Jan 292013
 

Car batteries, even when they are not used, they self-discharge over time. Car batteries have a standard level of natural self-discharge and it says that battery discharge, without having a consumer connected to it, should not exceed 1% per day. This self-discharge is due to several chemical and physical reasons, some of which are the following: when part of the active mass falls to the bottom of the battery, this could result in partial short circuits between the anode and the cathode plate, which causes discharge. Another common cause for natural self-discharge is the occurrence of cracks in the partitions between adjacent cells in the battery box, which leads to galvanic connection between the two electrolytes and partial self-discharge. Other causes are related to human negligence and errors during battery exploitation. Often distilled water is added without additional charge, which discharges the electrolyte and creates zones of unequal density. Moreover, ordinary water is added instead of distilled water, which significantly reduces the lifetime of the battery. If a battery is left for a long period without being charged, its capacity will fall below the critical minimum and no charging will work afterwards. The charging in cars is done by the car generator (alternator) when the car is in motion, so during a long idle time of the car, the battery must be additionally charged. Additional charging is also required when the car is used often, but for short distances with frequent stops of the engine. This is especially important in winter when temperatures are freezing, because the cold battery has less capacity than the warm one. If the battery is fully discharged, it needs to be replaced.
When replacing the battery, you need to pay attention to the following important parameters:
The battery must be chosen with dimensions corresponding to the place where it must be put. The more powerful batteries are also larger in size and do not always fit into the place of the old battery. Check whether the battery has left or right plus and make sure the new battery has the same polarity. For this purpose, you have to check the battery from the side where its clamps are. Check the capacity of your old battery and choose a new one with the same or higher capacity. If the old battery has an indication 55Ah, it is better to buy a battery with 55, 60 or 62Ah. However, if the car is second hand, the old battery may have not been selected correctly, therefore you should check what kind of battery is needed for this specific car model. This can be done by visiting the website of the manufacturer and complying with their requirements. The battery is also chosen depending on the way of use of the vehicle and on the presence of electrical consumers which consume a lot of electricity. For example: if you have a car with a low fuel consumption, which you drive almost daily for more than an hour each day without frequent stops of the engine and without additional consumers, you can purchase a smaller battery. However, if you have a powerful engine, if you drive in urban conditions, if you often have to stop the engine and re-start it, if you have GPS navigation, powerful audio system, strong halogen lights, etc. you need to choose the most powerful battery that fits into the designated place.
The plates of the battery have lattice structure of active material – lead and lead oxide. After charging the positive plates, lead dioxide is accumulated, and when charging the negative ones – spongy lead, and the electrolyte is composed of diluted sulfuric acid. When the battery functions, the lead interacts with the lead oxide and the sulfuric acid and they form lead sulfate and water. The charging activates the reverse reaction.
This means that during long term usage of the battery, the electrolyte is diluted, so one indication of battery discharge is the density of the electrolyte. The density can be measured by а densitometer (areometer) as long as the battery is not sealed. If it is not hermetically sealed and has plugs on the individual cells which may be removed, this must be done before the charging starts. Тhe areometer is an encapsulated glass tube, at the bottom of which there are lead balls to achieve the calibration of the specific weight of the device and in the elongated upper part there is dimensioning which indicates the density of the electrolyte. The reading is done by immersing the device in the electrolyte and the level to which it sinks is reported. There are areometers, which are filled with electrolyte, like a pipette. Often areometers are combined with thermometers, because the density of the electrolyte depends on the ambient temperature. The default value of the density of the electrolyte is 1.28-1.26g / sm3. The process of discharging the battery is called sulphation. This is a process in which the lead is converted into lead sulphate. Charging is carried out to restore the capacity of the battery and this process is called desulphation. When performing cycles of sulphation and desulphation, after a number of cycles of charging and discharging the battery runs out of its active substance and must be replaced. If a discharged battery is left at freezing temperatures for a long time, the liquid might freeze because of the prevailing water quantity in it. It is very important that when we open the battery, the electrolyte level is above the level of the plates. If there are parts in which the plates are above the liquid level, we need to add electrolyte or at least distilled water before the charging is started.

