Making backup power for a router
In fact, we will make backup power supply for ONT (Optical Network Terminal), but since the method is suitable for all similar devices with 12v power supply, I generalized it to “router”.
Target: make a device that can power the router for some time when the power goes out. In this case: the device should have an indication of the battery voltage, when switching from the main power circuit to the backup, the main power circuit should be completely broken and vice versa.
It was decided to implement the plan as follows. Switching of the circuits will be carried out by a 3-position (1 – power from the router, 0 – no power, 2 – power from the battery) 6-pin switch. Voltage indication via a voltmeter connected to the battery and the minus of which is broken by a mechanical switch for the keyboard. Battery (hereinafter referred to as ACB) lead-acid (AGM) for UPS. 3D printing of the case based on the dimensions of the battery case and all other components. Battery charging from a charger for car batteries.
We collect the necessary components
Battery
The battery was chosen CSB GP 1272 based on good reviews. Cost $20 at the time of purchase. (You can choose any other UPS battery)
Frame
Was modeled in FreeCAD and printed on a 3D printer. Thickness 3mm.
Download links: FCStd And stl
Switches and voltmeter
3-position switch link on AliExpress, cost $1.5 at the time of purchase.
Voltmeter link on AliExpress, cost $1 at the time of purchase. (I took the green one)
I already had a mechanical switch for the keyboard. Gateron, sold on AliExpress. Let's assume the price is $ 1 per piece. The main thing is not to accidentally buy an optical switch, otherwise nothing will work.
Rest
You will have to cut the wire from the power supply to the router in the middle. You will also need a soldering iron, flexible wires, terminals or “crocodiles” for the battery. I bought the wires here.
Assembling the device
We cut the router power supply cable in half and strip the ends.
We pass the wires of the part on which the connector for connection to the router remains through the hole opposite the inscription Šabaš, then we pass them through the hole of the 3-position switch and connect them in any way available to you with contacts 1 and 2 (middle) of the 3-position switch. In the future, all the pluses and minuses should be connected according to the same numbers.
In the same way as in the previous paragraph, we connect the wires from the router power supply to contacts 1b and 2b, passing them through the hole to the left of the Šabaš inscription.
We pass the wires from the battery through the hole to the right of the Šabaš inscription and connect them to contacts 1a and 2a of the switch.
We connect a voltmeter to the same contacts, while breaking the minus of the voltmeter with a switch for the keyboard.
We push the switches and voltmeter back into place.
We attach the case to the battery (I used double-sided tape + Velcro tape).
Done. I soldered all the contacts. Places where there are bare sections of wires and contacts can be insulated with silicone sealant just in case. The voltmeter and switches fit tightly into their places, millimeter by millimeter (I left a 0.5 mm reserve for holes when modeling the holes in the case).
Total
We have received a simple and reliable device for backup power supply of the router. Cost $23.5 + cost of 3D printing and charger for the battery, if you do not have one. The device allows you to switch the router to backup power supply and vice versa or turn it off completely with one touch. We see the battery voltage when pressing the mechanical switch from the keyboard.
It is worth mentioning that AliExpress has ready-made boards that automatically switch power to backup power when the regular power is disconnected. This option does not suit me, but know that this option also exists, and the router will still have time to turn off.
Charging and discharging the battery
Here I will try to dot all the i's and cross all the t's in the issue of operating such batteries. I will rely on the documentation from the manufacturer.
We will get a capacity of 7.2 Ah when discharging from 13.3 Volts to 10.5 Volts.
10.5 Volts corresponds to 0% battery charge, 13.3 100%.
The higher the percentage you discharge the battery, the shorter its life. If you always discharge it to 0%, it will last for 260 charge/discharge cycles. If you discharge it to 70% (12.5 V), it will last for 1200 cycles.
Storing the battery in a discharged state is harmful (it will degrade), the battery must be stored in a charged state.
The battery can be charged with a voltage of 13.5 to 15 volts. The higher the voltage, the faster the charge. At the same time, the faster the charge, the higher the temperature. The higher the temperature, the shorter the battery life.
The charging current is also important. The maximum charging current is 0.3C.
The full document can be found at this link.
My router (optical terminal) consumes 15 W according to the data sheet, and it discharges the battery from 13.3 to 12.5 volts in 5 hours. Based on this, we can conclude that the actual consumption is lower. I can assume that when discharged to 10.5 volts, the battery will last for 8-10 hours.
