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| en:av:technology [2020/06/12 06:31] – new fig9 kristaps.vitols | en:av:technology [2020/07/20 09:00] (current) – external edit 127.0.0.1 |
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| <figure label> | <figure label> |
| {{:en:av:bfig_4_pba_dch.png?300|title}} | {{:en:av:bfig_4_pba_dch_2.png?500|title}} |
| <caption>Discharging process of a lead-acid cell.</caption> | <caption>Discharging process of a lead-acid cell.</caption> |
| </figure> | </figure> |
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| <figure label> | <figure label> |
| {{:en:av:bfig_5_pba_chg.png?300|title}} | {{:en:av:bfig_5_pba_chg_2.png?500|title}} |
| <caption>Charging process of a lead-acid cell.</caption> | <caption>Charging process of a lead-acid cell.</caption> |
| </figure> | </figure> |
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| <figure label> | <figure label> |
| {{:en:av:bfig_8.png?400|title}} | {{:en:av:bfig_8_2.png?700|title}} |
| <caption>Generic chemical reactions of a Li-ion cell.</caption> | <caption>Generic chemical reactions of a Li-ion cell.</caption> |
| </figure> | </figure> |
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| == Discharging == | == Discharging == |
| All LIBs are characterized by relatively low self-discharge and no memory effect as opposed to the NiMH chemistry which requires occasional full discharge. In case of LIBs, full discharge is to be avoided to maximize battery lifespan. As for most batteries, the discharge rate affects the voltage of the cell – at high rates the voltage will drop more, in some cases it is beneficial to decrease the cut-off voltage to achieve desired end DoD. In most LIBs the discharge curve (vertical axis represents battery voltage while horizontal axis represents SoC or DoD) is linear with a drop at the final stage of discharge (90-100% DoD) when discharged at low rate. However, as the rate is increased, the drop at high DoD becomes flatter while the voltage drops faster at the opposite end of the curve, at low DoD (Fig. 9). The discharge performance is heavily affected by the temperature of the cell - in figure 9, the 6.6C rate curve does not reach 2.0V cut-off voltage because temperature of the cell has risen to the max limit. The nominal curve is given at 20, 23 or 25°C. At 45°C ambient temperature, the voltage curve of the cell is increased by less than 100mV, hence increased temperature minimally affect voltage under discharge. The situation is different if ambient temperature is decreased. At 0°C the voltage of a cell can a couple hundred mV lower (fig. 10). At negative temperatures the voltage decreases further limiting the discharge rate – if rate is too high the voltage drops below cut-off voltage and discharge should be terminated. This effect is somewhat mitigated if cell is operated at moderate discharge rate – due to internal losses the cell can self-heat and thus improve its performance. For most Li-ion types the available capacity rapidly decreases at low temperature (below -15°C). However, there exists a wide variety of different types and special purpose battery models which are designed to operate at high rates or low temperatures. | All LIBs are characterized by relatively low self-discharge and no memory effect as opposed to the NiMH chemistry which requires occasional full discharge. In case of LIBs, full discharge is to be avoided to maximize battery lifespan. As for most batteries, the discharge rate affects the voltage of the cell – at high rates the voltage will drop more, in some cases it is beneficial to decrease the cut-off voltage to achieve desired end DoD. In most LIBs the discharge curve (vertical axis represents battery voltage while horizontal axis represents SoC or DoD) is linear with a drop at the final stage of discharge (90-100% DoD) when discharged at low rate. However, as the rate is increased, the drop at high DoD becomes flatter while the voltage drops faster at the opposite end of the curve, at low DoD (Fig. 10). The discharge performance is heavily affected by the temperature of the cell - in figure 9, the 6.6C rate curve does not reach 2.0V cut-off voltage because temperature of the cell has risen to the max limit. The nominal curve is given at 20, 23 or 25°C. At 45°C ambient temperature, the voltage curve of the cell is increased by less than 100mV, hence increased temperature minimally affect voltage under discharge. The situation is different if ambient temperature is decreased. At 0°C the voltage of a cell can a couple hundred mV lower (fig. 11). At negative temperatures the voltage decreases further limiting the discharge rate – if rate is too high the voltage drops below cut-off voltage and discharge should be terminated. This effect is somewhat mitigated if cell is operated at moderate discharge rate – due to internal losses the cell can self-heat and thus improve its performance. For most Li-ion types the available capacity rapidly decreases at low temperature (below -15°C). However, there exists a wide variety of different types and special purpose battery models which are designed to operate at high rates or low temperatures. |
