Backyard Aquaponics
A place to discuss aquaponics

I'm feeling a bit silly but I suppose my excuse is I've never had much need to mess with most of this stuff before.

Here is the question I can't remember how to solve for.

I have an air pump. Rated for 100-120 input voltage with a rated output power of 44-58 watts.

For the ease of math lets call it 120 V and 50 W

Now I do know W=VA or for our use here W/V=A so 50/120=.4166667 A

So at 120 V this 50 watt pump is going to draw under half an amp.

My question is, when it come to inverting and running off batteries, how do I figure out all this? I think it's the watts that remains constant and the current that changes?

If that is the case then 50W/12 V=4.16666667 amps Is that right?

If so, I'll use the figure of 5 amps times how ever many hours of backup I want and then I should probably double it so I'm not expecting the batteries to drain down past half way. Is that the right way to do it?

Then I just need to figure out the best place around here to get deep cycle or at least decent marine batteries.

(I had originally intended to get a little 12 volt air pump but got talked out of it since it would provide only about 1/4th of the amount of air my tanks would really need in the case of a summer overnight power failure. (Not that we often get power outages for more than a few hours but we are in hurricane territory.)

Your pretty well spot-on with the calcs ,, just need to allow a little for power loss in the inverter ( which you already did by rounding up 4.16 to 5).

Yes also with battery ,, you want 8hrs back up ,, 8 times 5 is 40 amp-hours ,, so look for an 80 odd amp hour battery.

Cheers

Well seeing as how my Float charger is good for batteries rated from 5-125 amp-hours. I will aim for a battery near the top end of that which (if I find such) would give me about 12 hours of backup. Then I just have to hope that some one is around to do something if we have an issue beyond that.

Something I hadn't really checked. Will a float charger (meant for hooking up to batteries for storage) re-charge a battery if given long enough or am I going to need a real battery charger to re-charge things if I do use the backup.

Most Trickle chargers will charge at full rate if the voltage is detected low enough. So I would imagine it will charge your battery fine.

BTW, connecting batteries in series increased the voltage. connecting them in parallel increases the Ah... so you may only need 3 x 20Ah batteries, or 8x10... an 80Ah battery will hurt your hip pocket i Imagine.

Also, guess how long you want it to last, buy your batteries, charge them and test. If it doesn't last as long as you want, just add some more.

A decent car battery ( low maintenance ) depending on the space you have may be a good option , cheap because they're produced by the 10's of millions. $60 should get you a good one and it will be more than adequate for your needs. If you want something smaller , have a look at motorbike batteries once again very cheap because mass produced.

Float or trickle charger will be fine unless you have power outages every second day.


Cheers

I'm willing to spend a little extra for something that will survive longer (Spending $60 every other year isn't much cheaper than spending $120 every four years and I would rather not have to dispose of batteries too often either.) I was thinking a marine battery would be a little more deep cycle than a car battery and boating is a major pastime around Florida.

nother way of looking at it are you going to automate the fail safe if not just get a small generater[ here in aus $100.00] that will run air a small pump and a light or two

TCLynx , agree with buying quality to save battery replacement . Saying that , the $60 battery in my car is still going strong ( manufactured 8 years ago).
This is a situation where a solar panel is very suitable ,, used as a trickle charger. Maybe a $60 battery and a $60 solar panel would make it all green and fuzzy but also very effective.

Now that I think about it a little more , a car or marine battery is probably not the perfect one for this ( but would certainly do the job), as they are designed for large short outputs ( stating the motor). I'll have a look around tonight ,, during my LONG LONG nightshift at work

Car batteries are fine for 50 % discharges occasionally. They will last a heck of a long time in this application. Just keep them topped up, and dont discharge them often.

Agreed.
Draining car batteries totally, tends to cause the plates to delaminate, and then not hold charge.

I use the trolling motor batteries. They are deep cycle and you can run them down without damaging them. They are mass produced here in the states for fishermen who use a small electric prop to silently creep through the water without scaring the fish.

They would be good, but at what cost?

I'm finding some 90 amp hour marine batteries for the around $200-$220 USD range With free shipping. They are AGM SLA making them safer to ship and handle. I think the main drawback there is you can't fast charge a AGM battery.

What's up, where did the edit button go?

This place seems to have some good stuff here in the US, I'm sure I'll find some even better deals if I do a little more searching.
http://www.apexbattery.com/

As to the comment about a generator. This backup is meant to perhaps rescue the fish from dying while we are away for a vacation or if the power goes out all day while we are working or at night once we have a full load of fish. It also includes a low water pump shut off and will kick in the air in this situation too. (This should save the nice new pump from burning out if we have a major water leak somewhere.)

An inexpensive generator would require some one to be here to start it ans switch over pump power to it. That might be fine when we are here or if we are suffering an extended outage due to hurricane but it wouldn't actually be a backup system.

A nice big generator that could automatically kick in and power selected circuits for the house would be nice but those are generally pricey systems.

Looking at that battery link, it looks like I could get more than enough battery power for at least 8 hours plus a solar battery charger for under $300.