Backyard Aquaponics
A place to discuss aquaponics

Don't use a car battery if you want it to last very long, they are not designed to be run flat- only for short duration heavy discharge, and to withstand vibration. Running to near flat just a few times will leave it with much reduced capacity, but batteries designed for standby use can withstand hundreds of discharges to almost flat.

try an AGM deep cycle battery as Gungulla says "...can withstand hundreds of discharges to almost flat". If you buy a bigger one and only run it down 25 to 50% then the discharge and charge cycle can run into the 1000 to 1500 times. Aussie Batteries and Solar is where I buy my AGM batteries for my caravan and fish. They currently have a sale on with free delivery and they offer 5 year warranty on the batteries.
Even if you don't buy the batteries they have some great info

I'm looking at this again because we had another power cut on Wednesday when something tripped a switch on the power board.

Lucky I was home and managed to flick the switch and get the power back online but if we weren't home or were on holidays etc it could have been disastrous.

I've found this battery on eBay



How long could this power a 25W Hailea DC Air pump?

And what other bits do I need to buy in addition to the pump and the battery?

About 10 hours, but there isn't enough info there to be more precise. Also lacking is the number of cycles to 100% discharge, useful to know when you are using it for a run to flat backup.

Also required: wires, connectors and a washing machine/dishwasher pressure switch capable of switching at a bit under the pressure your current pump is delivering. If your air stones are 1m under the surface, the switch operating pressure will need to be under ~10kPa/1.5psi, assuming you aren't adding much back pressure with small air hoses.

Thanks Gunagulla! Is this the extra info you require?

I used an Eaton UPS (sine) and removed the small battery inside and strapped in a couple of deep cycle batteries. I only had to support 100W but one power outage we had it lasted for 5 hours. I got my UPS and batteries for free so it was a no brainer.

umm, nope

It's the 10 hour discharge capacity that I am after. Most reputable battery manufacturers give the capacity for a number of discharge rates, 20 hour, 10 hour, 5 hour etc.

That listing only gives the 20 hour rate, ie 1.4A, however, your pump will be discharging at over 2A, meaning the battery capacity will be significantly less, due to the Peukert Effect.

Running an AGM much below 50% discharge will see very short life plus as the volts drop the amps drawn can go up or load performance will drop off.
Be sure your comparing same performance for AC and DC air pumps, can be confusing. Make sure the flow is AT the depth of water your using. Normally its free flow out of the water.

Other option is a second pump the same as your main one and small inverter, although not terribly efficient the pump can be used if you main one dies.

I'm starting to think the UPS is just the better option for someone like me who doesn't understand batteries.

Won't I also need to buy a charger for the 12V battery? It looks like chargers alone are $100 plus...

That's pretty awesome! What is the minimum requirements for the UPS units to be able to handle this?

You can put a 20 - 40W panel and 5A regulator on the battery if you dont want to buy a 240V charger. Panel + regulator would cost less than $100.
A UPS is one more component that can fail, and they use power all the time, so it will cost money to run, although not too much for a small one. Whilst it may protect against the grid going down, it does not offer any protection against pump failure.

Ok. Interesting... I could be wrong - but I'm guessing that you have two air pumps and you run your main air pump continuously?

So the washing machine pressure switch wouldn't work if you were just using an air pump when the power goes out?

Yes, my 240V air pump runs all the time, by necessity with 400 trout in the FT.

The washing machine pressure switch will work just fine if your water pump runs continuously- just measure the pressure in the water pipe instead of the air pipe. If you only run it some of the time, the air pump will come on whenever it isn't running.

Oh cool! Didnt realize I could connect the washing machine pressure switch to the water pump...

Could I beg you for a diagram on how everything is connected? :notworthy:

You can beg, but it wont help

Trying to draw anything with a mouse pad is an exercise in frustration.

Basic description, electrical: Use ~2.5mm flexible (automative) cable, battery negative to negative terminal on air pump, battery positive to an inline 5A fuse then to NC contact on pressure switch. Common from pressure switch to positive terminal on air pump. That way the normal condition is for the backup air pump to run. When you blow into the pressure switch, the pump should turn off.
Mount the pressure switch so that it will drain automatically if any water gets into it somewhere handy with an air hose to a fitting on the water pipes that drain away when the pump is off, so there is no pressure in the line when the pump is off.

If you want to use PV panels to maintain charge in the battery, just wire the regulator according the the instructions. If using a 240V trickle charger (you should be able to find one for way less than $100), put it on a timer, so that it charges the battery say once per week for 15 mins, which should make up for self-discharge. If you use a battery of say 50AH and a 40W PV panel, that should just about keep the pump running full time, in case no one is home and the grid is out for an extended period.