Accumulators – General information
Car batteries are rechargeable battery packs. It is almost always lead-sulfuric acid, although in some cases it is steel – alkali or nickel – alkali. The battery consists of six 2 Volt elements, connected in series so that their terminals have a 12 volt potential difference. The actual battery voltage is not always rated at 12V. It varies from 14.5 V immediately after a full charge to 10.8 V if it is totally uncharged.
The basic features of a battery are:
- It can be recharged as long as it has given some electricity.
- It has low internal resistance and can give high currents for starter start without causing significant voltage drop.
- Its electrolyte is dilute sulfuric acid and as it is corrosive, it should not be allowed to come into contact with the eyes, skin, clothes or parts and parts of the car (If some acid drops, it must be rinsed with plenty of water).
- During charging, the electrolyte produces hydrogen and oxygen. This gas mixture is explosive and care must be taken to avoid sparks, cigarettes or flames in the area where battery is charged.
- Part of the water dissolving sulfuric acid evaporates during discharge and requires replenishment with distilled or deionized water.
In order to have chemical storage and power release, we need to have two different conductive materials in close proximity to each other immersed in a conductive liquid, the electrolyte. The automobile batteries use lead-antimony alloy plates coated with lead oxide paste. There are several plates in each item. After charging, we have a change of lead oxide to lead dioxide (which has a chocolate color) on the positive plates and porous lead (gray color) on the negative plates. So when the battery is fully charged, we have two different materials (lead dioxide and porous lead) immersed in dilute sulfuric acid. As long as the battery is discharged then the plates’ surfaces are converted into lead sulphate. Between the plates there are chemical inert separators. Previously it was wooden or porous rubber, but modern materials such as Kieselguhr inorganic (KG) with glass fiber reinforcement are used today. The dividers must be durable, because during the loading and discharging (especially in high currents) the plates are heated and stoned. They must also be porous so as to allow the electrolyte to pass. If their pores are small then the internal resistance of the battery increases and the voltage drops to its terminals. The plates are grouped and are so arranged that at the beginning and end of the group there are always negative plates so that they are always more than positive. The dividers are so positioned that their rib side is facing the positive plates. Thus, the electrolyte is concentrated in the positive plates. The battery shells were previously made with bitumen and asbestos, but are now made of polypropylene which is translucent and thus the level of the electrolyte is shown. It also has great strength and light weight.
Here it is to be noted that manufacturers to achieve a small volume make the plates and partitions very thin. This results in the battery being overheated because the plates can easily break and shorten. Also, as the volume of the battery decreases, the amount of electrolyte present in each cell decreases, resulting in increased battery sensitivity and easy electrolyte depletion.
Charging and discharging
When the battery is charged with polarity opposite to that provided by the battery, it passes through it.
The battery has a constant polarity and requires a constant polarity current to charge it. It may not be continuous but intermittent but never alternate. If the electric power (ie the unladen voltage) of the battery is 12V then to charge it we need a current of 14-16 V depending on the charge rate we want but also on the internal resistance of the battery (ie the voltage at the ends of the charger must exceed the battery’s electric power and also the voltage drop inside the battery due to its internal resistance). If we could see in a battery when we charged it then we would see that the current breaks down the electrolyte and the oxygen produced moves to the positive plate, whereby the charging progresses to form lead dioxide.
Sulfate radicals are formed from the two plates which form sulfuric acid in the electrolyte (increasing its concentration) and at the same time the negative plate is converted into porous lead. When discharging the stream is reversed due to the lead’s sulphurous lead porosity, sulfuric acid breaks down and the sulphate radicals released form lead sulfate on both plates, so that there is no difference between them and thus current flow. Also oxygen escapes from the positive plate and returns to the electrolyte to form water. Thus the electrolyte is diluted. The Beaume Densometer is an instrument that measures the density of the electrolyte, giving information on the battery charging state.
It consists of a float in a transparent container, which at one end has a thin nozzle and on the other, a suction bucket. The nozzle is placed on the element and suction electrolyte on the boomer. Depending on its density, the float balances in a position. Its indication informs us of the battery charge, provided that proper density electrolyte was used to charge the battery. A relationship for the correction of density for very hot or cold conditions is the following:
S20 = St + 0.007 (t-20)
St: measured density
t: Electrolyte temperature (degrees C)
S20: Corrected density (at 20 degrees C).
So if the density is 1,200 at 30 degrees C, then the corrected density is:
S20 = 1,200 + 0,007 (30 – 20) = 1,200 + 0,007 => S20 = 1,27.
From the above relationship, it appears that as the temperature drops, the electrolyte densifies.
The following values show the battery charge status
1.11 – 1.13
1.23 – 1.25
1.27 – 1.29
The nominal battery capacity is the measure of the amount of electrical charge that can be offered by a battery when it is discharged from full charge to the minimum allowable voltage (1.8 V per element or 10.8 V for a 12-volt battery). Typically, the battery capacity is for discharge in 10 hours and 25 ° C. Then we assume that the discharge is at a constant rate and with a current that will bring the battery from its original state to its final state (as defined in previous paragraph) in 10 hours. If the discharge rate is faster then the battery capacity decreases. As an example, we can take the case of a 36 Ah (ampere) battery that should be able to give 3.6 A for 10 hours or 7.2 A for 5 hours etc. In fact, if we discharge it with 7.2 A it will reach in the final state in less than 5 hours, but the difference will be small for a good battery. The battery capacity shows how much time we need to charge. Assuming we have a fully discharged battery, the theoretical charge is:
6A for 6 hours = 36Ah or
3A for 12 hours = 36Ah etc.
