I was not sure if I should make this little write up, because I was concerned it would be taken as pedantic. It is not intended to be, it is just that I saw many confusing statements in the posts, so I thought it might be a good reminder for what we all already know. If I get a chance before this thread is completely derailed, I can add some posts as to what this means for charging batteries and running electric motors. And maybe a part 2: Distance, Speed, Acceleration, Torque, Power and Energy.
Part 1: Voltage, Current, Power, Energy and Resistance
Voltage is used to express an electric potential difference. You can compare this to the pressure on a water pipe. The potential (we usually say voltage) is usually abbreviated as a capital U, and the unit of measurement for the potential is the Volt, abbreviated as a capital V. A capital V is used because this was defined by Allessandro Volta.
Current is used to express the flow of electric charge. You can compare this to the flow from a water pipe. The current is usually abbreviated as a capital I, and the unit of measurement for current is the Ampere, abbreviated as a capital A. Again a capital A is used because this was defined by André-Marie Ampère.
Power (electric) is used to express the rate of transfer of electricity. The power is usually abbreviated as a capital P, and the unit of measurement for the power is the Watt, abbreviated as a capital W. Guess what, a capital W is used because this was defined by James Watt. Electric power is simply calculated as the product of the voltage and the current (P = U x I).
Energy is used to express the capacity to do work (as in make your car move). Energy is usually abbreviated as a capital E, and the standard unit of measurement for energy is the Joule, but for electric energy we prefer to use the kilo-Watt-hour, abbreviated as kWh. Electric energy is calculated as the product of power and time (E = P x t), to express the energy in kWh, the power has to be in kW and the time in hours. Energy is not power divided by time so the unit is not kW/h.
Resistance is used to express the opposition to the flow of electricity. Resistance is abbreviated as a capital R, and the standard unit measurement for resistance is the Ohm. The Ohm (or Ω) was defined by Georg Ohm. The resistance is calculated as the ratio between the voltage and the current (R = U / I). This is Ohm's law, usually written as U = I x R. Above we saw that the power is calculated as P = U x I, so we can combine the two equations to calculate the power loss in a resistor as P = R x I x I.
Notes:
The small k in kW or kWh is not a unit, but simply a multiplier. In the case of the small k it is a multiplier of 1,000. Other examples are the capital M for 1,000,000 and the small m for 0.001. A large K is reserved for the unit of temperature, yes defined by Lord Kelvin. So if you see KW instead of kW that is a (non-sensical) Kelvin-Watt.
The above is simplified for usage on DC systems (like our batteries) and averaged AC systems (like our motors). It can get a lot more complicated for AC systems when considering the phase shifting between current and voltage.
Part 2: Batteries
Batteries are complicated. No, let me rephrase that, batteries are very, very, complicated.
A battery is used to store electric energy. In a battery the electric energy is actually changed into chemical energy when charging and back into electric energy when discharging. So you could state that a battery is used to store chemical energy. And that is the difference with a capacitor. A capacitor is also used to store electric energy, but it accumulates the electrons when charging and releases the electrons when discharging without transformation. Therefore a capacitor is more efficient and can charge and discharge faster. But even todays super capacitors cannot store enough energy (per liter) for use in an EV. Hence we stick with batteries.
The cell has a cell voltage. There are many different battery chemistries and the chemistry determines the cell voltage. Although the cell voltage of cells in a battery pack in use is also depended on the state of charge, the current, the temperature and a few more factors. It's complicated.
An EV battery is build from cells, these cells are made into modules, and the modules into a pack. The modules are put in a string (in series) and the strings are connected in parallel. The number of cells in each string determines the pack voltage U [Volt], the total number of cells determines the pack energy capacity E [kWh].
An important performance measure for battery technologies is the energy density. This is either the gravimetric energy density, expressed as the ratio of energy and volume (Wh/liter), or the volumetric energy density, expressed as the ratio of energy and mass (Wh/kg). For EV applications, here is where much of the development is, looking for lighter, smaller batteries for the same energy. Lead-acid batteries gravimetric density was too low to be practical for electric vehicles, the much higher gravimetric density of Li-Ion batteries enabled 'real world use' electric vehicles.
The State of Charge (SoC) of a battery is the ratio of the energy in battery and the capacity of the battery. So the SoC has no unit of measure and is usually expressed as a percentage. However, both the energy contained in a battery and the capacity of the battery can be defined in different ways, the SoC calculation depends heavily on those definitions. For instance the manufacturer defines bottom and top buffers, that is capacity that is not used to extend battery life as a trade off. Did I mention that it is complicated?
