During outages, providers' fibre nodes often have their own backup power for several hours, and the internet goes down not at the provider but at home: the router and fibre terminal switch off. A small 12 V DC UPS is enough to prevent that. You can buy one ready-made, or build one in an evening — for the same money you get more capacity and a design you can repair. Below: both options, the circuit, a parts list and runtime calculations.
How much your equipment draws
| Device | Typical draw |
|---|---|
| Wi-Fi router | 6–12 W |
| Fibre terminal (ONT) | 4–8 W |
| Router and ONT together | 10–18 W |
| 5–8 port switch | 3–5 W |
| IP camera | 3–6 W |
The most accurate way is to measure with a USB or DC meter. The label on the power adapter shows the maximum: "12 V 1 A" means up to 12 W; the router usually draws less.
Ready-made mini UPS or DIY
Ready-made mini UPS units with 5, 9 and 12 V outputs are convenient: plug in and go. But watch how their capacity is stated.
Important: "10,000 mAh" on the case is capacity at a single cell's 3.6–3.7 V, i.e. about 37 Wh. After conversion to 12 V roughly 30 Wh remain — 2–2.5 hours for a router plus terminal, not 10 as the numbers suggest.
A DIY LiFePO4 UPS gives several times more capacity for the same money, lasts thousands of cycles and can be repaired by swapping a single board.
The circuit: a UPS with no switchover
The most reliable design is "online": the router is always powered from the battery bus, while the mains adapter, through a charge module, both charges the battery and covers the load.
- A power adapter of 15–19 V supplies energy.
- A CC/CV charge module limits the voltage to 14.2 V and the current to 2–3 A: about 1 A goes to the router, the rest charges the battery.
- The battery bus: a 4S LiFePO4 pack is connected to it through a BMS board and a fuse.
- A DC-DC converter (buck-boost) delivers a stable 12.0 V to the router.
When the power goes out, nothing switches: the router simply keeps running from the same bus, now fed only by the battery. It doesn't even reboot.
Why the DC-DC converter if the battery is "12 volts" anyway? A 4S LiFePO4 pack swings from 14.2 V fully charged to around 11 V at the end of discharge. Most routers are designed for 12 V with about a 10 % tolerance, and the regulator protects them from both extremes.
Parts list
| Part | Specs | Purpose |
|---|---|---|
| 4 LiFePO4 cells | 3.2 V: 32700 cylindrical at 6 Ah, or 20–30 Ah prismatic | energy storage |
| 4S BMS board with balancing | 10–20 A | protection from over-charge, over-discharge and shorts |
| CC/CV charge module | 14.2 V output, 2–3 A (up to 5 A for 20–30 Ah cells) | charging and powering from mains |
| Power adapter | 15–19 V, 45 W or more (a laptop adapter works) | energy source |
| Buck-boost DC-DC | 12 V output, 2–3 A | stable 12 V for the router |
| In-line fuse | 5 A | protects wiring from shorts |
| Wires and connectors | 0.75–1.5 mm², 5.5 × 2.1 mm plug | connections |
| Enclosure | non-flammable, with ventilation holes | safety |
Capacity and runtime
Usable energy accounts for depth of discharge (0.9) and converter efficiency (0.9). The load is 12 W.
| 4S pack | Energy | Usable | Runtime at 12 W |
|---|---|---|---|
| 32700 cells, 6 Ah | 77 Wh | 62 Wh | about 5 h |
| 2 × 32700 in parallel (12 Ah) | 154 Wh | 124 Wh | about 10 h |
| 20 Ah prismatic | 256 Wh | 207 Wh | about 17 h |
| 30 Ah prismatic | 384 Wh | 311 Wh | about 26 h |
For typical 4–6 hour outage schedules the first option is enough; for long ones, go prismatic.
Assembly step by step
- Check the cells. All four should be within 0.05 V of each other, ideally from the same batch.
- Connect the cells in series (4S). Cylindrical cells with nickel strip and spot welding, prismatic cells with bolted busbars. Don't solder directly to a cell case: the heat damages it.
- Wire the BMS according to its diagram. Usually B− goes to the pack negative, the balance leads go in order from negative to positive, and P− is the output for load and charging. Plug the balance connector in last.
- Set the charge module with no load: 14.2 V, current limit 2–3 A.
- Set the DC-DC to 12.0 V before connecting the router — verify with a multimeter.
- Assemble the chain: adapter → charge module → bus (battery through BMS and fuse) → DC-DC → router.
- Test it. On mains the router runs and the battery charges. Unplug the adapter — the router must not reboot. After an hour, measure the battery voltage.
Why 14.2 V and not 14.6 V
The maximum LiFePO4 cell voltage is 3.65 V, i.e. 14.6 V for a 4S pack. But in this circuit the battery sits at charge voltage all the time. Holding 3.55 V per cell (14.2 V) gives a nearly full charge and a noticeably longer life than holding it at the maximum. More about voltage levels in LiFePO4 charge settings.
Safety: the BMS is mandatory — without it one cell will sooner or later be overcharged. Put the fuse right after the battery. Don't use cells of unknown origin, and don't charge LiFePO4 below 0 °C. The enclosure must not be airtight.
Summary
For a router and fibre terminal, a 12 V LiFePO4 UPS of 60–300 Wh is enough: it keeps the internet up from a few hours to a full day, with no switchover moment at all. If you need power for more than connectivity, move on to calculating battery capacity for the whole apartment, and see why LiFePO4 in LiFePO4 vs AGM.