Due to the fact that the battery does not have a 100% efficiency to reach full charge, we have to give it a load of 1.3 times the nominal. So for the example battery we need to give it.
36 X 1.3 = 46.8 Ah
So if we want to charge it in 6 hours, the charging current must be:
46.8 / 6 = 7.8A
Rate of charge
In a vehicle, the charge rate is set by the automatic. It depends on the battery charge level, which in turn depends on the recent loads to which the battery has been subjected, but also on its age and condition. However, if we want to charge a battery with a charger, then if there is no need for fast charging it is good to charge it with a current of 1/10 to 1/30 of its capacity. In our example, if the 36 Ah battery was totally uncharged, then it would need 46.8 Ah to charge (36 x 1.3) and if we charged it at a rate of 1/10 it would have to be charged at a current of 1/10 x 46.8 = 4.68 ie about 5A (for ten hours). It is advisable not to charge the batteries at a faster rate than 1/10 because they are stressed, overheated, they may shorten their components, the electrolyte level may drop, etc.
Filling of liquids
There is a case that we want to fill a new, dry battery with liquid. In this case, dilute sulfuric acid should be diluted to the correct density. We must be very careful during the process and wear safety glasses and protective clothing (gloves, apron etc.). Before diluting the acid we need to make a calculation of the amount we need. It is important to pour the acid in the water and never the water into the acid. During the dilution a significant amount of heat is produced and the acid has to be added slowly and stirred until the correct density is achieved.
The electromotive force of a battery is the voltage measured at its terminals when there is no external load. The ODE depends on the density of the electrolyte and is expressed by the above approximation:
ODD = density + 0.84 / cell
So if you measure with the densimeter a density of 1.25 we will have:
Element voltage: 1.25 + 0.84 = 2.09 V
Battery voltage: 2.09 x 6 = 12.54V
(we multiply by 6 because a 12-cell battery has 6 items in a row).
Batteries are also conductors. So they show some resistance to the current flow. This is called internal resistance. In the case of car batteries it is very low. One of the reasons why the lead batteries in cars are preferred is their low internal resistance. As the starting current is too high, if the battery had a high internal resistance, it would have an unacceptable voltage drop. Figure 8 shows a battery with its equivalent internal resistance. A numerical example shows that if the internal resistance was 0.05 Ω and the VoD 12V then if there was a current of 10 A we would have:
V = RED – r I,
V = voltage at the poles
ODD = electromotive force
r = internal resistance
I = current passing through
V = 12 – (0.05 x 10) => V = 115V
with the same calculations as follows:
ODD (V) current (A) internal voltage drop (V) voltage to the poles
12 10 0.5 11.5
12 20 1.0 11.5
12 50 2.5 9.5
12,100 5,0 7,0
This example shows us a battery with a high internal resistance. A current of 60A when the starter is turned is not unusual. We see that with such a battery the pole voltage would be less than 9.5 V. In fact, a good lead battery has an internal resistance of 0.005 Ω, so for a 60A current there is a voltage drop:
V = 0.005 x 60 = 0.3 V.
That is, when the starter works, the current is:
12V – 0.3V = 11.7V.
The internal resistance of the battery is due to several individual resistances, such as resistance between electrodes and electrolyte, plate resistance, resistance to internal connections, electrolyte resistance to ion flux (the ions are electrically charged particles which move to the electrolyte). Additionally, the internal resistance depends on the charge level and battery temperature. As long as the battery is discharged or the temperature drops, the internal resistance increases. Manufacturers can alter the internal resistance by varying the surface of the plates. Batteries with a large number of plates (and therefore larger capacity) have lower internal resistance. As the batteries age, one of the problems that arise is to increase their internal resistance. Clearly there is a point where there is not enough polar tendency to turn the starter fast enough to start the engine. For a start on a cold morning it needs more torque to rotate on the crank and the minimum engine speed to start is around 100 r.p.m. Under such circumstances, it seems if the battery has reached the end of its life.
Typically, vehicle manufacturers place batteries that are large enough to rotate the engine under certain conditions, usually at -30o assuming a 70% charge. Testing on cars determines the minimum current required by the starter in these conditions and with these elements a battery of sufficient capacity is selected.
Effect of temperature
At low temperatures, the electrolyte will be dense and will have a higher specific gravity but nonetheless the chemical reactions in the battery are slower and the end result is the decrease of the battery capacity in relation to the temperature. Figure 9 shows a typical characteristic curve showing the intersection (at -20 ° C) between the supplied and the required current for starting a given vehicle. The battery should be protected from freezing, especially when it is at a low charge level, because then the amount of water in the electrolyte is higher and there is a greater chance of freezing. A good charge is a good anti-freeze measure. The dependence of the freezing limit on the density of the electrolyte is shown in Fig. 10 as well as dependence on the charge level. A frozen battery gives a very low current, but it does not suffer permanent damage, although cracks may occur in its shell. This is because the frozen electrolyte does not expand and remains in a gel condition. It should be noted that recharging when the battery is frozen is difficult since only a very low current can pass. We do not have to fill liquids in batteries when their temperature is below the freezing point (0 ° C).