Markowb said:
I am going to be dry camping at a National Park for a week. I have a 29? Winnebago 5th wheel and a 1 year old Interstate 12V deep cycle battery. I will only be using the lights sparingly and whatever the Dometic fridge uses.
LED lights draw much less power than incandescent so it's worth the trouble to retrofit if you can.
I have a volt meter and have been told that the full charge is 12.7 and not to go below 11.9
12.78V at 77F. It will be a little higher when colder, a little lower when warmer.
The "not to go below" number depends on a number of factors. 11.9V even as a static voltage would be a fairly conservative cutoff. When the battery is being used, i.e. connected to the refrigerator and turning lights on and off the "not to go below" number is 10.5V. Some equipment might complain about diminishing battery voltage so it would be a good idea to test that out and know the low limit before you go.
1 About how many days will I have before I go from 12.7 to 11.9?
To realize all the storage the battery has, this would be down to 10.5V. So that factors significantly into your run time. Propane refrigerators don't draw a lot, mine is like 350mA at 12.5V. Your battery is probably in the neighborhood of 75Ah so that would be about 9 days. Lights can draw more but if they're not on for long that power average is usually pretty low too. A week might be possible with frugal use. Throw a solar panel on it or a few boost charges along the way and that would seal the deal.
2 If I connect the RV to my vehicle sitting idle about how long will it take to charge the battery from 11.9 to 12.7?
It's not a linear function, and alternators aren't the best at restoring the charge on a storage battery very quickly. It will bring up the "bottom" 2/3rds relatively fast but restoring the last quarter to a third can take a few hours. So you may have to be content with doing this a few times and restoring~70% a few times vs 100% once or twice with the charge time you get.
3 If I take the battery out of the RV and connect it to an average battery charger where there is an electric outlet about how long will it take to charge from 11.9 to 12.7?
Depends a lot on the charger. Some are only good for a few amps and would take a full day or more. A "fast" charger or RV converter will do it a lot faster but in this context you're still talking 4 or 5 hours to >90%.
4 I am open to any other suggestions.
When I had a popup camper I did the same exact thing you're proposing here. I didn't have a fridge though, just lights, radio and device charging. I'd either run the popup right off the car battery or using a wheelchair battery inside. I'd let the car idle a half hour or so in the morning and the evening and that covered my modest power needs. I also had a portable solar panel I'd throw on the roof to grab a few Ah during the day which didn't hurt.
What I'd want to know before I tried this is exactly what power the refrigerator and other devices used. I'd test the battery to see what it was capable of, and then have some idea how many hours/days I could realistically go between charges. I would charge at any opportunity I got regardless of how much was left, rather than run the battery completely down and expecting a speedy recharge.
Mark B.
Albuquerque, NM
