DIY UPS for Home Assistant Green & Xfinity XB7 Modem

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 Shelly Plus Uni.

Honest context: A $75 APC BE600M1 would do the same job out of the box. This build costs roughly $200 more over 10 years than that option. The engineering rationale — longer battery life, faster switchover, direct HA integration, no DC-DC converter voltage regulation — is documented in https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/design-rationale.md. Build this if those tradeoffs matter to you.

System Overview

AC grid powers a Mean Well LRS-100-12 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. A Shelly Plus Uni reports battery voltage and temperature to Home Assistant.

AC Power (Wall)
                 ↓
┌────────────────────────────────────┐
│  Mean Well LRS-100-12 PSU          │
│  Input: 85-264VAC  Out: 13.3V      │
└────────────────┬───────────────────┘
                 │ 13.3V (float)
                 ↓
┌────────────────────────────────────┐
│  Pololu Ideal Diode (4-60V, 10A)   │
│  Prevents PSU backfeed from batt.  │
└────────────────┬───────────────────┘
                 │
                 ↓
┌────────────────────────────────────┐
│  Cyclenbatt 12V 10Ah LiFePO4       │
│  Built-in 10A BMS, 5000+ cycles    │
└────────────────┬───────────────────┘
                 │
                 ↓
┌────────────────────────────────────┐
│  Victron BatteryProtect BP-65      │
│  LVD: 11.8V (Setting 7)            │
│  Reconnect: 12.8V, 90s delay       │
└────────────────┬───────────────────┘
                 │ 11.71–13.16V at device terminals
            ┌────┴─────┐
         [F3:2A]    [F4:5A]
            │          │
         HA Green   XB7 Modem
           (3W)      (~14W)

┌────────────────────────────────────┐
│  Shelly Plus Uni (Monitoring)      │
│  ADC → Battery Voltage             │
│  GPIO → DS18B20 Temperature        │
│  WiFi → Home Assistant             │
└────────────────────────────────────┘

Key Specifications

Parameter	Value
Battery	Cyclenbatt 12V 10Ah LiFePO4
Float voltage	13.3V (set on PSU trimmer)
LVD cutoff	11.8V (Victron BP-65, Setting 7)
LVD reconnect	12.8V (30s delay after threshold met)
Device voltage envelope	11.71–13.16V at terminals
Switchover time	<1ms (MOSFET ideal diode)
Typical load	~17–18W combined
Runtime at typical load	~6.3 hours
Enclosure	LeMotech IP65 ABS, 9.6″×7.6″×4.5″
Monitoring	Shelly Plus Uni → Home Assistant
Total build cost	~$234

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 (~12.2V): Shelly detects threshold via ADC. Home Assistant initiates graceful HA Green shutdown; modem continues on battery.

Mode 4 — LVD Cutoff (11.8V): Victron BP-65 disconnects loads after 90-second hold-off.

Mode 5 — AC Restoration: PSU resumes 13.3V, ideal diode switches source, BP-65 reconnects after 30s hold-off.

Assembly Layout

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

Documentation

File	Contents
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/bom.md	Full bill of materials with vendor, price, and notes
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/wiring.md	From/To wiring tables, fuse locations, component mounting
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/specifications.md	Electrical, performance, and protection spec tables
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/design-rationale.md	Build-vs-buy analysis, float charging strategy
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/component-selection.md	Per-component selection rationale
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/cost-analysis.md	Build cost breakdown and 10-year lifecycle comparison
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/safety.md	AC and battery safety procedures
https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/voltage-drop-analysis.md	Wiring resistance and voltage drop calculations

Datasheets

Status

Design complete
Components specified
Components ordered / received
Build
7-day power measurement baseline (in progress — Kill-a-Watt, devices measured separately)
Home Assistant automation documentation

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 https://github.com/wkcollis1-eng/DIY-LiFePO4-UPS/blob/main/docs/design-rationale.md.

Project Details