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Just jumping in on this thread.
What you need is a good energy audit of your rig. There are plenty of other threads and web pages about doing an energy audit. Basically you need an accurate means to measure the watts or amps every thing on your rig uses. All of this involves Ohms Law, that in short says that Watts = volts x Amps, or Amps = Watts/Volts. The voltage is fairly constant at about 12.2 to 12.6, depending on state of charge (SoC). So if you measure either of the two, you get the third value.
If you have a good battery monitor like a TriMetric 2030 or similar you have what you need. If not you could and should probably add one. There's no other good way to really monitor your batteries. Another option is to use a 12 volt dc amp meter like ESI 687 80 Amps DC/AC Low Current Probe/DMM. This meter is good down to a few milliamps. If you use an amp meter it has to be sensitive at fractional amp readings. The jaws on the meter clamps around a wire and measures the amps passing through it. For loads where you can't easily clamp around the wire you can use a fuse buddy device like: ESI 308B Fuse Buddy ATC Current Loop. This plugs into a standard ATO style fuse base and the current passes through a loop which the meter clamps around.
Either way, you need to ensure every 12 volt device is disabled except the one you are going to measure. Basically pull every 12 volt fuse in the entire rig and turn off any 12v DC breakers. If you don't know where all of your fuses are located this is a good time to find them. Some may be glass type fuses.
Make a list of every 12 v item and turn on one and only thing at a time, measure and record the watts or amps it draws. This also means turn on only light at time where multiple lights are on the fuse. It is likely you'll find that different light fixtures of the same type and same bulbs will draw different amounts. Then pull its fuse and go on to the next item. Don't forget items like your propane alarm and any other 12 volt powered alarm devices.
Once you have all this data, you'll make it into a table like a spread sheet, or just on paper if you prefer. Next to each item list the number of hours or decimal hours you think each device is used in a typical 24 hour period. Then multiple these two numbers to get either WattHours or AmpHours. Total these to to get a total power used in 24 hours.
If you have an inverter to produce 120 volt power from the 12 volt system, test each 120 volt device you would use in such a mode. Don't forget the inverter loses some power in the conversion process.
This will give you an accurate estimate how much 12 volt power you'll use in 24 hours. The big kicker is the furnace fan and is highly variable in usage. Keep in mind what others have said that for lead acid batteries you should only draw down the top 50% of the capacity at most. So if you have say 4 6 volt batteries in a typical series/parallel configuration you have total AHrs of 2 x the AHr rating of one battery. (if rated at 220AHrs each then you have 440 AHrs total capacity or 220 Ahrs of useable power). If your total from the energy audit is less than the useable battery capacity you may be good. You really do not want to draw down the batteries that much every day. I like keeping mine no lower than about 80% and most commonly about 97 to 92% in the morning.
The next consideration is the time required to recharge how ever much power you've used. This depends on may factors. Are you on shore power, generator and/or solar and if you're stationary the following day or on the move. This is all another discussion.
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Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & Coach House 272XL E450
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB
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