How to charge a car battery

Every car owner must know about the need to periodically charge the battery. The longevity and stable operation of the battery depends on this over the entire life of the battery, as well as the safety of the vehicle's on-board network.

How to determine if the battery is dead or not?

How to charge a car battery

It is quite easy to determine the battery discharge for direct and indirect reasons. But as a rule, the first signs are the dim headlights and a sluggish starter. Among other things, there are the following reasons:

  • inadequate operation of the alarm system, opening and closing the car with a delay, the central locking actuators work every other time;
  • when the engine is turned off, the radio is also turned off;
  • the headlights dim; interior lighting; when the engine is running, the brightness of the light changes;
  • when the engine starts, the starter initially grasps, then stops turning, then it spins at normal speed;
  • floating revolutions during engine warming up.

How to prepare the battery for charging

How to charge a car battery

To prepare for charging the battery, use the following algorithm:

  • remove the battery from its place by first disconnecting the negative terminal after the positive terminal, or depending on which terminal the quick-detacher is installed on. If the ambient temperature is less than + 10 ° C, then the battery should first warm up;
  • clean the terminals by removing sulfation products, grease, and wipe the battery case with a cloth moistened with a 10% solution of ammonia or soda;
  • If the battery is serviced, then you need to unscrew the jams on the banks and put them next to it. It is advisable to check the electrolyte density with a hydrometer. If the battery is maintenance free, remove the vent plug for free reagent vapor release;
  • for a serviced battery, distilled water must be added if the plates in the jar are less than 50 mm submerged; in addition, the level should be the same everywhere. 

It is extremely important to observe safety precautions, familiarize yourself with them before the charging process, especially if you do this at home:

  • the charge is carried out only in a ventilated room, preferably on the balcony, as harmful chemicals evaporate from the battery;
  • next to open banks when charging, you can not smoke, do welding work;
  • remove and wear terminals only when the charger is turned off;
  • Do not charge the battery with high humidity;
  • unscrew and tighten the lids of the cans only in protective gloves and glasses, in order to avoid the ingress of acid on the skin of the hands and eyes;
  • keep 10% soda solution near the charger.

Charger or generator - which charges better?

How to charge a car battery

It should be understood that with a serviceable generator and related parts, you do not need to charge the battery. It is designed to be charged by a generator (direct current charge).

The task of the stationary charger is to partially restore the battery, after which the generator will charge it up to 100%. A modern charger has a number of functions that prevent the electrolyte from boiling in the battery, and interrupt their work to achieve a charge of 14.4 volts.


A car generator charges the battery in the range from 13.8 to 14.7 volts, while the battery itself determines what current is needed to provide voltage to all power systems. Therefore, the principle of the generator and the stationary memory are different. In the ideal case, it is better to rarely use third-party battery charging.

What current and how long does it take to charge a car battery

The current is determined by the capacitive characteristics of the battery, calculated individually. The labels of all batteries indicate the rated capacity, indicating how to charge the battery. The optimal value of the charging parameter is about 10% of the battery capacity. If the battery is more than 3 years old or it is very discharged, then 0.5-1 amperes should be added to this value. 

If the starting current parameters are 650 Ah, then you need to charge such a battery at 6 amperes, but provided that it is only a recharge. 

If necessary, quickly charge the battery, in emergency situations, you can choose the value of 20 Amperes, while keeping the battery under charge for no more than 5-6 hours, otherwise there is a risk of acid boiling away.

How to charge the battery

Before you charge your battery with a charger, you need to know that the voltage is measured in Volts (V), and the current in Amperes (A). It is possible to charge the battery only with direct current, we will consider in detail. 

DC Charging

A simple way to provide constant current is to connect in series with a charged battery of a variable rheostat, however manual adjustment of the current is required. You can also use the special current regulator, which is also connected in series between the charger and the battery. The current strength at which the 10-hour charge is carried out is equal to 0,1 of the total battery capacity, and at 20-hour 0,05. 


DC voltage charging

How to charge a car battery

Constant voltage charging is somewhat simpler than direct current. The battery is connected observing the polarity when the charger is turned off from the network, after which the “charger” is turned on and the value at which the battery is charging is set. Technically, this method of charging is simpler, because it is enough to have a charger with an output voltage of up to 15 volts. 

How to determine the degree of battery charge

There are several methods available to measure the state of charge of the battery, which indicate the state of the battery. Let's consider in detail.

By measuring the voltage at the terminals without load

For a 12-volt acid battery, there is data that indicates the degree of discharge and other characteristics. So, the following is a table of the degree of charge of a 12-volt battery at an ambient temperature of 25 ° C:

Voltage, V12,6512,3512,1011,95<11,7
Freezing point, ° С-58%-40%-28%-15%-10%
The degree of charge,%-58%-40%-28%-15%-10%

In this case, the voltage at the terminals must be measured in the idle state of the battery and not earlier than 6 hours after its last operation on the machine.

 Electrolyte density measurement

An electrolyte is filled in a lead-acid battery, which has a variable density. If you have a hydrometer, you can determine the density in each bank, and in accordance with the data in the table below, determine the degree of charge of your battery:

The density of the electrolyte, g / cm³1,271,231,191,16
Freezing point, ° С-58%-40%-28%-15%
The degree of charging,% 100755025

Density measurement is carried out no earlier than one hour from the last moment of battery operation, only in the state of its rest, be sure to turn it off from the car circuit.

