Solar Panel input vs. Battery output Question

[featherliteCT1]

Assume the following:

1. a 250 amp hour, 12v battery bank, fully charged,

2. an array of Solar Panels, a charge controller and an inverter capable of outputting 50 amp hours,

3 a 50 amp hour, DC load (say, a 115v AC air conditioner).

Question: Will the battery discharge relatively quickly because the battery cannot accept the potential 50 amp input as fast as the 50 amp draw?

I am asking this question because I was wondering whether it would be practical to run an air conditioner under the above assumptions.

I have plenty of space on the roof of my trailer to install enough solar panels to output 50 amps, however, I do not want to install more batteries.

[John61CT]

First off, fix your units and terminology.

Battery 250Ah, correct.

Solar panels and SC, you mean rated peak 50A?

An inverter is a load, nothing to do with the solar, but 50A @12V would be 600W rated, that right?

An AC aircon is only a DC load if run through the inverter, also Ah not relevant but is amps at 12V what you mean?

If an input source of 50A is active same time as a 50A load, then nothing goes into nor out of the battery.

The scenario you outline would mean you could only run the aircon when the solar is putting out enough to power it without help from the bank.

Fine, but I would not call it practical.

Use a genset to power aircon.

Lots of solar + lots of batteries, might reduce runtime per day of the genset, by a little bit.

LFP batteries would help a lot.

Maybe $5-8 grand to do it right for a hot location.

Best is to move to cooler ground, park in the trees, let your body acclimate.

[RJ Howell]

Question: Will the battery discharge relatively quickly because the battery cannot accept the potential 50 amp input as fast as the 50 amp draw?

I am asking this question because I was wondering whether it would be practical to run an air conditioner under the above assumptions.

I have plenty of space on the roof of my trailer to install enough solar panels to output 50 amps, however, I do not want to install more batteries.

Biggest issue with an AC (once past the battery bank) is the startup surge. Get past that and you have a shot. Unless you're in a area that doesn't have much clouds, the array will require being much bigger that the require usage ah, with clouds (trees or or obstacles) then even double the size. The battery bank really does need to be increased to meet the new demand. The batteries are supplying the power to the AC unit, not the solar panels, they are replacing the power drawn from the battery. You will be beating the hell out of those batteries.

I've been reading about folks going the mini-split route for AC. Much lower draw, but.. you need the space to mount the thing.

My personal opinion is this will be tough (and expensive) to accomplish on battery.

[flboy]

You can run it with the right size inverter, but need to watch state of charge on battery bank. There are a lot of variables so "mileage will vary". I have 800W of solar panels and 425aH battery bank with 3000W pure sinewave inverter. I can run my 8000BTU AC that uses about 630W when compressor is running in full sun for many hours. However, I still have a draw on batteries since the Solar panels are not 100% depending on sun's angle to panels and the inverter is also not 100% efficient. Last week I was out inthe CTC doing some work with good sunlight (but partly cloudy) and I ran it for 3 hours off inverter and batteries were at about 75% and since I have Lead Acid batteries, I would not go below 50%.

Bottom line, it would be very costly to buy an array and batteries to handle the AC 100% and then you still have the issue at night with no sun or on cloudy days.

My approach is to be able to run off solar for a few hours to keep camper cool for afternoon naps in rest stops or parking lots without using the generator. Also for keeping the dog cool while I am out for a few hours shopping, eating, or etc....

That being my intent, I also designed system capacity to run off a Honda EU2000i generator so if I am present and want to run at night and charge batteries or etc..with no power issues.

You need a generator to backup solar in any case as the sun doesn't shine at night, you can have a cloudy day, or the only campsites you can get are in partial sun or shade.

Good luck.



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[flboy]

BTW.. to answer this question "Question: Will the battery discharge relatively quickly because the battery cannot accept the potential 50 amp input as fast as the 50 amp draw?" , the batteries will respond to (or regulate) any surges, but in theory, if the Solar Panels were indeed putting out 50A and the AC is using 50A (includes inverter and any inefficiencies), the battery would not be discharging or charging. If the panels were putting out 40A, then your batteries would be discharging at 10A. So if you look at your batteries as supplying all the current needs, the solar panels will help slow down the discharge of the batteries based on how much current they are supplying. If you had more than 50A available from the panels, in theory, they could be charging the batteries slowly while running the AC.

So bottom line... you should always look at it like you are running off the batteries which will discharge either faster or slower depending on solar panel contribution. Does that make sense?

[featherliteCT1]

John61CT, RJ Howell, and Flboy,

Thanks for the replies.


Please bear with me but here is what I am thinking (keep in mind I get confused easily):

Note: all numbers approximate and rounded.

If Flboy’s AC draws 630 watts/12= 52 amps per hour, then in 3 hours it would draw 157 amps (157= 52 x 3).

