Electrical engineering: low volt solar pannel/battery/air pump question.

This question is out of scope BUT- I thought I would give it a try:
I have a solar panel : ( http://sunelec.com/solar-panels/12v-solar-panels/sun-15-watt-solar-panel-12v.html ), currently powering up a 12v air pump (Measured Draw at 13.2volts is 2.4 Amp) to aireate a fish pond. I'd like to run the air pump 24/7 by introducing a battery to this sytem, but do not know how to set this up so it works automagically, after the setup.
There is no 110AC close by, just FYI, or I would not be using the panel.
Thank you kindly for reading this post, and considering it.
If no-one knows, any ideas as to what would be a good forum to ask this question?
Thanks again.
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I am puzzled by your data.
If the solar panel is 15W, 12VDC, then 15Ww/12V=1.25A.
So, how could you have measure 2.4A?
Have you measured it on another power supply or on a battery with 13.2V?

What you need is simple, but if you need 24/7 to work continuously and the air pump really takes 2.4A then you need a solar panel able to provide a bit more than 2.4A, maybe 3A or more. It is not enough your 15W, 12V solar panel.
What you need is a battery charger with DC input voltage from solar panel and in the same time to be able to switch automatically from solar panel to battery used as redundant, second source of power.
During the day the solar panel should provide direct current to the air pump plus current to charge the battery, that means 2.4A + charging current for the battery. Over the night, when there is no more power from solar panel, the circuit should switch automatically on battery.
Deepening where are you on the globe, how much sun you get and how long is the day, you may recalculate  the data below.
Assuming 12h night and 12h day, we may consider then the air pump to run on battery at least 12h, that means 12h*2.4A=28.8Ah the minimum battery capacity, but should be more.
As you r existing solar panel is able to provide only 1.25A, the air pump will not run as it suppose and actually the solar panel is overloaded. There is not enough current to charge also a battery.
You need a charge controller which together with the solar panel and the battery will do the job.
For example a battery 35Ah 12V will be enough:
The 100W panel with a charge controller:
These are only as example. Recalculating and choosing lower values for the power of the solar panel and charge controller will be a bit cheaper, because are different other brands and variants to choose.

Basically you start from your load, the air pump, you calculate the battery capacity, you choose the solar charge controller and finally the solar panel. Each time you have to increase a bit more the current/power in order to not stress the components to the maximum.
For example, as I considered above as battery backup for 12h, then I got 28.8Ah, which leads to a bit higher battery capacity. A 35Ah is good enough roughly for 14h. Then the solar charge controller may be something like:
Now, how big should be the solar panel? It depends how fast you want to charge the battery.
For example if we consider only 10% constant current charging of its capacity, then we get 3.5A and will take 10h for full charge. Do you have 10h sun on the sky? Let’s suppose yes. Then you need 3.5A for charging the battery and 2.4A for air pump, in total around 6A at 12V, which leads to minimum 72W solar panel. In the end is good one 100W solar panel, which at 12V can provide around 8.33A, let’s say 8.4A, from which we subtract 2.4A for air pump and remain 6A for battery to be charged.
With 6A we can charge a 36Ah battery in 6h.
Conclusion: you need a higher wattage solar panel, recommended at least 100W and a lead acid battery at least 35Ah to which you add a solar charge controller.
also - consider cloudy days too - do you have 100% of the days full sunshine? if not, that can lower the power output of the solar panel, so you may need a bigger one
There are couple more things to consider about your solar panel:
- you need to make sure you get enough light energy on the panel as long as possible  time during the day. For that you need to face the panel properly to the sky and find the proper tilt angle for your position on the globe. The links below might help you:
- you may boost your solar panel with more light, if it is not installed on the roof, especially when the sun is not anymore perpendicular to the panel if you lay down in front of the solar panel a mirror, but that does not help too much on a cloudy day:
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HardwareDudeAuthor Commented:
Thank you for your help. You are absolutely right, I copied the "Measured Draw at 13.2volts is 2.4 Amp" from posted user data on Amazon, not from a measurement I took.
This is the air pump product: http://www.marinemetal.com/products/12-vt-air-pumps/power-bubbles/item/power-bubbles
From that website: "12 V DCw, very low current +/- .46 amps"

