DIY UPS for Routers: No More Internet Drops!

Power cuts are incredibly common in many parts of India. Like everyone else, I was exhausted by my Wi-Fi dropping the exact moment the electricity went out. I had an old 12V 7.5Ah UPS battery lying around—still in great shape—and I decided: why not build my own DIY UPS for my router and ONT?

In this post, I will take you through the complete journey. I’ll share the step-by-step process, the problems I faced, how I solved them, and how you can replicate this build. The best part? No advanced electronics skills are needed!

🎯 The Goal: A Seamless Backup System

I wanted a setup that behaved exactly like a commercial UPS, but tailored for my network devices. My core requirements were simple:

Continuous Charging: When AC power is on, the battery should continuously charge.
Instant Switchover: When the power goes out, the router and ONT should switch to battery power instantly.
Zero Intervention: No manual switching or relays—just seamless, automatic backup.

Most importantly, it needed to be safe, straightforward, and cost-effective.

🧰 What You Need

Here is the complete parts list. You can source most of these from old electronics or local hardware stores for a few rupees.

12V 7.5Ah SLA battery (salvaged from my old UPS)
14V 1.5A smart charger (ensure it is the auto-cutoff type)
1N4007 diodes (I used these temporarily, later replaced with 1N5822)
2A fuse (optional but highly recommended for safety)
DC barrel jack cables (cut and reused from old adapter cables)
Wires, electrical tape, and a wire stripper

🚀 Step-by-Step DIY Wiring Guide

Follow these steps to wire everything up securely.

1. Prepare Your Components

Ensure your battery has a healthy resting voltage (ideally 12.5V or more). Strip the ends of your DC cables and gather your smart charger, diodes, wires, and fuse.

2. Connect the Charger to the Battery

Wire the charger’s positive + directly to the battery’s positive + terminal. Connect the charger’s negative - to the battery’s negative - terminal.

Tip: This keeps the battery charged whenever AC power is present.

3. Add Diodes for Protection

From the battery’s positive + terminal, connect a Schottky diode (like the 1N5822) in the forward direction.

Important: Diode direction matters! The plain side is power in (from the battery), and the banded side is power out (towards your router).

4. Connect the Router and ONT

Wire both devices’ positive + DC jacks to the output (banded side) of the diode. Connect the negative - wires directly to the battery’s negative - terminal.

Warning: Always double-check polarities before plugging in to avoid frying your devices!

5. Test the Setup

Plug in the charger and turn on the AC. Your charger’s LED should temporarily go red (charging) and then green (full). Unplug the AC to simulate a power cut. If wired correctly, your router will stay ON seamlessly!

🔌 Understanding the Wiring Logic

Initially, I was very confused about where the charger should connect. Does it go to the battery, the router, or both? Here is the simplified logic that finally clicked for me:

The Charger connects only to the battery.
The Battery powers the router and ONT through the diodes.

This configuration ensures the router always receives power. When AC is on, the charger tops up the battery while simultaneously powering the router. When AC drops, the battery instantly takes over.

Here is a quick schematic to visualize the setup:

[Smart Charger] 
      |
      | (14V DC)
      v
[12V 7.5Ah Battery] --(+)--> [1N5822 Diode] --(+)--> [Router / ONT]
      |                                                |
      |-----------------------(-)----------------------|

⚠️ Mistakes Made and Lessons Learned

The Diode Bottleneck

Initially, I used 1N4007 diodes simply because I had them on hand. However, these can only handle 1A of current and cause a voltage drop of around 0.7V. While it worked for a temporary 10-day setup, they run hot and are not ideal for long-term use.

The Ghost Green LED

When I turned the AC off, my charger’s LED bizarrely stayed green! It turns out, the battery was sending power backward into the charger circuit. Cheap chargers often lack internal reverse-protection diodes.

To fix this, I used Schottky 1N5822 diodes—they have a much lower voltage drop, support higher current, and successfully block the backflow into the charger.

📊 Pros & Cons of a DIY Router UPS

✅ Pros:

Cost-Effective: Reuses old UPS batteries. Total cost of new parts is just ₹100–₹150!
Zero Delay: True seamless backup with no switching lag.
Customizable: Easy to upgrade by adding a voltmeter, a fuse, or an ABS enclosure.

⚠️ Cons:

No Auto-Cutoff: Lacks automatic low-battery cutoff (unless you manually wire a BMS or cutoff module).
Heat Generation: Using undersized diodes (like the 1N4007) can cause them to overheat.
Maintenance: You still need to monitor the SLA battery for aging and voltage drops.

💡 Final Thoughts

I used to lose my connection every time the electricity blinked. Now, my internet is rock solid—even during heavy load shedding. This DIY UPS is compact, silent, and performs far better than I expected.

If you have an old 12V battery sitting around and want to build your own UPS, jump right in. With about ₹30 worth of extra parts and a little patience, you can eliminate internet drops for good.

Enjoyed this build? Share this post with anyone struggling with annoying internet cuts during power outages!