Battery Bank, Sizing and Capacity?

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Battery Bank, Sizing and Capacity?

Postby Rooftop Wind Turbine » Tue Mar 16, 2010 7:44 pm

>> Hello, I am trying to figure out how much back up time
>> I will get from the 8 battery bank, to know what exactly
>> that power will support in a black out. Also I'm trying
>> to figure out how large a battery bank that I may expand
>> to if the 8 batteries won't be enough back-up power.

OK, for starters, here's the basic way you figure out how much actual back-up time is in this 8 battery bank. Of course same method can scale up to figure out any size battery bank.

So, we multiply the amp-hours by battery voltage. The result will give you watt-hours. Then, we divide by 1000 for kWh because a kilowatt is 1000 watts. This is a good exercise because part of understanding all this stuff is understanding the relationship between watts, amps, volts and the like. If this doesn't make sense, read it through 2 to 3 times so you get it. This is really good fundamental stuff that will help you allot.

So, 8 interstate U2200 6v batteries are 232aH, aH is amp-hours. So that battery is actual charge capacity of..

aH x DCV = wH

amp-hours X DC volts = watt-hours

232 x 6v = 1392... 1392wH in each battery.

8 batteries x 1392wH = 11,136wH

a kilowatt is 1000 watts, so 11,136 divided by 1000 = 11.1kwH in that battery bank

11.1kwH in that battery bank as storage.

NOW, most inverters will shut down to protect, BEFORE depleting batteries completely, like a 24v system for example, at about 20v it will shut off and protect itself and your loads from low voltage. Also preserving your batteries from getting zero'd out.

So, for that I would say 80% or so is actually useable, some will be left and you can't get to it.

11.1kwH x .80 = 8.88kwK in this 8 battery bank of actual useable power stored.

DON'T FORGET. While the power is out, your wind and solar continue to keep making power in addition to the stored power.

The live charging per day will depend on whether you have clouds there, time of season, angle of roof to solar south? Panel ratings are "high noon, direct sun." Most of the time, you are getting less. You probably get less power over more hours, but you are lucky to get the equivalent of full power for 3-5 hours a day. Wind will vary as well, based on your site, orientation of roof, proximity to obstructions, and above all season and weather conditions.

Still, in most cases when the power goes out, it's stormy and wind does well in storms. Whatever the conditions, this working capacity adds to your ability to sustain a power outage.

At any rate, you will not be getting a battery bank that can carry your whole residence in power outage. That's REALLY expensive. What our goal is usually is an emergency subset of basic necessity. Usually it's like this. A RoofMill™ Kit kit with 8 batteries will sustain the following with careful management.

Refrigerator
Water Well Pump
Sump Pump if Basement
Emergency Lighting
Phone, TV, Radio
Sattelite, Cable, Internet

With this on a sub panel, as your backed up loads, when power goes out, you can manage these loads to sustain a power outage. The bigger your battery bank and system, the easier you can sustain an extended outage. The larger a system you put in, the more loads you can add to the backed up loads, which we call the GREEN CIRCUIT.

The more you want to run in a black out, the larger battery bank and wind/solar system you should put in. To run any heavy user loads like 240v cooking, heating, air conditioning, it will require that you measure this system and let us help you design it to satisfy your requirements. Hope that helps.
.
Sam

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