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FingerScore Hardware 5 — Power, Battery, and Circuit Design

🇰🇷KO🇺🇸EN🇯🇵JA
Split View필사 모드
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Introduction

In Part 4 we made the FingerScore ring send scores to the phone smartly over BLE. But there was an assumption baked in: the ring must have power. Fitting a battery into a finger-sized device, making it last days to weeks, and charging it safely is harder than it sounds. Sloppy power design means the battery dies in half a day, the device runs hot against your finger, or — worst case — the LiPo swells.

This Part 5 is about power, battery, and circuit design. We cover how to compute a power budget and predict battery life, choose a battery type, charge safely with a charging IC and protection circuit, make a clean voltage with a regulator, and handle safety issues like heat and ESD. There are equations and a schematic, but everything is spelled out so non-EE readers can follow along.

1. Power Budget — How Long Will the Battery Last

The first task is computing "how much current does this device draw on average." This is the power budget.

The average of sleep and active

The FingerScore ring spends most of its time asleep. Occasionally the accelerometer senses an impact, the device wakes (active), processes the score, sends it over BLE, then sleeps again. So average current is a weighted average of sleep and active current.

average current (weighted average)

I_avg = (I_sleep × t_sleep + I_active × t_active) / (t_sleep + t_active)

example values:
  I_sleep  = 10 uA      (most of the time)
  I_active = 8 mA       (BLE TX + MCU running)
  one wake duration t_active = 5 ms
  wake frequency = 6 times/min (a score/advert every 10s)

over 1 minute (60s):
  total active time = 6 × 5 ms = 30 ms = 0.03 s
  total sleep  time = 60 - 0.03 = 59.97 s

I_avg = (10uA × 59.97 + 8000uA × 0.03) / 60
      = (599.7 + 240) / 60
      = 839.7 / 60
      = about 14 uA

We got an average of 14uA. Even though active current is 800x sleep current, the active duration is so short that the average stays close to the sleep current. The key to low-power design is not "how little it draws" but "how often it wakes up."

Estimating battery life

Dividing battery capacity (mAh) by average current gives a rough lifespan.

battery life estimate

life (hours) = battery capacity (mAh) / average current (mA) × derating

CR2032 coin cell = about 220 mAh
average current = 0.014 mA (the 14 uA computed above)
derating = 0.7 (margin for self-discharge, temperature, voltage drop)

life = 220 / 0.014 × 0.7
     = 15714 × 0.7
     = about 11000 hours
     = about 458 days

(in reality it varies greatly with advertising frequency, connection
 hold, temperature. conservatively, call it a few months to a year.)

We multiply by a derating factor of 0.7 because a real battery cannot deliver 100% of its catalog capacity. Self-discharge, capacity loss at low temperatures, and cutoff voltage make a conservative estimate wise.

2. Battery Choice — Coin Cell vs. Small LiPo

In a ring form factor the battery takes up the most volume. Let us compare the two main options.

ItemCoin cell (CR2032)Small LiPo
Voltage3.0V (non-rechargeable)3.7V nominal, 4.2V full
Capacityabout 220 mAh20-110 mAh (by size)
Chargingno (single-use)yes
Thickness/shapethick, roundthin, rectangular, can curve
Safetyhigh (stable)needs care (overcharge/puncture risk)
Ring fitthickness burdencurved LiPo is favorable

Which one

  • Coin cell: no charging circuit needed, so the circuit is simple and safe. But you must replace it once spent, and its thickness (~3.2mm) makes the ring bulky. Good for early prototypes or extreme low-power designs.
  • Small LiPo: rechargeable for a better user experience, and curved variants can wrap around a ring. But it needs an extra charging IC, protection circuit, and safety design. Production devices usually pick LiPo.

FingerScore's final product is rechargeable, so the rest of this article assumes a small curved LiPo, while recommending that you first validate firmware with a coin cell during the prototype stage.

3. LiPo Charging IC and Protection Circuit

LiPo is convenient but dangerous. Overcharge it and it swells; over-discharge it and it dies; pull too much current and it heats. So two protection mechanisms are essential.

Charging IC (e.g., the TP4056 family)

Lithium batteries must be charged on a defined curve (CC-CV: constant current, then constant voltage). A charging IC does this automatically. The TP4056 family is popular with beginners.

