All materials are made up from atoms, and all atoms consist of protons, neutrons and electrons. Protons, have a positive electrical charge. Neutrons have no. For each state of charge, there is a line in fig. that shows the relation between battery voltage Vb and charging current I. So if the charger supplies a. 1. Current, Voltage, and Power. Peter Mathys. ECEN Electric Charge. • The concept of electric charge is the physical basis for describing electrical.
It is characterized by the Meissner effectthe complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics. Semiconductor In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron.
This is the case in a p-type semiconductor. A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator. In the classic crystalline semiconductors, electrons can have energies only within certain bands i. Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material.
The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy closer to the nucleus are occupied, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few semiconductor or virtually none insulator of them are available in the conduction band, the band immediately above the valence band.
The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on the band gap between the bands.
Charge, Current & Potential Difference
The size of this energy band gap serves as an arbitrary dividing line roughly 4 eV between semiconductors and insulators. With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension — that is in a nanowirefor every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other.
For a net current to flow, more states for one direction than for the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: However, as a semiconductor's temperature rises above absolute zerothere is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band.
The current-carrying electrons in the conduction band are known as free electrons, though they are often simply called electrons if that is clear in context. Current density and Ohm's law Main article: Current density Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI unitsthe current density is measured in amperes per square metre. To be more precise, one could take into account that the voltage increases slightly as the battery becomes more and more charged.
This would mean that the charging current will decrease slightly. However, the difference with the constant current of The voltage setting of the regulator: Also it is assumed that the regulator works perfectly: The total resistance between the battery poles and the point where the regulator senses the voltage: The charging characteristic is represented by the lines connecting the points A, B, C and D in fig.
Voltage regulator starts reducing field current. Fully charged, battery can be disconnected. Absolute end point of charging. Then the current will be the normal charging current of So the first point to be drawn in fig. This is starting point A. As charging continues, initially the current will remain the normal charging current of Meanwhile the battery becomes more and more charged so the lines representing higher states of charge are crossed.
This means that from starting point A in fig. To find where this vertical line stops, one has to calculate when the voltage regulator will start reducing the field current. Between the point where the regulator is connected and the battery poles, there is a voltage drop of So if the regulator starts reducing the field current when it senses a voltage Vr of This is point B in fig. Now charging continues and battery voltage will rise further, but the current decreases.
To find out how the current decreases, one has to look at the voltage drop mentioned above, which represents the difference between the voltage Vr sensed by the regulator and battery voltage Vb. Voltage Vr will remain constant at This means that the equation representing the relationship between I and Vb is: This is the end point D of the charging process.
Even charging a battery for a week would not make the current drop further and voltage Vb reach By now, the energy that is still pumped into the battery is not used for the chemical reaction that goes with battery charging see annex C: Electrolysis of water, meaning that water is transformed into hydrogen gas and oxigen gas the explosive gas mixture that is formed during battery charging.
The water that is lost this way, has to be replenished by topping up with destilled water once in a while. Corrosion of the grid of the positive plate, see annex C: These undesirable reactions go only slowly because the current is so low. Still they go faster than in a car because the voltage regulator has been readjusted to a slightly higher voltage.
Therefor a battery must be disconnected once point D is reached. The charging characteristic in fig. When battery voltage reaches Getting just this little into the corner of the danger zone won't harm the battery at all.
But it shows how important it is that the charger complies with the data that were used in calculating the charging characteristic. If the regulator would be adjusted slightly higher and the resistance between battery poles and regulator would turn out to be a bit lower, the charging characteristic could end up well into the danger zone.
Another interesting point is point C where the battery is just fully charged and charging can be stopped. Then all the electricity that has been taken from the battery, has just been charged back into it. Generally, they will not measure battery voltage Vb but use the indicator on the switchboard, thus measuring voltage Vi.
Charge, Current & Potential Difference
A battery can be considered fully charged when: The current has dropped below 4 A see note below ; and: The voltage Vi as measured with the indicator has risen above So then the point at which the battery is fully charged, will determined by the criterium that the current should drop below 4 A.
Still voltage Vi must be checked to make sure that the current has dropped to less than 4 A because the battery is charged, since there could be other reasons why the current is so low: The charger runs short of water so its power output is just enough for a few amperes charging current.
The nozzle is partially blocked, with the same effect on the power output. There is something wrong with the field current, maybe too many or too few lamps are switched on, the connections are poor or the brushes are wet.
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The current indicator is wrong and underestimates the actual current. The most likely cause for this is corrosion of the surfaces in the indicator switch operate the switch a few times to see if it goes back to a normal value. In this case, the voltage measurement part probably still functions. So to prevent that batteries that are not yet charged, are disconnected because the current is below 4 A for one of those reasons, also voltage Vi should be checked.
The graph of fig. For smaller batteries, the state of charge lines would end up closer towards the Y-axis and such batteries might not be fully charged when the current has decreased to 4 A. Therefor small car batteries can be considered charged when the current has dropped to 2 to 3 A instead of 4 A. So for those batteries, point C should lie at a current of 2 to 3 A.
For small batteries, the other points on the charging characteristic remain virtually the same. Still the choice of having point C at a charging current of 4 A seems wise considering: At a higher temperature, a battery accepts charging current more easily, so when reaching point C, it will be charged a bit more than fig.
Suppose the operator will check the battery only once every hour.
This means that on average, batteries will be disconnected half an hour after they have passed point C and then they will be charged considerably more see next paragraph. So point C is the minimum at which a battery can be considered charged enough. On average, batteries will be charged quite a bit more because the operator will only check once in a while. Allowing batteries to be disconnected a bit early means that total charging time see next paragraph will be shorter, which is important because: It means more convenience for users.
They can bring their battery late and still have it recharged the same day. It also means that the charger can serve more users without having to charge during the night. Allowing that batteries are disconnected a bit early reduces the risc that they will be seriously overcharged, which would damage them just like undercharging does.
It is important that operators have a simple and clear criterium for when a battery is charged well enough and point C provides this.
Another way of analysing the charging process is by of looking how charging current, battery voltage and some other variables vary over time. This comes down to calculating how much time it takes to go from one state of charge line to the next.
As long as the current is constant at