Why Smart Ring Battery Life Is a Physics Problem, Not a Marketing Problem
TL;DR
Most smart ring manufacturers advertise 7-day battery life. That number comes from a test mode where the heart rate monitor is off and the wearer is asleep. In real use, the gap between marketing and physics is about 40 percent. Pulsyn does not promise 7 days. We promise an honest number and explain why it is lower.
The 7-day lie is a test mode
Oura says the Ring 4 lasts 7 days. RingConn says 10 to 12. Ultrahuman says 6. What none of them say plainly is that the advertised number is a best-case scenario measured under conditions no real user replicates.
The standard test mode for wearable battery certification looks like this. The device sits on a stationary stand. The Bluetooth radio is connected to a phone but only pings occasionally. The heart rate monitor is disabled or set to the lowest sampling interval. The SpO2 sensor is off. The accelerometer is on but not processing motion algorithms because there is no motion. The temperature sensor is sampled once every 10 minutes. The screen, if there is one, is off. The device is in a room at 22 degrees Celsius with no thermal load.
Under those conditions, yes, the ring lasts 7 days. Under the conditions of a human who walks, sleeps, showers, and wants their heart rate checked every 5 minutes, it lasts about 4. User reports on r/ouraring and r/ringconn consistently show 4 to 5 days for Oura and 6 to 7 for RingConn. The gap is not a defect. It is the difference between a lab test and a human body.
This is not unique to smart rings. Apple Watch rates itself for 18 hours. Garmin Fenix says 14 days in smartwatch mode. Those numbers are also measured under controlled conditions. But the smart ring has a unique problem. The ring is the smallest wearable form factor that still tries to do everything a watch does. The battery is smaller by an order of magnitude, and the physics of power density does not care about marketing slides.
What actually draws power in a smart ring
To understand why the battery dies fast, you need to look at the power budget. A smart ring is a computer with a battery, and every subsystem consumes milliwatts. The total budget is fixed by the battery capacity. When the consumption exceeds the capacity, the ring dies.
The photoplethysmography (PPG) LED is the biggest consumer. This is the green and red light that shines into your finger to measure blood volume changes. A typical PPG LED in a smart ring draws 0.5 to 2 milliwatts when active. If the heart rate is sampled every 5 minutes, the LED is on for about 1 second per sample. That sounds trivial. But the LED driver circuit, the analog front end, and the ADC all have to wake up. The total power cost of one heart rate sample is closer to 3 to 5 milliwatts for a 2-second window. Multiply that by 288 samples per day and you have spent 1.4 to 2.4 watt-hours just on heart rate. For a ring with a 20 to 25 milliamp-hour battery at 3.7 volts, that is 20 to 25 percent of the total capacity.
The SpO2 sensor is worse. Blood oxygen measurement requires red and infrared LEDs at higher intensity because the absorbance difference between oxygenated and deoxygenated hemoglobin is small. The LED power for SpO2 is roughly 2 to 3 times higher than heart rate. Most rings run SpO2 only during sleep to save power. If you ran it continuously, the battery would last about 2 days. This is why Oura and RingConn both gate SpO2 behind a sleep mode. It is not a feature choice. It is a power budget necessity.
The Bluetooth Low Energy radio is the next major consumer. BLE was designed to be low power, but "low" is relative. A BLE connection event at 7.5 millisecond intervals draws about 10 milliwatts when active. In practice, smart rings use a connection interval of 100 to 200 milliseconds to save power. Even so, the radio consumes 1 to 2 milliwatts average during an active connection. If the ring is syncing data to the phone every few minutes, the radio is on a lot. Add in the occasional firmware update download or a long sync after a flight, and the radio budget can hit 3 to 5 percent of the total battery.
The accelerometer is cheap. A modern MEMS accelerometer like the Bosch BMI270 draws about 10 microamps at 50 hertz. That is negligible. The problem is not the sensor. The problem is the processor that has to stay awake to process the motion data. Step counting and sleep stage detection both require the main MCU to run at 10 to 30 megahertz for bursts of 10 to 50 milliseconds. The MCU in a smart ring typically draws 3 to 5 milliamps when active. If the MCU wakes up every second to process motion, the average power draw is still small. But during sleep stage detection, the MCU runs continuously for minutes at a time. That is where the power goes.
