DIY UPS (Emergency Battery) for Routers So It Won't Turn Off When the Breakers Are Off

Hi, today we are building a UPS or backup battery for the router / modem so that it does not turn off if there is a power failure at home.

You can find this article also on the PCB way community here

Materials:

• 3x 18650 cells
• Holders for 3 cells (3D printed)
• 15cm of wire (two of different colors)
• Male DC connector
• Female DC connector

Tools used:

• Tester
• Welder
• Tin for soldering
• Vice

The system is very simple.

As you know a single 18650 cell goes from a voltage of 3V when it is at 0% to a voltage of 4.2V when it is 100% charged

So if we put three fully charged 18650 batteries in series we will have a voltage of 12.6V when it is at 100%, so little more than the voltage at which my modem works which is 12V.

By connecting the material as shown in the photo, the modem will draw current from the wall adapter and, only if this is not available, will the modem draw current from the batteries until the power returns.

For those who write to me that it is not healthy to make a lithium ion battery pack like 18650 cells work without a Battery Management System (BMS) I answer that they are absolutely right.

The cells, despite being made by the same factory, at the same time and under the same conditions of temperature, humidity etc. however they can be chemically different.

Given this difference that they can present, in a battery pack with cells in series it is not right to let them work without a BMS with a balancing function because you could risk an overvoltage or undervoltage in which the battery voltages rise above the level relative to the range 0 % - 100%.

But I'll tell you one thing ... almost every BMS you've tried in your life didn't have balance. Yes, because even if on the site it says that they have the balance, it is not true! Because having a 30 milliamp balance (like most cheap BMS) is the same as not having it.

Furthermore, the modem will charge the battery up to 12V and not 12.6 so even if there were any voltage excursions they would still probably be within the limit of 3.00V and 4.20V

Check that the cells have the same voltage

Before starting with the construction phase it is essential to start with the balancing of the cells.

Although I said earlier that it is not essential that the cells are perfectly balanced, it is still very important that when we go to create the battery pack they all have the same voltage in order to start from a stable situation.

How to balance the cells (if they don't have the same voltage)

If the cells do not have the same voltage, you can balance them in several ways. The first way is to connect them to a power supply and set 4.2V with all the cells and the second is to use an object that we created in this article and that you see in the picture that does it automatically and with signaling LEDs and with constant current.

Arrange the cells

To arrange the cells one after the other like all other classic 3s1p setups, I used a 3D printed cell maintainer from this link . If you do not have it or cannot print it, you can always hold the cells with some masking tape or Kapton tape.

Obviously, since the cells are in series, they must be arranged by spacing the poles.

Solder the cells

Once you have positioned the cells you have to weld them in series (therefore a positive with the negative of the next and so on). As you can see from the video, I welded them with a normal soldering iron, slightly scraping the surface to make the tin adhere.

ATTENTION - This method is highly not recommended as the cells are not predisposed to receive such a high temperature and this damages them. However, since it was only seconds and that the device must not have high performance, this time it is okay but be aware that it is absolutely not recommended.

Check the polarity of the modem power supply connector

Before connecting the battery pack to the modem, make sure with a multimeter that the positive is in the inner (middle) part of the connector and the negative is the round outer part (as in most cases).

Add female and male connectors

Once you have checked the polarity of the modem, connect the female connector (which houses the male connector of the modem) to the battery pack and then continue connecting this to the male connector that must enter the modem.

Done! Now when you unplug the modem (perhaps because you need it for a short period of time) or the power goes out, you will continue to have internet on your laptops and phones.