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| en:av:autonomy_and_autonomous_systems:technology:power_sources [2020/12/20 18:55] – agrisnik | en:av:autonomy_and_autonomous_systems:technology:power_sources [Unknown date] (current) – external edit (Unknown date) 127.0.0.1 | ||
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| ===== Power sources ===== | ===== Power sources ===== | ||
| - | There are two main battery types: single-use - **primary** and rechargeable - **secondary**. Electric vehicles (EV) and most of the other autonomous systems use secondary batteries (except for small toy vehicles and special applications) hence in this chapter the term battery mean secondary battery unless noted otherwise. From an economic perspective, | + | There are two main battery types: single-use - **primary** and rechargeable - **secondary**. Electric vehicles (EV) and most of the other autonomous systems use secondary batteries (except for small toy vehicles and special applications) hence in this chapter the term battery mean secondary battery unless noted otherwise. From an economic perspective, |
| - | From everyday knowledge, it is known that batteries have different voltages. A wall clock typically uses an AA or AAA size 1.5V battery while a car has 12V lead-acid battery under the hood. There are two reasons for different battery voltages: chemistry and series connection. The chemical composition of battery materials determines the voltage in the range of 1.2V to 3.9V. How come a car lead-acid battery has 12V? It actually has multiple smaller batteries inside and they are series-connected (mind the polarity) to sum up their voltages. These individual internal batteries are called cells. Figure 1 shows some multi-cell batteries. It would be technically correct to say that a battery is in fact two or more series-connected cells of the same kind. Hence a battery composed of just a single cell would not be a battery but rather just a cell. However, to not cause confusion it is accustomed in everyday language to use the term battery for any number of cells while a cell means a single element. This notation will be used here as well. One of-the-shelf battery is the car lead-acid battery which has six 2.1V cells inside (the voltage is rounded to 12V for convenience), | + | From everyday knowledge, it is known that batteries have different voltages. A wall clock typically uses an AA or AAA size 1.5 V battery while a car has a 12 V lead-acid battery under the hood. There are two reasons for different battery voltages: chemistry and series connection. The chemical composition of battery materials determines the voltage in the range of 1.2 V to 3.9 V. How come a car lead-acid battery has 12 V? It actually has multiple smaller batteries inside and they are series-connected (mind the polarity) to sum up their voltages. These individual internal batteries are called cells. Figure 1 shows some multi-cell batteries. It would be technically correct to say that a battery is in fact two or more series-connected cells of the same kind. Hence a battery composed of just a single cell would not be a battery but rather just a cell. However, to not cause confusion it is accustomed in everyday language to use the term battery for any number of cells while a cell means a single element. This notation will be used here as well. One of-the-shelf battery is the car lead-acid battery which has six 2.1 V cells inside (the voltage is rounded to 12 V for convenience), |
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| - | In electrical engineering, | + | In electrical engineering, |
| ==== Technical parameters ==== | ==== Technical parameters ==== | ||
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| **Voltage** | **Voltage** | ||
| - | The chemical composition of electrodes defines the voltage of a single cell. All types of battery cells have a certain nominal voltage Unom. As previously noted, the nominal voltage of different chemistries is in the range of 1.2 V to 3.9V. The nominal voltage is somewhere between maximal voltage Umax (charging voltage) and minimal voltage Umin (discharge cut-off voltage, end-of-discharge). The nominal voltage is used for calculations to determine the voltage of the battery pack if cells are series-connected. Discharge cut-off voltage is the voltage beyond which discharge should be terminated to prevent damage to the cell. A battery discharge voltage curve is given in the figure below. For primary batteries, it is desirable to have a flat curve which translates to the stable supply voltage. | + | The chemical composition of electrodes defines the voltage of a single cell. All types of battery cells have a certain nominal voltage Unom. As previously noted, the nominal voltage of different chemistries is in the range of 1.2 V to 3.9 V. The nominal voltage is somewhere between maximal voltage Umax (charging voltage) and minimal voltage Umin (discharge cut-off voltage, end-of-discharge). The nominal voltage is used for calculations to determine the voltage of the battery pack if cells are series-connected. Discharge cut-off voltage is the voltage beyond which discharge should be terminated to prevent damage to the cell. A battery discharge voltage curve is given in the figure below. For primary batteries, it is desirable to have a flat curve which translates to a stable supply voltage. |
