Energy storage is one of the most difficult problems to solve in this project, few applications have such tight volume, self discharge, burst current and longevity requirements. Throughout the development of the project there has been many twists and turns in the storage and topology. First I will outline the options for energy storage in this size class.

Energy Storage Type.RechargeableNominal VoltageDuplicates in Ring.CapacityLife time CapacityPeak CurrentForm FactorNotes
Silver Oxide 337No1.55V2-4~8mAh~8mAh> 10mA4.8mm Diameter 1.6mm thicknessRequires end user replacement.
ML414HYes3V2-41mAh300mAh20uA4.8mm Diameter 1.4mm thicknessMax current not enough for lighting LEDs.
CerachargeYes1.5V4-100.1mAh> 100mAh~1mA4.4 x 3.0 x 1.1mmA good option!
Super Capacitor CPH3225AYes3.3V10-200.011mF (4uAh)>300mAh10mA3.5 x 2.8 x 1mmBest longevity, easily supplies max current.


Replaceable Batteries

Initially the ring was designed to use the silver oxide batteries (Ring Version 1.0). These allow for a very simple design, as there is no need to recharge the batteries, they would last many months before needing to be replaced. However the need for the user to replace the batteries necessitates careful design of a very small water proof battery housing. This adds bulk and complexity.

User Charging

It is possible to combine different options to get the best of both worlds, for example the ML414 could be used for bulk energy storage, then the CPH3225A could be used to supply burst current for lighting the LEDs. However, the high energy density options like the ML414 have very poor cycle life. Assuming a charge cycle every few weeks, the ring would have a life span of only a few years before the battery no longer holds adequate charge. This also ends up complicating the circuit and using up lots of space.

The Ceracharge batteries are a good middle ground. With enough duplicates they can comfortably provide enough current, and their capacity is much greater than the super capacitor (CPH3225A), enough for about a couple of weeks of runtime. However, these have a similar lifecycle issue. The datasheet cannot guarantee useful lifespan beyond 10 years of usage.

Self Charging

Wanting to remove the user worry about maintaining adequate charge in the ring, I moved to designing self-charging designs. Even for energy storage options that have poor cycle life, self charging would allow the maximum degradation of the energy storage to be much worse before the ring is unusable.

The two reasonable options for energy harvesting are: Light, Thermal Gradients.

Thermal gradients seem like they could provide enough total energy to support this project. However, the physical size of the system would be hard to keep small. This is because there would have to be a “cold side”, likely a section of the ring with fins to dissipate heat. This would add unacceptable bulk for the sleek design intensions for this project. (It is an interesting enough idea that I am pursuing it separately though!)

Light / Solar is the other good option. This allows for a very compact design, as solar cells can be made thin. An additional benefit is that it is easy to make a series array of arbitrary voltage, which simplifies the harvesting electronics. The main downside however is that there is not very much surface area and light intensity to work with. My calculations suggest that it is marginal that the ring could survive in a well lit room. This would still allow the energy storage to “coast” for longer. This is despite the circuitry for harvesting displacing about 25% of the space that could be used for more storage.

However, if the user goes outside even for a few minutes a day, the whole calculation becomes much more favourable. Even an overcast day is typically 50-100 times brighter than inside.

Charging Circuits

Charging CircuitMPPT MethodQuiescent current in mild light.Efficiency in low light.Efficiency In Bright Light
bq25504Fixed ratio of open circuit voltage.1-2uABadVery Good
LDO based.Fixed by absolute value based on storage element voltage.< 100nAGoodVery Bad
R1800KFixed by absolute value based on R1800K SKU.144nAGoodGood

I initially used the bq25504, I was drawn in by its low quiescent current values. Later I discovered this was misleading, as these are quoted when in shutdown (due to a full storage element etc), however it is significantly more when the converter is actively trying to extract energy, it seems to be about 15uA in that case.

This issue led me to thinking about a very low quiescent current method, even when energy is being actively extracted. If the photodiode string is fed directly into a LDO, which then charges the storage element through a diode, the quiescent current can be very low. The power point tracking would be done very naively, where the photodiode string length would be matched to the storage element voltage such that the system is near its maximum power point in “normal” conditions. If this was calibrated for typical indoors lighting efficiency could be quite good. The poor efficiency when in very bright light is usually less important, as in that situation you are likely to have enough energy to fairly quickly saturate the storage element.

I then found the R1800k series of chips. This is basically a switch mode version of the above. The “maximum” power point is set to a fixed voltage via selecting a specific chip SKU. Again you would specify this for your “normal” operating conditions. The fact that is is a buck regulator allows somewhat independent selection of the photodiode string voltage and storage element nominal voltage. Whenever the input voltage rises above the MPP, the converter moves the energy to the output, until the terminal charge voltage is reached. The chip is then deactivated, allowing the input voltage to rise to the photodiode string open circuit voltage. This chip has a very low quiesent current of only 144nA, it uses more than this while switching, though it will not switch often in very low light, limiting the practical amount of energy it wastes. This chip will break even at 720nW, compared to the bq25504 at around 15uW, which in this application where there is typically ~3uW available in normal indoor lighting is quite an important difference.