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Battery chargers
There are different chargers with different output voltages (6, 12, 24VDC), different output currents and protections. Typically, these devices are equipped with overload protection, reverse voltage protection and short circuit protection. The devices are equipped with an ammeter which displays the charging current at any time of the charging. At the beginning of the charging the current is high but it gradually lowers.

Most modern devices for battery charging automatically regulate the charging current and voltage. The correct charging current does not exceed 10% of the battery capacity (if the battery has capacity of 70Ah, then the charging current mustn’t exceed 7A).
The lower the current, the longer is the charging time, but the battery is charged more efficiently. There are devices in which when the battery is charged, a green light is activated and the device turns off automatically.
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Battery charging
It has to be carried out in a ventilated room, because the charging releases gas, which is flammable and poisonous. We should also avoid contact with the electrolyte because it contains diluted sulfuric acid and it can damage your clothes. Even if the battery is relatively new and its parameters are good, it needs recharging, especially during the winter period, which will provide the necessary balancing charge and desulphation.
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The battery has to be disconnected from the electric circuit of the car and then placed on the charging spot. The negative terminal must be disconnected first, and then the positive one, and when placing the battery back – the other way round. This is done because if you start to disconnect the positive terminal first, the wrench may contact the metal body of the car and provoke short circuit and a spark. The attempts to charge a battery without previously disconnecting it first, can cause damage to the electronics of the car. The battery terminals must be cleaned well of residue and oxides using a wire brush or a sandpaper.

If the battery has removable caps, they must be removed. The clips must be attached to the battery terminals, observing the polarity – the red clip is connected to the positive terminal and the black one – to the negative terminal. Then the charger needs to be plugged into the mains. You must always keep an eye on the charging. After a while you can see bubbling of the electrolyte. If it is very strong, the charging current needs to be reduced, because charging a battery through very high charging current shortens its life. The total charging time depends on the exhaustion of the battery and the magnitude of the charging current.

The friendliest method of charging is the continuous charging where the charging voltage must not exceed 14.4VDC for 12VDC battery. If the battery gets too heated or swollen, you must temporarily turn off the charger or reduce the charging current. If the voltage is greater than 14.4VDC, hydrogen and oxygen are formed in the battery. As they are explosive gases, the presence of a spark can cause explosion. The overcharging of the battery is as harmful to it as the undercharging. Overcharging leads to a rapid process of active material loss, fluid loss and the reduction of the battery life.

If the battery hasn’t been charged for long time and has remained in a discharged condition, this would cause irreversible chemical reactions and impossibility to be fully charged, regardless of the duration of charging. Moreover, a heavy discharged battery mustn’t be charged by a charging current equal to 1/10 of its capacity, according to the rule, but by current which doesn’t exceed 1/20 of its capacity.
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Battery testers
There are various ways to check if a battery needs charging. One of them is checking the density of the electrolyte, but sometimes, with good density indicators, the battery cannot maintain sufficiently strong starting current. Another way is using a DC voltmeter, often as part of a portable multimeter, to measure the voltage of the electrodes, which for 12VDC battery must be above 12VDC (about 12.4VDC). We can also use an ammeter to measure the inrush current, but the most reliable and complete results are obtained when using specially designed equipment, called battery testers.

These are portable devices with the size of a multimeter, but made of plastics, which is resistant to acids. The device has two clamps connected with the tester through flexible cables, crocodile type, that are connected to the respective battery terminal – the red one to the positive terminal and the black one to the negative terminal, an electronic display which shows the read results and protections from overvoltage, reversed polarity of the device, etc. Some professional devices are able to print the results. The main advantage of battery testers is that they measure the voltage of the battery, the start voltage, the inrush current, the voltage at idle running and under load, all of which can be measured without disconnecting the battery. Most of these testers do not require external power supply and draw power from the battery itself.

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