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tutorials:capacitor_selection [2026/03/25 13:53] – [Batteries vs. Capacitors: A Brief Introduction] ibchadmintutorials:capacitor_selection [2026/03/25 14:46] (current) – [References and Further Reading] ibchadmin
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 Most IoT devices (and electronic devices more generally) are built around the assumption of a constant, steady supply of power, which has traditionally been the domain of batteries.  They cannot be casually swapped for capacitors, which have on average lower power density and significantly different I-V characteristics over their discharge cycles: if using capacitors as the primary energy storage, a device will need to be designed around these limitations.  So if batteries are tried and tested and the “default” for most applications, why bother with capacitors? Most IoT devices (and electronic devices more generally) are built around the assumption of a constant, steady supply of power, which has traditionally been the domain of batteries.  They cannot be casually swapped for capacitors, which have on average lower power density and significantly different I-V characteristics over their discharge cycles: if using capacitors as the primary energy storage, a device will need to be designed around these limitations.  So if batteries are tried and tested and the “default” for most applications, why bother with capacitors?
  
-Sustainability: in most electronic devices, the battery is typically the component with the lowest working life (around a few years at most).  Most batteries also contain heavy metals that are environmentally toxic: even if replacement is viable, disposal is a constant concern.  In a world with millions (or billions or even trillions) of small IoT devices, the quantity of potential battery e-waste goes from an inconvenience to a significant problem. +  * **Sustainability**: in most electronic devices, the battery is typically the component with the lowest working life (around a few years at most).  Most batteries also contain heavy metals that are environmentally toxic: even if replacement is viable, disposal is a constant concern.  In a world with millions (or billions or even trillions) of small IoT devices, the quantity of potential battery e-waste goes from an inconvenience to a significant problem.  Capacitors, by contrast, generally have significantly longer active lives than capacitors do, reducing the need for replacement: the composition of most capacitors also makes e-waste less of a concern than with batteries (though not entirely irrelevant). 
- +  * **Maintenance**: Most batteryless devices are intended to be deployed in conditions where access may be difficult (remote wilderness) or impossible (microsatellites).  Even when accessible, the human effort required to regularly replace and dispose of batteries may make certain deployments impractical.  The goal of batteryless devices is to not only reduce the need to dispose of e-waste from batteries, but the human labor required to maintain them as well. 
-Capacitors, by contrast, generally have significantly longer active lives than capacitors do, reducing the need for replacement: the composition of most capacitors also makes e-waste less of a concern than with batteries (though not entirely irrelevant). +  * **Quick charging**: Many UAV/robotics platforms rely on batteries for power storage, but the batteries used in most platforms take significantly longer to charge than discharge: this results in a relatively low duty cycle (period where a robot/UAV is active vs. recharging) as recharging becomes a primary bottleneck.  As capacitors can be charged significantly quicker (seconds vs. minutes or hours) than batteries, they are being explored as an alternative energy storage method to reduce downtime. 
-Maintenance: Most batteryless devices are intended to be deployed in conditions where access may be difficult (remote wilderness) or impossible (microsatellites).  Even when accessible, the human effort required to regularly replace and dispose of batteries may make certain deployments impractical.  The goal of batteryless devices is to not only reduce the need to dispose of e-waste from batteries, but the human labor required to maintain them as well. +  * **Batteries may be infeasible**: Some applications may prevent the use of batteries at all.  Devices deployed to monitor conditions in the [[https://dl.acm.org/doi/10.1145/3384419.3430722|Mithraetum of Circus Maximus]] in Rome were forced to forgo batteries, as environmental conditions ran the risk of causing battery leakage and damage to the site. Capacitors can also be made at scales far smaller than any battery, and (barring any breakthroughs in battery technology) will likely be the only practical method of energy storage for micro-scale systems like [[https://en.wikipedia.org/wiki/Smartdust|smart dust]].
-Quick charging: Many UAV/robotics platforms rely on batteries for power storage, but the batteries used in most platforms take significantly longer to charge than discharge: this results in a relatively low duty cycle (period where a robot/UAV is active vs. recharging) as recharging becomes a primary bottleneck.  As capacitors can be charged significantly quicker (seconds vs. minutes or hours) than batteries, they are being explored as an alternative energy storage method to reduce downtime. +
-Batteries may be infeasible: Some applications may prevent the use of batteries at all.  Devices deployed to monitor conditions in the Mithraetum of Circus Maximus in Rome were forced to forgo batteries, as environmental conditions ran the risk of causing battery leakage and damage to the site. Capacitors can also be made at scales far smaller than any battery, and (barring any breakthroughs in battery technology) will likely be the only practical method of energy storage for micro-scale systems like smart dust.+
  
