EMP-Resistant Solar Bank / Block / Kiosk — From-Scratch Operational Build

0) What We’re Building (Scope & Capabilities)

Goal: A portable, 24 V LiFePO₄ solar kiosk in a gasketed steel enclosure:

• ~2.5 kWh battery (24 V × 100 Ah) using two 12 V LiFePO₄ in series
• MPPT solar input (≤100 V PV Voc; 400–800 W arrays)
• Optional 120 VAC pure-sine inverter; DC outputs (SB50, 5.5 mm barrel, USB-C PD)
• EMP-aware: steel Faraday shell with EMI gaskets, honeycomb vent, line-entry feedthrough EMI filters, cascaded surge protection (GDT → MOV/TVS → EMI filter), tight single-point bonding, and nested spares in Faraday bags.

1) System Architecture

Why 24 V LiFePO₄? Lower current than 12 V (smaller copper, cooler lugs), stable charging, solid support from MPPTs and inverters.

• LFP charge setpoints: Absorb ≈ 28.4 V, Float ≈ 27.0 V. Disable temperature compensation and equalize; follow your battery manual.
• MPPT range: Choose a controller with 100 V PV max (e.g., 100/30 or 100/50). Always connect battery first, then PV.
• PV wiring: Use UL 4703 PV wire and genuine MC4 connectors. Provide a PV DC disconnect.
• AC side (optional): If you include an inverter, add a UL 1449 Type 2 SPD and a chassis-mounted EMI/RFI line filter.
• EMP/EMI line filtering: Use panel-mount feedthrough EMI filters (Pi/C filters) for every conductor crossing the enclosure.
• Ventilation without losing shielding: Use aluminum honeycomb EMI vent panels with conductive gaskets.
• Bonding: Single-point bond bar tying enclosure, SPDs, filter bodies, DC- bus, and AC safety ground. Keep straps short and wide.

2) Bill of Materials (BOM — with Amp Notes)

Size wire via ampacity charts; fuse near sources; keep high-current runs short.

3) Cut & Drill Sheet (Printable)

• Door perimeter: Apply EMI gasket fully around seam; miter corners; firm latch pressure.
• Left wall (top): PV DC disconnect cutout per model template (~6″ from top; 3″ clearance above/below).
• Left wall (below): PV gland plate (4× 20 mm holes) for MC4 pigtails or fixed entry.
• Top panel: Honeycomb vent opening ~116 mm square + 4× M4 holes on 105 mm square; place toward rear.
• Right wall (mid): AC EMI filter rectangular cutout; below it an L5-15 receptacle plate (if AC out used).
• Bottom wall (front-right): 2× SB50 bulkhead plates (24 V out; utility DC in).
• Bottom wall (rear): Ground/bond lug (3/8″ stud) to bare steel.
• Interior backplate: Drill for busbars, Class-T fuse block, shunt, MPPT, inverter.

Scrape paint where metal must bond (filter flanges, vent frame, bond lug). Use star washers.

4) Wiring & Assembly — Step by Step

Safety Preflight

• Remove jewelry; wear safety glasses & Class 0 gloves; keep CO₂ extinguisher nearby.
• Tape over battery terminals until needed. Verify SPD/filter ratings before power-up.

Step 1 — Prep the Shielded Enclosure

1. Deburr cutouts; scrape paint at all bond points (door landing, filter flanges, vent frame, ground lug).
2. Install EMI gasket fully around the door seam; ensure even compression.
3. Mount honeycomb vent with conductive gasket & star washers; measure continuity (<0.1 Ω) to enclosure.
4. Install panel-mount feedthrough EMI filters for PV+, PV−, 24 V+, 24 V−, control lines; torque per datasheet.
5. Install AC line EMI filter and UL1449 Type 2 SPD brackets (wire later).
6. Bolt in the copper bond bar close to penetrations; keep SPD/filter grounds short & wide.

Step 2 — Place Power Hardware

1. Set batteries (two 12 V in series or one 24 V) on mounts; strap and pad; leave 10–20 mm air gap.
2. Mount Class-T fuse within 7–8″ of battery +; then positive busbar.
3. Mount negative shunt between battery − and negative busbar; all returns must pass the shunt.
4. Mount MPPT with airflow from the vent; keep battery leads short & stout (e.g., 6 AWG for ≤50 A).
5. Mount inverter (if used) with clear airflow; bond chassis to the bar.

Step 3 — EMP/Surge Stack at Each Penetration

Outside → Inside order: GDT → MOV/TVS → Feedthrough EMI filter → device. Bond grounds at the same bolt on the bond bar.

