Topics:
- Introduction
- Materials and specifications of different types of batteries
- Lead-acid battery
- Nickel-cadmium (Ni-Cd)
- Nickel-metal hydride (Ni-MH)
- Lithium-ion (li-ion)
– Structure of a battery pack: series and parallel connections
– Voltage range of a lithium-ion cell - Super capacitor (supercap)
- Battery cell balancing
Introduction:
The hybrid or fully electric car has larger, heavier batteries than cars with only an internal combustion engine. In hybrid cars, high voltages are used, which can be life-threatening during repairs if handled by unqualified people. For example:
- A starter motor that is operating uses around 1.2 kW (1200 watts)
- A hybrid car that drives fully on electricity uses around 60 kW (60,000 watts)
Work on hybrid cars may only be carried out by people who have completed special training for this. There is a 12-volt on-board electrical system for powering accessories (such as the radio, etc.) with its own small battery, and there is a high-voltage on-board system that (depending on the brand) operates at 400 volts. The 400 V voltage is converted to 12 V by a special DC/DC converter and charges the relevant battery.
High demands are placed on the hybrid traction batteries. They must have a very large storage capacity. Large energy reserves are stored, and very high voltages are drawn when supporting the internal combustion engine (hybrid), or when supplying energy for the complete drivetrain (BEV).
The image below shows a battery pack of a Toyota Prius. In this Nickel Metal Hydride (NiMH) battery there are 28 modules, each consisting of 6 cells. Each cell has a voltage of 1.2 volts. In total, the voltage of this battery pack is 201.6 volts.
Materials and specifications of different types of batteries:
When developing the electric drivetrain, a choice is made between different types of batteries. Their properties, performance, construction possibilities, and costs play a major role. The most commonly used battery types in hybrid and fully electric vehicles are Ni-MH (nickel-metal hydride) and li-ion (lithium-ion) batteries.
In addition to the Ni-MH and li-ion types, there is development of electrolytic capacitors, which we group under the heading “super capacitor”, or “supercaps”.
The table shows the materials of the different batteries with their specifications.
Lead-acid battery:
The table also mentions the lead-acid battery (gel and AGM versions are not considered). Because the lead-acid battery has the longest service life when discharged by a maximum of 20%, suffers from sulfation as it ages, and has low energy density and capacity, it is not suitable for use in electric vehicles. However, we do find the lead-acid battery as an auxiliary battery; the low-voltage consumers such as lighting, comfort systems (body) and infotainment function at a voltage of around 14 volts.
Nickel-cadmium (Ni-Cd):
In the past, Ni-Cd batteries suffered from a memory effect and for that reason were already unsuitable for use in electric drivetrains: constant partial charging and discharging takes place. Modern Ni-Cd batteries have virtually no memory effect anymore. The biggest disadvantage of this type of battery is the presence of the toxic substance cadmium. This makes the Ni-Cd battery extremely environmentally unfriendly. The use of this battery is therefore legally prohibited.
Nickel-metal hydride (Ni-MH):
The Ni-MH battery can be charged faster than a lead-acid battery. During charging, both heat and gas are generated, which must be dissipated. The batteries are equipped with a cooling system and a vent valve. Thanks to its long service life and high energy and power density, the Ni-MH battery is suitable for use in electric vehicles. However, this type of battery is sensitive to overcharging, excessive discharging, high temperatures, and rapid temperature changes.
The image below shows the Ni-MH battery pack of a Toyota Prius. This battery pack is located in the trunk, behind the rear seat backrest. When the temperature sensors register a high temperature, the cooling fan is activated (visible in the photo on the right by the white housing). The fan draws air from the interior and blows it through the air ducts in the battery pack to cool the cells.
Lithium-ion (li-ion):
Due to the high energy and power density of the lithium-ion battery (compared to Ni-MH), a li-ion battery pack is usually used in plug-in hybrids and fully electric vehicles. The li-ion battery performs reasonably well at low temperatures (although power then decreases, so many EV batteries have a heating system) and a li-ion battery has a long service life. The expectation is that, in the coming years, techniques will improve thanks to further development of, among other things, battery management, so that both range and service life will increase even further.
In the next image we see the (li-ion) battery pack of a BMW i3. The lid has been unscrewed and is standing behind it. When installed, the lid seals airtight.
The i3’s battery pack is mounted under the vehicle. The space in the floor area between the front and rear axles has been used as much as possible to provide as much space as possible for the battery pack.
In the image we see the eight separate blocks, each with twelve cells. Each block has a capacity of 2.6 kWh, which makes a total of 22 kWh. For comparison: the current generation i3 (as of 2020) has a battery with a capacity of 94 Ah and an energy content of 22 kWh. The size of the battery pack has remained the same since its introduction in 2013, but performance (and therefore its range) has improved significantly.
