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Have you ever reached for your calipers to take a quick measurement, only to discover the battery is dead? We set out to solve this common frustration in the simplest way possible, while also exploring the latest advancements in small-scale energy harvesting technology along the way.
In this blog, you will learn more about…
We have many sets of calipers around the office (example1, example2), some that are very low cost and run on a single LR44 battery (also called G13, A76, and L1154). Our solution will work with most of these options.
The estimated cost of goods sold (COGS) for this design is approximately $6 per unit at a production quantity of 100. The solar cell accounts for the majority of this cost. LR44 batteries cost roughly $0.21 each when purchased in bulk. Based on these figures, this design would require over 20 years to recoup its initial investment, which is a considerable length of time This estimate doesn’t factor in the added convenience and reduced frustration of never dealing with a dead battery.
What would the breakeven point be if we included the total atmospheric carbon impact or other “true cost” metrics?
This was an internal effort, but we followed 219’s process for new product development as if it were a product we were developing for a client; see sections below. We’ve made good progress and achieved many of our key objectives, reaching roughly 80% completion. We’re pausing temporarily to focus on other project work, but will revisit this and do some of the fine detailed iterations in 2025.
Goal: To explore unconventional solutions for powering digital calipers.
Goal: Learn about the target device and the energy harvesting solutions.
Mechanically we did not want to require modifying the existing calipers assembly, just adding parts. We also want to retain access to the inside jaws, outside jaws and the depth measurement. Our electronics will need to get access to the battery contacts, and we conceived of a battery shaped electrical ‘biscuit’ that would install into the battery compartment, use the battery spring tabs to increase stackup tolerance, and expose the contacts such that it could interface with a flat PCB that is present in the location of the battery door.
Red & Green represent keep-aways, Blue represents the available volume for electronics.
We disassembled three calipers all of which shared the same basic appearance and performance specs, although they did have slightly different design files as evidenced by trivial differences in the PCB and Housing. So it is a mass-produced commodified design that seems pretty optimal in most regards, except for the battery management, as we will see. The calling-cards of this design include: the red, blue and yellow button, 150mm measurement length, 1mil precision, and LR44 battery and sliding battery door.
We could theoretically extend the battery life by tenfold just by lowering the sleep current of the calipers via an off switch or a logic chip (e.g. Greepak SLG47512, may draw 1uA and still provide auto-power-off). However we continued the exercise with the goal of getting away from primary batteries. This chart only captures sleep current and self-discharge. We presume an extremely low usage ‘duty cycle,’ the calipers spend most of their life in sleep current.
We could get 10 minutes of operation from a 2.5V 10 mF EDLC, as it discharges from 2.5V ~ 1.3V with an LDO regulating to 1.5V until the voltage rail collapses. We found some new solid state electrolyte lithium batteries in chip form factor! These have an energy volumetric density that is 10x greater than the standard EDLC (TDK cera charge available for purchase from Digi-Key, and samples from ITEN). These new batteries are safer than the liquid electrolyte and are more tolerant to deep discharge, but do operate at non-standard lithium-ion voltages.
Type | Solution Candidate | Volume (mm3) | Energy (J) | Cost (Qty 1) | Energy Density (mJ/mm3) | Energy Cost Efficiency (mJ/$) | Self-Discharge per Month (% of SoC) |
Electrolytic Capacitor | 603 | 0.02 | $0.63 | 0.03 | 32 | 99% | |
Electric Double Layer Capacitor (EDLC) | 521 | 1.5 | $5.30 | 2.9 | 285 | 90% | |
Lithium Ion Rechargeable with SSE | 29 | 1.9 | $13.00 | 66 | 145 | 25% | |
NiMH 2s1p | 720 | 151 | $10.00 | 210 | 15,120 | 20% | |
LR44 | 610 | 706 | $0.21 | 1156 | 3,360,000 | <1% | |
SR44 | 610 | 756 | $3.58 | 1238 | 211,173 | <<1% |
EDLC have ~100x volumetric energy density of electrolytic, Lithium SSE have ~10x the volumetric energy density of EDLC, Alkaline have ~1000x volumetric energy density of Lithium SSE
With power magnitude in mind we did a survey of some of the energy harvesting space, it is a fragmented market that straddles several burgeoning developments, ultra low power, low cost manufacturing, and also various material science disciplines. From this survey we began to see the vast operating regimes of these energy harvesting “transducers.” For context, we believe that a green LED can illuminate for a human detectable pulse with approximately 20 uJ (1mA at 2V for 10 ms) or a continuous 2 mW.