Batteries degrade over time. Degradation depends on the battery technology and the use (abuse) over time. The degradation means lower battery capacity. So over time you will loose some of the ability to store energy, and thus range.
The State of Health (SoH) of a battery is usually considered as the ratio of the current capacity of the battery and the original (new) capacity of the battery. Thus also the SoH has no unit of measure and is expressed as a percentage.
So the energy stored at 100% SoC for a new battery is not the same as at 100% SoC of a used battery. The actual energy stored is the original capacity multiplied by the SoC and the SoH.
Notes:
There is much more to be said about batteries. We can talk about the difference between kWh and Ah, we can talk about the mechanisms of battery degradation, etc. Its complicated, but I think I mentioned that.
Part 3: Charging
Batteries charge and discharge with DC (directed current). Electric power is transmitted as AC (alternating current) (thank you Nikolai, indeed Thomas was wrong), so we need a conversion from AC to DC to be able the charge.
So an AC charger is not a charger at all. It is an AC power source for your onboard charger. The onboard charger converts the AC power into DC power to charge the battery. The maximum power is limited by either the AC power source (the outlet, your wall box or the public 'charger') or by the car's onboard charger, whichever is lower.
A DC charger delivers the DC power directly to the battery. Therefore the two heavy contacts in a DC charger connector. The maximum power is limited by the charger's voltage, current or power limit, or by the battery. When charging on 350 kW DC charger, the car may very well control the voltage to limit the power to say 150 kW or less depending on the battery type, the SoC, the temperature, ... The DC charger can deliver a current up to 400A, that explains the hefty cables and connectors these use.
In both cases the DC charge voltage is controlled by the car to optimize the charge process without damaging the battery. For an AC charger the onboard charger controls the DC charge voltage, and for a DC charger the desired charge voltage is communicated by the car to the external charger.
A battery cell and thus a pack has an internal resistance. So when charging or discharging a battery pack you will lose some energy. It is transformed into heat. Also your onboard charger is not 100% efficient. And the charging cable has losses too. The total charging loss is typically about 5 to 10%. So charge equipment may report more energy delivered than you effectively added to the battery.
Notes:
Your wall box is actually an implementation of Electric Vehicle Supply Equipment (EVSE).
Part 4: Inverters and Motors
In the early days, I mean before the invention of the starter motor (by Charles Kettering, in 1911), made ICE engine cars popular, electric cars used DC motors. Nowadays electric cars almost exclusively use different types of AC motors. Since the energy you carry around is stored in a battery, and a battery delivers that energy as DC power, you need an inverter to transform the DC power from the battery into AC power for the motors.
Without explaining the details of the differences between various types and many variations of electric AC motors (synchronous, induction, reluctance, permanent magnet, …) it is enough to note that each type has it advantages and disadvantages in terms of mass, volume, efficiency, performance, etc. Therefore, different manufacturers opt for different technologies, sometimes even for different technologies on the same vehicle to optimize for different circumstances.
Some praise must go James Clerk Maxwell, who defined every electrical engineer’s favorite set of differential equations that lay down the fundamentals for all these types of electric motors.
The most interesting property of electric motors in general is the ability to delivery high torque over a wide speed range. This gives your EV it’s pizazz that differentiates it from an ICE vehicle, without the need for a transmission switching different gear ratios.
For each AC motor in our vehicle, we need an inverter to provide the (usually 3 phase) AC power. An inverter does that by switching the DC voltage from the battery in alternating directions at a high frequency, thus creating an AC voltage. Inverters are complicated pieces of equipment combining microelectronics and software for controls with power electronics for high current output.
Controlled by software, the inverter varies the frequency and the voltage of the power output to control the motor speed and torque. Settings in the software keep things within limits for battery wear, power electronics loading, motor thermal stress, etc. Different settings, compromising those limits, can give you a software enabled power upgrade.
The energy conversion in the inverters (DC to AC) and in the motors (electric to mechanical) come with efficiency losses that transform into heat. Therefore, the inverters and motors are (usually liquid) cooled.
Development steps in battery technology receive a lot of press, but developments in power electronics for inverters have been equally important. The development of reliable, high power IGBTs (Insulated Gate Bipolar Transistor), as an alternative for MOSFETs (Metal Oxide Semi-conductor Field Effect Transistors), for inverters lead to a development push in the 90’s (GM EV1), just like the development of Lithium batteries lead to a push around 2008 (Tesla Roadster). Meanwhile new MOSFET technologies are making an entry seeking higher efficiency and better reliability.
