Introduction:
The battery’s role is to supply energy to the consumers when the alternator provides no or insufficient energy, for example, when starting the engine. The battery acts as a buffer where energy is stored. The energy supplied by the alternator is stored in the battery and must be discharged when needed. Because electrical energy is hard to store, the electrical energy provided by the alternator is converted into chemical energy. When the battery needs to supply electrical energy to consumers, the chemical energy is converted back into electrical energy.
If the car’s battery is in good condition, but still drains after standing idle for a few hours, there may be a parasitic drain.
Operation:
Inside the battery, there are multiple thin lead plates in a container of sulfuric acid. The lead forms a bond with sulfur, triggering a chemical reaction. The lead is converted into lead sulfate (PbSO4).
Diluted sulfuric acid is a mixture of sulfuric acid and demineralized (purified) water. This diluted sulfuric acid is often called electrolyte. When the lead plates are connected to a charging device, they undergo a transformation. The plate connected to the negative terminal releases sulfur to the electrolyte. The lead sulfate converts to porous lead. The plate connected to the positive terminal absorbs oxygen from the electrolyte and releases sulfur to the electrolyte. After charging, this plate contains lead dioxide (PbO2). The aforementioned process causes a voltage difference between the positive and negative plates.
When a consumer is connected to the lead plates charged in the aforementioned manner, current flows. The lead dioxide on the positive plate is once again converted to lead sulfate. The porous lead of the negative plate is also converted to lead sulfate. Thus, during charging and discharging of the batteries, a transformation occurs in the positive and negative plates (chemical action). The electrolyte also undergoes a transformation during charging and discharging. When the battery is discharged, the positive and negative plates consist of lead sulfate. The sulfur used to form lead sulfate is extracted from the electrolyte. Thus, the electrolyte of a discharged battery has a low sulfur content. In a charged battery, the lead sulfate of the plates is transferred to the electrolyte, which then has a high sulfur content. Since the sulfur particles are the heavier particles in the electrolyte, the specific gravity of the electrolyte increases as the state of charge of the battery increases. A fully charged battery has an electrolyte with a specific gravity of 1280 kg/m3. When the battery is fully discharged, the electrolyte has a specific gravity of 1140 kg/m3. For comparison: water has a specific gravity of 1000 kg/m3.
Battery Construction:
Batteries are composed of several cells, each containing several positive and negative plates. Each cell has a voltage of approximately 2V. A 12V battery has 6 cells connected in series. The positive and negative plates are separated by separators.
Positive and Negative Plates:
The positive plates are connected to the positive terminal, and the negative plates to the negative terminal. To prevent connection errors, both terminals are marked, and the positive terminal always has a larger diameter than the negative terminal. The positive and negative plates are interconnected using a bridging piece. The plates consist of a grid made of lead alloy. The grids are filled with paste (a mixture of lead powder, sulfuric acid, and various applications). The separators are made of plastic and cellulose. During the energy conversion in the battery, more heat is developed at the positive plate than at the negative plate. To prevent warping of the positive plate, the positive plate is always placed between two negative plates.
Battery Cells:
All cells of the battery are filled with the so-called electrolyte, a mixture of distilled water and sulfuric acid. Distilled (also known as demineralized) water is water from which contaminants like lime and chlorine compounds have been removed. Older batteries have cells equipped with filling openings. Through these openings, demineralized water can be refilled. The filling opening can be sealed with a filler cap. In newer batteries, filling is no longer possible. These are maintenance-free batteries where water consumption is so low that refilling is not necessary.
Charging / Discharging:
The state of charge of a battery can be measured with a hydrometer. A good battery charger automatically reduces the current when the charging voltage exceeds 2.35 V per cell (about 14 V for a 12 V battery). If this value is exceeded, water molecules decompose into oxygen and hydrogen, producing hydrogen gas, which in large amounts forms an explosive mixture (oxyhydrogen gas).
- Normal Charging:
In normal charging, the battery’s capacity is restored to 100%. The charging current is 5 to 10% of the capacity. A battery with a capacity of 40 Ah is charged with a current of 2 to 4 A during normal charging. - Rapid Charging: Batteries that are quickly fully discharged can be partially recharged using rapid charging. The charging current amounts to 30 to 50% of the battery’s capacity. For a 40Ah battery, the charging current is 12 to 20 A. Rapid charging is not often used. Many fast chargers can also be used as starting aid and normal chargers.
- Trickle Charging: When a battery is not used for an extended period, voltage loss occurs due to self-discharge. By continuously connecting a trickle charger to the battery, it stays fully charged. The charging current is about 0.1% of the battery’s capacity. A battery with a capacity of 40 Ah is then charged with a current of 0.04 A. Some battery chargers automatically switch to trickle charging at the end of normal charging.
