Bill Collis
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... |