Part 5: ???
Part 1: Voltage, Current, Power, Energy and Resistance
Voltage is used to express an electric potential difference. You can compare this to the pressure on a water pipe. The potential (we usually say voltage) is usually abbreviated as a capital U, and the unit of measurement for the potential is the Volt, abbreviated as a capital V. A capital V is used because this was defined by Allessandro Volta.
Current is used to express the flow of electric charge. You can compare this to the flow from a water pipe. The current is usually abbreviated as a capital I, and the unit of measurement for current is the Ampere, abbreviated as a capital A. Again a capital A is used because this was defined by André-Marie Ampère.
Power (electric) is used to express the rate of transfer of electricity. The power is usually abbreviated as a capital P, and the unit of measurement for the power is the Watt, abbreviated as a capital W. Guess what, a capital W is used because this was defined by James Watt. Electric power is simply calculated as the product of the voltage and the current (P = U x I).
Energy is used to express the capacity to do work (as in make your car move). Energy is usually abbreviated as a capital E, and the standard unit of measurement for energy is the Joule, but for electric energy we prefer to use the kilo-Watt-hour, abbreviated as kWh. Electric energy is calculated as the product of power and time (E = P x t), to express the energy in kWh, the power has to be in kW and the time in hours. Energy is not power divided by time so the unit is not kW/h.
Resistance is used to express the opposition to the flow of electricity. Resistance is abbreviated as a capital R, and the standard unit measurement for resistance is the Ohm. The Ohm (or Ω) was defined by Georg Ohm. The resistance is calculated as the ratio between the voltage and the current (R = U / I). This is Ohm's law, usually written as U = I x R. Above we saw that the power is calculated as P = U x I, so we can combine the two equations to calculate the power loss in a resistor as P = R x I x I.
Notes:
The small k in kW or kWh is not a unit, but simply a multiplier. In the case of the small k it is a multiplier of 1,000. Other examples are the capital M for 1,000,000 and the small m for 0.001. A large K is reserved for the unit of temperature, yes defined by Lord Kelvin. So if you see KW instead of kW that is a (non-sensical) Kelvin-Watt.
The above is simplified for usage on DC systems (like our batteries) and averaged AC systems (like our motors). It can get a lot more complicated for AC systems when considering the phase shifting between current and voltage.
Part 2: Batteries
Batteries are complicated. No, let me rephrase that, batteries are very, very, complicated.
A battery is used to store electric energy. In a battery the electric energy is actually changed into chemical energy when charging and back into electric energy when discharging. So you could state that a battery is used to store chemical energy. And that is the difference with a capacitor. A capacitor is also used to store electric energy, but it accumulates the electrons when charging and releases the electrons when discharging without transformation. Therefore a capacitor is more efficient and can charge and discharge faster. But even todays super capacitors cannot store enough energy (per liter) for use in an EV. Hence we stick with batteries.
The cell has a cell voltage. There are many different battery chemistries and the chemistry determines the cell voltage. Although the cell voltage of cells in a battery pack in use is also depended on the state of charge, the current, the temperature and a few more factors. It's complicated.
An EV battery is build from cells, these cells are made into modules, and the modules into a pack. The modules are put in a string (in series) and the strings are connected in parallel. The number of cells in each string determines the pack voltage U [Volt], the total number of cells determines the pack energy capacity E [kWh].
An important performance measure for battery technologies is the energy density. This is either the gravimetric energy density, expressed as the ratio of energy and volume (Wh/liter), or the volumetric energy density, expressed as the ratio of energy and mass (Wh/kg). For EV applications, here is where much of the development is, looking for lighter, smaller batteries for the same energy. Lead-acid batteries gravimetric density was too low to be practical for electric vehicles, the much higher gravimetric density of Li-Ion batteries enabled 'real world use' electric vehicles.
The State of Charge (SoC) of a battery is the ratio of the energy in battery and the capacity of the battery. So the SoC has no unit of measure and is usually expressed as a percentage. However, both the energy contained in a battery and the capacity of the battery can be defined in different ways, the SoC calculation depends heavily on those definitions. For instance the manufacturer defines bottom and top buffers, that is capacity that is not used to extend battery life as a trade off. Did I mention that it is complicated?
