to 20 V. That is, to a value that is exactly higher than the voltage of a standard 12 V battery in any of its states.
3. Connect the probes – the ends of the wires – to the battery terminals. It is important not to confuse the signs: the red wire is needed for the plus terminal, the black wire is for the minus terminal.

After that, the device will show the current voltage of the battery. The battery is considered fully charged if its voltage is 12,7 V. A decrease of 0,1 V is a loss of 10% of the energy reserve. Therefore, if the instrument shows 12,4 V or less, the battery must be charged. 

The second method of verification is most often used in service centers, but nothing prevents the car owner from using it on his own. To do this, you need a load plug – this is a device with a resistor that includes the functions of a voltmeter. With its help, a short-term load on the battery is created, at the end of which a voltage measurement is taken. 

The measurement option with a load fork gives a more accurate result. 10,2 V is the ideal indicator, it indicates the integrity of the battery and its full charge. A voltage of about 9 V means that the battery needs to be recharged – the energy reserve is at an average level. If the value is below 8V, the battery is completely discharged or defective. 

Pay attention to the important features of the second method:

• It is not recommended to use the load plug to the battery in the cold. The air temperature should be at least 20 degrees.
• The current during the load should be 100 A, sometimes 200 A.
• You need to load the battery for 5 seconds, the measurement at the end of the fifth second is the most accurate. 
• After loading, the voltage should return to its normal value. That is, the voltage will change from 10,2 V to 12,6 V or higher.

If there was no load plug with you, you can turn on the headlights of the car – this is also a load on the battery. But in this case, you need to focus on other voltage values: 11,8 V is a high battery charge, 10 V and below – the battery is discharged or almost discharged. However, this method cannot be called accurate.

How to measure charge without a voltmeter or hydrometer

If there were no devices for measuring density or voltage at hand, alternative methods will come to the rescue:

1. Indicator check. Some batteries have special “eyes” by the color of which you can determine the charge of the battery. If it is green – the battery is charged, white – the charge level is low, black – the device is discharged or malfunctioning. But it is worth noting that this indicator is not a light bulb, but a ball. That is, a lens that shines through a hole in the lid. Therefore, the accuracy of such a measurement is not very high, and the green glow can be maintained even at 70% of the energy reserve – when the battery is actually time to charge.
2. Charger check. This is a simple method – some memory has a special button for checking. You need to connect the device to the battery terminals and press it, and then fix the readings.
3. Checking with an alarm. Some alarm remotes have a dedicated button. By briefly pressing this button, you can see the voltage level of the battery on the display. If the system is remote, all the same data can be found in the alarm application with a GSM module.
4. Checking the level of the solution. The measuring glass tube must be lowered into the liquid until it comes into contact with the solid part of the battery. A value of 10-15 mm on the scale indicates that the battery is charged.
5. On-board network voltage indicator. This is a special device that connects to the cigarette lighter. With it, you can monitor the voltage.

There are many more ways to check the battery charge and its health. But many of them are dangerous or not accurate enough. 

How to prepare the battery for charging

Charging a battery is a simple process, but you need to take it responsibly and seriously. Inside the battery contains a solution of sulfuric acid, and it is very dangerous for humans. In addition, gases, including explosive hydrogen, are released from the electrolyte during charging. 

Safety is not just a formality. The health or lives of many people around may depend on this knowledge. Therefore, it is extremely important to follow the safety rules when charging a car battery:

1. First you need to wear overalls or ordinary things that cover the body as much as possible. Cotton clothes will not work, because the acid easily “eats through” them. The main thing is to protect your eyes with glass or plastic glasses, and your hands with rubber gloves.
2. Then you need to check the battery cover in the car for damage and electrolyte smudges. But if acid is found, you should not try to remove it mechanically – wipe it off with something or rinse it off with water. For these purposes, soda is used. If the device is in order, it can be removed from the car.
3. The battery must not be charged in a poorly ventilated room or in an apartment with children. It is best to do this in the garage. The reason lies in the dangerous gases that are released during charging.
4. Do not smoke in a place where the battery is being charged. One spark may be enough to cause an explosion.
5. If the battery has been in the cold for a long time, it must first be allowed to warm up to room temperature.
6. Before connecting the charger, it is necessary to clean the battery case from dirt. Also, oxidation must be removed from the battery contacts.
7. The battery must be charged away from other electrical devices. It is better to remove metal accessories from your hands, and put electronic gadgets away.
8. If the battery has a filler hole, it must not be shaken or turned over. Care must be taken when working with such batteries, otherwise the acid solution may spill.
9. Remove the covers from the battery containers, if any. For maintenance-free batteries, remove the plug from the gas outlet. This is necessary so that the evaporation of substances does not accumulate inside the container.

When working with the battery, you should keep a solution of soda (10%) with you – they can neutralize the acid. If liquid from the battery gets on the skin, and there is no soda at hand, the burn area can be washed under running tap water. After charging, wash your hands with warm water and soap.

Choosing how to charge your car battery

There are two ways to charge a car battery. In both cases, the value of one of the two main electrical quantities is fixed. That is why it is recommended to buy a charger with which you can control both the current and voltage. The owner of this charger can choose the charging method himself. 

How to properly charge a car battery? There is no unequivocal answer to this question, but the method with constant voltage is considered more correct. However, both methods have a number of advantages and disadvantages. We will describe them in detail below.

