In theory, a batteryless device can completely forgo energy storage and rely entirely on ambient energy harvested from the environment. In practice, most devices require some form of energy storage buffer between the harvester and the device, if only to smooth out the overall power curve and provide somewhat more predictable operation.
So, why capacitors over batteries?
Broadly speaking, batteries and capacitors both function as power storage devices: both store a potential difference (or voltage), which can be discharged to power a device or circuit. How they do so varies, however, with significant implications on capabilities and behavior.
Batteries store their potential difference via electrochemical reaction. One portion of the battery (the anode) releases electrons that are accepted by the other portion (the cathode): this movement of electrons generates current (and by extension, power). Chemical operations limit how quickly batteries can discharge (or charge) their energy, but results in a comparatively energy dense storage method with a constant voltage over most of its charge/discharge cycle.
Depending on the materials used for the anode and cathode a battery will be either disposable (the chemical reaction is irreversible) or rechargeable (the chemical reaction can be reversed by an outside power source). However, even rechargeable batteries have a finite lifetime, losing effectiveness over multiple recharge cycles: chemical composition plays a significant role in base lifetime, but environmental conditions and how often the battery is charged and discharged also limit a rechargeable battery’s practical lifetime.
Capacitors store power via electromagnetism, rather than a chemical reaction. Most capacitors contain two conductive plates separated by a non-conductive layer (called the dielectric). When connected to a power source, the dielectric blocks the normal flow of electrons between the plates, leading to one plate accumulating electrons while the other loses them: this results in the polarization of the dielectric as the plates exert equal but opposite electrostatic force on the dielectric, storing a potential difference in the field. Once disconnected from a power source, the plates will seek to equalize charges, and the resulting electron flow from one plate to another creates a current (and power). This behavior allows for rapid charge and discharge times relative to a battery, though with the caveat that voltage will fluctuate to a greater extent.
Storing power via electromagnetism has limitations, however. All dielectric materials have a breakdown voltage, or a point where the resulting electric field becomes strong enough to act as a conductor: lighting is a visible example of this behavior in nature, occurring when the potential difference between clouds and the earth exceeds the breakdown voltage of air. This breakdown voltage places a practical ceiling on the maximum voltage of a capacitor, and so the only other way to increase power storage is to increase the size of the plates: due to the physics involved, a capacitor will usually be significantly larger than a battery that can store a comparable amount of energy.
Capacitors are also more limited in terms of long-term storage compared to a battery. In theory, if an ideal capacitor is charged and then disconnected from a circuit, it will retain a charge indefinitely: in practice, the physics of real-world capacitors results in some amount of discharge even when disconnected from a circuit or otherwise inactive. Combined with their generally lower overall capacity, energy stored in a capacitor tends to be “use it or lose it” in a way that it is not with batteries.
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?
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 on a capacitor is expressed as:
Q = CV
Where Q is charge, C is the capacitance, and V is voltage. By extension:
V = Q/C
The larger the capacitor, the higher the charge accumulation necessary to reach a specific voltage value (and the longer it takes to accumulate it).
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.
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.
Because of leakage and ESR, it should not be taken as given that the extra power gained from a larger capacitor will be converted entirely to useful work.
Given the above, we can come up with some general guidelines when selecting capacitors for a batteryless device:
A number of capacitor setups have been explored, ranging from simple single capacitor setups to more complex power storage and management. The most common setups and examples are explored below.
The simplest implementation merely replaces a device’s battery with a capacitor (or a single fixed bank of capacitors in series and/or parallel with each other): this capacitor then serves as the primary energy storage for the device. This design has an advantage in relative simplicity, and energy management is likewise relatively straightforward as there is only one source to manage. However, using only one capacitor limits the device to, at minimum, the capacitance required for its most intensive task: this can potentially be an issue if the device’s other tasks would have more optimal performance with a smaller capacitor.
Batteryless devices often include multiple peripherals with their own individual energy requirements, and these conflicting requirements often mean that selecting an ideal capacitor for one means compromising the performance of others.
Federated storage arises from the observation that if capacitors cannot provide the steady power of a traditional power source, then there is no need to restrict energy storage to a single capacitor or bank: separate individual capacitors can be used to store energy and power for individual peripherals or tasks, allowing each capacitor to be optimally sized for what it is powering. Separate energy banks also allow for more flexible operation: a sensor can gather readings without impacting the processor that parses them, which in turn does not use energy that the radio needs to send the readings to another node.
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
Reconfigurable storage setups approach the capacitance issue from a different angle. Capacitors connected in parallel have increased capacitance (and reduced capacitance when in series): by using switches to (dis)connect or adjust connections between an arbitrary number of capacitors connected in parallel (or series) with each other, the overall capacitance can be dynamically adjusted to best meet the current demands on the device. This provides (from the circuit’s perspective) a single power source, but with the flexibility of being able to adjust capacitance to best match current demands.
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