Temperature sensing is the cheapest sensor. A thermistor or infrared sensor draws microamps. But the ring also has a charging circuit, a battery management system, and a DC-DC converter. Those always-on circuits draw a baseline of 0.5 to 1 milliwatt. Over a week, that baseline alone is 5 to 10 percent of the battery.
Add it all up. A realistic daily power budget for a smart ring with all-day heart rate, sleep SpO2, motion tracking, and temperature looks like this:
- PPG heart rate: 20 to 25 percent
- SpO2 during sleep: 15 to 20 percent
- BLE radio: 5 to 10 percent
- MCU and motion processing: 10 to 15 percent
- Baseline circuits: 5 to 10 percent
- Margin for temperature variation and battery aging: 10 percent
That is 65 to 90 percent of the battery used by core functions. The remaining 10 to 35 percent is your headroom for notifications, firmware updates, and the fact that lithium polymer batteries lose 20 percent of their capacity in the first year.
Why the ring form factor makes everything worse
The battery is the problem. A smart watch has 300 to 500 milliamp-hours of lithium polymer. A smart ring has 15 to 30. The Oura Ring 4 is reported to have a 22 milliamp-hour cell. The RingConn Gen 2 is around 25. The Pulsyn Rune 1 prototype uses an 18 milliamp-hour cell.
The reason is volume. A finger ring has an interior diameter of 16 to 22 millimeters. The wall thickness is 2 to 3 millimeters. The electronics must fit inside that torus. The battery is the largest component. It is a custom curved lithium polymer cell that wraps around the inner circumference. The energy density of lithium polymer is about 250 to 300 watt-hours per liter. At 18 milliamp-hours and 3.7 volts, the Pulsyn battery stores about 0.066 watt-hours. In a volume of 0.3 milliliters, that is 220 watt-hours per liter. That is at the low end of lithium polymer density because the curved form factor sacrifices packing efficiency.
The ring also has a thermal problem. A battery generates heat during discharge. A watch has a large back plate and surface area to dissipate that heat. A ring has a small surface area and is surrounded by skin, which is an insulator. If the ring draws too much power continuously, the battery warms up. Lithium polymer efficiency drops at higher temperatures. The ring's own body heat makes the battery work harder. This is a feedback loop that watches do not have to deal with.
Then there is the charging problem. A ring battery charges at 0.2C to 0.5C for safety. At 0.5C, an 18 milliamp-hour battery takes 2 hours to charge. Oura and RingConn both use charging cases with a small internal battery. The case charges the ring in 20 to 80 minutes. The case itself holds 300 to 500 milliamp-hours and charges via USB-C. This is a good user experience but it adds cost. The Pulsyn charging case uses a 400 milliamp-hour cell and a pogo pin connector. The case is a separate product with its own battery, its own charging circuit, and its own failure modes.
How the numbers actually work
Let me walk through the math for the Pulsyn Rune 1 prototype. This is a real device, not a theoretical exercise.
The battery is an 18 milliamp-hour lithium polymer cell at 3.7 volts nominal. That is 66.6 watt-hours of total energy. The usable capacity is about 80 percent because we stop discharge at 3.0 volts to protect the cell. So usable energy is 53.3 watt-hours.
Our current power budget:
- Heart rate every 5 minutes, 24 hours: 1.8 watt-hours per day
- SpO2 during sleep only, 8 hours: 0.9 watt-hours per day
- BLE sync every 10 minutes: 0.3 watt-hours per day
- Accelerometer + MCU motion processing: 0.4 watt-hours per day
- Temperature every 10 minutes: 0.05 watt-hours per day
- Baseline always-on: 0.4 watt-hours per day
Total: 3.85 watt-hours per day.
53.3 watt-hours usable divided by 3.85 watt-hours per day equals 5.0 days. That is our honest number. With battery aging after 6 months, capacity drops to 90 percent. So 4.5 days. In cold weather, the battery is less efficient. So 4.0 days. That is the number we will print on the box.
The only way to get to 7 days is to cut features. Disable heart rate during the day. Disable SpO2. Sync once per hour. Reduce motion processing. We will not do that because the product would be worse. We would rather ship an honest 4-day ring than a dishonest 7-day ring that only works in airplane mode.
