Unreasonable air conditioner plan?

[WalkaboutTed]

I'm in the middle of planning my son's solar system for his cabin.  I'm giving him our older 450ah (will be 24v) battery bank. He'll be working nights and is where the days are in the high 90s to low 100s, with cool nights (down into the 70s and 60s). So he needs air conditioning to sleep during the day. 

We just bought for our tiny house a Frigidaire 12,000BTU window air conditioner. I checked the a/c with the WattsUp and it draws about 158w on startup and about 115w running on high.  We tried running the a/c unit on Hubby's 450ah bank (12v) with a modified sine wave 1500/3000 inverter and it ran just fine,  including starting up from unplugged and "off". But we didn't run it long, just to see if it worked.

My son would be running it on the 24v system with a pure sine wave inverter.  He'll be getting a Victron 100/50 solar charge controller,  matched with 1400w of panels. I know 450ah of battery isn't much for running an a/c, but if he runs the a/c during the day with maybe several hours after sunset would that be too much of a burden? It doesnt appear so to me, but I may be missing something. This will be his starter system,  eventually he'll be upgrading.  Comments, please?
Thanks,
Ted

 

[Malamute]

 But we didn't run it long, just to see if it worked.

  Comments, please?
Thanks,
Ted

 How long is not long? They can take about 3 minutes before the compressor kicks in, which is the large surge power requirement, not just initial plugging it in and the fan running.

 Also consider getting a hard start capacitor on the AC unit, it eases the compressor startup power requirement. They are cheap and easy to buy and install.

 

[WalkaboutTed]

Wow, Malamute, thank you! I guess I didn't run it long enough! I thought those numbers were too good to be true. After four minutes, the compressor did kick in and the a/c consumption went up to about 900watts. Bummer!

Using the above numbers, with a 25% fudge factor for the inverter, that brings it to about 50amps an hour just for running the a/c (continuously) . Now my question is, if he just runs it during the day and an hour or two after sunset, will that setup (1400w solar panels and 450ah of battery) work?
Ted

 

[mjbeam64]

I'm running a 5000 BTU air conditioner in a van on 300 AH AGM batteries with 700 watts of panels parked in direct sun and it works fine. The solar fully powers the air conditioner when it starts getting hot around 10-11am, and when the sun gets lower in the sky the batteries are able to pick up the slack. The next morning the solar is able to top off the batteries before the air conditioner is needed again. Your mileage may vary.

 

[B and C]

So 225 Ah at 24 volts? Not sure. That gives you a little over a 100 Ah's to use and with the sun setting, the panels won't be putting out as much.

 

[VanFan]

I'm running a 5000 BTU air conditioner in a van...

Rear window mount?  Thanks.

 

[mjbeam64]

Rear window mount?  Thanks.

I use my van as an office during the day. I hung moving blankets from the ceiling in such a way that there is an air gap between the wall and the blankets. There is a 4 foot heavy duty zipper so I can access the cab area. So there is an area about 5' by 7' that I work in at a desk, enclosed by moving blankets. I have a Maxxair fan in the ceiling at the back of the van. The air conditioner is at the rear, mounted below the fan, the front sticks though a cut out in the moving blankets. I roll the front windows down about six inches and the Maxxair fan pulls in air from the cab, between the blanket and van walls to the back of the van, over the AC and out the vent. So far it is working well.

 

[WalkaboutTed]

Duplicate post, sorry

 

[WalkaboutTed]

So 225 Ah at 24 volts? Not sure. That gives you a little over a 100 Ah's to use and with the sun setting, the panels won't be putting out as much.

I'm not understanding how I only have 100ah available with a 450ah battery bank (four 6 volt batteries) at 24volts.
I thought I would have 225ah available.
But maybe if he pre-chills the cabin down when there's plenty of sun, would that help. Also, now my question is, will such a high rate of discharge (900w), damage the batteries as far as the Peukart effect?
If I could give my son specific instructions on how to make it work without destroying the batteries, it would be great if he could live with it. The air conditioner does have a timer.

What about overpaneling the charge controller so he can get more wattage in in the morning and evening when the sun is low? I've read that you can over panel up to 1.3X a charge controller before causing too much of a problem.
Ted

 

[tx2sturgis]

First, I would buy a lower capacity A/C, in the range of 5000 to 8000 BTU, and set it up so that it only has to cool the bed with maybe a nylon screened canopy around and over the bed to keep out the heat, and help retain the cool air, rather than trying to cool the entire living space.