CC-CV charge curve

voltage/current
  ^
  |        CC region        CV region
  |  current --------        ‾‾\___
  |  constant                    \___  (current tapers off)
  |
  |  voltage  ___/‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾  (constant after reaching 4.2V)
  |     /
  +------------------------------------> time
     charge start              near full
  • CC (constant current): charge fast at constant current at first.
  • CV (constant voltage): on reaching 4.2V, fix the voltage and slowly reduce current. When current is small enough, charging is complete.

Charge current is usually set to about 0.5C-1C of capacity. For a 50mAh battery, that is 25-50mA. Pushing too much current into a small battery is dangerous, so limit charge current with the charging IC's program resistor (R_PROG).

Protection circuit (protection IC)

Separate from the charging IC, you need a protection circuit guarding the battery itself. A protection IC (e.g., the DW01 family + dual MOSFET) cuts off:

  • Over-charge: stop charging if voltage gets too high
  • Over-discharge: stop discharging if voltage gets too low
  • Over-current/short: cut off immediately on large current

Many LiPo cells ship with a protection circuit already built in (PCM). When using a "raw cell" without protection, you must add a separate protection IC. This is a safety matter — never omit it.

4. Regulators — LDO vs. Buck Converter

Battery voltage varies with charge state (4.2V full to 3.0V depleted). But the MCU and BLE chip want a stable voltage (e.g., 3.3V or 1.8V). The regulator turns that varying voltage into a steady one.

ItemLDOBuck converter
Principledumps excess voltage as heatswitches for efficient conversion
Efficiencylow (worse as voltage gap grows)high (80-95%)
Noisevery cleanhas switching noise
Part countfew (2 capacitors)many (inductor, capacitors, etc.)
Sizesmalllarge (due to inductor)
Quiescent currentultra-low-power parts availablevaries by part

FingerScore's choice

The ring is extremely cramped, and the gap between input (3.0-4.2V) and output (3.3V) is small. A small gap means small LDO efficiency loss. Plus the BLE radio likes a clean supply, so a low-noise LDO is favorable.

So FingerScore defaults to an ultra-low quiescent-current LDO. You must pick a part whose own current draw (Iq) in sleep is tens of nA to a few uA. If the LDO's Iq is 100uA, that dwarfs the earlier average current (14uA) and ruins battery life.

why LDO quiescent current matters

device average draw = 14 uA
LDO Iq = 1 uA   -> 15 uA total, life nearly preserved (good)
LDO Iq = 100 uA -> 114 uA total, life cut to 1/8 (bad)

lesson: for sleep-heavy devices, LDO quiescent current is a key spec

5. Decoupling and Power Integrity

When a chip suddenly demands a lot of current (the moment of a BLE transmission, etc.), the supply voltage momentarily sags. This sag can make the chip misbehave or corrupt the BLE signal. Decoupling capacitors tame this.

A decoupling capacitor is a small capacitor placed right next to a chip's power pin, acting as a tiny energy reservoir. When the chip suddenly pulls current, the nearby capacitor supplies it instantly instead of the distant battery, reducing the voltage sag.

decoupling capacitor placement (conceptual)

[battery]---[LDO]----+----+----[MCU/BLE VDD pin]
                     |    |
                  100nF  1uF   <- as close to the chip pin as possible
                     |    |
                    GND  GND

key: place capacitors physically close to the chip's power pin.
     farther away, trace inductance reduces the effect.

Usually you combine one 100nF (for high-frequency noise) and one 1uF-10uF (for low-frequency / bulk transient supply) per chip power pin. Their placement matters even more in Part 6 (PCB design).

6. Low-Power Design Techniques

As the power-budget section showed, lifespan hinges on "how often and how long you wake up." Techniques you can apply in both firmware and hardware:

  • Use deep sleep: the MCU's deepest sleep mode (e.g., System OFF) drops draw below a few uA. Wake triggers are accelerometer interrupts or a button.
  • Clock gating: turn off the clocks to unused peripherals (timers, ADC, UART) to cut current.
  • Power down peripherals: fully cut unused sensors, LEDs, and regulator channels via GPIO. A single lit LED can make your sleep current meaningless.
  • Optimize advertising interval: BLE advertising too often eats power. Stop advertising once connected.
  • Choose sensor modes: keep the accelerometer in a low-power "motion wake" mode so it only wakes the MCU on impact. The MCU sleeps fully otherwise.
power consumption priority (catch the big thieves first)

1. a lit LED / always-on sensor   <- biggest thief, top priority
2. frequent BLE advertising / short interval
3. high LDO quiescent current
4. ungated peripheral clocks
5. inefficient code (busy-wait, etc.)

principle: before you save uA, catch the things eating mA.

