Perpetual Wearable Biosensing with Andy Kong

Chris welcomes Andy Kong to discuss “Chargerless” and how to build ultra-low-power wearable health trackers that never need wall charging. (00:00:15)

From Hacker Mindset to Startup Founder: They discuss Andy’s background in biosensing, staying with Samy Kamkar, and bringing a pragmatic “get things done” hacker mindset to founding Chargerless in San Francisco, as documented on Andy’s personal website. (00:02:10)

Solving the Wearable Charging Bottleneck: Andy explains how frustration with daily Apple Watch charging led him to design a wearable requiring only one minute of daily sunlight exposure, noting that modern wearables are bottlenecked by battery volume rather than logic chip size. (00:04:30)

Deliberately Underpowered MCUs: Chris and Andy explore system architecture trade-offs, focusing on Chargerless’s choice of an underpowered microcontroller paired with lean firmware rather than feature-heavy, power-hungry processors. (00:07:15)

Intelligent Sampling vs. Continuous Monitoring: They contrast Whoop and Apple Watch’s continuous PPG power drain with Chargerless’s periodic sampling approach, which reserves high-frequency tracking for active workout modes. (00:09:45)

The Limits of Commercial Sensor ICs: Why off-the-shelf health ICs from TI and Analog Devices consumed 100 times too much power due to unoptimized ADCs, inefficient amplifiers, and fixed pulse timings. (00:12:20)

EKG vs. Wrist PPG: Explaining how multi-lead chest EKGs measure electrical heart potential vectors, whereas wrist Photoplethysmography (PPG) uses green light reflection to detect oxygenated blood volume changes. (00:15:05)

Badge Life and DIY Biometrics: Chris and Andy reminisce about community hardware projects like Chaos Camp’s Card10 EKG badge, Kerry Scharfglass’s IR-synchronized dragonfly badge, and Chris’s earlobe PPG sensor that flashed LEDs inside a Solo cup based “badge”. (00:17:50)

Custom Discrete PPG Frontend: How Andy replaced commercial ICs with a discrete circuit that strobes a green LED 50 times per second for microsecond bursts, cycling both the LED and transimpedance amplifier. (00:20:40)

Eliminating Decoupling Lag: Stripping decoupling capacitors removed turn-on delays, while using two amplifier stages avoided high-capacitance digital potentiometers. (00:23:10)

Pivoting Away from BCI: Andy describes his college research on brain-computer interfaces (BCIs) with OpenBCI and why he abandoned non-invasive EEG due to microvolt-level signal noise from cable movement. (00:25:55)

Quantified Self & Personal Informatics: Discussing how self-trackers capture environmental data to identify root causes of complex health symptoms. (00:28:30)

Low-Power Bluetooth Data Batching: How Bluetooth Low Energy accounts for less than 5% of overall power draw by batching 10 KB payloads every 10 minutes for fast burst transmissions to the phone. (00:31:15)

Form Factors and Ring Safety: Comparing Whoop, Oura, Garmin, and Fitbit Air trackers, while covering smart ring battery constraints, Samsung battery recall testing, and degloving risks. (00:34:00)

Whoop’s Slide-On Battery: How Whoop designed a slide-on battery pack to charge while being worn, avoiding the user drop-off caused when devices are taken off to charge. (00:36:45)

Solar Harvesting & Laser Dicing: Chargerless uses boost converters to harvest power from amorphous solar cells, which Andy initially custom diced with laser cutters at MIT. (00:39:20)

Bare-Metal Registers & 28-Hour Days: Transitioning from dev boards to custom PCBs required low-level MCU register programming, custom Makefiles, and a 28-hour daily cycle sleep study in tinfoiled rooms. (00:42:10)

Pricing & Open Data Access: Chargerless is rolling out a pre-production run of a few hundred units for $199 with no subscription fees on getchargerless.com and open data export capabilities. (00:45:00)

Power Over Skin (Body Channel Communication): Andy breaks down his Power over Skin research project, which sends 40–100 MHz RF power safely through human skin using the body as an antenna. (00:47:45)

Microwatt Energy Harvesting: Utilizing Skyworks low-forward-drop diodes, tank circuits, and rectifiers to harvest microwatts for powering LED earrings, ring joysticks, and skin patches. (00:50:30)