Need help with some math please.

[WorldwideDave]

Thanks in advance for reading!

Question #1 - discharge duration: I am trying to figure out how long I can run inverter/pump/heater on the pool before needing to stop it.
Question #2 - discharge time of day: I am also trying to figure out when is the best time to run my loads aka discharge battery.

On Tuesday end of day at 4 PM, I had no sun, and had brought my LiFePO4 battery down to 12.87 V which according to my victron smart shunt was 12% state of charge. I disconnected all loads from battery with kill switch, and went out to vote (live in the USA).
On Wednesday morning, I hooked up four (4) 200 W panels in 2 series, 2 parallel to a victron smartsolar 100V/20A solar charge controller. By 8:30 AM, I was at 13.39 volts on the battery, could see it was charging, and left overnight for business trip.
Today this morning (Thursday), I went and checked and battery was at 14.19V and 84% state of charge at 9:41 AM. By 10:30 (45 minutes alter) I was at 91% state of charge. 15 minutes later at 10:45 I checked, and battery was at 92% state of charge, but 13.93 V - less volts than 15 minutes ago.
Question #3 - why voltage goes lower when SOC higher: Not sure how or why voltage goes down when SOC goes up when zero loads connected.
I then went out again 30 minutes later, and SOC was 100%, but battery voltage was at 13.79 V (still less than 30 minutes or 45 minutes prior). Zero current, zero power, zero consumed Amp Hours on shunt.

Then I fired up my inverter, and started my pool pump and pool heater at 11:15. Battery reports 100% SOC, but voltage is 13.11, current is -20 amps, and time remaining indicates 7 hours 51 minutes (love this feature, but don't trust it so much yet). On solar charge controller, it shows max PV being delivered (290 for 12v), and battery at 13.11 (matching the shunt), and current at 20 A in bulk state.

I imagine that the state of bulk will change to Absorbtion very soon if not already.

Within 2 minutes of running pump, the state of charge reads 99%, voltage is 13.05, current is negative 21 amps, and time remaining is 9 hours 2 minutes (longer than prior).

Question #4 - How do others manage loads during the day? Do most people charge all day, then at night run loads off battery, or for my setup is it better to run the pool gear during the time the sun is out charging the panels?

My concern is that I hope to run the pool for longer than there is sun out to charge the panels, which requires more battery, bigger SCC, more panels, etc. I am getting those things, but for now, what is the longest I should run the inverter/pump/heater, and what time of day should I run them.

Thank you for reading!

 

[robbob2112]

Convert everything to watt-hours and the math becomes simple

Pool filter/heater - how many watts does it actually consume in 1 hour - that is the watt hours of consumption - assume 1000w/hr

Power in - how many watts go in over/times how many hours - i.e. on a perfect clear day you generally get 5hrs sun - 6 x 200w panels = 1200w, * 5hrs =6000Whrs

if your battery can take power at a rate of 14.4v * 40amps = 576watts * 5hrs = 2880whrs to the battery at max, but the load will consume whatever it takes and only leave the leftovers for charging.

So... assuming a 5hr period - no real numbers so just guesses - get real data and plug it in
6000whrs generated by the panels
1000whrs * 5hrs = 5000whrs used by pool
this only leaves 1000whrs to go into the battery.

So this says the hypothetical array is undersized to both run the pool and charge the battery. To do both you need the 5000whrs the pool consumes and the 2880whrs the battery holds.

How long will the battery last once the sun is down
2880Whrs / 1000w = 2.88hrs

once you have common units it should make sense. What I have left out is the losses due to less than perfect efficience - 94% MPPT, 90ish Inverter, etc... those are all in the docs and just figure the losses will make it take longer to charge the battery, will make the battery last not as long

Give me real numbers if this doesn't make sense and we can do some real examples.

The voltage fluctuates at the end as the charge current slows and the cells balance out.

 

[Offgrid Jungle]

I just helped my neighbor put his system on line. His goal is to power his pool free off of solar. We have the pool pump timer set to run from 9 AM to 3 PM. In "theory", it will give the PV a chance to charge the battery partially before 9 am, trickle a little in between 9 to 3, and get an hour or two of charge before dark. I have my doubts he will be successful with 4500 watts of solar unless he has perfect sunny days LOL. I tossed a spare conduit in the trench to his ground mount solar rack so when he adds the second string, he will meet his goal easily.

(Did I mention that he has the entire house, guest house, and pool on a single EG4 battery? LOL)

 

[robbob2112]

Shrug, if you have a simple life with only lights, a tablet and router, and tv powered then I can see it. It is all those things running all the time that eat power for breakfast

 

[Offgrid Jungle]

4 mini splits, micro, washer dryer, a bunch of ceiling fans.... and it's a BNB so you know the ACs will run 24/7 with the windows open. He's dreaming.

