How to calculate how long a car battery will last as backup

[StirCrazy]

5 am would actualy be the bar minimum I would feel comfterable running with in the winter. that will cover all my pumps and my heaters and my actinic lighting. the reason I need both heaters is that if I lose power I have no heat in my house so the aquarium heaters will be on quite often. and if this is summer then you can skip the heaters and add the chiller.

I am actualy looking into one of these for the house. just trying to decide which model.
http://www.generac.com/PublicPDFs/E21590SBY.pdf

Steve

[Willow]

if you need that much power steve then your going to need a generator. the batteries won't last long at all and with a generator you can still have stuff like a computer, tv...

[r_wellwood]

Now that would be a serious backup system. I would consider something like the CanTire system, but going gas? (The portion of Scottish blood in my veins is whispering cheap, cheap, cheap...)

If you get this setup, make sure to post some pictures. Heh, do a test and videotape the whole thing. I would grab some popcorn to watch.

RW

[Chin_Lee]

Originally Posted by r_wellwood

5 amps, seems like a lot of equipment could run on that. Wouldn't the idea behind the backup be to maintain temperature, water flow (PH or two, return pump from sump if heater is there), and areation? Maybe some minor lighting could be powered, but why bother with keeping MH, VHO, all PowerHeads, etc going?

Strange things happen randomly in nature, and a power outage for a Fish/Reef tank for even a day should be tolerable for the system (with temperature, basic flow, and aereation maintained).

I could see something heavy-duty being necessary on a larger/complex system if say a chiller was also involved (closed loop pump and compressor).

Exactly. I think our objective is to keep our livestock alive in the event of a blackout as oppose to letting them continue thriving. So to keep them alive, give circulation and warmth. I would keep one 6060 tunze which is 13 watts and a 300w heater just to keep the display tank water circulating and heated. I will probably set the heater a ~75 which is just a little below my norm.

So if its 6600 total watts, how long will that 6600 watts last? If I wanted to know how long my 13w tunze will stay on, what formula do I use to calculate time into that equation? (sorry i'm asking you all to do my math and research )

[Pansy-Paws]

Here's the generic formulas to use:

Let's assume equipment of 400 watts (or 3.3 amps @ 120V), so Steve's request is not far off. I'm also assuming that the heater is on 100% of the time.

3.3 amps x 120 volts = 400 watts

To keep this running for 8 hours,

400 watts / 12 volts x 8 hours = 267 amp-hours from the battery.

We also have some efficiency factors to consider,

- the typical inverter efficiency is 90%
- battery ratings are based on a 20 hour discharge, and they provide less energy output when discharged faster. Using a battery discharge curve, a typical battery might provide 85% if discharged in 8 hours (if discharged in 1 hour, the rating drops to around 50%).

With these corrections,

267 amp-hours / 0.90 / 0.85 = 350 amp-hours

.... much more than the Canadian Tire unit.

If we only have the 60 amp-hour unit, we don't even get 1 hour to discharge the battery.

i.e.

400 watts / 12 volts x 1 hour = 33 amp-hours
33 amp-hours / 0.90 / 0.50 = 75 amp-hours required

Further, Andy's concern about the sine wave is correct. Depending on the quality of the wave output (square wave, modified square wave, sine wave), some devices may not work at all and others (typically motors and pumps) will consume 25% more amp-hours than their rating, potentially running hotter, and shortening their life.

I'd definately check out the type of sine wave output you'd be getting.

[Chin_Lee]

Wow thanks Lyle. So if A+B=C what's D? JKing.

Thats helps a lot. At least now I know maybe a heater is not such a good idea but I would imagine that two Tunzes at 13 and 23 watts will last:

13 + 23 = 36 watts which equals 0.3 amps

So if the battery is 60 amp hour and factor in the efficiency factors, that battery really has about 46 amp hours.
60 amp hour * 0.9 * 0.85 ~ 46

So to calculate how long the two tunzes will run, the hours will be the variable

36 watts / 12 volts * h = 46

So breaking it down, it becomes:
36h/12 = 46
3h=46
h=46/3

Looks like that battery will run both my tunzes for about 15 hours. Wow my grade 8 math is coming back to me.

I think to keep my tank warm in the event of a prolonged black out, I will consider boiling some tank water with my gas stove attached to my BBQ and pouring it back into the tank (slowly of course)..... won't be easy but it will work.

Thanks again Lyle

[Quinn]

I still think a small gasoline generator is the way to go.

[G1GY]

Originally Posted by teevee

I still think a small gasoline generator is the way to go.

I have a 4 KW Onan generator in my moterhome that I would hook up to the house if the power ever went out for any lenth of time.

[StirCrazy]

Originally Posted by teevee

I still think a small gasoline generator is the way to go.

thats what I am doing when I upgrade the power to the house this spring. I am getting a 1500 watt gen from a friend to run the fish tanks. these are nice but most have a 2 to 4 hour run time be for you have to fill the gas tank so you can't really go out for dinner. this is why I am looking at the total house back up gen, if I am on holidays or work ect.. no worries, automatically senses the power loss and fires and after 10 seconds switches the house to back up power.

Steve

[mike miles]

The first thing you need to do is find out how many amps your pumps and heaters need to operate.then you need the power inverter as per amp draw from pump/heaters.then you need to find deep cycle batterys.
calculate reserve power and amps per hour.this will give you an estimated time your system will run and to be safe i would go .75%

Mike