Above is an image of the first prototype. Wires will go away and it will be built in layers rather than a large pile of stuff. Some more electronics might be added to warn when running on battery.

The TP4056 charge the battery to 4.2V. Keeping the battery fully charged 24/7 will not only kill it in a couple of months, the risk of fire and explosion is rather great.

The buck converter feeds 3.9V to the TP4056 charger, limiting the charge of the Li-Ion battery to ~3.8V. This cripples the capacity but make the battery lifetime more or less unlimited.

The boost converter ensure that the router get a stable 5V power no matter if the power comes from the battery or the 3.8V source. It also ensure that small spikes in the power is regulated.

This setup gives a UPS with about 1.2Wh capacity. At 0.6W and 20% power loss in the extra electronics (in reality it’s more like 5%) we end up at 90 minutes of battery power. Not enough to handle the mean black out, but more than enough to handle someone unplugging the power from the outlet when vacuuming the floor or what not.