How to charge a car battery?
Lead batteries must be charged from a constant (rectified) current source. You can use any rectifiers that allow adjustment of the charging current or voltage. At the same time, a charger designed to charge one 12-volt battery should provide the possibility of increasing the charging voltage to 16.0-16.5 V, since otherwise it will not be possible to fully charge a modern maintenance-free battery (up to 100% of its actual capacity).
The positive wire (terminal) of the charger is connected to the positive terminal of the battery, the negative to negative.
In operating practice, they usually use one of two methods of charging the battery: charge at constant current or charge at constant voltage. Both of these methods are equivalent in terms of their effect on battery life. When choosing a charger should be guided by the information below.
Constant current charge
The battery is charged at a constant charge current of 0.1 x C20 (0.1 of the nominal capacity in a 20-hour discharge mode). This means that for a battery with a capacity of 60 A • h, the charge current should be 6 A. To maintain a constant current throughout the entire charging process, a regulating device is required.
The disadvantage of this method is the need for constant (every 1-2 hours) monitoring and regulation of the charging current, as well as abundant gas evolution at the end of the charge.
To reduce gas evolution and increase the degree of battery charge, it is advisable to stepwise decrease the current strength as the charging voltage increases. When the voltage reaches 14.4 V, the charging current is reduced by half (3 Amperes for a battery with a capacity of 60 A • h) and at this current the charge is continued until gas evolution begins. When charging the latest generation of batteries, which do not have openings for adding water, it is advisable to reduce the current by half again when charging voltage is increased to 15 V (1.5 A for 60 Ah batteries).
The battery is considered fully charged when the current and voltage during the charge are stored unchanged for 1-2 hours. For modern maintenance-free batteries, this condition occurs at a voltage of 16.3-16.4 V, depending on the composition of the alloys of the lattices and the purity of the electrolyte.
Constant voltage charge
When charging with this method, the degree of charge of the battery at the end of the charge directly depends on the magnitude of the charging voltage that the charger provides. So, for example, in 24 hours of continuous charge at a voltage of 14.4 V, a 12-volt battery will be charged at 75-85%, at a voltage of 15 V – at 85-90%, and at a voltage of 16 V – at 95-97%. It is possible to fully charge the battery for 20-24 hours at a charger voltage of 16.3-16.4 V.
At the first moment of turning on the current, its value can reach 40-50 A or more, depending on the internal resistance (capacity) of the battery. Therefore, the charger is equipped with circuit solutions limiting the maximum charge current to 20-25 A.
As the charge increases, the voltage at the terminals of the battery gradually approaches the voltage of the charger, and the value of the charging current, respectively, decreases and approaches zero at the end of the charge (if the value of the charging voltage of the rectifier is lower than the voltage at which gas evolution starts). This allows you to produce a charge without human intervention in a fully automatic mode. Usually, the criterion for the end of charge in such devices is the achievement of a voltage at the terminals of the battery when it is charged equal to 14.4 ± 0.1 V. In this case, as a rule, a green signal lights up, which serves as an indicator of the achievement of a given final voltage, i.e. end of charge. However, for a satisfactory (90-95%) charge of modern maintenance-free batteries using industrial chargers with a maximum charging voltage of 14.4-14.5 V, it will take more than a day.
Battery charge on a car
When the battery is used in a car, its charge occurs at constant voltage. Car manufacturers, in agreement with the battery developers, set the charging voltage level to 14.1 ± 0.2 V, which is lower than the intense gas emission voltage. With decreasing temperature, the charge efficiency at constant voltage decreases due to an increase in the internal resistance of the battery. Therefore, the battery in the car does not always restore its capacity after discharge completely. Typically, the degree of battery charge in winter is 70-75% if the voltage at the battery terminals is 13.9-14.3 V with the engine running and the main beam on. Therefore, in severe winter conditions (at low temperatures, frequent and long starts of a cold engine and short runs) it is advisable to periodically (preferably at least once a month) charge the battery from a stationary charger and at a positive temperature.
For a fully charged battery, the electrolyte density is 1.28 ± 0.01 g / cm3, linearly decreasing, as the battery discharges, it is 1.20 ± 0.01 g / cm3 for batteries, the charge level of which has decreased to 50%. A fully discharged battery has an electrolyte density of 1.10 ± 0.01 g / cm3.
If the density value in all batteries is the same (with a spread of ± 0.01 g / cm3), this indicates the degree of charge of the battery and the absence of internal short circuits. In the presence of an internal short circuit, the density of the electrolyte in the defective battery bank will be significantly lower (by 0.10-0.15 g / cm_) than in other cells.
To measure the density of liquids, hydrometers with replaceable densitometers are used to measure the density of various liquids, for example, antifreeze with a density of 1.0 to 1.1 g / cm3 or an electrolyte with a density of 1.1 to 1.3 g / cm3.
When measuring, the float should not touch the walls of the cylindrical part of the glass tube. At the same time, it is necessary to measure the temperature of the electrolyte. Density measurement result in + 25 ° C. To do this, add or subtract the correction indicated in the special literature to the readings of the densitometer.
If during the measurement it turns out that the NRC is below 12.6 V and the electrolyte density is below 1.24 g / cm3, the battery must be recharged and the charging voltage at its terminals checked while the engine is running.
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