Power cuts are common in many parts of India, and like everyone else, I was tired of my Wi-Fi dropping the moment electricity went out. I had an old 12V 7.5Ah UPS battery lying around — still in great shape. That’s when I decided: why not build my own DIY UPS for router and ONT?
In this blog, I’ll take you through the full journey — step-by-step, every problem I faced, how I solved them (with help from ChatGPT!), and how you can do it too. No advanced electronics skills needed!
🔧 What I Wanted to Build
I wanted a setup that behaves like a real UPS:
- ⚡ When AC power is on, the battery should charge
- 🔋 When power goes out, the router and ONT should switch to battery instantly
- 🔁 No switching, no relays, no manual stuff — just seamless backup
Also, I wanted it to be safe, simple, and cost-effective.
🧰 Components I Used
- ✅ 12V 7.5Ah SLA battery (from my old UPS)
- ✅ 14V 1.5A smart charger (auto-cutoff type)
- ✅ 1N4007 diodes (temporarily; later replaced with 1N5822)
- ✅ A 2A fuse (optional but highly recommended)
- ✅ DC barrel jack cables (you can cut and reuse from old devices)
- ✅ Wires and electrical tape
🪛 Step-by-Step DIY UPS Wiring Guide
Step 1: Prepare Your Parts
Ensure your battery has a decent resting voltage (12.5V or more). Strip the ends of DC cables and gather your charger, diode, wires, and fuse.
Step 2: Connect the Charger to the Battery
Connect the charger's positive (+) to the battery’s + terminal, and charger negative (–) to battery – terminal. This will keep the battery charged while AC is present.
Step 3: Add Diodes for Load Protection
From the battery’s + terminal, connect a Schottky diode (like 1N5822) in forward direction (band side goes to the router).
Step 4: Connect Router and ONT
Wire both devices’ DC jacks to the output of the diode and the – wire to battery –. Make sure polarities match to avoid damage.
Step 5: Test It!
Plug in the charger and turn on AC. Your charger LED should go red briefly, then green. Unplug AC to simulate power cut — router should stay ON!
Optional:
- ✅ Add a 2A fuse on battery + for safety
- ✅ Install a voltmeter to monitor battery health
- ✅ Put everything in a plastic or metal enclosure
⚙️ The First Confusion: Where to Connect What?
I thought the charger connects to both battery and router, but that caused confusion. After clarification, here's what I did:
- 🔌 Charger connects only to the battery
- 🔋 Battery powers the router and ONT through diodes
This setup allows the router to always receive power — from charger when AC is present, and from battery when it's not.
🔀 Why Diodes Are Important (And What I Got Wrong)
Initially, I tried using 1N4007 diodes, because they were available. But they can only handle 1A and drop around 0.7V — not ideal.
Still, for a temporary 10-day setup, they worked. I added them:
- 📥 From charger + → to router + (via diode)
- 📥 From battery + → to router + (via diode)
The white band on the diode must face toward the router — this was confusing at first, but easy once I learned it:
Plain side = power in (from battery or charger)
Band side = power out (to router)
⚠️ Green LED on Charger Even When AC Was Off?
This puzzled me! My charger’s LED glowed green even when AC was disconnected.
Turns out, the battery was sending power backward into the charger circuit — because cheap chargers don’t have reverse protection diodes inside.
This wasn't dangerous for short-term use, but once I install Schottky 1N5822 diodes, this backflow will stop.
🚧 What I’ll Improve Later
- 🔄 Replace 1N4007 with 1N5822 diodes (lower voltage drop, higher current support)
- 🔒 Add fuse and enclose everything in a small ABS box
- 🔌 Add voltmeter to monitor battery health
But even with this basic setup, it works great!
✅ Final Wiring Summary
Here’s the final connection layout:
- 🔌 Charger + and – → Battery + and –
- 🔋 Battery + → Diode → Router/ONT +
- 🔋 Battery – → Router/ONT – (direct)
Charger is always connected, and battery powers router silently during cuts.
📊 Pros and Cons of DIY Router UPS
✅ Pros:
- Low-cost — uses old UPS battery
- Seamless backup (no switching delay)
- Simple, efficient, and easy to build
- Customizable (add voltmeter, fuse, case, etc.)
⚠️ Cons:
- No automatic low-battery cutoff (unless added manually)
- Diodes can heat up if undersized — use Schottky
- Battery needs monitoring for aging
🧠 What I Learned
This project was simple but packed with small learnings:
- ✔️ Diode direction matters!
- ✔️ Not all chargers protect against reverse flow
- ✔️ You can use old batteries smartly with just ₹30 worth of parts
📦 Total Cost
If you have an old battery, your total cost is around ₹100–₹150:
- Diodes (₹5–₹20)
- Fuse (₹10)
- DC wires and jacks (reuse)
Even if you buy a new battery, it’s still cheaper than a branded DC UPS!
💬 Final Thoughts
I used to lose connection every time electricity blinked. Now, my internet is rock solid — even during load shedding. This DIY UPS is compact, silent, and works better than I expected. Huge thanks to ChatGPT for guiding me through the logic, safety, and every small mistake I made.
👉 If you're using a 12V battery and want to build your own UPS — just do it! It's easier than you think.