DC Charging

For charging, a constant current value is set. This does not mean that nothing needs to be changed until the very end – quite the contrary. The fact is that the charging current (measured in amperes – A) will need to be reduced in the process. This is done manually, with the help of memory. The sequence of actions is as follows:

1. The charger is set to current. Its value should be 10 times less than the battery capacity. For example, if 70 A * h is written on the battery case, you need to set the current to 7 A.
2. The battery will start charging and its voltage will gradually increase. When it reaches 14,4 V, the current strength will need to be reduced by 2 times.
3. When the voltage reaches 15 V, you need to once again reduce the current strength by 2 times. This incremental reduction reduces the risk of device failure.
4. The battery is fully charged if the current and voltage do not change within 1-2 hours.

DC charging is relatively fast – in about 10 hours, but it has its drawbacks:

• The battery wears out faster.
• A lot of gases are released, especially when the battery is almost fully charged.
• You need to regularly monitor and control the process: switch the current strength in time and check the battery temperature (if it is above 40 degrees, the battery must be disconnected and charging should be continued later).

It is better not to use this method on an ongoing basis. It is suitable in cases where there is no time for constant voltage charging, but the battery needs to be recharged.

DC voltage charging

This method is the opposite of the previous one. A constant voltage is set on the device (in volts – V), while the charging current will change. It will be more “strong” at first, especially if the battery is low. Therefore, it is imperative to make sure that the charger has a limiter (usually up to 20 A). Then the current will gradually decrease. After all, the greater the battery charge, the less the device needs to be recharged. 

How to charge a battery with constant voltage:

1. The charger is set to a fixed voltage of 14,5 V, then charging is turned on. 
2. The process does not need to be controlled as carefully as in the first method. The voltage will always stay the same, but the current will drop. 
3. If the current drops to almost zero, charging is complete. You can also focus on a value that is 100 times smaller than the battery capacity. That is, if the battery has a capacity of 70 A * h, and the current in the process has decreased to 0,7 A, the battery is already almost fully charged.
4. For this method, a memory with a digital display is best suited. Because on outdated screens with an arrow it is more difficult to see the exact value of the current strength.
5. At the end, you can let the battery “rest” and check the voltage at the battery terminals. If it corresponds to the specified, that is, 14,5 V or higher, then the battery is charged. 

This method is considered correct, because the battery wears out more slowly. But it is not without its shortcomings:

• The battery is not fully charged, but 80-85%. If you charge the battery with a constant voltage of 15 V, and not 14,5 V, it will be charged to 90%.
• The second way is long – the battery can be charged for longer than a day.
• At low temperatures, an acid battery cannot be charged correctly with a constant voltage. This is due to the increased resistance inside a cold battery.

The option with constant voltage is considered correct. If there is no need for urgent charging, you should choose it. 

How to charge a car battery with a charger

When the terminals and body of the device are cleaned, the covers are removed from the battery containers, and overalls are put on, it’s time to start charging the car battery. To do this, remember a simple procedure: 

1. First, the charger terminals are connected to the battery contacts. It is important to remember the signs: the “red” wire is connected to the plus, and the “black” to the minus.
2. Only now the charging device is connected to the network.
3. Using special buttons on the charger, you need to set the parameters – current or voltage. It depends on the charging capacity. 
4. After setting the parameters, you need to press the charging power button, and if it is not there, the process will start automatically.
5. When the battery is charged, you need to turn off in the reverse order: first turn off the charging on the device, then remove the plug from the outlet, and only at the very end – remove the charger terminals from the battery contacts.

If you follow all the steps according to the instructions, there will be no problems. The main thing to remember is that there should be no fire or sparks in the room with the battery. That is why the terminals are connected to the battery at the very beginning, even before being connected to the network – this is the most important point.

How to understand how much to charge a car battery? It depends on the charging method. Many modern chargers have an indicator that will let you know that the battery is charged. Most often, the process takes about a day.

How to charge a car battery from another car

If the car’s battery is dead, and its power is not enough to start the starter, you can charge the battery from the generator of another car. This method is also called “lighting”. For recharging, you only need a car of an indifferent motorist and special starting wires with large clips – “crocodiles”. They most often have standard colors: black and red. 

It is also worth paying attention to the fact that to recharge a standard 12-volt battery, you also need a 12-volt battery. The battery voltage of trucks and heavy equipment is 24 V or 48 V, they are not suitable for lighting a car.

A mistake in lighting is sometimes very expensive for car owners. If you do something wrong, you can lose all the electrical equipment or the car battery. Therefore, it is worth knowing the correct sequence of actions:

1. First you need to put the cars so that the wires of the “crocodiles” can reach both batteries.
2. On the donor car, that is, on a serviceable car, you need to turn off all electrical equipment and ignition.
3. Next, you need to connect the pluses of the batteries on the cars with a red starting wire.
4. You have to be careful with cons. One clip of the black wire is attached to the negative terminal of the donor machine. And the second clip is connected to a bare metal part on the body or engine of another car. This ensures that the battery of the donor car will not run out in the process.
5. Make sure that the wires do not get stuck anywhere and are not near moving parts of the engine.
6. After that, you need to start the engine of a serviceable car and do not turn it off for at least 5 minutes. The colder it is outside, the longer the engine needs to run. After that, it is necessary to turn off the engine and turn off the ignition.
7. Now you can check if the car starts, the battery of which has been discharged.
8. After all the steps, you need to carefully disconnect the start wires, while not touching the other contacts with the clamps.

This method is a last resort. After lighting the car, the car will start and work properly for some time, but this is not a reason to refuse normal charging. It is advisable to fully charge the battery so as not to reduce its capacity and other important characteristics.

Popular questions and answers

We asked an expert to answer the most common questions about car battery charging – Nikolai Baranov, General Director of the club car service “Details”.