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DIY-LiFePO4-UPS

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DIY UPS for Home Assistant Green & Xfinity XB7 Modem3 child itemsAssembly

DIY UPS for Home Assistant Green & Xfinity XB7 Modem

12V LiFePO4-Based Uninterruptible Power Supply

A 12V LiFePO4-based uninterruptible power supply for keeping a Home Assistant Green and Xfinity XB7 cable modem running during grid outages. Built into an IP65 enclosure with Home Assistant monitoring via ESPHome INA260 current sensor.

Honest context: A $85 APC BE600M1 provides comparable backup capability. This build costs roughly the same over 10 years as that option (based on battery replacements and electricity usage). The engineering rationale — longer battery life, faster switchover, direct HA integration, no DC-DC converter voltage regulation — is documented in design-rationale.md. Build this if those tradeoffs matter to you.

⚠️ Safety Warning

[!CAUTION] This project involves lithium batteries and AC mains voltage. Improper handling can cause fire, electric shock, or equipment damage.

✅ Disconnect AC power before any work inside the enclosure
✅ Use appropriate fusing on battery connections
✅ Never charge LiFePO4 below 0°C (32°F)
✅ Keep a Class B or ABC fire extinguisher accessible
✅ Review docs/safety.md before building

Disclaimer: Information provided for educational purposes only. Build at your own risk.

Performance Summary

Validated Runtime: ~3.6 hours to hardware LVD (11.8V) under a sustained 14.9W load.

Monitoring upgrade note: The Shelly Plus Uni has been replaced by an ESPHome XIAO ESP32-C3 + Adafruit INA260 current sensor. All capacity figures below are now coulomb-counted by INA260 rather than inferred from voltage profiles. The May 2026 figures supersede the March 2026 D3 estimates.

Key Finding

The UPS delivers a reliable ~3.6 hours of runtime at typical HA Green + XB7 loads. This is ~42% of the theoretical maximum runtime at full charge (10 Ah / 1.18A = 8.47h).

This result is not due to battery degradation, measurement error, or software issues. It is a direct consequence of the single-rail 13.3V CV architecture.

Why the Runtime is Limited

The system uses a single shared rail for both charging and load. To keep the voltage safe for the connected equipment, charging is capped at 13.3V. This holds the battery at approximately 65% SOC, preventing proper absorption charging at 14.4V. As a result, approximately 3.5 Ah (~35%) of the battery's rated 10 Ah capacity is never accessible.

Capacity zone	Ah	Reason
Float ceiling — never loaded	~3.5 Ah	13.3V float ≈ 65% SOC; requires 14.4V charger to recover
Rate/LVD penalty	~1.3 Ah	IR drop at 1.18A strands charge above LVD threshold
Delivered (INA260 measured)	4.18 Ah	Coulomb-counted, May 6 2026
Protected at LVD cutoff	~0.8 Ah	Intentional; prevents over-discharge
Below LVD to rated-empty	~0.8 Ah	Intentional; cell protection

Discharge Characteristics

Extremely flat voltage plateau from 13.0V down to 12.85V — delivers the bulk of usable energy
Knee region (12.40–12.85V) shows accelerating decline; ~53 min at production load
Cliff begins at ~12.40V: voltage slope accelerates from −30 mV/min to −58 mV/min
12.4V warning provides approximately 15–20 minutes of action time before hardware LVD

Conclusion

The system performs exactly as designed. The layered protection (voltage warnings → automated shutdown → BP-65 hardware LVD) is robust and all three tiers are now fully live-validated.

Current validated specification: ~3.6h runtime at 14.9W average under the existing single-rail topology, based on INA260 coulomb counting (May 6 2026).

A two-stage charging architecture (dedicated 14.4V charger + DC-DC regulator) would be required to approach the full ~7.1–8.5 hour theoretical runtime.

Reports and data can be found in docs/. Last validated: May 2026 (INA260 coulomb counting)

Commissioning Results

The chart records a full two-cycle discharge test performed March 25–26, 2026 using a Netgear R6400 router (~7W DC) as a substitute load, with battery voltage logged via the Shelly Plus Uni at 5-second intervals.

Discharge 1 (7.76h): Voltage held the characteristic LiFePO4 flat plateau from 13.2V to 13.0V over 7.76 hours — a drop of only 0.2V at light load, confirming textbook cell behavior. Recharge (10.1h): AC restored; PSU returned the battery to float within minutes, peaking at 13.28V and holding stable through a 10-hour recharge window. Discharge 2 (9.36h): A second full discharge ran the battery to LVD cutoff. The Victron BP-65 tripped at 11.77V — within 0.03V of the 11.8V design target — confirming protection circuit accuracy. Post-LVD OCV rebound to 12.13V confirms healthy cell chemistry with no permanent capacity loss from the deep discharge.

Key findings:

Discharge plateau variance under 0.2%/hr
BP-65 cutoff accuracy ±0.03V
Internal resistance ~260mΩ at low SoC (derived from OCV recovery)
Bulk recharge from 12.9V to 13.2V completed in under 12 minutes after AC restoration

System Overview

AC grid powers a Mean Well HDR-60-12 PSU set to 13.3V float, which charges a 12V 10Ah LiFePO4 battery through a MOSFET ideal diode. On grid failure, loads switch directly to battery in under 1ms. A Victron BatteryProtect BP-65 disconnects loads at 11.8V to prevent over-discharge. An ESPHome XIAO ESP32-C3 with Adafruit INA260 current sensor and DS18B20 temperature sensor reports battery voltage, current, power, and accumulated Ah/Wh to Home Assistant.

