Supercapacitor Backup Power Design for 5V Electronic Door Locks:

Complete Implementation Using Two Volfpack 10F Graphene Pouch Cells

Electronic door locks must remain operational during power outages, typically supporting 8–15 lock/unlock cycles plus microcontroller housekeeping. Volfpack Energy’s graphene-based 10F pouch cells (2.7 V nominal, ultra-low ESR <1 mΩ, >1 000 000 cycles, operating range –30 °C to +70 °C) provide a compact, maintenance-free solution. Using two 10F cells in series (2S configuration) creates a 5.4 V bank with 5 F equivalent capacitance. This delivers ample energy while fitting slim lock enclosures (typical pouch size per cell approximately 6–7 cm × 3–4 cm).

A 12 V rail is unnecessary. Readily available 5 V solenoids and DC gear motors provide sufficient force and torque for bolt retraction in residential, cabinet, and light commercial locks. Operating directly at 5 V eliminates the boost converter, reduces component count, lowers cost, and improves overall efficiency.

Volfpack 10F Cell Bank Topology and Energy Budget

Bank configuration: Two Volfpack 10F, 2.7 V pouch cells in series.

• Equivalent capacitance: C_bank = 5 F

• Maximum voltage: 5.4 V (charge safely to 5.0 V)

• Minimum usable voltage: 2.5 V (ensures stable supply for 3.3 V LDO or direct actuator drive)

Usable energy calculation:

E = 1/2 × C × (V_i² - V_f²) = 1/2 × 5 × (5.0² - 2.5²) = 1/2 × 5 × (25 - 6.25) = 46.875 J

Apply conservative deratings for real-world conditions (Colombo ambient temperatures 30–40 °C):

• 20 % end-of-life capacitance fade over 8–10 years

• 8–12 % losses in driver, wiring, and minor ESR effects

• 0.5–1.5 J for MCU housekeeping (15–30 mA at 3.3 V for 30–60 s)

Net usable energy: approximately 35–40 J

Actuator Power Profiles at 5 V

Two common actuator types operate directly from the supercap bank:

Solenoids (pull-type or latching):

• Typical draw: 0.8–1.2 A peak for 150–400 ms pull-in

• Energy per cycle: 1.0–3.0 J (peak-and-hold drive reduces this significantly by applying full voltage briefly then dropping for any hold phase)

• Advantage: sub-50 ms response time; latching versions require only short pulses in each direction with zero continuous hold power.

Motorized actuators (5 V DC gear motor with worm or lead-screw self-locking):

• Running current: 150–300 mA

• Stall/peak: up to 400–500 mA for 0.8–1.5 s

• Energy per cycle: 0.6–2.5 J

• Advantage: quieter operation and zero hold power due to mechanical self-locking.

With 35–40 J net energy, the system supports 12–20+ cycles depending on actuator efficiency and drive technique (conservative estimate: 12–15 cycles with a solenoid or 15–20+ with a self-locking motor). This comfortably exceeds typical safety/egress requirements of 5–10 cycles.

Voltage drop during a 1.2 A pulse:

ΔV = I × ESR_bank ≈ 1.2 A × 2 mΩ = 2.4 mV (negligible; bank voltage remains essentially flat).

Charging Circuit

The main 5 V rail (from USB-C, PoE, or regulated adapter) continuously trickle-charges the bank using a dedicated supercapacitor charger IC.

Recommended device: Analog Devices LTC3225

• Charges two cells in series to selectable 4.8 V or 5.3 V (use 5.0 V target)

• Programmable charge current up to 150 mA with integrated active cell balancing

• PGOOD flag for charge-complete indication

• No balancing resistors required; low noise and low quiescent current

Charge profile: constant-current / constant-voltage (CCCV).

Approximate full charge time from 0 V at 150 mA:

t ≈ (C × V) / I = (5 × 5.0) / 0.15 ≈ 167 seconds (under 3 minutes).

The lock becomes operational within 30–60 seconds after power restoration.

Firmware handling (on MCU such as ESP32 or STM32):

- On power-up: enable charger until PGOOD asserts.

- Monitor main VIN via ADC or comparator (brown-out threshold ~4.5 V).

- On power loss: disable charger and switch to supercap-powered mode.

Power-Path and Backup Handover

The architecture is intentionally simple due to 5 V operation:

• Main 5 V rail → ideal diode (LTC4412 or P-channel MOSFET controller) → system 5 V bus (MCU + actuator driver)

• Supercap bank → same 5 V bus via low-dropout path with protection

Handover occurs in <10 µs with zero reverse current from supercap to main supply. Add 220 µF low-ESR ceramic capacitance on the 5 V bus to absorb actuator inrush current.

Actuator drive circuit:

• H-bridge driver (e.g., DRV8833 or discrete MOSFETs) with soft-start PWM to limit inrush

• Current-sense resistor (0.05 Ω) for stall detection and protection

• Snubber network (0.1 µF + 10 Ω) across actuator terminals to suppress EMI

Protections, Thermal, and Environmental Considerations

- Over-voltage: charger clamps at safe level per cell; add 2.8 V TVS/zener per cell

- Under-voltage lockout: disable actuator drive below ~2.8 V to preserve MCU last-gasp functions

- Temperature: in Colombo conditions, limit charge voltage to 4.8–5.0 V maximum for minimal long-term fade

- Leakage current: total bank <5 µA → negligible self-discharge (<1 J per month)

- Short-circuit: polyfuse on bank output

- Reverse polarity protection: Schottky diodes during assembly

- MCU last-gasp routine: power-fail interrupt triggers BLE beacon (“power outage – lock operational”), event logging, and limited actuation attempts

Bill of Materials (Core Supercapacitor Section)

- 2 × Volfpack 10F, 2.7 V graphene pouch cells

- LTC3225 supercapacitor charger IC

- LTC4412 ideal diode controller

- DRV8833 or equivalent H-bridge driver

- Passives: 220 µF low-ESR ceramics, TVS diodes, snubbers, current-sense resistor

Estimated cost for supercap core at 1k volume: $5–9.

Prototype Validation Steps

1. Assemble 2S bank and verify balancing and total ESR (<2 mΩ).

2. Perform 5 000–10 000 charge/discharge cycles at realistic peak current while monitoring capacitance and ESR.

3. Simulate power outages using a relay; scope voltage rails to confirm clean handover and >12 cycles.

4. Test in thermal chamber at 45 °C to validate performance under local ambient conditions.

This design with two Volfpack 10F cells in a 5 V architecture provides robust, zero-maintenance backup power. It outperforms traditional batteries in cycle life, temperature resilience, and simplicity while keeping the solution compact and cost-effective. Volfpack prototypes are locally available in Sri Lanka, facilitating rapid iteration.

The bank offers comfortable margin for most residential and light commercial door locks. If higher force actuators or >20 cycles are required, parallel additional 2S strings or upgrade to higher-capacitance cells. Provide the specific actuator datasheet (current draw and pulse duration) or MCU platform for further schematic or firmware optimization.