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Old 07-24-2021, 02:38 PM   #1
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Solar battery sizing to run portable air con unit) cooling mattress pad

Hi,

I am looking at a 2008 34ft Newmar coach so well insulated.

If I wanted to run a portable air unit and a heating/cooling mattress pad how much Lithium/solar and inverter to run both off batteries for at least 1 night.? I would be ok with maybe an hour of generator in 24 hours but would rather not.

I also might take my current coaches 440ah of perfectly good Lifeline Agm,s and have them as and backup bank on a manual switch.

I will be dropping close to 80k on the coach so a couple of grand more on the loan is not a big deal for more of each.

Thanks,
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Old 07-24-2021, 02:56 PM   #2
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Figure maybe $10,000 to do what you want and even then it might be iffy.
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Old 07-24-2021, 03:16 PM   #3
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Figure maybe $10,000 to do what you want and even then it might be iffy.
That's doable. Even 2 hours would do me for air. Just enough to cool the coach before bed. We now sleep at home at 74f using the cool mattress pad. So say from 80f to 74f and then maintain the temp.

If it's over 80f at night I will be in a camp ground on 50amp :-)
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Old 07-27-2021, 04:07 PM   #4
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This question is impossible to answer as it is - you need to know the wattage of the things you want to run, and an air conditioner is probably the most power-hungry of all. I've never heard of a cooling mattress, so no clue what that would want for power, but it should have a rating sticker on it somewhere - Watts equals volts times amps, so you are going to get at least 1 of those on a rating sticker, and can usually learn the second by the power cable, then you can calculate the third if it isn't wattage. Watts never change, voltage and amperage vary opposite each other.

Then you need to know by time, and finally... The type of batteries. Unless you have VERY low power requirements, "one hour in 24" of generator is not going to do you much of recharging, solar systems can give you whatever the power of your array is for about 5 hours per day (for calculation purposes) but certain battery chemistries - namely anything OTHER than lithium - cannot charge at max-rate for their entire charge.

Most battery options charge in an 80/20 method: 80% of the charging can be done in 20% of the total time.... But that last 20% WILL take 80% of the total time! Lithium don't do this, you can slam them at full charge rate right to the finish line. Solar can only charge batteries, so if there's no place for the power to go, then the panels aren't capturing anything useful even in full sun. Lead chemistry batteries also should not be considered according to their ratings as "I have that much power available" because that will damage them to drain them fully. Trying to size the system to only need 50% of the rated capacity will keep the batteries healthy, and will give you some reserve since you CAN safely go below 50% remaining, but not below 20% - that WILL damage the batteries.

So if your need is 200 watts (just example numbers) continuously per hour for 10 hours - that is 2 kWh or 2000 total watts. A 200 watt load into a 12v source (with no losses) is a 16 amp load. Most batteries are rated with a "reserve capacity" time, that is how long the battery can supply a 20 amp load before it is depleted. For every battery you add to the array, the load is divided equally among them. I have 8 GC2 golf cart batteries, for an array size of 880 total amps (at 12 volts) or 10,560 total watts. That's TOTAL, running them dead. I don't want to do that, so on a theoretical 200 watt continuous load (about what my tv and AppleTV and local network setup uses) I can watch for 25 hours down to a 50% rate. In practice it doesn't last this long b/c there are always power conversion losses to the equipment.

So how much power do you need, and for how long?
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Old 07-27-2021, 04:34 PM   #5
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Thanks for the reply.

I had initially thought a 5000btiu air conditioner but decided against it because of space. I definitely won't be trying to run a roof one.

So this is now canceled

Thanks,
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Old 07-27-2021, 11:48 PM   #6
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The power difference between a roof air and a store-bought portable isn't that much - and the portable will deliver far less actual cooling for the power used. They rarely seem to live up to their ratings.

What MIGHT be better on power is a ductless mini-split, but again the power considerations are that you aren't likely to run that on an inverter and battery for long - certainly not with any lead chemistry batteries, you'd need a ton of them (quite literally) to get any decent runtime.
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Old 07-28-2021, 05:58 AM   #7
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Quote:
Originally Posted by geordi View Post
The power difference between a roof air and a store-bought portable isn't that much - and the portable will deliver far less actual cooling for the power used. They rarely seem to live up to their ratings.

What MIGHT be better on power is a ductless mini-split, but again the power considerations are that you aren't likely to run that on an inverter and battery for long - certainly not with any lead chemistry batteries, you'd need a ton of them (quite literally) to get any decent runtime.
Thanks Geordi,
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