• PV pair: 3-electrode GDT across PV+/PV−/chassis (nominal ≥350 V) → MOVs to chassis → feedthrough filters → MPPT PV in.
• 24 V DC out: GDT (~75–150 V) + TVS (standoff 33–36 V) to chassis → feedthrough filters → SB50/DC distro.
• AC out: UL1449 Type 2 SPD line-to-ground/neutral + Corcom line filter as the wall penetration.

Step 4 — DC Power Bus & Protection

1. Series link: Batt1+ → Class-T → Bus+, Batt1− → Batt2+ (series jumper), Batt2− → Shunt → Bus−.
2. Land MPPT output to busbars; configure LFP profile (Absorb 28.4 V; Float 27.0 V; EQ off; temp-comp off).
3. Run 24 V DC outputs from busbars through feedthrough filters to SB50/aux; fuse each branch.
4. Inverter leg: dedicated fuse sized per manual; short, fat cables; bond chassis.

Step 5 — PV Side & Disconnects

1. Size strings for cold Voc within the MPPT’s 100 V max. Typically 1–2 modules in series.
2. Install a lockable PV DC disconnect; label “PV SYSTEM DISCONNECT”.
3. If paralleling strings, fuse each string in a combiner. Use UL 4703 PV wire & genuine MC4 connectors.

Step 6 — Bonding & Ground

• Tie enclosure, SPD grounds, filter bodies, device chassis to the single bond bar with wide straps.
• For longer outdoor deployments, add an earth electrode; keep strap to the bar short. If soil is poor, prioritize equipotential bonding.

Step 7 — Commissioning

1. Battery-only test: PV open. Verify DC outputs, inverter startup, and cool SPDs.
2. Program MPPT LFP profile (Absorb 28.4 V; Float 27.0 V; EQ off; temp-comp zero).
3. PV test: With disconnect open, verify polarity; then close. Confirm charge current and behavior.
4. Shield sanity check: Compare RF noise around the box door closed vs open with a handheld radio/SDR.

5) Capacity & Runtime Planning

• Battery ≈ 2.56 kWh (24 V × 100 Ah). Usable ≈ ~2.3 kWh (≈90% DoD on LFP).
• Example loads:
  • Router + ONT + AP ≈ 35–40 W → ~50–65 h
  • Laptop 65 W + monitor 30 W ≈ 95 W → ~20–22 h
  • 500 W AC tool (25% duty) avg ~125 W → ~18 h
• Solar 400–800 W array: ~2–5 kWh/day depending on season, location, and tilt.

6) Printable Single-Line Diagram

[PV Array]--MC4----[PV Disconnect]--[GDT]--[MOV]--[Feedthrough EMI]--+-->[MPPT]
|
[Bond Bar]
MPPT + --> [Busbar +] --[Class-T Fuse]---> [Battery +]
MPPT - --> [Busbar -] --[Shunt]---------> [Battery -]

[Busbar +]----[Fuse]----[Feedthrough EMI]----> [DC OUT SB50]----> Loads
[Busbar -]-----------------------------------> [DC OUT SB50]----> Loads

[Busbar +/-] ---> [Inverter 24V] ---> [AC EMI Filter] ---> [UL1449 SPD] ---> [AC Outlet]

All chassis/enclosure/filters/SPD grounds --> [Bond Bar] --> [Enclosure Ground Lug]

7) Hazards & Hard Rules

• Do not backfeed buildings. Use only with listed transfer equipment; never via a cord to a panel or a meter base.
• DC arcs don’t self-extinguish. Cover PV when wiring. Fuse close to sources. Protect against abrasion and strain.
• LiFePO₄ charging: Follow the battery manual; disable temp-comp; no equalize.
• Shielding: No open holes—only honeycomb vents and filtered feedthroughs.
• Bonding: Short, wide ground straps; bare-metal contact. Inspect seams and torque regularly.

8) Why This Is EMP-Aware (Summary)

• Shield first: Steel enclosure + EMI gasket + honeycomb vents for high attenuation of fast fields.
• Filter every conductor: GDT → MOV/TVS → feedthrough filter at the wall stops conducted energy at the boundary.
• Bonding discipline: Single-point bar, short/wide straps reduce internal resonances and ground bounce.
• Nesting spares: Spare MPPT/DC-DC stored in Faraday bags provides rapid recovery after a severe event.
• Standards-aligned: Applies the spirit of MIL-STD-188-125 and classic DoD grounding/bonding practices.

