I saw this crazy article about how to build your own Uninterruptible Power Supply. I’ll not put a link to it as I think it is possibly the most irresponsible thing we could do. It came complete with warnings such as:
- The AC Output Voltage Can kill You
- The DC Current from the battery can burn you
- There is enough DC current in a battery bank to stop your heart
- Shorting batteries can cause blinding flashes, blow wrenches into splinters, even cause the batteries to explode and spray sulphuric acid and hunks of plastic everywhere.
And my favourite:
- A ring that gets between ‘hot’ wires can amputate your finger
An Uninterruptible Power Supply consists of three main stages:
- A source of power – usually a battery.
- A Battery Charger – to charge the batteries
- An Inverter – to convert DC power to AC power
This article is proposing that you buy all the bits an pieces and assemble them yourself. And while you’re on, make sure that you’ve selected the correct type of battery, make sure that the battery charger is adequate for the battery bank, ensure that the inverter is adequately rated, then connect it all together with heavy duty cable, and there you have it – your very own Uninterruptible Power Supply.
Why would anybody in their right mind do this? Never mind the inherent danger with somebody with a little knowledge putting high voltage AC and DC together, but you can buy off-the-shelf Uninterruptible Power Supplies with all the design worked out at a fraction of the price that it will take to build your own.
Another misconception is that people assume that you can just use car batteries for the UPS. There’s two drawbacks for this. Firstly a car battery is not designed for deep cycling – that is constant charging and discharging. It is designed for providing instantaneous power to crank your engine. A UPS battery is designed to be discharged slowly. Secondly, no appreciation of the voltage conversion has been taken into account. For example, I often get asked for a UPS to power an entire house. What is the power consumption of an entire house? Well, excluding things like electric showers and electric ovens, and just covering some lighting and some electronic systems we could probably get by with 3KVA or so. 3KVA is about 13 Amps of current at 230V, but to generate this from a 12V battery you will need to draw 250A! This is why 3KVA UPS systems usually have a DC battery string of 96V.
My point is, is that articles like this which may be of interest to the hobbyist are actually giving people impractical and dangerous advice. People who may be a dab hand at wiring a few things together probably do not understand the complexities of the Uninterruptible Power Supply and how the individual parts should fit together to produce a fully functioning system that provides you with the backup system that you require, and probably at a fraction of the cost they would otherwise spend.
