Help- Inverter n Battery Question

I'm not into electronics and don't know how to do the calculations.. Hope someone will be willing to help me.
I have an AC heating device that draws 500 watts intermittently. I'd like
to be able to use that on a camping trip using an inverter and a couple of Deep Cycle batteries.
How many hours of service can I get out of the batteries before they drop too far to use? Let's use a battery that has 800 Cold Cranking Amps as an example? Regarding the intermittent drawer from the heater.. I don't know what the OFF time is when it reaches temperature and starts to cycle.. but, I'd guess the actual ON time is what I'm interested in.
THANKS for your help
--
Bill Splaine, Healdsburg, CA
Some Of My Woodcarvings: www.picturetrail.com/chips
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hey Baldy, It works like this. It's not the "cold cranking amps" that count but the amp-hour rating of the battery.
So if the heater is 500 watts and the battery 12 volts, you find that you'll be pulling about 40 amps when the heater is on. Since the inverter has some losses (which also show up as heat so in that sense are helping you out) let's figure on 50 amps. Now you need the amp- hour rating of your batteries. For example, lets just say that each battery is 100 amp-hours. This means the battery can put out 1 amp for 100 hours or 10 amps for 10 hours etc. That means that with one battery you can run the heater for about two hours. If you have two batteries, it would be four hours. Now that is total ON time. The "off" time doesn't count as little current is drawn. With larger batteries the longer they can run.
Allow me to suggest that a deep-cycle battery is NOT a very efficient way to carry heat on a camping trip (unless it's with an ATV!) These babies are HEAVY! A chemical method (kerosene heater etc.) is FAR lighter for the heat produced. For that matter, did you ever consider not carrying fuel at all? There is some thoughtful logic behind the traditional CAMPFIRE!
Benj =====================================Baldy wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thank you very much for taking the time to help. The question is answered.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Baldy wrote:

Not quite, perhaps.
There are inverters and inverters.
Some are designed to run off the normal car battery. These often have circuitry that shuts the inverter down before the battery is discharged to the point that it couldn't start the vehicle.
Such inverters will only use a fraction of the stored AH in a deep discharge battery.
Of course this also has the advantage that the battery life will possibly be a lot longer. Fully discharging a battery and then leaving it in that discharged state is a fairly certain way of permanently damaging it.
--
Sue




Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.