Estimating Battery Level — Voltage Gauge vs. Coulomb Counting

To show "battery 30%" in the phone app, you must know the remaining charge. Two methods:

  • Voltage gauge: read the battery voltage with an ADC to estimate level. Simple, but LiPo has a flat discharge curve (small voltage change in the middle), so it is inaccurate. Voltage also dips momentarily under load, adding error.
  • Coulomb counting: integrate current in and out to track accurately. Using a dedicated fuel-gauge IC (e.g., the MAX17048 family) is accurate but adds a part.
LiPo discharge curve and the pitfall of level estimation

voltage
 4.2 |‾‾\
     |   ‾‾‾‾‾‾‾‾‾‾‾\        <- flat region: voltage alone can't
 3.7 |              ‾‾‾‾\       distinguish remaining level
     |                  ‾‾\
 3.0 |                     \_
     +------------------------> usage (%)
     100%                  0%

in the flat region, a voltage gauge confuses 60% with 40%.
if accurate level matters, consider a fuel-gauge IC.

FingerScore only needs to show the level in about 4 steps (100/70/40/10%), so to save cost it is reasonable to start with a voltage gauge plus a correction table, and upgrade to a fuel-gauge IC if user experience becomes important.

Battery Aging — Lifespan Is Not Forever

LiPo loses capacity with repeated charge cycles. Typically after 300-500 cycles it drops to about 80% of initial capacity. Storing it long at full charge (4.2V) or in heat ages it faster.

  • Not charging to 100% every time, but stopping at 80-90%, extends lifespan.
  • For long-term storage, charge to about 50%.
  • Setting a slightly lower charge-termination voltage in firmware (e.g., 4.1V) reduces capacity a little but can greatly extend lifespan.

This kind of care decides whether a daily-worn finger device "still lasts a day a year later."

7. Current Measurement — Measuring the Invisible Draw

Once the design is done, you must measure what it actually draws. The catch: sleep current (uA) and active current (mA) differ by 1000x, so a regular multimeter struggles to measure both accurately.

  • Regular multimeter: sees the average but misses short active pulses and reads sleep current inaccurately.
  • Dedicated current analyzer (e.g., Nordic PPK2, Joulescope): shows the wide uA-to-mA range simultaneously, over time. You can even see the shape of wake pulses, helping chase down "why is my average high?"
current waveform measurement (as seen on a dedicated analyzer)

current
  ^
  | mA      ┌┐        ┌┐        ┌┐
  |         ││        ││        ││   <- BLE transmit pulse
  | uA  ____┘└________┘└________┘└___  <- sleep baseline
  +-----------------------------------> time

check points:
 - is the sleep baseline higher than expected (uA)?
 - are the pulses too long or too frequent?

Without measurement, "it'll probably be fine" almost always hides a draw somewhere. Measuring is not optional — it is mandatory.

8. Schematic — Power Section ASCII

Drawing the elements so far as a single power-section block:

FingerScore power section block diagram

 USB-C  ---> [ESD protect] ---> [charging IC TP4056 family] ---+---> [LiPo + protection IC]
 (VBUS)                              (CC-CV charge)            |
                                                              |
                                                              v
                                                    [power/load switch]
                                                              |
                                                              v
                                              [LDO 3.3V (ultra-low Iq)]
                                                    |       |
                                                 100nF     1uF  (decoupling)
                                                    |       |
                                                    v       v
                                              [MCU + BLE SoC VDD]
                                                    |
                                                    +--> [accelerometer VDD]
                                                    +--> [status LED (GPIO control)]

To summarize the flow: power entering via USB-C passes ESD protection, goes to the charging IC, which safely fills the LiPo. In use, LiPo to load switch to LDO produces a clean 3.3V, which passes the decoupling capacitors and feeds the MCU/BLE/sensor.

9. Charging and USB-C

These days charging via USB-C is standard. But for a small device you do not need to implement the full USB-C spec.