And, lets try to get some better numbers.

Have you got a way to power the A/C unit from grid power with a kill-a-watt unit tracking the kwh's used for about 10 hours of run time when the place is in maximum heat?

If it is pulling 900 watts for 10 hours....nope. But if the duty cycle is say, 30%, that's like 300 watts for 10 hours...much more likely to work well.

 

[WalkaboutTed]

I noticed that the Victron 100/30 will do up to 880w of panels at 24v. So if I used two charge controllers instead of one, his system can be up to about 1800watts of panels. Let's work on that supposition?
Ted

 

[WalkaboutTed]

Okay, I just reset the wattsup with the unit until 2100 tonight as we have grid power in our plave. I have the a/c set at "eco"and 75F. I'll report my results when I have them.

Unfortunately, my son would not be willing to live with a smaller unit. Especially when Mumsy comes to visit and she can't tolerate such high temperatures....that would mean I couldn't ever visit him in the Summer during the day.
Ted

 

[B and C]

"I'm giving him our older 450ah (will be 24v) battery bank" is what threw me. Whatever the Ah rating of one battery in that configuration is what you have to work with. So you have found a 6V battery that is 450Ah? Most are 225 Ah. You will have a 225 amp battery bank but at 50% usable it will be just a little over a 100 Ah usable.

Batteries in series, the voltage goes up the amperage stays the same.
Batteries in parallel, the voltage stays the same and the amperage goes up.

 

[WalkaboutTed]

B and C,
It's four 225ah six volt batteries. Currently we have them in a 12v bank, but will be rewiring it into a 24v bank. Thats still 450ah, right, or not? I've only worked with 12v systems so far. So far I've missed a lot of details, I'm so glad I asked!

But since the batteries are already three years old, I can probably trust him to kill them sooner than later. But at least they should get him through the summer.

 

[XERTYX]

I'm not understanding how I only have 100ah available with a 450ah battery bank (four 6 volt batteries) at 24volts.
I thought I would have 225ah available.  

The 6v batteries are 225ah then? All 4 ran in series for a total of 24v? 

That would be a 24v bank of 225ah yes. I think they were referencing the 50% depth of discharge rule of thumb. To keep the batteries alive. So 50% of the 225ah is 112.5ah. So for about a 10% fudge factor around 100ah of useable capacity yes.

If ran 2 sets of series that are paralleled for 12 volts then 450ah with 225ah usable. But the puekert effect is gonna hurt more on 12v for a heavy load.

 

[mjbeam64]

Yeah, 125 AH (3000 watt hours at 24 volts) of usable power is going to be tough with a 1000 watt appliance. That's only 3 hours, probably a good bit less as drawing a lot of current on lead acid batteries will lower the number of AHs available even more.

 

[WalkaboutTed]

Hmmm. I noticed that on some 24v inverters that they have the option to start a generator if the voltage goes too low. Maybe we need to investigate that, but I have no idea where to start. It's starting to get mighty complicated.

 

[B and C]

If you can beg or borrow a clamp on DC ammeter, it will give you the real number of what the A/C draws from the batteries.

 

[bullfrog]

Just a thought. Is it possible to insulate the cabin or bedroom area? My foam filled bright white camper is 11’ x 7’ and has two beds. Where I am it cools off at night enough to stay cool in the camper till mid day with temps up to 90 degrees. Maybe dig him a cellar to sleep in. My dad did just that when he worked nights. It was both quieter and cooler. AC and solar is a difficult act to manage.

 

[XERTYX]

I think the key component is that the huge load will be during the day. When the sun is out. Personally I factor on 4 hours of sunlight for my needs. Many people factor on 5 hours of sunlight. I try and over budget under utilize.

What about running the big boy in a WELL insulated cabin 4 hours a day with full batteries so the solar is powering it. Then the rest of the time using the small 5k btu window unit air conditioner?

Do the math once you finish your test on shore power and see how much it uses. I want to say there is a maytag?? Energy star unit that averages about 300-400 watt hours. That's a lot on batteries but I guess it depends on how long he would run it.