What Pulsyn is doing differently
There are three ways to improve battery life in a smart ring. Make the battery bigger. Make the hardware more efficient. Make the software smarter. The battery is fixed by your finger size. The hardware is already near the efficiency limit. The software is where we are working.
Adaptive sampling is the first change. Most rings sample heart rate at a fixed interval. We are building a system that adjusts the interval based on activity. If the accelerometer shows you are still, the ring samples heart rate every 10 minutes. If you are walking, it samples every 2 minutes. If you are running, it samples every 30 seconds. The average interval across a day is about 5 minutes, but the power is saved during the 80 percent of the day when you are not moving much. This should save 15 to 20 percent of the PPG power budget.
On-device processing is the second change. Most rings stream raw PPG data to the phone for processing. The phone does the heart rate calculation, the sleep staging, the anomaly detection. That means the BLE radio is active for long transfers. Pulsyn processes everything on the ring itself. The MCU runs the algorithms locally. The phone only gets the final scores and summaries. The BLE transfer is a few kilobytes instead of a few megabytes. The radio is on for milliseconds instead of seconds. This saves 5 to 10 percent of the total power budget.
Sleep mode is the third change. When the ring detects you are asleep, it shuts down non-essential systems. The BLE radio disconnects and only advertises once every 30 seconds. The accelerometer drops to 12.5 hertz. The temperature sensor runs once per minute. The SpO2 sensor runs, but at a lower LED intensity because sleep is stable. The MCU drops to a low-power state between samples. In our testing, sleep mode draws about 40 percent less power than awake mode.
These three changes together should push the Rune 1 from 5.0 days to 5.5 or 6.0 days in real use. That is still not 7. But it is an honest number under honest conditions, and that is the point.
The honest truth about battery life
I want to be clear about what we are not promising. We are not promising a ring that lasts a week with every sensor on. That is physically impossible with current battery chemistry and the ring form factor. Anyone who says otherwise is measuring in a lab or lying.
We are also not promising a ring that lasts forever. Lithium polymer batteries degrade. After 300 charge cycles, capacity is down to 80 percent. After 500 cycles, it is 70 percent. For a ring you charge twice a week, 300 cycles is about 3 years. This is why most smart rings are designed to last 2 to 3 years. The battery is sealed inside. You cannot replace it. The ring is a disposable computer with a titanium shell.
This is the part of the hardware business nobody talks about. The business model depends on the battery dying. If the battery lasted 10 years, you would not buy a new ring. Planned obsolescence is not a conspiracy. It is a design constraint of lithium chemistry and sealed enclosures.
Pulsyn is not going to fix battery chemistry. We are not going to invent a new form factor. What we can do is be honest about the numbers. We can tell you the ring lasts 4 to 5 days with all features on. We can tell you it will degrade to 3 to 4 days after 2 years. We can tell you the charging case takes 60 minutes for a full charge. And we can tell you that if you want 7 days, you should buy a watch, because watches have bigger batteries and the physics allows it.
The smart ring is a tradeoff. You trade battery life for comfort. You trade a screen for a form factor that does not wake you up. You trade a big battery for a device that fits under a boxing glove. The tradeoff is worth it for some people. It is not worth it for others. We are building for the people who want the tradeoff and want to know what they are trading before they pay.
That is the honest number. And that is why we print it.
About the author
James Hoffmann is the founder of Pulsyn. He has been designing the power management firmware for the Rune 1 prototype since 2024. He believes the most honest thing a hardware company can do is print the battery life number that includes all the sensors you actually bought the device for.
References
- Oura Ring 4 battery specifications: Oura Help Center, "Battery life and charging" (accessed June 2026).
- RingConn Gen 2 technical specifications: RingConn product page, "Battery" section (accessed June 2026).
- User-reported battery life on r/ouraring: Reddit threads, May to June 2026, aggregated from user posts.
- User-reported battery life on r/ringconn: Reddit threads, May to June 2026, aggregated from user posts.
- Bosch Sensortec BMI270 datasheet: Power consumption specifications, 2022.
- Lithium polymer battery energy density: Battery University, "Lithium-ion battery" technical summary (accessed June 2026).