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| **Capacity and energy** | **Capacity and energy** | ||
| - | The second most important quantitative battery parameter is capacity Qbat. Capacity determines how much charge a battery can store. It is measured in amp hours (Ah). Higher Ah rating means the battery will be able to run longer before requiring a recharge. If the load current Iload is known then the runtime t can be calculated as follows: | + | The second most important quantitative battery parameter is capacity Qbat. Capacity determines how much charge a battery can store. It is measured in amp-hours (Ah). A Higher Ah rating means the battery will be able to run longer before requiring a recharge. If the load current Iload is known then the runtime t can be calculated as follows: |
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| The next electrical parameter is current. A good battery datasheet will provide at least a few current values at different conditions. Common parameters are standard charge current, rapid charge current, max. continuous discharge current and standard discharge current. Often the charging current ratings are significantly lower than discharge ratings. | The next electrical parameter is current. A good battery datasheet will provide at least a few current values at different conditions. Common parameters are standard charge current, rapid charge current, max. continuous discharge current and standard discharge current. Often the charging current ratings are significantly lower than discharge ratings. | ||
| - | In engineering and battery datasheets there is another battery-specific parameter | + | In engineering and battery datasheets there is another battery-specific parameter |
| **Cycle life and ageing** | **Cycle life and ageing** | ||
| - | Battery lifetime is a critical parameter of secondary batteries. Depending on the chemistry battery lifetime is affected by ageing mechanisms: cyclic ageing and calendar ageing. As the name suggests calendar ageing is related to the absolute age of the battery: as battery ages, its performance will deteriorate – capacity will decrease and internal impedance will increase leading to decreased current capability. | + | Battery lifetime is a critical parameter of secondary batteries. Depending on the chemistry battery lifetime is affected by ageing mechanisms: cyclic ageing and calendar ageing. As the name suggests calendar ageing is related to the absolute age of the battery: as the battery ages, its performance will deteriorate – capacity will decrease and internal impedance will increase leading to decreased current capability. |
| - | The other ageing mechanism – cyclic ageing, is related to the intensity of battery usage. A full battery cycle is a full charge followed by a full discharge. Battery manufacturers in battery datasheets give an estimated cycle life – typically few to several hundreds of cycles. For this cycle number to be true it is of importance | + | The other ageing mechanism – cyclic ageing, is related to the intensity of battery usage. A full battery cycle is a full charge followed by a full discharge. Battery manufacturers in battery datasheets give an estimated cycle life – typically few to several hundreds of cycles. For this cycle number to be true it is important |
| A key fact is that batteries degrade with each cycle even if the cycle is not full. However, this degradation rate and linearity are not the same for all models. | A key fact is that batteries degrade with each cycle even if the cycle is not full. However, this degradation rate and linearity are not the same for all models. | ||
| **Battery pack** | **Battery pack** | ||
| - | As previously described, a battery pack consists of cells and a set of auxiliary components. Both in literature and practice, the word “pack” is often omitted as is here as well. For stationary applications, | + | As previously described, a battery pack consists of cells and a set of auxiliary components. Both in literature and practice, the word “pack” is often omitted as is here as well. For stationary applications, |
| - | Each battery module or a small battery pack consists of individual cells. All cells are of the same model and are preferably parameter-matched to provide maximum performance utilization. There are two types of connections | + | Each battery module or a small battery pack consists of individual cells. All cells are of the same model and are preferably parameter-matched to provide maximum performance utilization. There are two types of connections |
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| As the capacity rating is increased, the C-rate is increased proportionally as well, resulting in higher permissible | As the capacity rating is increased, the C-rate is increased proportionally as well, resulting in higher permissible | ||
| - | One of a battery pack’s description parameters is the cell configuration: | + | One of a battery pack’s description parameters is the cell configuration: |
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| + | **Battery management system (BMS)** | ||
