I've got a small aerator running for a fish pond but its only solar powered and now its coming into winter I'm thinking of adding a battery to it so it can run for longer (also means during the summer it should hopefully run over night)

The specs on the pump are:
Peak-power: 2.5W, Max-power voltage: 7.68V, Max-power current: 325mA

Is something as simple as the circuit below enough for it or do I need anything else, the issue I can see is that both the solar panel and battery will provide current during the day so that may be a problem?

Thanks

 

Is this your first winter with the pond? Fish need an air hole to let out the "bad air" while you pump in nice O2 rich air. For a small pond just about any air pump will do. Note: if the hole ever freezes over, don't chop a new one: use a (battery powered?) drill to cut another hole. Banging on the ice is not a good thing for the fish.

OK, so .325A is nearly 8 AH a day off a 6V battery. Guestimating 4 hours of sunlight to get the daily charge yield a panel of 8AH/4H = 2A; 2A * 6V = 12W,so a tiny panel is able to do this.

Batteries shouldn't be let to drawn anywhere near full discharge, 1/4 to 1/10 is more common. So a 32AH to 80AH is needed. Perhaps more as I assume this battery is out in the cold by your pond.

A simple diode is all that's used on solar garden lights, but those have a careful balance between the cell's output and the battery. You need more going in then out just to take care of cloudy days so a better charge controller is called for; a simple PWM type would do fine, you will find inexpensive units on EBay. Do note these may not survive a winter.

 

http://www.ebay.co.uk/itm/10A-12V-24V-waterproof-solar-charge-controller-regulator-for-boat-yacht-marine-/111003170652?pt=UK_Gadgets&hash=item19d84df75c

Would something like that do and will that switch between the solar panel and the battery, or is that just for charging the batteries?

 

My first reaction is to never advise someone to charge a battery with just a diode. I'm told that you can do this with Lead-Acid batteries but that they don't like a big depth-of-discharge (DOD), which Ernie eluded to by suggesting a large capacity battery.

If you do take the lead-acid approach then I too would suggest a 6V battery with a 6V/~2.5A solar array.

I wouldn't suggest any other battery chemistry for what you're wanting to do without some sort of charge control.