Battery backup DIY

Hi all. Been long time since I've been on here and got a new little project I want to do.

I have a couple of 5v trigger relays left over from my Arduino fish tank controller project and want to make a battery backup unit for one of my power heads in the fish tank. I have just had a power outage and need to ensure there is at least a little bit of water movement and more importantly gas exchange within the tank.

The power head comes with a 12v 1a power plug.

I was thinking of something along the lines of the following two ideas...

My theory is that the mains power will keep the relay open and allow the 12v PSU to power the motor/power head. If the power fails, both the 12v AND the 5v will fall to 0 meaning the relay will switch to closed and the battery power can then pass through to power the fan until its stored energy is depleted.

I plan on using either normal batteries (Ie not huge car batteries or other larger 12v kinds, more of a collection of 1.5v ones in series)

  1. Do either of these look viable?
  2. What type and how many batteries would I need to run this. It won't have to be long, maybe just a couple of hours.

I assume that for 2 hours I would need a total of 12v and 2000mAh. I have a bunch of rechargeable 1700mAh batteries. If I linked 8 of these in series, would I get 12v and 1700mAh?

The easiest route is to use a charger that can be left on with your battery , and connect the battery to your Arduino. The charger keeps it charged and when the mains goes off the battery takes over .

I have just plugged the power head plug into a power reader and it shows as using the following:

Watts: 16.5w
Amps:0.134A

Looks like you are measuring on the 120VAC side, that won't give an accurate reading of fan power, is there a data plate on the fan showing Volts and Amps or Watts?

outsider:
Looks like you are measuring on the 120VAC side, that won't give an accurate reading of fan power, is there a data plate on the fan showing Volts and Amps or Watts?

Yes you do have a point. I suppose if I was looking at making a mains power back that dat would be right, but I'm not so thanks for pointing that out.

I don't have any data no and cant find any. And I guess I won't know without splicing the wires and measuring with a multimeter. :-/

Why don't you just use a UPS?

Riva:
Why don't you just use a UPS?

Firstly because I'm tight.
Secondly because I have left over stuff lying about doing nothing.
Thirdly like most people on here I like to and prefer to build things for myself.

However things have changed slightly. I have been looking into what someone suggested about trickle charging a 12V battery and having that power the fan... however I have discovered that the fans are actually 24V so I would need to link two 12V batteries in series to get the power but then I have to buy a 24v charger which are not as cheap and easy to come by.

UNLESS there is any way that anyone knows how to charge two 12v batteries in series using one 12v charger?