System Architecture

Key Specifications

Parameter	Value
PSU	Mean Well HDR-60-12, 13.3V float
Battery	Cyclenbatt 12V 10Ah LiFePO4
Float voltage	13.3V (set on PSU trimmer, lacquered)
LVD cutoff	11.8V (Victron BP-65, Setting 7)
LVD reconnect	12.8V (30s delay after threshold met)
Device voltage envelope	11.71–13.11V at terminals (2A worst-case)
Switchover time	<1ms (MOSFET ideal diode)
Typical load	16.0W AC wall / 14.9W DC at devices + monitor (INA260 measured)
Accessible capacity	4.18 Ah / 53.3 Wh (INA260 coulomb-counted, May 2026)
Runtime at typical load	~3.6 hours to LVD
Battery lifespan	10–20 years (calendar aging at 13.3V float, 63–75°F)
Enclosure	LeMotech IP65 ABS, 9.6″×7.6″×4.5″
Monitoring	ESPHome XIAO ESP32-C3 + INA260 + DS18B20 → Home Assistant
Total build cost	~$224 + ~$25 ESPHome monitor board

Operating Modes

Mode 1 — AC Present: PSU outputs 13.3V through ideal diode; battery held at float equilibrium. Loads draw from PSU, not battery.

Mode 2 — AC Failure: PSU output drops to 0V. Ideal diode blocks. Battery feeds loads directly, no interruption.

Mode 3 — Low Battery (~12.4V): ESPHome Cliff phase alert. HA notification: Critical Warning.

Mode 4 — Low Battery (~12.2V): ESPHome detects threshold. Home Assistant initiates graceful HA Green shutdown; modem continues on battery.

Mode 5 — LVD Cutoff (11.8V): Victron BP-65 disconnects loads after 90-second hold-off.

Mode 6 — AC Restoration: PSU resumes 13.3V, ideal diode switches source, BP-65 reconnects after 30s hold-off. PSU immediately enters CC mode at 4.5A (~60W) bulk recharge, tapering to CV float over approximately 10–15 minutes.

System Architecture

Automation Validation Status

Automation	Threshold	Mar 26	Mar 28	May 6	Status
Early Warning	< 13.0V for 2 min	✅	✅	✅	PASS
Critical Warning	< 12.4V	✅	✅	✅	PASS
Graceful Shutdown	< 12.2V for 1 min	⚠️ Not tested	⚠️ Not tested	✅ Confirmed	PASS
BP-65 hardware LVD	~11.8V	✅ Direct (11.77V)	✅ Inferred	✅ Confirmed (11.675V)	PASS

All four tiers are now fully live-validated. The graceful shutdown automation was confirmed on May 6, 2026 — the final outstanding commissioning item.

Assembly Layout

Component Layout

Elevation (top) and plan (bottom) views of LeMotech junction box. Interior Dimensions in inches.

Assembly Pictures

Assembly Pics

Documentation

File	Contents
docs/bom.md	Full bill of materials with vendor, price, and notes
docs/wiring.md	From/To wiring tables, fuse locations, component mounting
docs/specifications.md	Electrical, performance, and protection spec tables
docs/design-rationale.md	Build-vs-buy analysis, float charging strategy
docs/component-selection.md	Per-component selection rationale
docs/cost-analysis.md	Build cost breakdown and 10-year lifecycle comparison
docs/safety.md	AC and battery safety procedures
docs/voltage-drop-analysis.md	Wiring resistance and voltage drop calculations
docs/supplemental-analysis.md	Thermal Analysis & Enclosure Heat Budget, Runtime Analysis, Failure Modes & Effects Analysis (FMEA)
docs/design-specification.md	Formal requirements, design decisions, and verification matrix
docs/commissioning.md

Datasheets

Related Projects

LiFePO₄ Battery Bank Study — 500Ah DIY battery bank with 130+ days of monitoring
Home Assistant Config — HVAC monitoring implementation

Status

Notes

Device input voltage tolerance (±10%) is inferred from IEC 62368-1 design practice; neither Nabu Casa nor Comcast publish explicit DC input voltage ranges for these products.
The PSU output trimmer is set to 13.3V and lacquered. Drift analysis shows expected aging drift of 3–5 mV/year — negligible relative to the 500mV headroom before OVP.
No DC-DC converter is used. The direct battery feed strategy is documented in design-rationale.md.
The INA260 monitor board (XIAO ESP32-C3 + Adafruit INA260 + DS18B20 on SBB400 breadboard) replaced the Shelly Plus Uni in April 2026, adding coulomb counting, phase detection, and runtime estimation. Monitor draws ~30–40 mA from the 12V battery bus via Pololu D24V7F3 3.3V regulator.
The Shelly Plus Uni ADC-based capacity estimates (D3, March 2026: 4.85 Ah / 62.5 Wh) are superseded by the INA260 coulomb-counted measurement of 4.18 Ah / 53.3 Wh from May 2026. The D3 figure was computed from voltage profile timing × assumed constant load and should no longer be used as the capacity baseline.

💝 Support This Project

If this build log or the design rationale saved you time or money on your own UPS project, consider sponsoring continued measurement and documentation work.

12V LiFePO4 UPS for Home Assistant and network gear. First full outage validation (May 6, 2026): 217 min AC-off, 4.18 Ah / 53.3 Wh delivered, graceful shutdown via slope-based cliff_imminent trigger — confirmed end-to-end. Includes design docs and outage reports.

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