With a load fork

The easiest way to determine the degree of charge is with a load plug, while the battery does not have to be disconnected from the power systems and removed from the car.

The load plug is a device with a voltmeter and leads connected in parallel. The plug is connected to the battery terminals and the readings are taken after 5-7 seconds. Using the table below, you will find out the state of charge of your battery, based on the data of the load plug:

Voltage at battery terminals, V  10,59,99,38,7<8,2
The degree of charge,% 1007550250

According to the voltage under the load of the electrical equipment of the car

If there is no load plug at hand, the battery can be easily loaded by switching on the headlights and the stove. At the same time, using a voltmeter or multimeter you will get accurate data that will indicate the performance of the battery and generator.

How to charge a car battery

If the car is equipped with a voltmeter (GAZ-3110, VAZ 2106,2107, ZAZ-1102 and others), then when you start the motor, you can determine the degree of charge by observing the arrow of the voltmeter. At the same time, the operation of the starter should not drain voltage below 9.5V. 

By built-in hydrometer indicator

How to charge a car battery

Most modern batteries are equipped with a hydrometric indicator, which is a peephole with a color indicator. With a charge of 60% or more, the eye will show in green, which is enough to confidently start the engine. If the indicator is colorless or white, it means insufficient electrolyte level, topping up is required. 

Rules for charging a car battery

How to charge a car battery

Using the rules of proper battery charging, you can efficiently and correctly charge the battery, while ensuring the safety of yourself and others, and also extend the battery life. Next, we will answer frequently asked questions.

Is it permissible to charge the car battery in cold weather

Most car owners do not suspect that in winter the degree of battery charge may not exceed 30%, which is affected by the negative outside temperature, which affects the discharge. If the battery freezes in the cold, then this is fraught with failure, especially if it freezes water. On a vehicle, the battery will only charge efficiently when the temperature is above 0 ° C under the hood. If we are talking about the use of a stationary charger, then let the battery warm up at room temperature + 25 ° for several hours. 

To prevent freezing of the battery, if in winter the average temperature varies from -25 ° to -40 °, then use a heat-insulating cover.

Can I charge a car battery with phone charging

Unfortunately, it is not possible to charge the battery with a charger for a mobile phone. The first reason is the characteristic of the phone’s charger, which rarely exceeds 5 Volts and 4 Ah. Among other things, with a 100% probability, you risk provoking a short circuit in the banks of the battery and knocking out plugs in 220V machines. That is why there are special chargers for batteries.

Can I charge a car battery with a laptop power supply?

As practice shows, using a laptop power supply, you can recharge the car battery. To do this, you must follow the sequence for connecting the PSU, car bulb and battery. Despite the fact that many managed to charge their battery in this way, it is still recommended to use the classic method. Any of the alternative methods is dangerous because the charger and battery may behave inappropriately. If you are interested in this method, be sure to watch the video below.

Charging a car battery with a laptop power supply

Is it possible to charge the battery without disconnecting from the on-board network of the car

Theoretically, this method of such charging is possible, but subject to certain rules, otherwise it can lead to failure of the entire on-board network of the car. The rules of such a charge:

  • connect the terminals of the charger and only after that plug it into the 200V network, and at the end of charging, the terminals are first removed, after which the charger is disconnected from the network. If you mix up the sequence, then this is fraught with the formation of overvoltage;
  • connection of terminals starts with positive; disconnection with negative;
  • Before connecting the memory, all power consumers of the machine must be turned off, in no case leave the keys in the ignition;
  • The charging process should only be carried out in a well-ventilated area.

Can I light from another car?

How to charge a car battery

A frequent and effective method of charging is “lighting” from another car, but only if the starter turns sluggishly. Technically, this process is easy, but neglecting the simplest rules can lead to failure of the engine control unit, BCM units, and so on. Sequence:

  • we connect the wires to the donor battery, first, with the help of crocodiles, it clamps the “+” terminal after the “-”;
  • the other end of the wires is connected to the battery of the recipient, in the same sequence, observing the polarity, otherwise as the minimum you and the donor will lose the battery;
  • the recipient’s engine starts, it is desirable that the operating speed range is from 1500 to 3500 revolutions, after 20 seconds you can disconnect the wires from the patient, and try to start the ICE. Usually this time is enough.

Remember, in no case do not connect to the patient’s battery when the engine is running, otherwise the generator and a number of electrical equipment are more likely to fail. 

How charging affects battery life

The average life of a more or less quality battery is from 3 to 5 years. If the generator is always in good condition, the drive belt changes in time, and its tension is stable, then there is no need to charge the battery for a long time, but only if you use the car at least 2 times a week. Charging the charger itself does not affect the reduction of battery life compared to the following list:

  • undercharge or overcharge. Both of these adversely affect the further life of the battery, for example, because overcharging leads to boiling of the acid and destruction of the cans;
  • salt deposits and deep discharge, because of which it will be necessary to reanimate the battery, and this is a significant reduction in battery life;
  • Fast or express charging is extremely detrimental to the battery, since in this case there are high currents, so this method can only be used in the most extreme cases. 


Proper charging of the battery significantly determines its service life and overall performance. Always use the charging rules, monitor the technical condition of the generator and drive belt. And also, as a preventive measure, charge the battery every six months with small currents of 1-2 Amps. 

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  1. How many hours does the new Tomtom Easybike battery need to be charged?

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