Batteries degrade over time. Degradation depends on the battery technology and the use (abuse) over time. The degradation means lower battery capacity. So over time you will loose some of the ability to store energy, and thus range.
The State of Health (SoH) of a battery is usually considered as the ratio of the current capacity of the battery and the original (new) capacity of the battery. Thus also the SoH has no unit of measure and is expressed as a percentage.
So the energy stored at 100% SoC for a new battery is not the same as at 100% SoC of a used battery. The actual energy stored is the original capacity multiplied by the SoC and the SoH.
Notes:
There is much more to be said about batteries. We can talk about the difference between kWh and Ah, we can talk about the mechanisms of battery degradation, etc. Its complicated, but I think I mentioned that.
Part 3: Charging
Batteries charge and discharge with DC (directed current). Electric power is transmitted as AC (alternating current) (thank you Nikolai, indeed Thomas was wrong), so we need a conversion from AC to DC to be able the charge.
So an AC charger is not a charger at all. It is an AC power source for your onboard charger. The onboard charger converts the AC power into DC power to charge the battery. The maximum power is limited by either the AC power source (the outlet, your wall box or the public 'charger') or by the car's onboard charger, whichever is lower.
A DC charger delivers the DC power directly to the battery. Therefore the two heavy contacts in a DC charger connector. The maximum power is limited by the charger's voltage, current or power limit, or by the battery. When charging on 350 kW DC charger, the car may very well control the voltage to limit the power to say 150 kW or less depending on the battery type, the SoC, the temperature, ... The DC charger can deliver a current up to 400A, that explains the hefty cables and connectors these use.
In both cases the DC charge voltage is controlled by the car to optimize the charge process without damaging the battery. For an AC charger the onboard charger controls the DC charge voltage, and for a DC charger the desired charge voltage is communicated by the car to the external charger.
A battery cell and thus a pack has an internal resistance. So when charging or discharging a battery pack you will lose some energy. It is transformed into heat. Also your onboard charger is not 100% efficient. And the charging cable has losses too. The total charging loss is typically about 5 to 10%. So charge equipment may report more energy delivered than you effectively added to the battery.
Notes:
Your wall box is actually an implementation of Electric Vehicle Supply Equipment (EVSE).
Part 4: Inverters and Motors
In the early days, I mean before the invention of the starter motor (by Charles Kettering, in 1911), made ICE engine cars popular, electric cars used DC motors. Nowadays electric cars almost exclusively use different types of AC motors. Since the energy you carry around is stored in a battery, and a battery delivers that energy as DC power, you need an inverter to transform the DC power from the battery into AC power for the motors.
Without explaining the details of the differences between various types and many variations of electric AC motors (synchronous, induction, reluctance, permanent magnet, …) it is enough to note that each type has it advantages and disadvantages in terms of mass, volume, efficiency, performance, etc. Therefore, different manufacturers opt for different technologies, sometimes even for different technologies on the same vehicle to optimize for different circumstances.
Some praise must go James Clerk Maxwell, who defined every electrical engineer’s favorite set of differential equations that lay down the fundamentals for all these types of electric motors.
The most interesting property of electric motors in general is the ability to delivery high torque over a wide speed range. This gives your EV it’s pizazz that differentiates it from an ICE vehicle, without the need for a transmission switching different gear ratios.
For each AC motor in our vehicle, we need an inverter to provide the (usually 3 phase) AC power. An inverter does that by switching the DC voltage from the battery in alternating directions at a high frequency, thus creating an AC voltage. Inverters are complicated pieces of equipment combining microelectronics and software for controls with power electronics for high current output.
Controlled by software, the inverter varies the frequency and the voltage of the power output to control the motor speed and torque. Settings in the software keep things within limits for battery wear, power electronics loading, motor thermal stress, etc. Different settings, compromising those limits, can give you a software enabled power upgrade.
The energy conversion in the inverters (DC to AC) and in the motors (electric to mechanical) come with efficiency losses that transform into heat. Therefore, the inverters and motors are (usually liquid) cooled.
Development steps in battery technology receive a lot of press, but developments in power electronics for inverters have been equally important. The development of reliable, high power IGBTs (Insulated Gate Bipolar Transistor), as an alternative for MOSFETs (Metal Oxide Semi-conductor Field Effect Transistors), for inverters lead to a development push in the 90’s (GM EV1), just like the development of Lithium batteries lead to a push around 2008 (Tesla Roadster). Meanwhile new MOSFET technologies are making an entry seeking higher efficiency and better reliability.
Part 5: ???
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