Greetings, 3horn!
I've been playing with this stuff for about the past 10 years or so,
mainly in the avenue of making too much noise with pro audio gear out
in the middle of nowhere, powering it off of a car battery.
I assume this will be A/C powered equipiment, and not specialty D/C
powered equipiment.
Given that assumption, you have two problems. 1) How to get A/C out
of a battery, and 2) how big or how many batteries you need to sustain
that power for 3 hours.
The first problem is an easy one to solve: A DC to AC Power Inverter.
These are devices that covert 12 volts DC to 120 volts AC, 60 HZ.
There are two kinds on the market: ones that produce a modified sine
wave and ones that produce a pure sine wave.
Modified sine wave inverters have more noise on the line, and will
require extensive filtration to clean the power up for sensitive
electical components.
Pure sine wave inverters make an AC power that is most similar to the
power from a wall outlet.
Laser printers, in particular, are sensitive to 'dirty' power, and
will benefit most from a pure sine wave inverter.
A good description of square wave, modified square wave and sine wave
power output is at this page:
http://www.almac.co.uk/proven/PAGES/invert.htm
To determine what size you will need, just add up the power
consumption of all of your equipiment. If you are using a laser
printer, you may be looking at over 600 watts just for that. The
laptop and accessories could run 20 watts to over 100 watts.
Once you have the total wattage draw, take a look at some of the
following offerings:
Majorpower.com offers serious power inverters, some of the best I've
seen:
http://www.majorpower.com/inverters/?source=google
I particularly like this, the Pro Sine series:
http://www.majorpower.com/xantrex/prosine.html
They provide very clean AC power, with less than a 3% variance from a
60hZ sine wave. However, it may be overkill for your project; they
start at 1,000 watts and go up, as well as having a starting price tag
of over $700.
Less expensive power inverters can be found at dcacpowerinverters.com:
http://www.dcacpowerinverters.com/power_inverters.html
Also, NexTag offers a large list of power inverters and pricing from
various online merchants:
http://www.nextag.com/serv/main/buyer/OutPDir.jsp?nxtg=459437_C83ACF7A2CE79A3E&node=&otherForm=n&doSearch=y&advanced=n&searchnode=-1&search=power%20inverter
Now that you have the power inverter in hand, you need batteries.
Deep-cycle batteries will provide the longest service life and number
of charge-discharge cycles. A deep cycle battery will outlast a
regular car battery by a factor of 2 to 10 times or more. An
excellent FAQ on deep cycle batteries is found at the Deep Cycle
Battery FAQ:
http://uuhome.de/william.darden/dcfaq.htm
The time a battery will last is measured in amp-hours; how many amps
can be drawn for how many hours. If a battery is rated at 10 amp
hours, it can have 10 amps drawn for one hour, or one amp for 10
hours. What you'll want to know is how much amperage the power
inverter will draw at peak load.
Most power inverters do not list their amp draw at peak load, just an
efficiency rating. For example, the ProSine 1000 lists an 89%
efficiency. At max load, 1,000 watts, that would equate to a draw of
about 1100 watts from the battery. 1100 watts at 12 volts is 91 amps.
So, for a 3 hours use, you would need 275 amp-hours of reserve
capacity.
1000 watts / .89 efficiency = ~1100 watts.
1100 watts / 12 volts = 91 amps.
91 amps * 3 hours = 275 amp-hours.
One maker of deep cycle batteries is the Trojan Battery Company
located at:
http://www.trojan-battery.com/default.htm
They have very good resources there, as well as a battery picker that
can help you to choose the battery you will need.
I hope this has provided you with enough information to complete your
project. Good luck, and if you need further clarification, just ask!
Further Reading:
DonRowe.com Power Inverter FAQ:
http://www.donrowe.com/inverters/inverter_faq.html
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