Hi all. Please can someone tell me if this will work. I know that I am essentially trying to build a UPS and that these are available ready made. I would like to know if my idea will work or if there is a flaw in my idea that I am missing. I have some of the bits I need already so I'd like to use them.

I want to provide an ac current from a deep cycle 12v battery via an inverter. I have a 300w inverter and I will only draw about 130w so that is ok.

I will build a switch using a relay to control the UPS on/off. My plan for this part is to connect the relay coil (SPDT I have is rated for dc on the coil and 220v ac on the load side so it should be fine) to a 12v off-the-shelf transformer and this will be used to hold the relay in an open position as long as the mains/grid power is on. The other side of the relay will be connected as a switch on the positive supply from my 12v deep cycle lead acid battery so if the mains power goes out the relay switches to the NC position completing the circuit from battery to inverter.

I will also connect a battery charger to the battery so while the mains electricity on on the battery charges. This will keep it topped up or if the electricity goes out it will start recharging the battery once it comes back on.

I think in my simple view this will work....charger, battery, control switch connedted to mains to control the UPS, and finally the inverter connected to my devices. By the way...the devices I will be running are a small aquarium heater and a water circulation pump.

I'd really like to hear from anyone who can say if my idea sounds feasible.

Thanks.

 

Does it matter if the power briefly cuts out when it switches?

 

Hi Tom. No...that would be OK. It just needs to start up to provide power for water circulation and to maintain the aquarium temperature. So it does not need to be a seamless switchover.

 

Hi Tom. No...that would be OK. It just needs to start up to provide power for water circulation and to maintain the aquarium temperature. So it does not need to be a seamless switchover.

Then it seems like it would work. Be wary of inrush currents. When the pump shuts down for half a second or so, it will draw more than the nameplate power or current. Which could trigger your inverter to overload. It might be best to get a bigger inverter.

 

Hopefully the motor nameplate has a spec for "LRA" or Locked Rotor Amps.

This is the amount of current the motor would draw when the rotor is at 0(zero) RPM.

When the motor is first switched on, the rotor is usually at 0 RPM unless acted on by an outside force.

 

Thanks both very much....I feel more confident about giving this a go.

Inverter spec: it is a pure sine inverter. The pumps I am using are inductive pumps therefore pure sine inverter.

• 300 watts'' continuous power with 500 watts'' peak output
• Automatic alarm and shutdown protection against overload, over temperature and low-battery conditions
• Eliminates electrical interference that may cause annoying humming sounds if products are powered using a modified sine wave invertor
• Ideal for inductive loads such as pumps, power tools and refrigerators that cannot be powered by a modified sine wave invertor

 

PS...I also redid the math.....the heater is a 50w job...just the backup (my normal heater is 200w) and the pump is 18w. So only 68w total....and the heater only comes on when the temp drops so with summer on the way it will be less often.

 

That should be absolutely ok on the inverter.

 

Thanks very much everyone. I'll give this a go and let you know how it works out.

 

Get a relay with a mains voltage coil and you can skip the 12v walwart.