In adition, the solar panel SAYS 12v on the name, but the specs are:
Watts       15      
Cells       36      
Panel Vmp       17.70      
Panel Imp       .85      
Panel Voc       22.0      
Panel Isc       .91
This data is greek to me, does it help you, does it change anything?
HardwareDudeAuthor Commented:
Are you sure is the manufacturer? Or is just another trader?
First line “12 V DCw, very low current +/- .46 amps”
That is not the way how you write the electrical parameters. This device has no proper made datasheet.
It is unknown to me “DCw”. I just suppose comes from “Direct Current working”. It would have been enough “12V DC”.
Then “+/- .46 amps”. Why + and -? Because  you can connect it either ways and is not important + and -? I have no other explanation. It would have been enough and better to write “0.46A” and then to specify that the pump can be connected regardless to + and – , if that is what is suggested by “+/- .46 amps”.  Usually when you refer to “+/-” in this way, then you refer to tolerance of the value.
Here are 3 more websites selling the same air pump and they claim it takes only 1/4A, which means 0.25A.
Now is 3:1 statements about air pump current, probably they copy one from another, because they are just sellers without technical background. They should make up their mind.

However, to be the safe with your solar panel, let’s consider that will take 0.5A.
Now, compared with your initial data, everything changes.
12h providing continuously 0.5A for the pump means 12h*0.5A=6Ah. That is a cheaper and smaller battery. Assuming that you want to charge it also in 12h, then you need 0.5A to charge it and additional 0.5A for the air pump, so in total 1A, 12V, 12W for the solar panel. But is good to not load it to 100%. Therefore your 15W, 12V panel providing 1.25A will do the job. You do not need another panel. What you need now is just the battery, I would take a 8Ah or even 10Ah battery, and the charge controller.
Regarding the charge controller, must be checked one which is able to limit the battery charge current to 0.5A. I might be able to check it later when I have time.
Her is a glossary for the solar panel parameters:

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HardwareDudeAuthor Commented:
This guy is awesome.
Regarding your last post above, I would go with 10Ah battery. The 14.5Ah is too big and you have no time to charge it with your 15W solar panel and in the same time to run the air pump.
Let’s just consider the 10Ah battery.
If the air pump takes 0.46A (let’s say 0.5A) and the solar panel can provide 1.25A, let’s say we take only 1A to not stress it to 100%, then we get from 1A only 0.5A to charge. If the summer time allows you 14h sunshine then you may charge it to 7Ah, but not more. So, it makes no sense a higher capacity.
If the air pump takes only 1/4A, which is 0.25A, then remains 0.75A to charge the battery, which in 14h results 10.5Ah, but I do not think you get 14h true sunshine, let’s limit ourselves to 12h or even 10h. So the 10Ah battery is good enough for your air pump and your 15W solar panel.
Remain only to find a proper charge controller.
I have no time to look now into the details, but here is the website with proper datasheet and operation manual of the charge controller which you mentioned:
Ok, I checked one more time.
From my point of view is not good.
You need one 12V which is able to limit the charging current to 0.5 or 0.75A and not 6A or 10A, because the solar panel cannot provide more than 0.5A or 0.75A for charging, depending by your real current of the air pump.
HardwareDudeAuthor Commented:
Wow, ok, I will try to find a current limiting charge controller. Hopefully such a device is available.
HardwareDudeAuthor Commented:
Viki- I am going to go with the second option as it makes the connections idiotproof since it has 6 connectors, all with a little picture, taking the guesswork out of who has to be connected to what.
I will report back results upon installation.
Thanks again. You are extremely helpful and effective. They must worship you at work.
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