 ===== Capacitor Sizing: Why Bigger is Not (always) Better ===== ===== Capacitor Sizing: Why Bigger is Not (always) Better =====
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 If using a capacitor in place of a battery, it might be tempting to use a capacitor with the highest capacitance possible to minimize the difference in storage capacity.  However, a larger capacitor may end up being detrimental overall if two main factors are not taken into account: If using a capacitor in place of a battery, it might be tempting to use a capacitor with the highest capacitance possible to minimize the difference in storage capacity.  However, a larger capacitor may end up being detrimental overall if two main factors are not taken into account:
  
-Charge Time and Voltage: Charge on a capacitor is expressed as:+==== Charge Time and Voltage ==== 
 + 
  
-Q = CV+Charge on a capacitor is expressed as: 
 + 
 +//Q = CV//
  
  
 Where Q is charge, C is the capacitance, and V is voltage.  By extension: Where Q is charge, C is the capacitance, and V is voltage.  By extension:
  
-V = Q/C+//V = Q/C//
  
  
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 All electrically-powered devices have minimum voltage thresholds necessary for operation (and may have restricted operations at those minimums): the longer it takes for the capacitor to reach those thresholds, the longer the downtime.  In some applications this may be acceptable if a larger amount of power is needed for specific work: for other applications (e.g. sensors trying to detect specific events), longer downtimes may interfere with the device’s primary tasks. All electrically-powered devices have minimum voltage thresholds necessary for operation (and may have restricted operations at those minimums): the longer it takes for the capacitor to reach those thresholds, the longer the downtime.  In some applications this may be acceptable if a larger amount of power is needed for specific work: for other applications (e.g. sensors trying to detect specific events), longer downtimes may interfere with the device’s primary tasks.
-Leakage and Resistance: As mentioned previously, real world capacitors must contend with some level of leakage.  There are multiple contributing factors, from minor flaws in the plates from manufacturing to some small level of current passing through the dielectric, which larger capacitors exacerbate. + 
 +==== Leakage and Resistance ==== 
 +  
 + 
 +Real world capacitors must contend with some level of leakage.  There are multiple contributing factors, from minor flaws in the plates from manufacturing to some small level of current passing through the dielectric, which larger capacitors exacerbate. 
  
 Real world capacitors also have an equivalent series resistance (ESR) which is a product of the resistance of their material composition: larger capacitors will often have a higher ESR simply due to having more material (and thus, resistance) for current to travel through.   Real world capacitors also have an equivalent series resistance (ESR) which is a product of the resistance of their material composition: larger capacitors will often have a higher ESR simply due to having more material (and thus, resistance) for current to travel through.  
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 Given the above, we can come up with some general guidelines when selecting capacitors for a batteryless device: Given the above, we can come up with some general guidelines when selecting capacitors for a batteryless device:
  
-Assume short bursts of energy in most cases: it is difficult to match the steady power of a comparable battery device short of some supercapacitors or a source of harvested energy capable of providing more power than is actively consumed.  If either option is not available, then selecting purely for high capacitance may provide no benefit or even be counterproductive, and many batteryless devices will need to be designed with an appropriate checkpointing strategy to best make use of these short bursts. +  * **Assume short bursts of energy in most cases**: it is difficult to match the steady power of a comparable battery device short of some supercapacitors or a source of harvested energy capable of providing more power than is actively consumed.  If either option is not available, then selecting purely for high capacitance may provide no benefit or even be counterproductive, and many batteryless devices will need to be designed with an appropriate [[tutorials:checkpointing_overview|checkpointing strategy]] to best make use of these short bursts. 
-Have enough energy to complete the largest discrete task: At minimum, the capacitor should be able to provide enough charge to complete the largest discrete (i.e. must be done in one power cycle) task: for example, if the device has a radio that it uses to transmit data, it should have enough power to complete the transmission without encountering a power failure. +  * **Have enough energy to complete the largest discrete task**: At minimum, the capacitor should be able to provide enough charge to complete the largest discrete (i.e. must be done in one power cycle) task: for example, if the device has a radio that it uses to transmit data, it should have enough power to complete the transmission without encountering a power failure. 
-The smaller the capacitor, the more reactive the device: the less time a device spends charging, the more often it will be active (and capable of detecting/reacting to events, if necessary).  This is often in tension with the previous point, as many devices have a small number of tasks or peripherals with significantly higher energy demands than the others. +  * **The smaller the capacitor, the more reactive the device**: the less time a device spends charging, the more often it will be active (and capable of detecting/reacting to events, if necessary).  This is often in tension with the previous point, as many devices have a small number of tasks or peripherals with significantly higher energy demands than the others. 
-Be aware of non-ideal behaviors: like batteries, a capacitor’s paper performance can be degraded by various factors such as temperature, humidity, and frequent charge/discharge cycles.  What may work on a test bench or simulation may fail in a practical deployment if these factors are not properly accounted for.+  * **Be aware of non-ideal behaviors**: like batteries, a capacitor’s paper performance can be degraded by various factors such as temperature, humidity, and frequent charge/discharge cycles.  What may work on a test bench or simulation may fail in a practical deployment if these factors are not properly accounted for.
 ===== Capacitor Setups ===== ===== Capacitor Setups =====
  