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| It can be said that the type of chemistry plays a critical role in the discharge performance. The most obvious initial difference is the nominal voltage (figure 11). It is commonly assumed that a single Li-ion cell has 3.6V nominal voltage although 3.7V are prevalent as well – these values are for the dominant group of LCO, LMO, NMC and NCA. On top of these two numbers, values around them can exist as well, for example, //LG Chem// produces 18650-size INR18650MJ1 cell (NMC type) whose datasheet’s nominal voltage is 3.635V. However, 3.6 and 3.7 values are close together and difference is not critically important in most cases. | It can be said that the type of chemistry plays a critical role in the discharge performance. The most obvious initial difference is the nominal voltage (figure 12). It is commonly assumed that a single Li-ion cell has 3.6V nominal voltage although 3.7V are prevalent as well – these values are for the dominant group of LCO, LMO, NMC and NCA. On top of these two numbers, values around them can exist as well, for example, //LG Chem// produces 18650-size INR18650MJ1 cell (NMC type) whose datasheet’s nominal voltage is 3.635V. However, 3.6 and 3.7 values are close together and difference is not critically important in most cases. |
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| <figure label> | <figure label> |
| == Charging == | == Charging == |
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| In general, the CCCV charging method is used to charge LIBs similarly to lead-acid chemistry. Hence, there are two main charging phases: the faster constant-current phase and the slower constant-voltage phase as shown in figure 12. If a battery is deeply discharged (below minimum voltage) then a pre-charge phase should be introduced before full current CC phase. The pre-charge current should be 10% or less than the nominal charging current (given in the datasheet) of the battery. Once the voltage of the battery is higher than minimal discharge voltage, charger can switch to full current charging in CC phase. In normal operation, pre-charge phase should be omitted as BMS (battery management system) has to prevent deep discharge and associated damage to the LIB. However, if the battery voltage is indicating deep discharge then pre-charge should be carried out to pre-condition both electrodes for effective lithium ion transport. Immediately applying full current (or even worse fast-charge current) to a deeply discharged LIB can result in additional heating (risk of thermal runaway and associated danger) and permanent damage to the electrodes. | In general, the CCCV charging method is used to charge LIBs similarly to lead-acid chemistry. Hence, there are two main charging phases: the faster constant-current phase and the slower constant-voltage phase as shown in figure 13. If a battery is deeply discharged (below minimum voltage) then a pre-charge phase should be introduced before full current CC phase. The pre-charge current should be 10% or less than the nominal charging current (given in the datasheet) of the battery. Once the voltage of the battery is higher than minimal discharge voltage, charger can switch to full current charging in CC phase. In normal operation, pre-charge phase should be omitted as BMS (battery management system) has to prevent deep discharge and associated damage to the LIB. However, if the battery voltage is indicating deep discharge then pre-charge should be carried out to pre-condition both electrodes for effective lithium ion transport. Immediately applying full current (or even worse fast-charge current) to a deeply discharged LIB can result in additional heating (risk of thermal runaway and associated danger) and permanent damage to the electrodes. |
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| Most of the charge to the battery is delivered during the CC charging phase. The controllable parameter is current. The standard charging rate commonly is 0.5C which results in approximately 2-hour 0-100% battery charging (including CV phase). Older Li-ion chemistries were quite sensitive to charging current – higher rates would result in metallic lithium plating on electrodes, electrode expansion (package deformation) and overall performance deterioration. In worst case it would result in thermal runaway and venting with flame. Progress of technology and development of new types (NMC, NCA) have resulted in more robust cells with higher allowable charging rates. Now, faster charging can be achieved by using 1C or even 2C rates. However, fast charging has its limits. In standard charging, most time of charging is spent in CC phase, when battery is charged to 80-90% SoC. CC phase is terminated when charging voltage level is reached and charging transitions to CV phase during which the remaining charge is delivered to the battery. Charging during CV phase happens much slower due to ever decreasing