171 vs 177

 

Batteries will accept the biggest charge during their "bulk" stage, slurping down the amps in just an hour or so, which takes you up to 20% DOD.  When charging from a generator usually you only want to get to this point.  Trying to get through the "absorbtion" stage to get to the final "float" stage will take many, many more hours and is a waste of fuel.  If your trip is less than a week long, shoot for a bulk charge every other day or every 3rd day (depending on your current DOD), then do a full charge at home.  If you'll be out more than a week you should try to get it above 10% DOD at least once a week to prevent damage.

 

This is an excellent FAQ which explains all you want to know about lead acid batteries:  http://www.windsun.com/Batteries/Battery_FAQ.htm

 

To find your DOD let the battery sit for a few hours - no charging or loads - then take a voltage reading.  Look at the chart in that FAQ to find your State of Charge (SOC is exactly opposite DOD, so 70% DOD = 30% SOC).

 

Sorry I don't know how many amps the furnace uses, a quick look online said anywhere from 4 to 11 amps while running, figure on a 50% duty cycle so anywhere from 2 to 5.5 amps-hour.  I took a lot of measurements of energy useage of things in the 'pod and didn't check the furnace fan (http://www.rpod-owners.com/forum_posts.asp?TID=1723&PID=14737&title=creating-an-instruction-maintanence-manual#14737)

 

Since you have two batteries in parallel you'll experience something called the Peukert Effect.  Bottom line, two lead acid batteries in parallel have more energy than two of them used separately.  If your batteries are rated at 100 amps @ 20 hours, two of them is 200 amps total, or 100 amps if you only go to 50% DOD.  However, in reality you'll get slightly more than 100 amps.  Maybe 110 or 115.  Here is a good link with explanations of the Peukert Effect, and a calculator which you can download.  http://www.smartgauge.co.uk/technical1.html