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Old 04-05-2015, 06:36 PM   #29
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I am posting a chart I have posted in the past, but don't know just where it is now. It may help answer your question about LED lighting and some other loads. My 2014 20' FC has only LED lighting.

Awesome list. Thanks a heap, idroba. Your measurements can't be too far off our 2012 19' Flying Cloud. Much appreciated.

Not that it makes a whole lot of difference but the group 24 Interstates in the trailers are aprox 84ah each, not 80 and 75 as has been quoted in this thread.
I know ,picky, picky!

Thanks, George. I appreciate the precision. It's very apparent from this thread that knowing what we're working with will help us stay out of trouble. I think I'll assume 80ah each, just to be on the safe side, and give us some cushion in the event we push the limits of the batteries again.

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Old 04-05-2015, 07:13 PM   #30
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I expect that there is a variation in the AH capacity of group 24 batteries, from brand to brand, and then battery to battery. I like 75 as a conservative number which will catch most of the variations. When it comes to batteries + or - 10% is probably about as close as you will ever get. The important thing to remember is that there is not a lot of energy stored in any group 24 battery, but with care you can live with it.

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Old 04-05-2015, 09:12 PM   #31
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Dropped wrench across battery terminals once. it turned bright red. Knocked it off with a hammer. Let it cool on the floor and when cooled it was sort of a blue color. it was the hardest wrench I ever owned! Drop it on the floor and it would ring a high pitch. Yes, the battery was fried.
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Old 04-05-2015, 09:18 PM   #32
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A friend and coworker of mine once got his ring caught between a truck fender and a wrench connected to the + terminal. His ring was literally fused to his finger and took surgery to remove it. A nasty reminder to lose the jewelry before working on cars.

"If the women don't find you handsome, they should at least find you handy." - Red Green
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Old 04-06-2015, 12:45 PM   #33
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@idroba, et al,
Hoping not to belabor the topic, this is what I think I’ve learned, applying the various posts, especially idroba’s chart, and comments on WindyJim’s post. We started with two fully charged Group 24 Interstates (conservatively 75 a/h each, or 150 a/h total). Our first afternoon/evening in Death Valley we used:

2 fantastic fans (on low 15 hours): (2 x .93 x 15) = 27.90 a/h
Refer w/fan (15 hours): ((.32 + .55) x 15) = 13.05 “
Bath fan (15 mins.): (.25 x 1.5) = 0.38 “
Lights (various, throughout the evening): = 5.25 “
Propane detector (15 hours): (.06 x 15) = 0.90 “
Pump (15 mins): (.25 x 3) = 0.75 “
Water heater (one hour) = 0.74 “

The next day we left the two fantastic fans running for 24 hours (bad idea); the refer with fan for 24 hours (necessary in DV, hottest spot in the nation for four of the six days we were there); we ran an electric kettle (1050 watts) off the inverter for 15 mins (25.50 a/h!!); and increased usage of the bath fan, lights, propane detector, pump, and water heater. My guesstimate is that we drew the batteries down another 83 - 85 a/h, or more than the equivalent of one battery. While it wasn’t summer, the sun was intense, so I’m guessing that we put back (a max of) 70 a/h that day.
So, in our first two days in DV, we drew down roughly 135 a/h, and recharged 70 a/h, for a total discharge of 65 a/h, or 43.33% of our battery capacity. Each day (and night) thereafter we used progressively less power, especially the fantastic fans, which we turned off during the day, then only ran one in the last three evenings. We could not cut back on powering the refrigerator, so it was the primary draw. We used a blender to make smoothies one day, but it only draws 25% of the power of the electric kettle (maybe 7.00 a/h total). I think the real damage was done the first two days in DV; after that, we never recovered our power.
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Old 04-06-2015, 02:23 PM   #34
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I find it's easier to work/think in watts watt/hours.

Especially when there are 5V,12V and 120VAC via Inverter devices around.