10) Sizing Notes (Wire & Fusing)

• PV strings: OCP ≥ Isc × 1.25 per string (if paralleled); UL 4703 PV wire; MC4 rated as per datasheet.
• Battery main fuse: Class-T sized for cable protection and inverter surge.
• AC line: Filters and SPDs matched to inverter output; observe local electrical code.

11) Parts to Buy (Example References)

• MPPT 100/50 (or 100/30) with LFP profile; manuals specify battery-first wiring and 24 V setpoints.
• Pure-sine inverter 24 V, 1000–2000 VA (short, large-gauge leads; fused).
• UL 1449 Type 2 SPD for 120 VAC; Corcom-style line EMI filter (10–20 A).
• Panel-mount DC feedthrough EMI filters (20–50 A Pi/C type).
• 3-electrode GDTs (PV ~350 V; 24 V rail 75–150 V); MOVs with thermal protection; 33–36 V TVS.
• Honeycomb vent panel (~120 mm) with conductive gasket; genuine Stäubli MC4; UL 4703 PV wire.
• Anderson SB50 bulkhead & pigtails; copper bond bar & braid; gasketed steel enclosure.

12) Maintenance & Resilience Tips

• Store a spare MPPT and DC-DC in nested Faraday bags inside the enclosure (not connected).
• Quarterly torque check on lugs; replace MOV/SPD modules that show heat or after major surges.
• Keep the door gasket clean and conductive; remove paint from all bond landings.
• Use MPPT logs to track yield; unexpected drops flag array or wiring issues early.

13) Standards & Rationale (Plain English)

• Threat model: EMP has fast E1, lightning-like E2, and slow E3 components; our layers address conducted/radiated E1/E2 paths.
• Hardening playbook: Shield → Bond → Filter → Surge; copy the proven moves from defense/telecom even without certification testing.
• Vents & filters: Honeycomb vents act like waveguides below cutoff; feedthrough filters keep the enclosure’s integrity at the wall.

14) Full “From-Nothing” Build Sequence (Condensed Timeline)

1. Cut & fit the box: penetrations, gasket, vent, filters, bond bar.
2. Mount battery hardware: fuse, shunt, busbars.
3. Mount MPPT and (optional) inverter; preserve airflow lines.
4. Install GDT/MOV/TVS stacks at each penetration; bond to the bar.
5. Wire battery → bus → loads; set LFP charge values; battery-only test.
6. Wire PV via disconnect → filters → MPPT; PV test.
7. Add AC SPD + AC EMI line filter if using AC output.
8. Final: seal door, verify seam continuity, label everything.

Appendix A — Minimal Runtime/Capacity Table

Solar backfill: 400 W ≈ 1.6–2.4 kWh/day; 800 W ≈ 3.2–4.8 kWh/day (season/latitude dependent).

Appendix B — Printable Cut Sheet Notes (Give to Fabrication)

• Door seam landing: 12 mm conductive fabric gasket, corners mitered; closed gap <0.5 mm.
• Vent: 116 mm square opening; 4× M4 holes on 105 mm square; conductive gasket under frame.
• AC filter: cutout per datasheet (e.g., ~48×28 mm rectangle) with 2× M4 fixings.
• Feedthrough filters: threaded bushings (e.g., M12–M20) with wrench clearance; metal-to-metal seating.
• Bond lug: 3/8″ stainless stud through bare steel; serrated/star washers both sides; label “GROUND/BOND”.

Sources & References

• EMP/HEMP Guidance: CISA — EMP Protection & Resilience Primer; DOE/CESER — HEMP Waveform & Mitigation notes; MIL-STD-188-125 (High-Altitude Electromagnetic Pulse Protection); MIL-HDBK-419 (Grounding, Bonding & Shielding).
• PV Code/Best Practices: NEC 690.13 (PV DC disconnect), 690.9 (OCP), arc-fault context; NREL O&M Best Practices for PV.
• Hardware Datasheets: Stäubli MC4 connectors; UL 4703 PV wire; Corcom EMI filters; Littelfuse TMOV (thermally protected MOV) and TVS diodes; Bourns 3-electrode GDTs; Spectrum Control feedthrough EMI filters; Leader Tech honeycomb vent panels.
• LFP Charging: Victron Energy lithium manuals and defaults for 24 V LFP; Battle Born charging guidance (scaled to 24 V).
• Safety (No Backfeed): OSHA/CPSC guidance on generator/backup system connection hazards; use of listed transfer equipment only.

Always follow your specific product manuals and local electrical codes. Values here are typical for 24 V LiFePO₄ and may require adjustment to match your exact components.