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 With this flexibility comes increased complexity.  A separate energy management circuit is usually required to measure and distribute energy to the individual capacitors, which leads to increased overhead and circuit size.  The device itself must also be able to account for the current energy state of each individual peripheral or task: using the previous example, if a device wishes to transmit data it must be able to recognize whether or not the radio has sufficient power for operation and adjust its behavior accordingly. With this flexibility comes increased complexity.  A separate energy management circuit is usually required to measure and distribute energy to the individual capacitors, which leads to increased overhead and circuit size.  The device itself must also be able to account for the current energy state of each individual peripheral or task: using the previous example, if a device wishes to transmit data it must be able to recognize whether or not the radio has sufficient power for operation and adjust its behavior accordingly.
  
-Examples +**Examples** 
-United Federation of Peripherals (UFOP) http://dx.doi.org/10.1145/2809695.2809707 +  * [[http://dx.doi.org/10.1145/2809695.2809707|United Federation of Peripherals (UFOP)]] 
-Stash https://doi.org/10.1145/3641511 +  * [[https://doi.org/10.1145/3641511|Stash]]
 ==== Reconfigurable Storage ==== ==== Reconfigurable Storage ====
  
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 Like federated energy, there is a price in both circuit size and complexity, requiring multiple capacitors (even when some may be rarely active) and some control circuitry and logic to switch configurations as needed: there is also some energy loss when the configuration changes, due to equalization of charges across the capacitors.  While capable of a wider range of capacitances than single or federated storage methods, the range of capacitances is ultimately limited by the number of possible combinations, so the energy profile of the device should still be well understood to ensure proper capacitor sizing. Like federated energy, there is a price in both circuit size and complexity, requiring multiple capacitors (even when some may be rarely active) and some control circuitry and logic to switch configurations as needed: there is also some energy loss when the configuration changes, due to equalization of charges across the capacitors.  While capable of a wider range of capacitances than single or federated storage methods, the range of capacitances is ultimately limited by the number of possible combinations, so the energy profile of the device should still be well understood to ensure proper capacitor sizing.
  
-Examples +**Examples** 
-Capybara https://doi.org/10.1145/3173162.3173210 +  * [[https://doi.org/10.1145/3173162.3173210|Capybara]]
  
 ===== References and Further Reading ===== ===== References and Further Reading =====
  
  
-https://engineering.mit.edu/ask-an-engineer/how-does-a-battery-work +  * [[https://engineering.mit.edu/ask-an-engineer/how-does-a-battery-work|How Does a Battery Work?]] 
- +  * [[https://courses.lumenlearning.com/suny-physics/chapter/19-5-capacitors-and-dielectrics/|Capacitors and Dielectrics]] 
-https://courses.lumenlearning.com/suny-physics/chapter/19-5-capacitors-and-dielectrics/ +  * [[https://dl.acm.org/doi/abs/10.1145/3686138.3686144|The Future of Unmanned Aerial Vehicles Has No Batteries]] 
- +  * [[https://lirias.kuleuven.be/retrieve/804774|CapBot: Enabling Battery-Free Swarm Robotics]] 
-https://dl.acm.org/doi/abs/10.1145/3686138.3686144 +  * [[https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/|Capacitor Charge and Time Constant Calculator]] 
- +  * [[https://www.allaboutcircuits.com/industry-articles/understanding-capacitor-leakage-to-make-smart-things-run-longer|Understanding Capacitor Leakage to Make Smart Things Run Longer]]
-https://lirias.kuleuven.be/retrieve/804774 +
- +
-https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/ +
- +
-https://www.allaboutcircuits.com/industry-articles/understanding-capacitor-leakage-to-make-smart-things-run-longer+
  
tutorials/capacitor_selection.1774446783.txt.gz · Last modified: 2026/03/25 13:53 by ibchadmin
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