If Flboy’s solar panels of 800 watts/12= 67 amps per hour operated at 100% efficiency= 67 amps, and if there were no more efficiency losses in the system, then in 3 hours the solar system would theoretically produce 200 amps (200= 67 x 3).

Note: I use the term “solar system” to mean the solar panels, charge controller, inverter and wiring.

Consequently, in that 3 hour period, the solar system theoretically would cumulatively output 200 amps and the AC would cumulatively draw 157 amps.

If Flboy’s battery of 425 amps used 25% of its capacity, then in that 3 hour period the battery discharged 106 amps (106= 425 x 25%).

If the AC drew 157 amps and the battery only discharged 106 amps, then the battery received 51 amps from the solar system (51= 157 minus 106).

Consequently, the solar system operated at only 25% efficiency (25%= 51 actual solar system amps / 200 theoretical system amps).

I assume that the 75% loss in efficiency was due to a combination of:

1. solar panel inefficiency

2. inverter inefficiency

3. charge controller inefficiency

3. wire and connection losses

4. battery charging inefficiency.

5. clouds, temperatures and sun angles

6. other ?????

Questions for everybody:

1. Do my calculations make sense?

2. Do you think that the battery charging inefficiency factor is a significant factor, perhaps one of the largest factors?

This goes to my original question which was "Will the battery discharge relatively quickly because the battery cannot accept the potential …. amp input as fast as the … amp draw?"

Again, thanks for all your help!!!!!

[flboy]

Hi.. I think I understand what you are saying now... and you are correct if the panels cannot supply all the needs of the Air Conditioner while running and the battery is discharging. It is true that the battery when it is charging... it may not use all the power you might have available to charge depending on it state of charge, temperature, etc..


I am not going to go through all the math because it can be misleading without all the variables applied when actual data is taken. At best, it good for making purchase decisions and trade offs for designing your system.

Note also. that I have 225 aH available for use. You can only take lead acid batteries down to 50% so 450aH are not available to use (I know I said 425aH earlier. It is actually 450aH) so I would say I used 50% of the battery power... not the 450aH you assumed.. but in any case, yes, there are all sorts of inefficiencies. For one, three of the panels had partial shade for 2 of the 3 hours taking me down to 500W that was also under partly cloudy conditions. In those circumstances I was only onboarding 29A from the panels.. which is a real life scenario.

My point is that if you want to run an AC reliably and for extended periods night and day, make sure you have a generator or else you will need more solar panels and batteries than you will want to carry.

For my intended purpose of running for a few hours during the day at stops or for the pet while I am out, and starting with a full charge, this works and is a great compromise of cost, capacity and functionality.

If this was a permanent structure and I had the money, I'd add alot more panels and storage, but even at that with the cheap cost of electricity, it likely would never pay back because things will break about the time you recover costs if ever.

You have to be realistic on what you can do. My answer to your question is the same. The rate your battery can take a charge does not factor into the equation while AC is running. When it stops running, that does affect battery recovery time.. but it is on a curve depending in how far you are discharged. So, if your panels can handle the full need of the compressor, the battery charge rate again does not matter. It starts to play into the equation when panels cannot keep up alone and the battery is supplementing. In that case if I have 800W or 67A trying to charge the bank which is at 80%or 90%, I may only be using 50A or.much less of the 67 available under ideal conditions. I have seen my system pump in over 55A when I was discharged well and the sun conditions were good. I have never seen full capacity (marketing numbers).

Back again to, a lot of variables.. so carry a generator too for the best of both worlds. I hope this helps and does not confuse.. best wishes.

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[RJ Howell]

Ftboy is hitting some points here that are very important. Some of which I may disagree, yet the point is the same. You're battery bank is way under sized for what the draws are.

Okay, the controller looks at the battery and decides how many amps it will take from the panel, balance are tossed away. That controller is going to loose it's mind as it reads, determines and finally supplies the amperage the battery wants. In the interim that battery is taking a hit.. This is why we size the battery bank to our requirements and allow them to do the deed we wish.

I am of the opinion that you can't look at the panel to supply power to an appliance. It's the batteries doing the deed.

[flboy]

I absolutely agree on battery bank supplying the power with relative size to intended load being very important. The Solar system in the RV is effectively a battery charger which can determine battery bank recovery time. Didn't mean to imply to the contrary as I poorly tried to explain the offset when the battery bank is supplying power to the load and the panels are contributing power to the system. My apologies for creating confusion.

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[featherliteCT1]

I appreciate everybody talking the time to comment.

Thank you!

I think I will try to borrow a a large inverter from a friend and then do bench testing in my garage. In so doing, I will record actual performance data as I experiment with different large loads on the battery.