Your 84AH group 24 battery has nominally ~1KW/hr to go on and less in practice.

so if you have 2 batterues you have 2KW/hrs on the best conditions of the year and each 100W of nominal solar will probably only top up 0.4 to 0.5KW/HR of solar a day under good conditions adjusting for angles, etc. Or you can top it up same in 4HR of driving. (.4-.5KWH)

LED lights use very little, radio and such not too much, charging phones and devices, then fans, and then pumps (but pumps are on demand so not much unless you take an extended shower which you don't have power for anyways)

Your furnace fan is 4AH or about 50W when running. saving enough for that if cold and the fans and fridge fans if hot should be your main concern and not a problem if you DON"T RUN INDUCTIVE LOADS (Heaters/Incandescent lights of any kind, bug killers, whatever)

Inductive Heaters and Cookers will kill your battery. That's what gas is for.

The the AC inverter is also a gateway to using up your battery, it burns power just being on. If your main computing device(s) is USB charging(phone tablet) you'll be much better off than if you have a laptop,charging plugged into an AC, running off an inverter, drawing from your 12V !
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Old 04-06-2015, 05:44 PM   #35
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@ timhortons
I find it's easier to work/think in watts watt/hours.

Thanks, Tim,
That's a helpful way to look at it, and certainly one I'm more familiar with. Appreciate your input.
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Old 05-20-2015, 07:10 PM   #36
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Originally Posted by idroba View Post
Like most threads, this one has drifted, but if it helps people understand batteries and their use, that is OK.

One thing that many RVers have little concept of is just how little energy they have stored in their batteries. Let me try to put that into perspective.

Most Airstreams come with two group 24 batteries, with a storage capacity of about 75 amp hours each. An amp hour is what it sounds like. These batteries will supply a one amp load for 75 hours. They would supply a 5 amp load for 15 hours. Not to try to confuse things, but the more load you put on them, the lower the amp hour capacity would be. So, loading them hard, like running a 1000 watt inverter with a coffee pot would reduce their capacity considerably. But most RV loads are relatively small these days with LED lights and so on, so lets say that the 75 amp hour capacity is a good number.

With two batteries, you then have a total capacity of 150 amp hours, in theory, assuming they are fully charged and you can run them down to no charge.

But, that is a very bad idea, running down to no charge. With the charge varying between 40 to 100% you might get 300 to 400 cycles if promptly recharged to 100%. If you left them sitting at 40 to 50 % charge, never bringing them back to 100%, they soon will have a permanent capacity of 40 to 50%. In other words, damaged.

If you discharged them to 0% you might get 10 to 20 cycles before failure. If you ran them down to 0% and left them un recharged for a few days to a week or two, I doubt you would get 5 cycles from them. Flooded cell batteries hate to be left discharged. AGM batteries are somewhat better, but not much. This is battery abuse for any lead acid battery.

So, the common recommendation is to not discharge a lead acid battery to less than 40 to 50% of itís capacity, and then promptly recharge it to 100%. Prompt is a day or two, less than a week.

So, you have two 75 amp hour batteries in your Airstream. You donít discharge them lower than 40%. (you use 60%) Thus you have a capacity of 75 x 2 x .6 = 90 amp hours.

Lets put this into household perspective. We all get electric bills. The energy you pay for is listed in kWh (kilowatt hours, 1000 watt hours). Most households use 400 to 1400 kWh a month.

Your RV batteries were rated in amp hours, so how to convert? A watt is a volt x an amp. We had 90 amp hours of capacity and a 12 volt system. So the battery capacity of our two RV batteries, not used lower than 40% is 90 amp hours x 12 volts = 1080 watt hours, or a bit more than 1 kWh.

So, in your home you probably use 400 to 1400 times that amount of energy in a month. Think of 400 to 1400 pairs of 2 batteries sitting around your yard.

RV batteries have VERY limited capacity. That is one reason they are so often abused and fail regularly, and are replaced often. But if not abused, they can hold up quite well. You simply must not ask more from them than they can deliver, and replace it quickly. (recharge).

I hope that helps in your understanding of RV battery capacity.
Thank you! Using your formula I have been able to determine my maximum wattage of my panels.


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