Building an uninterruptable power supply

I'd like to build an uninterruptable power supply for an AC (110V) fan so that when the power goes out, the fan can continue running intermittently (say, 5 min every 2 hours, for 10 hours, for a total of 25 min). The fan is roughly 1/4 horsepower (186 W). Therefore, to run for 25 min requires 186 * 60 * 25 = 279,000 J of energy.

I'm thinking about using a bunch of 18650 batteries to provide enough energy. I also need an inverter of some sort to convert it to AC. Some kind of logic would also be needed to run the fan intermittently.

Can you give a high level guidance on what else I need and the general feasibility of this?

For the control of the fan I would try an SSR, Solid State Relay. That relay can be controlled by an Arduino,
Making 110 volt AC..... Wait for more helpers to join.

Use 12V lead acid batteries instead. The proper charger controllers and DC to AC inverters are cheap, efficient, have over-discharge protection, and are already on store shelves. For a fan, you can use the dirt cheap "modified sine wave" inverters.

My UPS uses 4 off 12V 12Ah lead acid batteries, a 50V PSU adjusted to 52.8V (2V2 per cell) and floated across the battery, and a 50V to 230V inverter. Simple.

Unless you need the small size and light weight of a lithium battery use lead acid.

I think the lead acid battery is a great choice.

Will the main electrical power be constantly charging the battery, and the battery is powering the fan through the inverter? That way, when there is a power outage, the fan can still draw power from the battery.

Is there a way to set it up so that the fan is powered directly by main power directly (bypassing charging the battery) as long as there is no power outage, and when there is a outage, the battery then gets kicked in?


So you have 4 12V batteries connected in series? What is the 50V PSU for?

I really like the lead acid battery idea. It easily has the energy capacity to run the fan for a long time without dealing with a bunch of Li-ion batteries.


Charging the 4 12V batteries.

My UPS uses a 100W solar panel and a Sunforce 10A charge controller for a 12V 135 Ah deep cycle power source. That and a 200W pure sine wave inverter have worked for several years now without a failure.

There are hundreds of inexpensive options along those lines!

The 50V PSU provides the power when the mains is on, supplying the inverter and keeping the batteries charged.

I like lead acid batteries because they don't need a fancy charge controller and they don't do nasty things if you abuse them. They are also fairly cheap. Their big disadvantage is weight and size, but in a lot of applications that does not matter.

One thing you should do is include a fuse in the wire to the battery, as close to the battery as physically possible. In the case of my battery the fuse is in the link between batteries 2 and 3 (so in the middle). Any decent battery can easily supply enough current to melt wires and create enough heat to start a fire. My batteries are also mounted on 2 sheets of plaster board both because it is fire proof and because it will absorb acid should the batteries split.

That is the definition of a "no break" or "uninterruptible" power supply

That is an "offline" power supply.

If you are only powering fans, that would seem to be acceptable for your purpose.

Now I'm learning a lot.

For "no break" uninterruptible power supply, wouldn't the batteries be constantly discharging and charging which would ruin the lifespan of the batteries?

And for the "offline" version, is there a way for the batteries to automatically kick in in case of power failure? I saw a Youtube video where a guy manually plugged in the battery after power failure, and this is not what I want. The ideal solution is to not drain the battery during normal times and have it automatically kick in during power failure. If that's not possible then I'll have to look into the "no break" solution more.


Not if the power supply/charger can supply full system power under normal operating conditions. Lead acid batteries can and should be kept at "float charge" conditions when not in use.

For that to be true there would have to be current flowing both ways in the wires to the battery, which is impossible. Either the batteries are charging, or they are discharging, or they are sitting there idle.

If you just float a PSU of 2V2 per cell across the battery then it will keep it charged safely, you then draw current from across the PSU / battery combination to supply the load. Use a PSU with enough current output to support the entire load. If the output from the PSU fails for any reason the battery will take over.

I am too lazy to reinvent commonplace technology:

Four times 12 V is 48 V and you need a slightly higher voltage to actually charge them. :face_with_raised_eyebrow:

That is clearly the whole point of the design. :grin:

Generally you have relays which monitor the incoming mains and on failure, do two things - start the inverter and switch the load from the mains to the inverter output.

It may be possible to use SSRs or for the control of the inverter DC power, a FET to do the switching.

The obvious answer was indeed, to just go buy the equipment designed for the job.

Mind you it is clear from the size of the device cited there, that it will provide the specified power for no more than a few minutes. The intention is that it sends a message to the PC causing it to shut down cleanly, not to continue operating as normal. :roll_eyes:

Years ago I had a UPS just like your design, but commercially made. As I recall it was 15KVA, 230 volts in and out, and powered a British-made pick-and-place machine. The least little problem with commercial power would shut it down. The UPS powered the machine and the mains power kept the batteries charged. No switching time. Yes, it was big.

Some of the reviews of that product on Amazon make grim reading.

I've been using APC products for decades with no complaints. The reviews mostly complain about the battery life. I've had some last only a couple of years and some lasted ten years. What people don't seem to realize is that they are supposed to give you a few minutes to shut down your PC normally in a power failure.

The model I pictured is running on my servers- NAS, MQTT Broker, plus some networking hardware. No displays or power-hungry desktops. It will keep my network and WiFi alive for about 30 minutes in a power outage. (Which are pretty rare here because we have underground utilities.)


Exactly. :+1:
If you want something that is going to supply normal mains load for even an hour, you need to spend big bucks.
For long backup times you need a UPS that has fan cooling, big heatsinks even if its MOSFET switching, and elaborate battery and power monitoring systems.
Because those sorts of UPS are designed to keep very expensive and vital equipment running.

Tom... :smiley: :+1: :coffee: :australia:

Do you still need relays for an online version where the battery is connected to the load at all times? If I understand correctly, the PSU plugs into AC main power and delivers DC output (say, 50 V), this powers the load through an inverter while also charges the battery. So the battery and the PSU is connected to the inverter in "parallel", such that when main power goes out, the battery automatically takes over without the need for any relays or ways of sensing when the main power goes out. During normal use, the battery is kept at "float voltage" and therefore discharges very little.

Is that correct?