current. When high rate is used in CC phase, the charging voltage limit is reached much faster due to cell heating and resistive drop (seen as voltage increase) similar to that of discharge curve (at high discharge current battery voltage drops, at high charge current voltage steps up). As a result, less charge is transferred to the cell, for example just 60 – 80% or even less. The remaining charge must be delivered in the slower CV phase. The other issue with fast charging is temperature rise. Both resistive losses and ionic conductivity losses produce heat which increase temperature of the battery. When max temperature threshold has been reached, charging current must be decreased hence fast charging transitions to standard charging. This problem can be alleviated if proper thermal management is used for the battery pack. A quality cooling system can keep temperature low (well below max limit, preferably not more that around 30°C) to allow fast charging while avoiding performance degradation. For some battery models, active cooling can allow to increase charging current even higher to achieve faster charging time. In general, it is commonly assumed that EV fast charging (CC phase) can charge battery only to 80% SoC level. | Most of the charge to the battery is delivered during the CC charging phase. The controllable parameter is current. The standard charging rate commonly is 0.5C which results in approximately 2-hour 0-100% battery charging (including CV phase). Older Li-ion chemistries were quite sensitive to charging current – higher rates would result in metallic lithium plating on electrodes, electrode expansion (package deformation) and overall performance deterioration. In worst case it would result in thermal runaway and venting with flame. Progress of technology and development of new types (NMC, NCA) have resulted in more robust cells with higher allowable charging rates. Now, faster charging can be achieved by using 1C or even 2C rates. However, fast charging has its limits. In standard charging, most time of charging is spent in CC phase, when battery is charged to 80-90% SoC. CC phase is terminated when charging voltage level is reached and charging transitions to CV phase during which the remaining charge is delivered to the battery. Charging during CV phase happens much slower due to ever decreasing current. When high rate is used in CC phase, the charging voltage limit is reached much faster due to cell heating and resistive drop (seen as voltage increase) similar to that of discharge curve (at high discharge current battery voltage drops, at high charge current voltage steps up). As a result, less charge is transferred to the cell, for example just 60 – 80% or even less. The remaining charge must be delivered in the slower CV phase. The other issue with fast charging is temperature rise. Both resistive losses and ionic conductivity losses produce heat which increase temperature of the battery. When max temperature threshold has been reached, charging current must be decreased hence fast charging transitions to standard charging. This problem can be alleviated if proper thermal management is used for the battery pack. A quality cooling system can keep temperature low (well below max limit, preferably not more that around 30°C) to allow fast charging while avoiding performance degradation. For some battery models, active cooling can allow to increase charging current even higher to achieve faster charging time. In general, it is commonly assumed that EV fast charging (CC phase) can charge battery only to 80% SoC level. |
| </figure> | </figure> |
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| The charging (SoC increase) speed gradually decreases during CV phase as the current rate declines. In standard charging, CC phase lasts less than 1 hour while CV phase can be in the range of 1 to 2 hours. As previously noted, majority of charge is delivered to the battery during the short CC phase while the long CV phase delivers remaining fraction. If it is required to fully charge a battery the additional step of cell balancing can increase charging time. The charging is terminated when charging current decreases below cut-off limit. This limit traditionally is 10-3% of the 1C rate. Charging cut-off conditions are not always provided by the battery manufacturer, hence there is some engineering freedom. Additionally, charging timeout can be introduced as well. For example, the datasheet of US18650VTC6 cell (manufactured by Sony) states CCCV charging to 4.2V at 2.5A (0.5C) with 2.5h cut-off - a current cut-off limit is not specified. The timeout criterion can be helpful when the battery reaches its end of life. In some cases, the self-discharge/leakage current increases as the battery ages. If the leakage current is higher than current cut-off level, then battery charging current will never decrease below set cut-off and charger will indefinitely continue charging the battery. A timeout can prevent this situation. | The charging (SoC increase) speed gradually decreases during CV phase as the current