| + | Sometimes even a single cell requires an obligatory BMS, which can have a variety of functions. The main task of a BMS is to maintain a safe operation of the battery – the safety of Lithium-based ell has always been an issue, which requires special care. | ||
| + | The functions can be divided into four groups: protection, monitoring, estimation balancing. | ||
| + | Safety essentially is protection. Some cells have some inherent safety features, such as overpressure, | ||
| + | The thermal management system can be a part of the overall BMS. Battery packs can be actively cooled (or heated) – the temperature of the coolant medium and its flow must be monitored as well. BMS monitoring functions might include data logging of all mentioned measured parameters and additional ones like total cycles, max and min discharge levels, total delivered energy and other time and charge related variables. | ||
| + | BMS and its functions can extend further. As mentioned, thermal management can be a part of BMS, especially if active temperature control is used: forced-air or liquid cooling/ | ||
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| + | ==== Fuel cell technology ==== | ||
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| + | Fuel cells (FC) are devices somewhat like batteries. Their purpose is to provide electrical energy by converting chemical energy. Same as batteries they are electrochemical devices. The main difference is that fuel cells use some sort of chemical compound (fuel), which is supplied to the cell to produce electricity by a controlled electrochemical redox reaction. The fuel and oxidizing material (oxygen from the air) is consumed during the process. On the other hand, the battery already had all components embedded in a closed package and no material was consumed during charging/ | ||
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| + | The most notable fuel cell technology is the proton exchange membrane fuel cell (PEM FC). The basic elements of a fuel cell are anode, cathode and electrolyte. A PEM layer contains an electrolyte and separates the anode from the cathode. Fuel is delivered to the anode side while oxygen is delivered to the cathode side. Popular fuels are hydrogen and methanol. As an electrochemical reaction takes place, protons from fuel are transferred through the PEM to the cathode side where waste is produced: water in case of hydrogen fuel and CO2 if methanol FC is used. As usual, the electrical load is connected to anode and cathode to deliver electrical energy. | ||
| + | Common efficiencies are in the 50% to 60% range which means that a significant amount of heat will be generated during power production – a cooling system like one of the common ICE vehicles is required as the temperature operating range of FCs is limited. | ||
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| + | A key issue for hydrogen FC adoption is the lack of refuelling infrastructure. Hydrogen gas is extremely flammable, it can diffuse in and through metals and it can cause metal embrittlement hence manufacturing, | ||
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| + | ==== Supercapacitors ==== | ||
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| + | In simplistic terms, supercapacitors (SC) are capacitors with extremely high capacity. In fact, they use special physical effects (electrochemical pseudocapacitance and/or electrostatic double-layer capacitance) to provide capacity. Depending on brand names and physical effects, supercapacitors are also called boost capacitors, ultracapacitors, | ||
| + | One must not confuse SCs with common high capacity aluminium electrolytic capacitors which are made with rated voltages from a few to hundreds of volts. The rated voltage of a single SC cell is in the range of 2.1 V to 3 V. | ||
| + | The capacity of single SCs ranges from hundreds of millifarads to a few kilofarads – they extend the capacitor capacity range as the largest electrolytic capacitors are just around 1F incapacity. However, they have not replaced batteries due to relatively minuscule specific energy (7.4Wh/kg for 3400F capacitor), which makes them inappropriate for bulk energy storage. | ||
| + | SC technology is evolving to improve the overall performance. Hybrid capacitors have been developed – they use both SC and Li-ion technology. The result is a so-called lithium-ion capacitor – as the name suggests, it is more like a capacitor with some features of the Li-ion battery. | ||
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| + | To conclude this chapter, see the figure below - a Ragone plot, which is an effective tool to graphically compare gravimetric energy density (specific energy) and gravimetric power density (specific power) of various energy storage elements. The lowest performance is at the bottom left corner while the highest performance is at the top right corner - a spot to be taken by future technologies. | ||
| + | As can be seen, fuel cell technology can provide the highest specific energy while capacitors can provide the highest specific power. However, Li-ion technology with its high specific energy and good specific power is the right choice for most mobile/ | ||
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