- Buffer Charging: In buffer charging, the consumers and the charging device are both connected to the battery. The charger supplies a current that keeps the battery practically full. The battery provides peak current to the users. Buffer charging occurs when the alternator charges the battery while simultaneously supplying current to the users. The alternator has a voltage regulator set to 14.4 V for a 12 Volt system. After starting, the alternator charges rapidly for some time. While driving, the charging current reduces greatly. When the battery is fully charged, the charging current becomes so small that the charger only keeps the battery charged.
When the car is in a garage, it is good to have the battery on the trickle charger. The battery then has a longer lifespan than a battery that is often significantly discharged and quickly recharged by the alternator. A battery will discharge if a consumer is left on with the engine off (such as the lights). If a battery is deeply discharged (completely empty), the battery is internally damaged. The lifespan is drastically shortened.
Capacity:
The capacity of the battery is the amount of electrical energy the battery can maximally contain. The capacity is expressed in Ah (ampere-hour). The capacity is determined based on the test results. Example: A battery has a capacity of 60 Ah. This battery can supply a current of 3A for 20 hours. (60Ah : 20h = 3A). The terminal voltage will not drop below 1.75V per cell.
Cold Cranking Amperage:
Generally, it can be assumed that the size of the cold cranking amperage is 4 to 5 times the battery’s capacity. The cold cranking amperage provides information about the speed at which the battery can deliver electrical energy. For starter batteries used in cars, the cold cranking amperage is more important than capacity. The cold cranking amperage decreases significantly with a drop in temperature. This is because chemical reactions occur much more slowly at lower temperatures. The conditions under which the cold start current is measured are predetermined.
According to DIN standards: the cold cranking amperage is the maximum current the battery can supply at a temperature of 255 K (-18 degrees) for a certain time, at a sufficient voltage:
- After 30 seconds of discharging with the cold cranking amperage, the terminal voltage must still be at least 1.5 V per cell.
- After 150 seconds of discharging with the cold cranking amperage, the terminal voltage must still be at least 1V per cell.
Disconnecting Battery Terminals:
For certain tasks (consider airbags, starter motor, alternator), the battery needs to be disconnected. Otherwise, there is a risk of short-circuit, or an airbag could be accidentally activated. It is sufficient in these cases to simply detach the negative terminal. The positive terminal can remain on the battery. Never remove only the positive terminal! If it touches the chassis (which serves as ground and is therefore connected to the negative terminal), a short circuit will occur. When the battery is removed, always disconnect the negative terminal first, and then the positive terminal.
A battery should never be disconnected with the engine running. Today’s engines are fully electronically controlled. The electronics can be severely damaged by the peak currents coming from the alternator.
In the past, a (non-electronic) diesel engine could be disconnected in this way because the fuel pump was mechanically driven and injectors opened at a certain injection pressure. Due to the mechanical operation, the engine could keep running without a battery after starting.
Jump Starting with Jumper Cables:
If the battery is dead, it must be charged to start the engine again. This is possible by connecting the battery to another car using jumper cables. It’s important to use good (thick) jumper cables. Thin cables create a lot of resistance at high currents and get very hot as a result. There’s a risk that a heavier/larger engine cannot be started with cables that are too light.
Connection order is important; never connect the positive (red) and negative (black) cables to one battery at the same time, as you might quickly cause a short by allowing the contacts on the other end of the cable to touch each other. Follow this order:
- Connect the negative cable to one car and the other end of the negative cable to the other car.
- Then connect the positive cable to one car, and then to the other. It doesn’t matter whether the positive or negative cable is connected first.
Now both batteries are connected in parallel. When batteries are parallel, the voltage remains 12V. It is not the case that the total battery voltage is now 24 volts. That would happen if the batteries were connected in series, as occurs in electric/hybrid vehicles. For more information about series and parallel connections (again using resistors as an example), see the page current, voltage resistance.
With the jumper cables connected, the alternator of the ‘charging’ car charges the dead battery. It’s best to leave this for about a minute, as the engine might not start right away otherwise. Especially if it’s a heavy diesel engine. After a minute (or longer), the car with the dead battery can be started.
Handling the removal of the jumper cables is also important; because the car providing a jump to the other car still transmits a lot of charging current through the jumper cables to the dead battery, it’s not good to suddenly remove the jumper cables. The charging current/voltage is high while charging, but when removing a cable, the current has nowhere to go, except into the vehicle’s own electronics. A current spike occurs, which can also reach the control units. This problem can be avoided by turning on all heavy consumers in the charging car (the car recharging the dead battery). Think of the rear window defroster, lights, possibly seat heating, etc. When removing a jumper cable, the current spike is distributed among these components that already demand a lot of electricity. The control units remain protected. The cables should be removed in the same order as they were connected: first the positive or negative cable on both cars, then the other. Never remove both from one battery at the same time.
It’s best to charge a dead battery with a battery charger, as an alternator charges with maximum current. A battery charger adjusts the charging current to the battery’s condition. When a battery is deeply discharged (thus when the battery voltage has dropped below 6 Volts), it becomes internally damaged. The lifespan is drastically shortened.
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