That's a good question and I initially didn't know, but it was a test I needed to do.
The batteries I used are 7 AH sealed lead acid batteries. Completely discharging lead acid batteries can damage them and shorten their life. Its best if you don't discharge the batteries past 50%. Based on the chart below, 12.2 volts is about a 50% state of charge. So I decided to run the test until the batteries reached 50% state of charge. If the power was actually out, I would be happy to run the batteries into the ground and buy new ones if it meant saving my reef. For a test, I would rather not risk damaging the batteries. Could you please help me to understand what wire go where on relay.
Thanks
For the test, I unplugged the outlets so it would switch over the battery backup just like in a power failure. The batteries started the test at 12.8 volts. Two days later (51 1/2 hours) running continuously on the battery backup, the batteries showed 12.2 volts where I stopped the test. Extrapolating out the remaining 50% battery life, the pump would run for ~103 hours or just over 4 days.
I knew that running one DC pump directly off of DC batteries would produce very long run times. The uninterrupted AC power supplies or computer backups will never get anywhere near the run times with the same pump, even with a bigger battery. That is because they have a power inverter that changes the DC battery power into AC power. Then the pumps power supply converts it back into DC power to run the pump. These conversion steps waste energy and reduce the amount of power available to run the pump. For me, it makes no sense to spend more money on an uninterruptible power supply (UPS) that runs the wave maker for less time. That's why I chose to build my battery backup this way, and would do it the same way again.
If I was going to make a change, my return pump is also 24v DC and will run on a single 12 volt battery. However, at about 5 times more amperage draw. So my current batteries would run the return pump for 1/5 the time or about 20 hours. A size 27 65 amp hour AGM battery is available at Costco for $88 and would run my return pump for ~4 days again. Running the return pump would keep the sump from being stagnant as well as provide superior surface skimmer, filtration, and oxygenation. I'm not seriously entertaining make this change though. The only reason is it's very unlikely my tank will ever go 4 days with no power. If the power goes out, and I know about it, I get out a small generator that runs my tank, fridge, freezer and window air conditioner. Even if we go out of town for a few days, I have a neighbor come over and feed my fish once a day and check on the tank. So I estimate that ~30 hours of battery backup is probably the longest I would need. However, for LESS money, the much longer run time is a good piece of mind. It's also more reliable having less converters and inverters that may fail.