Tesla has used small battery cells in models from 2013 onward (Model S and Model X) that are slightly larger than the standard AA batteries we know from the TV remote control. The battery cells (18650 from Panasonic) are 65 mm long and have a diameter of 18 mm. The most extensive battery packs contain no fewer than 7104 of these cells.
In the images below we see the individual battery cells on the left and a battery pack on the right in which the 7104 cells are housed.
The lithium-ion battery consists of four main components:
- the cathode (+) consisting of a lithium alloy
- the anode (-) consisting of graphite or carbon
- the porous separator
- the electrolyte
During discharging, the lithium ions move through the electrolyte from the anode (-) to the cathode (+), to the load and back to the anode. During charging, the ions move in the opposite direction and then go from the cathode (+) to the anode (-).
The electrolyte contains lithium salts to transport the ions. The separator ensures that the lithium ions can pass through, while the anode and cathode remain separated from each other.
Structure of a battery pack: series and parallel connections:
The battery cells are housed in modules, which are connected to each other in series. The following schematic diagram shows a battery pack that closely resembles those of a Volkswagen E-UP! and Renault Zoë. The only difference here is the number of cells: the battery pack of the E-UP! has 204 cells and that of the Renault Zoë has 192.
In this example, the battery pack consists of two packs of six modules. Each module contains two parallel-connected groups of 10 series-connected cells.
- Series connection: the battery voltage increases. With a cell voltage (li-ion) of 3.2 volts, one battery module delivers (3.2 * 10) = 32 volts.
The disadvantage of a series connection is that with one bad cell, the capacity of the entire series string becomes lower. - Parallel connection: the voltage remains the same, but current and capacity increase. A bad cell has no influence on the cells in the circuit connected in parallel with it.
Manufacturers can therefore choose to use multiple parallel connections per module. In the modules of the Volkswagen E-Golf, therefore, not (as in this example, two) but three groups of cells are connected in parallel to each other.
The images below show the battery modules and specifications of a Volkswagen ID.4, again showing the series and parallel connections in two different configurations. Both packs contain 24 cells, but the choice of the number of series and parallel connections determines the nominal voltage and the capacity per module.
| 62 kWh | 82 kWh | |
|---|---|---|
| Materiaal | Lithium-ion | Lithium-ion |
| Celtype | Prismatic | Prismatic |
| Fabrikant | LG | LG |
| Gewicht per module | 30 kg (66 lbs) | 30 kg (66 lbs) |
| Capaciteit per batterijcel | 78 Ah | 78 Ah |
| Aantal cellen | 24 | 24 |
| Aantal modules in pakket | 8 | 12 |
| Schakeling in module | 12 serie, 2 parallel | 8 serie, 3 parallel |
| Capaciteit per module | (2 * 78) = 156 Ah | (3 * 78) = 234 Ah |
| Nominale spannig per cel | 3,7 volt | 3,7 volt |
| Nominale spannig per module | 44,4 volt | 29,6 volt |
| Energie per module | (44,4 * 156) = 6,93 kWh | (29,6 * 234) = 6,93 kWh |
| Energie totale batterijpakket | (8 * 6,93) = 55,4 kWh | (12 * 6,93) = 83,2 kWh |
The calculated energy of the total battery pack differs from the energy stated for the packs. For example, for the 62 kWh pack it is calculated that it delivers about 55.4 kWh, while for the 82 kWh pack about 83.2 kWh is calculated. So there is a discrepancy.
Possible causes are the manufacturer’s marketing designation, rounding of values, the use of a different nominal voltage per cell (for example a higher average voltage within the operating range), and the discharge characteristics of a cell. After all, a pack with fewer cells connected in parallel has a lower capacity per module and discharges according to a different discharge curve.
Voltage range of a lithium-ion cell:
Lithium-ion cells have a service life of about 2000 discharge and charge cycles before their capacity is reduced to about 80% of their initial charge capacity.
The voltages of a li-ion cell are as follows:
- nominal voltage: 3.6 volts;
- discharge limit: 2.5 volts;
- maximum charging voltage: 4.2 volts.
Most Battery Management Systems (BMS) use a lower limit of 2.8 volts. If the cell is discharged further than 2.5 volts, the cell becomes damaged. The service life of the cell is shortened. Overcharging the li-ion cell also reduces its service life, but is also dangerous. Overcharging the cell can make it flammable. The temperature of the cells also affects service life: at a temperature below 0°C, the cells may no longer be charged. A heating function offers a solution in this case.
Super capacitor (supercap):
In the previous paragraphs, various battery types were mentioned, each with their applications, advantages and disadvantages. One disadvantage that everyone with such a battery has to deal with is charging time. Charging a battery pack can take several hours. Fast charging is an option, but it comes with more heat and possibly also faster aging (and damage) of the battery pack.
Currently, a lot of research and development is taking place on super capacitors. We also call these “super caps” or “ultracapacitors”. Using supercaps could offer a solution to this:
- Charging is very fast;
- They can deliver energy very quickly (discharge), so a substantial increase in power is possible;
- More durable than a li-ion battery thanks to an unlimited number of charge cycles (at least 1 million) because no electrochemical reactions occur;
- Partly in connection with the previous point, a supercap may be fully discharged without this having harmful consequences for service life.
