My 94 Regency has two Walmart deep cycle batteries for the house --- how long will they last with no hook up if I run a couple of roof vent fans? I just returned from a Boy Scout camp out for a couple of nights and it was very hot -- 95 to 98F during the day and the unit would get very heat soaked before bedtime. It would have been nice to use the vent fans for cool down but I was concerned about battery life.
Does anyone elso have much experience with dry camping?

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Originally posted by Frank Strong: Does anyone else have much experience with dry camping?

That is all our Barth has ever done. I took the super-long 50 amp cord out to get more space in the bin.

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My 94 Regency has two Walmart deep cycle batteries for the house --- how long will they last with no hook up if I run a couple of roof vent fans?

Short answer: It depends.

Long answer: Need some numbers.

As to your batteries, are they six or 12 volters? If they are 12 volters, add their amp hours at the 20 hour rate. If the batteries are rated at cranking amps or CCA, they are not true deep cycle batteries, and can not be drawn down as far as true deep cycle batteries. Or as often.

If they are six-volters, the amp hour capacity of one battery is what you have.

For practical usage and decent battery longevity, you should draw down to no lower than 50% state of charge. You can go lower, but battery longevity goes away. It is a trade-off, like everything else. Some of this compromise is determined by available space. Some is determined by how long you want your batteries to last and how much weight you want to carry. State of charge is best determined by a power meter, but a hydrometer or digital volt meter works, too. Trojan says wait a long time before using a digital voltage to determine SOC, but a half hour with no charge or discharge works pretty well. Some this will depend on the age and history of your batteries, too.

OK, on to the numbers. Lets say that you have 200 amp hours available. That means that you can use around 100 amp hours, or a little more.

Now, you need an ammeter, preferably a digital multimeter that will read up to 10 amps. Connect it and measure the current of each device. Fans on all speeds. Water pump, stove fan, TV, etc. Now, estimate or log how much each device is used per day. Lets say you come up with 30 amp hours of daily use. That means in three days you will draw 90 amp hours out of your batteries. No Hay Problema. You estimates are likely to be a little off, so just keep a log for a night or too, and you will have pretty good duration to plug into the formula. Taking your notes in tenths of an hour(six minutes) makes the calculation more convenient.

If your usage is a problem, consider changing to more efficient appliances. Our original Kool a Matic roof vents were power suckers, so we bought a Turbomaxx and a Fantastic Fan. The TM is a trifle more efficient on low. Both have thermostats so they only run when necessary. Both are also quieter, less rattly, and close up tight. We also have the portable box fan that FF sells. It moves a lot of air for a little juice, and blows on us at night in summer.

My reading light is an LED array, and is very efficient, but Susan cannot stand LED color, so she gets a halogen MR11 spot unit that is pretty efficient, too. West Marine has adapters for the MR11s. Boat stores also sell the bulbs. I got mine from Boaters' World. Either one is way more efficient than the OEM bulbs. Many people have too many or too bright lights burning. Our Barth was made with 19 incandescent and 18 fluorescent bulbs in the ceiling and under the upper cabinets, not counting bathroom and closets, so we have a lot of flexibility in bulb selection. Each dual eyeball fixture has one "useful" bulb and one "mood" bulb. The "mood" bulbs are very low current consumption, and allow us to not trip over the dog. They also glow with a golden hue that really complements the cherrywood cabinetry.

With the available battery space in a Regency, you might consider more or better or bigger batteries. Golf cart batteries are the most for the buck, but if you have the space, L16s usually last longer, so their higher price per amp hour is mitigated somewhat. The figures I have collected on the life of L16s are all over the map, as the care given by the owner is a prime factor, but they seem to last longer. We had them in some of our equipment at work, but even though I was the Battery Nanny, it was a 24/7 operation, so they got abused when I wasn't looking. I am on year eight of my Trojan golf cart batts, just to give you an idea. The cheaper golf cart batteries might not last as long, but, being cheaper, might still be a good value. MWrench reported that Costco golf cart batteries did not stand up to heavy current draw as well as Trojans, so if you are a heavy inverter user, give that some consideration, too.

I believe a frequent dry camper should consider a TriMetric or similar. I believe one will pay for itself by allowing you to manage your batteries properly and easily. This will avoid the expense and inconvenience of buying batteries you didn't want at a time and place not of your choosing.