rate declines. In standard charging, CC phase lasts less than 1 hour while CV phase can be in the range of 1 to 2 hours. As previously noted, majority of charge is delivered to the battery during the short CC phase while the long CV phase delivers remaining fraction. If it is required to fully charge a battery the additional step of cell balancing can increase charging time. The charging is terminated when charging current decreases below cut-off limit. This limit traditionally is 10-3% of the 1C rate. Charging cut-off conditions are not always provided by the battery manufacturer, hence there is some engineering freedom. Additionally, charging timeout can be introduced as well. For example, the datasheet of //US18650VTC6// cell (manufactured by //Sony//) states CCCV charging to 4.2V at 2.5A (0.5C) with 2.5h cut-off - a current cut-off limit is not specified. The timeout criterion can be helpful when the battery reaches its end of life. In some cases, the self-discharge/leakage current increases as the battery ages. If the leakage current is higher than current cut-off level, then battery charging current will never decrease below set cut-off and charger will indefinitely continue charging the battery. A timeout can prevent this situation. |
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| Charging is affected by temperature. Cell/battery datasheets provide information about ambient temperature for three situations: discharge, charge and storage. Traditionally one would expect that storage temperature range is the broadest. It is not so in case of LIBs. For short term storage (less than a month) it is the same as discharge operating temperature whose range can be -20°C to +60°C. As the storage time increases, ambient temperature should be kept within -20°C to 25°C range to maintain calendar life. Temperature during charging must be in 0 to 45°C range, preferably below 30°C. Already under 10°C standard charging rate should be decreased to 0.25C. It is generally assumed that LIBs should not be charged if temperature is below 0°C – if temperature is lower, ion mobility is restricted and charging will cause deformation of electrodes, which in turn will degrade performance and safety due to plating of metallic lithium. Both the LIB and the charger should be equipped with temperature monitoring to perform charging only if temperature is within safe operation range. If a battery will be required to be charged at freezing temperatures (an EV in northern countries where the winter temperatures are well below 0°C), then battery pack has to be equipped with thermal management which can provide heating. Of course, the temperature of the LIB will rise on its own during charging due to internal losses – BMS has to prevent temperature rise above operating point. Temperature can be controlled by controlling current or by using active thermal management. It can be noted that charging temperature can vary from model to model and from chemistry to chemistry. Again, LTO excels in operation at low temperatures. An LTO LT34 pouch cell made by Leclanche can be both discharged and charged at temperatures ranging from -20°C to +55°C. Research laboratories are working on improvements for all Li-ion chemistries to allow charging at temperatures below freezing point. There are reports that some LIBs (non-LTO) can be charged at freezing temperatures albeit at very low rates. | Charging is affected by temperature. Cell/battery datasheets provide information about ambient temperature for three situations: discharge, charge and storage. Traditionally one would expect that storage temperature range is the broadest. It is not so in case of LIBs. For short term storage (less than a month) it is the same as discharge operating temperature whose range can be -20°C to +60°C. As the storage time increases, ambient temperature should be kept within -20°C to 25°C range to maintain calendar life. Temperature during charging must be in 0 to 45°C range, preferably below 30°C. Already under 10°C standard charging rate should be decreased to 0.25C. It is generally assumed that LIBs should not be charged if temperature is below 0°C – if temperature is lower, ion mobility is restricted and charging will cause deformation of electrodes, which in turn will degrade performance and safety due to plating of metallic lithium. Both the LIB and the charger should be equipped with temperature monitoring to perform charging only if temperature is within safe operation range. If a battery will be required to be charged at freezing temperatures (an EV in northern countries where the winter temperatures are well below 0°C), then battery pack has to be equipped with thermal management which can provide heating. Of course, the temperature of the LIB will rise on its own during charging due to internal losses – BMS has to prevent temperature rise above operating point. Temperature can be controlled by controlling current or by using active thermal management. It can be noted that charging temperature can vary from model to model and from chemistry to chemistry. Again, LTO excels in operation at low temperatures. An LTO //LT34// pouch cell made by //Leclanche// can be both discharged and charged at temperatures ranging from -20°C to +55°C. Research laboratories are working on improvements for all Li-ion chemistries to allow charging at temperatures below freezing point. There are reports that some LIBs (non-LTO) can be charged at freezing temperatures albeit at very low rates. |