  • Power-only: connect just VBUS and GND, and add a 5.1kΩ pull-down on each of CC1/CC2 to tell the USB-C host "I am a power-only sink." Omit these resistors and charging may not start.
  • If you also need data: you can add USB for firmware updates, but a small device like a ring usually uses wireless (OTA) updates and keeps USB charge-only.

In a ring-sized device the USB-C connector itself takes up volume. So people use magnetic charging docks (exposing 2-4 contact pins) or wireless charging (small Qi). Contact-based docks have simpler circuits but require care with waterproofing and corrosion.

Power On/Off — Load Switch and Boot

A ring can hardly have a physical power button (waterproofing, space). So power management is a collaboration between firmware and hardware.

  • Load switch: a switch between the battery and the circuit (a P-MOSFET or a dedicated load-switch IC) that disconnects the whole circuit when unused, bringing leakage current near zero.
  • Button wake: implement "power on" with a small tact switch or capacitive touch. Long-press to turn on, longer-press to turn off.
  • Auto-boot on charge: make it power on automatically when a charger is plugged in, so it revives even from a fully-off state in storage.
power state machine (conceptual)

  [fully off]  --long press--> [boot]  --init done--> [running]
       ^                                                  |
       |                                                  |
       +------- longer press / low voltage ---- [shutdown] <--+

key: in "fully off," the load switch disconnects the circuit to stop leakage.
     the trick to keeping the battery alive through months of storage.

Thanks to this design, the battery does not leak even if the user does not use it for days, and it powers on immediately when worn again. A small detail, but it decides whether "the ring left in a drawer still turns on a month later."

10. Heat, Safety, and ESD

Since a small device sits on a finger, heat and safety are especially important.

Heat

  • During charging, the charging IC and LDO produce heat. Setting too high a charge current makes them hot, so keep it conservatively at or below 0.5C.
  • An LDO dissipates heat equal to input-output voltage gap times current. A small-gap design runs cooler (another reason we chose the LDO).

LiPo safety

  • On overcharge, puncture, or crushing, LiPo can swell or ignite. A protection IC is essential, and the mechanical design must leave clearance so the cell is not compressed.
  • Charging outside the temperature range (too cold or too hot) is dangerous. Using a charging IC with temperature protection (NTC) is wise.

ESD protection

A device touched by fingers is exposed to electrostatic discharge (ESD). The same static spark you feel grabbing a doorknob can kill a chip.

ESD threat points and defense

USB-C pins  ----[TVS diode]---- internal circuit
contacts/button ----[TVS diode]---- GPIO

principle: on every exposed conductor a human can touch, route excess static
           to ground with a TVS (transient voltage suppression) diode.

A TVS diode does nothing in normal operation, but when a momentary high voltage like static arrives, it shunts that energy to ground to protect the internal chip. Adding one to USB-C data/power lines and every externally exposed pin is wise.

11. Common Mistakes

  • Ignoring LDO quiescent current: picking a part without checking Iq in the datasheet, then sleep current is 100x higher and the battery dies in a day.
  • Omitting the protection circuit: a raw LiPo cell without a protection IC dies from over-discharge or swells from overcharge. A direct safety hazard.
  • Missing the CC pull-down resistor: without the 5.1kΩ on USB-C, the charger gives no power.
  • Decoupling capacitor placement: placing capacitors far from the chip kills their effect, and BLE dies intermittently.
  • Excessive charge current: pushing more than 1C into a small battery causes heat, reduced lifespan, and safety problems.
  • Skipping measurement: trusting only calculation and not measuring fails to catch hidden draws.

12. Closing — What's Next

In this Part 5 we covered the power design that actually keeps the FingerScore ring on. We predicted lifespan with a power budget, chose a battery (coin cell vs. LiPo), charged safely with a charging IC and protection circuit, made a clean voltage with an LDO, raised efficiency with decoupling and low-power techniques, and addressed heat, ESD, and safety.

We now hold an electronic circuit that communicates smartly (Part 4) and stays on for a long time (Part 5). But how do we place all these parts on a ring-sized board, route them, conform to a curved surface, and make the antenna work well?

The next Part 6, PCB Design and Miniaturization, covers the schematic-to-PCB flow, component placement and routing, ring form-factor constraints, antenna layout, design for manufacturing (DFM) and Gerber output, assembly, and even mass production and certification (FCC/KC) — the final step that turns the circuit into a real, touchable object.

References

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