A smart ring is the most constrained battery design challenge in consumer electronics. The battery must fit into a ring-shaped shell that is worn on a finger, which means: a maximum outer diameter of 22–24 mm, an inner diameter of 17–22 mm (depending on ring size), a channel cross-section of roughly 2–4 mm wide by 2–3 mm tall, and a circumferential arc length of about 70–75 mm for a full ring or 30–40 mm for a typical open-arc battery placement. The total available battery volume is typically 150–500 mm³. At standard LiPo volumetric energy density, that is 25–80 mAh.
The geometry problem in numbers
Consider a size 8 ring (inner diameter 18.2 mm). The battery channel in a typical smart ring design is approximately:
- Arc length available: 60 mm (∼270° arc, leaving space for the PCB module)
- Channel width: 3.5 mm
- Channel height: 2.5 mm
Maximum battery volume: 60 × 3.5 × 2.5 = 525 mm³ (theoretical). Realistic battery volume (after manufacturing margins and encapsulation): approximately 350–400 mm³.
At a volumetric energy density of 500 Wh/L (achievable with HV-LCO at 4.48 V), this yields a maximum cell capacity of approximately 0.4 cm³ × 500 Wh/L = 0.2 Wh ÷ 3.85 V = ~52 mAh. At standard LCO (4.20 V, ~400 Wh/L), the same volume yields ~40 mAh.
Why high-voltage LCO wins
In a ring, every mAh of capacity gain has outsized runtime impact because the total is so small. The difference between 40 mAh and 52 mAh at the same average current draw is a 30% runtime extension — the difference between a 18-hour battery and a 24-hour battery. This makes HV-LCO at 4.45–4.48 V the default chemistry choice for smart rings, despite the additional BMS complexity required.
HV-LCO requires a precision charge voltage reference: the tolerance on the 4.48 V ceiling should be ± 10 mV or less, otherwise chronic overcharge accelerates electrolyte decomposition. This is achievable with a dedicated charge IC (e.g., Microchip MCP73831 with external voltage trim, or TI BQ25100 configured for HV) but requires careful PCB layout and temperature characterisation of the voltage reference component.
Cell architecture for ring geometry
Two cell architectures are used in smart rings today:
Curved rectangular pouch (arc-shaped): A standard LiPo pouch is curved along its long axis to follow the ring arc. Tab exits at one short end. The cell is pre-curved during manufacturing to a radius matching the ring's inner radius + half the battery channel width. This is the simpler manufacturing approach and is used in most first-generation smart rings. Minimum curve radius for this application: R10–12 mm (tight, but within limits for a single-curvature 2.5 mm thick cell).
Annular or arc-segment cell: A custom-tooled cell where the electrode stack itself is curved circumferentially (not just the pouch). This requires dedicated electrode cutting tooling (arc-shaped electrodes rather than rectangular) and is significantly more expensive to develop. It achieves better volumetric efficiency than a curved rectangular cell (fewer dead corners) but is only cost-effective at volumes above 200,000 units/year.
Wireless charging and BMS design in a ring
Smart rings almost universally use wireless (Qi or proprietary) charging because the ring surface cannot accommodate a reliable contact charging solution for a device worn on a finger. This has two BMS implications:
1. Higher thermal management burden. Wireless charging at the coil generates heat in a very small enclosure. A 30 mW–50 mW receiver coil in a 3 cm³ ring shell can raise the cell temperature by 8–12 °C above ambient during charging. The BMS must include a temperature-based charge rate reduction that activates above 38 °C to protect both the cell and the wearer from a warm ring.
2. No mechanical charging connector failures. Wireless charging eliminates the most common mechanical failure mode in small wearables — connector fretting wear. For a device expected to last 2+ years with daily charging, this is a meaningful reliability improvement over contact charging.
Practical power budget
The power budget determines whether 40–52 mAh is enough. For a representative health-monitoring smart ring with continuous HR and SpO₂ sensing:
| Function | Typical average current | Duty cycle | Average contribution |
|---|---|---|---|
| MCU (active processing) | 4 mA | 5% | 0.20 mA |
| MCU (sleep) | 0.01 mA | 95% | 0.01 mA |
| HR sensor (continuous) | 1.5 mA | 100% | 1.50 mA |
| SpO₂ sensor (periodic) | 8 mA | 10% | 0.80 mA |
| BLE (advertising) | 5 mA | 4% | 0.20 mA |
| BLE (connected / data sync) | 10 mA | 2% | 0.20 mA |
| Total average current | ~2.9 mA | ||
At 2.9 mA average draw: 50 mAh ÷ 2.9 mA × efficiency factor (0.85) = ~14.7 hours between charges. This aligns with the overnight-charge pattern typical of smart ring products. Adding a 25% ageing margin (end-of-life at 75% of initial capacity): 50 × 0.75 ÷ 2.9 × 0.85 = ~11 hours at end-of-life — still a viable full-day product.