Off-Grid Smart Locks: Solar, 12 V Power & Real-World Reliability

No Wi-Fi. No grid. Still secure.

1) System Overview (What You’re Building)

• Power: 12 V DC bus from a battery bank (or a dedicated small pack) with inline fuse.
• Conversion: If the lock expects 5 V (USB), add a buck converter (12 V→5 V).
• Access: Keypad / NFC / local Bluetooth—no cloud required.
• Resilience: Mechanical override + surge/EMP hygiene + weather sealing.

2) Power Budget (Know Your Numbers)

Estimate daily consumption:

• Standby draw: 0.3 W × 24 h ≈ 7.2 Wh/day
• Actuation overhead: ~2–3 W for ~1–2 s per unlock (negligible in total)

Battery sizing rule-of-thumb: Target 7–14 days autonomy without sun. For 7.2 Wh/day, that’s ~50–100 Wh usable capacity. (Example: a modest 12 V, 10 Ah LiFePO₄ ≈ 128 Wh usable.)

3) Solar Sizing (Tiny Panels, Big Stability)

• Panel: 5–10 W 12 V nominal.
• Controller: Small PWM is fine; MPPT if panel is shaded/variable.
• Cabling: Keep runs short; aim <3% voltage drop (AWG 18–16 usually ok).

4) 12 V Wiring (Simple & Safe)

• Fuse: Inline 1 A (slow-blow) close to the battery or DC distribution block.
• Polarity: +12 V (red) → controller/buck → lock; Ground (black) common return.
• Serviceability: Add a quick-disconnect (XT30/DC barrel) at the door to remove the lock without rewiring.
• 5 V locks: Use a buck converter; set output to 5.1 V, verify under load.

5) Surge & EMP Hygiene (Because Remote ≠ Immune)

• Surge absorber: TVS diode or surge protector on the 12 V bus near the controller.
• Grounding: Follow your site’s grounding plan; avoid ground loops.
• Cable discipline: Twist positive/negative pairs to reduce induced spikes on long runs.

6) Weather Sealing & Condensation Control

• Gaskets: Use closed-cell foam or silicone gaskets behind exterior keypads.
• Penetrations: Route cables through glands; seal with neutral-cure silicone.
• Condensation: Desiccant packs inside cavities; consider a breathable membrane vent.
• Cold-weather: Lithium cells handle low temps better than alkaline; for extreme cold, power from the 12 V bus.

7) Fail-Safe Entry (When Power Isn’t There)

• Mechanical bypass: Retain a physical latch/lock cylinder hidden but reachable.
• External 5 V/12 V jack: Optional weather-sealed port to temporarily energize the system.
• Spare key plan: Store off-site or with a trusted neighbor; avoid obvious key safes.

8) Maintenance Cadence (Seasonal, Not Stressful)

• Quarterly: Inspect seals, check connectors, test actuation, verify solar/controller LEDs.
• Bi-annually: Tighten terminals, clean contacts, run a “low-voltage” test.
• Annually: Full system check: open/close cycles ×50, log any stalls or errors.

9) Quick Start Checklist

• [ ] Measure lock standby draw (multimeter inline)
• [ ] Size battery for 7–14 days autonomy
• [ ] Choose 5–10 W panel + small PWM controller
• [ ] Add 1 A inline fuse near source
• [ ] Weather-seal keypad & cable glands
• [ ] Add mechanical bypass + external service jack (optional)
• [ ] Document wiring with a photo inside the enclosure