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| == Battery management system == | == Battery management system == |
| === Supercapacitors === | === 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, pseudocapacitors and electrostatic double-layer capacitors (EDLC). One must not confuse SCs with common high capacity aluminum electrolytic capacitors which are made with rated voltages from few to hundreds of volts. The rated voltage of a single SC cell is in the range of 2.1 to 3V. While majority of SCs are available as single cells, commercial SC batteries (series/parallel) with voltages higher than 100V are available on the market. The capacity of single SCs ranges from hundreds of millifarads to a few kilofarads – they extend capacitor capacity range as the largest electrolytic capacitors are just around 1F in capacity. If compared to LIBs, SC main advantage is the high specific power density (up to 14kW/kg) and vast cycle life (1000000 cycles). However, they have not replaced LIBs or other batteries due to relatively miniscule specific energy (7.4Wh/kg for 3400F capacitor) which makes them inappropriate for bulk energy storage. SCs can be used to improve the power capability of battery systems, especially in applications where the bulk energy storage is a relatively low capacity battery (with low power) as in hybrid vehicles and fuel cell vehicles. SCs can be successfully implemented in applications requiring regenerative braking: the power handling capability of SCs is perfect for absorbing high power pulses of braking while the stored energy can be used for the following acceleration also requiring high power. | 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, pseudocapacitors and electrostatic double-layer capacitors (EDLC). One must not confuse SCs with common high capacity aluminum electrolytic capacitors which are made with rated voltages from few to hundreds of volts. The rated voltage of a single SC cell is in the range of 2.1 to 3V. While majority of SCs are available as single cells, commercial SC batteries (series/parallel) with voltages higher than 100V are available on the market. The capacity of single SCs ranges from hundreds of millifarads to a few kilofarads – they extend capacitor capacity range as the largest electrolytic capacitors are just around 1F in capacity. If compared to LIBs, SC main advantage is the high specific power density (up to 14kW/kg) and vast cycle life (1000000 cycles). However, they have not replaced LIBs or other batteries due to relatively minuscule specific energy (7.4Wh/kg for 3400F capacitor) which makes them inappropriate for bulk energy storage. SCs can be used to improve the power capability of battery systems, especially in applications where the bulk energy storage is a relatively low capacity battery (with low power) as in hybrid vehicles and fuel cell vehicles. SCs can be successfully implemented in applications requiring regenerative braking: the power handling capability of SCs is perfect for absorbing high power pulses of braking while the stored energy can be used for the following acceleration also requiring high power. |
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| 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 so called lithium ion capacitor – as name suggests, it is more like a capacitor with some features of the Li-ion battery. The main advantage is elevated voltage: 3.8V rated voltage increases the specific energy narrowing the gap between SCs and LIBs. Future improvements using graphene materials could increase performance of SC, the same is true for LIBs. | 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 so called lithium ion capacitor – as name suggests, it is more like a capacitor with some features of the Li-ion battery. The main advantage is elevated voltage: 3.8V rated voltage increases the specific energy narrowing the gap between SCs and LIBs. Future improvements using graphene materials could increase performance of SC, the same is true for LIBs. |
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| | To conclude this chapter see figure 14 - 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. Given figure represents generalized performance - higher performing application specific technologies exist and are under continuous development. 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/portable applications. |
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| | <figure label> |
| | {{:en:av:bfig_14_ragone.png?500|title}} |
| | <caption>General Ragone plot of energy storage elements.</caption> |
| | </figure> |
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