How to Build an Uninterruptible Power Supply

An uninterruptible power supply, or UPS, is crucial to protecting critical data that would be otherwise lost in the event of a power outage. Here's how to build your own uninterruptible power supply.

Difficulty: Moderately Challenging

Instructions

1. Choose your batteries. As weight generally isn't an issue, a UPS almost always uses lead acid batteries, due to their low price and high capacity. Sealed Lead Acid (SLA) batteries are easily found, cheap, and easy to use. However, wet cell batteries can give much superior performance. Due to their hydrogen venting and need for topping up, wet batteries should only be used by those familiar with their characteristics.

2. Select a power supply to charge the batteries with. Unless the charger is particularly sophisticated (and expensive), it will likely be a "float" charger that delivers constant voltage equal to the particular battery's fully-charged voltage. This means that you will have to manually top-up your batteries' charge by increasing the voltage for a few hours every 6 months.

3. Check to see if your power supply will allow a reverse current to run through it. This is what will occur when the power goes down. If the charger does not have reverse current protection, then you will need to get a battery isolator, either of the type meant for multi-battery vehicles or just a large diode.

4. Pick out an inverter, which will convert your battery's DC current to AC power. Ensure that both the volt-amps will be enough to run your appliances, and that the surge volt-amps are enough to handle the power spike many appliances require on start-up.

5. Get all your wires, cables and fuses that you will need to connect your system. Ensure that your power cable is thick enough to handle the amps. A bus bar will help you to organize your wires.

6. Put on your protective gear. You'll need protective eyewear when handling the batteries. Non-conductive gloves are an option if you want to be extra safe. Remove any jewelry.

7. Connect your system. Attach the charging cables to the batteries, ensuring that you've got the proper polarities connected. Plug in the charger to a household AC outlet. Hook your inverter up to your batteries, and connect your appliances to the inverter. Wait for the batteries to charge.

8. Test the system by disconnecting the charger from the wall.

Tips & Warnings

• Electricity can be deadly. Only attempt this procedure if you are a skilled electrician familiar with safety procedures.
• AC currents will easily kill you. DC currents can stop your heart, and a fully-charged battery circuit will certainly burn you.
• Batteries contain caustic chemicals. If they are wet cell, ensure they're properly ventilated , as hydrogen gas builds up during charging.