Source | Solution Candidate | Volume (mm3) | Power Avg. (uW) | Cost Qty 1 | Source Impedance | Power Avg. Density (nW/mm3) | Power Cost Efficiency (uW/$) | Comment |
Ambient RF | (requires clearance) | 336 | 1 | $5.00 | high | 3 | 0.2 | estimate, 50% energy conversion efficiency |
Piezo | 593 | 49 | $25.27 | high | 83 | 3 | Quoted peak power at 1% duty cycle, 70% energy conversion efficiency | |
Kinetic Switch | 3999 | 15 | $6.88 | low | 4 | 3 | Quoted peak power at 10% duty cycle, 70% energy conversion efficiency | |
TEG | 2970 | 110 | $4.98 | low | 37 | 22 | With 30C temp delta, 70% energy conversion efficiency | |
Solar | 71 | 26 | $4.05 | diode | 360 | 6 | 200 lux, 90% energy conversion efficiency |
Piezo power can be very peaky with frequency, Ambient RF is exciting but challenging, and Solar voltage can be adjusted by selected panel series configuration
We then explored some of the power management chips. The goal of these chips:
These were some of the leading integrated solutions we identified for the two broad classes of energy transducers, low voltage and high voltage:
Part | Output | Features | Application | EVM |
ADP5091 | Adjustable LDO, unregulated Sys | Adjustable Battery Thresholds | Low voltage, continuous sources (TEG & Solar) | |
AEM10941-QFN | Adjustable LDO x 2, Adjustable Buck | High integration, works from ~50 mV to 5 V sources | Low voltage, continuous sources (TEG & Solar) | |
SPV1050 | LDO x 2 | 550 mV and 30 μA to Cold start or 2.6V and 5 uA in buck-boost mode | Low voltage, continuous sources (TEG & Solar) | |
LTC3588 | Adjustable buck | Built-in rectifier, Over Voltage Protection (OVP) | AC or transient, higher voltage (Piezo, Dynamo) |
There are two broad types of PMU controllers, one for continuous energy sources and one for transient sources.
PMU parts for energy harvesting tend to have slightly different operating voltages for their energy sources, energy storage elements, and loads, as well as varying operating modes for charge transfer and regulation. Selecting the optimal solution requires a detailed evaluation of quiescent current in different modes, cold-start and keep-alive voltages, and energy and voltage thresholds for different mode transitions.
We selected the ADP5901 from Analog Devices. Perhaps we should have also looked to see active datasheet updates, as a proxy for excellent adoption and support. It was still at revision A and had a few obvious errors.
We purchased some EVMs and did some testing. From this we selected solar photovoltaic to power the calipers for power density and ease of use. We are going to use amorphous solar cells of three series cells. A discretely implemented bang-bang regulator and an EDLC for our storage.
We also tested other regulators, ideal diodes and load switches. Below are some of the detailed findings.
Bench test setup
The values at the y-axis intercept 77 uA / 500 lux = 32 uA / 200 lux
Open circuit voltage is linear with the log of irradiance
Fill factor is the ratio of the maximum power output to the theoretical maximum power, calculated as the product of the short-circuit current (Isc) and the open-circuit voltage (Voc), it is impacted by series and shunt resistance of the cell (graphic source)
Our first battery biscuit with copper tape and a solid pin as the axial electrode.
We aim to immediately power the calipers upon illumination first filling a small ceramic capacitor and then, if any excess charge generated by the solar panel accumulates it in a larger storage EDLC capacitor. We implemented a discrete bang-bang (hysteretic buck) utilizing a window comparator part with a built-in reference (MIC833YM5-TR). Load switches are implemented discretely with an RC circuit and mosfets allow adjustable dead-time and thresholds.
This approach ensures that we have enough short-term energy storage to power the calipers for a few seconds, even under fluctuating light conditions. A 10 mF EDLC (CPH3225A-2K) serves as the deep storage element.
Matched our energy source voltage and power knee with our load with the small amorphous solar cell (AM-1456CA-DGK-E). We found a Schottky diode with a very low Vf ~150 mV drop at 15 uA (BAT6302VH6327XTSA1).
UVLO (Under-Voltage Lockout): We oversized our solar panel, such that UV is much less likely to occur and this was in part because we had a hard time sourcing an UVLO.
OVP (Over-Voltage Protection): We found a low quiescent current low dropout linear regulator that could give us 1.5V output, which would keep the voltage seen by the caliper within its recommended range (STLQ015M12R).
We beveled the top and bottom of the biscuit to improve the reliability of the electrical connections. We had to create a ramp in the circuit board to prevent the spring pin in the battery biscuit from acting as a latch preventing disassembly. Iterating on such small parts is fantastically fast, especially with a core-xy printer.
Just a few examples of products that do or could exist using present-day energy harvesting technologies:
A few organizations focused on the energy harvesting space. Chip Makers & Integrated Solutions:
All of the performance figures are lacking context and utilize simplifying assumptions. It is a fascinating and challenging space. If you are looking to explore energy harvesting for your device, please contact us at getstarted@219design.com.
Date published: 02/04/2025