If you want to keep your computer running when the power goes out, you need a UPS. These units provide backup battery power to a computer so that you can save data, shut down the computer and avoid downtime. They also protect sensitive equipment from the dangers of blackouts and voltage surges.

They’re not the most attractive or convenient thing to have on hand, but they can be a lifesaver in the event of a power outage. You can hook a car battery up to most of them, and you’ll be able to power things like your TV, phone and laptop while you wait for the electricity to return.

But you need a lot of power to run one. And if you’re not careful, you could end up with a hefty lead-acid battery that takes forever to recharge.

Batteries that are rated to supply at least twice as much power as you’re using are the best bet. These are generally called “gel cells” – they’re cheap, they don’t leak and they have reasonable performance. They do have one major disadvantage, though: gas bubbles in the gel electrolyte can develop and cause damage to the battery plates if they’re not constantly removed from them.

This will eat away at your battery’s capacity very quickly, even if you just let it charge and don’t use the battery for long. This doesn’t mean you can’t add a larger battery to your UPS, but you should probably stick with something less than 35Ah for starters.

Buying a new battery is an expensive way to increase your UPS’s capacity, but you can also make your own. All you need is a suitable lead acid battery and some circuitry to convert it into an ups.

The first stage is to connect the battery to a charger circuit. You can use a normal DC charger or a power inverter. If you’re using a power inverter, you’ll need a transformer and a circuit to amplify the AC output of the charger.

Once you have this, it’s just a matter of connecting the inverter to the battery via a circuit that enables it to change over to inverter mode in the event of a power outage. The circuit uses a relay to flip its contacts and switch the battery to inverter mode.

It’s very easy to do this, all you need is a relay, a relay IC and some mosfets that switch the inverter’s input and output circuits. This simple circuit can be made by any new hobbyist, and you’ll have a handy UPS in no time at all!

Second, you need a way to switch the inverter back to charging mode if the battery’s discharge level reaches about 11V. The circuit is based on a pair of opamps, one set up to generate 200Hz pulses and another set up to break these into 50Hz pulses. This way you’re sure that the mosfets never conduct simultaneously.

Lastly, you need to find a good quality inverter that can handle your new battery’s load. These are usually a bit more expensive than cheap ‘off-the-shelf’ inverters, but they’re worth it in the long run.

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