Supercaps are capacitors with a capacitance and energy density that are thousands of times higher than standard electrolytic capacitors. The capacitance is increased by using a special electrolyte (insulating material) that contains ions and therefore has a very high dielectric constant between the plates. A separator (a thin film) is soaked in a solvent with ions and placed between the plates. The plates are usually made of carbon.
The capacitance of the capacitor shown is 5000 F.
With the supercaps, a combination can be made with a Li-ion HV battery; during short acceleration, the energy from the capacitors can be used instead of the energy from the HV battery. During regenerative braking, the capacitors are fully recharged again within a fraction of a second. Future developments may also make it possible to replace the Li-ion battery with a supercap pack. Unfortunately, with current technology the capacity and therefore the power density are too low compared to a lithium-ion battery. Scientists are looking for ways to increase the capacity and power density.
Battery cell balancing:
Through passive and active battery cell balancing (English: cell balancing), each cell is monitored by the ECU to maintain a healthy battery condition. This extends the service life of the cells by preventing deep discharge or overcharging. Especially lithium-ion cells must remain within strict limits. The cell voltage is proportional to the state of charge. The charge of the cells should be kept as balanced as possible with each other. With cell balancing, it is possible to control the state of charge accurately to 1 mV (0.001 volt).
- Passive balancing ensures an equal state of charge of all battery cells by partially discharging the cells with too high a state of charge (we will return to this later in the paragraph);
- Active balancing is a more complex balancing technique that can regulate the cells individually during charging and discharging. The charging time with active balancing is shorter than with passive balancing.
In the following image we see a battery module with eight cells.
The eight cells are charged to 90%. The service life of a cell decreases if it is constantly charged to 100%. Conversely, the service life also decreases if the battery is discharged below 30%: at a state of charge of <30% the cell is deeply discharged.
The state of charge of the cells will therefore always be between 30% and 90%. This is monitored by the electronics, but the vehicle driver does not see it.
The digital display in the dashboard indicates 0% or 100% upon reaching 30% or 90%.
Due to aging, some cells can become weaker than the others. This has a major influence on the state of charge of the battery module. In the next two images we see the state of charge when two cells have a lower capacity due to aging. In these situations, the battery cells are not balanced.
- Discharges faster due to bad cells: the two middle cells discharge faster due to their lower capacity. To prevent deep discharge, the other six cells in the module can no longer deliver energy, so it can no longer be used;
- Does not fully charge due to bad cells: due to the low capacity of the middle two cells, they charge faster. Because they reach 90% faster than the other six cells, charging cannot continue.
It is clear that cells with a lower capacity are the limiting factor both during discharging (while driving) and during charging. To make optimal use of the full capacity of the battery pack and to benefit a long service life.
There are two methods for battery balancing: passive and active.
- Without balancing: four cells all have a different state of charge. Cell 2 is almost empty and cell 4 is fully charged;
- Passive: the cells with the most capacity are discharged until the state of charge of the weakest cell (in the example cell 2) is reached. The discharge of cells 1, 3 and 4 is a loss.
In the example we see that the cups are discharged until they have reached the state of charge of cell 2; - Active: the energy from the full cells is used to fill the empty cells. There is no loss here, but rather transferring energy from one cell to another.
Below, the operating principle of passive and active cell balancing is explained.
Passive cell balancing:
In the example we see four battery cells connected in series with a switchable resistor (R) in parallel across them. In this example, the resistor is connected to ground with the little switch. In reality, this is a transistor or FET.
In the example we see that cell 3 is charged to 100%. From the earlier paragraphs we know that this cell charges faster because it is weaker than the other three. Because the state of charge of cell 3 is 100%, the other three cells are no longer charged.
The resistor that is connected in parallel across cell 3 is included in the circuit by the switch. Cell 3 discharges because the resistor absorbs voltage as soon as current flows through it. The discharging continues until the cell is at the level of the other cells; in this case 90%.
When all four cells in this module have the same state of charge, they can be charged further.
With passive cell balancing, energy is lost: after all, the voltage absorbed by the resistors connected in parallel is lost. Nevertheless, to this day many manufacturers use this way of balancing.
Active cell balancing:
Much more efficient, of course, is active cell balancing. Here, the energy from the overfull cell is used to charge the empty cell. An example of active cell balancing is shown below.
In the example we see two cells connected in series (3 and 4) with their voltages above them (4 and 3.9 volts respectively). Cell 3 is discharged by means of the transformer. The FET on the primary side makes discharge possible. The primary coil in the transformer is charged with this. The FET on the secondary side switches in the secondary coil of the transformer. The obtained charging current is used, for the excitation of the transformer, under another cell. The transformer under cell 4 is also switched on and off by FETs.
