DIY Beefier UPS Battery Back Up Project

[ron101]

With this season's storms and resultant power outages many of us were left with still, cold tanks waiting on hydro to turn the life support back on. I was one of the luckier ones who's power was restored after only a few hours but I figured it was nonetheless a wake up call to get some sort of back up power system in place.

There is a thread going in the Van Reefing Club forum that discusses things a bit. Basically there are two main options: battery power and generator power.

Battery power has the advantage of cost and automatic switch over. The serious disadvantage is run time. Most off the shelf consumer grade UPS's will only supply a couple hundred watts of power for about 5-15 minutes.

Generators of course can put out gobs of power relative to a UPS; enough to run a whole system, lights and heaters included. They can run as long as you have fuel so they will cover extended outages. Downside is cost and lack of automatic switch over (yes there are units that will do that, at the expense of another zero to the price).

My priorities were basic life support (circulation and maybe a small heater) and automatic switching. I want to ensure that the system is covered while I'm out of the house. So what I'm looking for is some sort of UPS with a much larger reserve.

I searched online and found a couple brief articles about using a computer UPS with deep cycle wet lead acid batteries. Consumer UPS's use a small 12V lead acid gel cell rated around 10 amp hours (Ah). The batteries are small due to packaging and weight considerations and gel style so they are spill proof. Both these considerations restrict the batteries capacity. So the idea is to get a 12V marine or RV battery and wire it in to the UPS. I picked up a 95 Ah deep cycle battery from Costco for $40, a second hand UPS for $15, and a couple terminals for $5.



The UPS is an APC 650 VA model; maximum output is 400W. According to the specs on all the UPS's that I have looked at. The duration of the power output is non-linear with change of output level. As in if you cut the power output in half (from 210W to 105 W), the duration will about triple. This model is rated for 37 minutes at 105 watts with its 12 Ah battery. If the same math holds for 2x25w powerheads and a 95 Ah battery, this setup should be good for about 14 hours.

Of course one can always add more batteries in parallel to add extra capacity, limited only by the UPS's charging ability. It might be possible to add in a dedicated charger like the one shown to increase the charging rate; I will have to see if it interferes with the UPS's battery metering circuits. Also more batteries would increase the need for ventilation, charging batteries give off hydrogen gas.

My goal was to get about half a day's worth of basic life support for the tank. Beyond that I figure I would be home and could use more crude methods such as manual stirring and heating water with a camp stove. Comments, suggestions, and questions are welcome.