 

[robbob2112]

delusional

 

[Offgrid Jungle]

Not to worry, I controlled the electrical design. It's all set up with a spare conduit for a second 6000XP and battery stack. I already posted that I ran a second conduit for the second PV string he needs...

 

[WorldwideDave]

Give me real numbers if this doesn't make sense and we can do some real examples.

The voltage fluctuates at the end as the charge current slows and the cells balance out.

Thanks again @robbob2112 for your replies to my post this year.
All I can tell is that when the inverter, pool pump, and pool heater run, it is discharging 45-50 amps according to the smart shunt. Not sure if accurate, but that's what it says.

Question: I assume that is DC amps? Not sure the difference between A/C and D/C amps, other than the flow of electrons (DC one way, A/C on Hertz back and forth 60 times a second). Otherwise amps the same?

Also, when the sun is out and the MPPT is at its max 290 watts or so, the 100/20 is giving at or close to 20 Amps to the battery. Smart shunt confirms what MPPT screen says, so believe it is accurate. I believe this is 20 DC Amps.

So 30 amps too little is coming from MPPT towards battery.

Over on the load side...

The pool pump has a surge for a moment or two. Does not run a 'prime' cycle at 3000 RPM - goes into normal run routine at 2400 RPM, which is enough pressure for the pool heater to work and not overheat.

Pool heater is 10,000 BTU, 120 V mastertemp. According to manual: "Electrical Rating: 60 Hz 120 / 240 Volts AC, single phase: Enclose the incoming AC power line to the heater in an approved flexible conduit connected directly to the junction box on the inside of the access door panel. Line voltage field wiring should be 14 gauge, with a circuit capacity of 15 Amps. Note: The heater operating AMP draw is about 5 AMPS at 120 VAC and 2.5 AMPS at 240 VAC. A 15 AMP circuit capacity is required for the inrush current at startup." It is at 120V, not 240. It is A/C amps, not DC.

The Pentair intelliflo variable speed pool pump shows me the watts in use and RPMs. It says under 400 watts once started up, and runs at 120 V, so call it 3 amps. Although I see elsewhere online it says that it is closer to 1000 watts at my speed. So call it 8 Amps.

With the pool pump at 8 amps A/C and the pool heater at 5 amps A/C, that's 13 amps A/C, under the 20 amps A/C I've wired it for. However, even with max inrush of 15 amps A/C on both (not saying they do that, but theory), that is still 30 amps A/C max, and only for a few seconds.

The 12 V 2000 W inverter probably uses amps to do its job, but if the shunt reporting 45-50 amps, and the max the pool heater and pump can pull is 30 amps, is it reasonable to expect an inverter to draw 15-20 amps just to do its 'conversion', even with 0/2 AWG wires (yes over wired per Will suggestion)? According to online data, 2000 W inverter, at 12V DC, would be 166 amps DC per hour, or 2.7 amps a minute (?).

Still confused how this equals 50 amps I normally see the system drawing when the PV is turned off/dark.

Any assistance here appreciated.

Bottom line is need another SCC to connect the other 8 panels up.

 

[WorldwideDave]

Shrug, if you have a simple life with only lights, a tablet and router, and tv powered then I can see it. It is all those things running all the time that eat power for breakfast

It is the pool that makes this an issue.
Similarly, if charging an EV it creates an issue.
Or my electric bikes.
Or the electric wall oven.
Or the electric clothes dryer.
Reducing the wall oven and clothes dryer is next.

 

[WorldwideDave]

4 mini splits, micro, washer dryer, a bunch of ceiling fans.... and it's a BNB so you know the ACs will run 24/7 with the windows open. He's dreaming.

Rentals are a funny thing. Wonder if he bought those appliances like the ceiling fans knowing what their wattage is before putting the solar in. He'll also probably start looking for ways to turn off things like the mini splits on a schedule...like forcing people to use them from 3 PM on, and shut them off at 8 PM sharp. He's going to be needing more batteries at least. Hope you were an honest friend and neighbor and helped him set expectations. Sounds like you may have - just read your other posts

 

[Steve777]

Comments about if your system is undersized seem basically right. But there is another aspect which will come into play, even before you expand the system. Making sure there is someplace for the PV energy to go when it is being generated.

So in answer to your #2, your will likely be better off running your loads during the day when the sun is out. This will ensure that there are loads to take in all the PV power available. If your loads are off during the day to charge the batt, then as it gets closer to 100% SOC, the CC will start feeding less power to the batt, as it should, wasting some potential output.

None of this will change if your system is undersized for your loads or not (although you might squeeze a few more W-H out of it by running loads during the sunny times). But to most efficiently use the system as it is now, this should help. Note though, this approach will effect other things, like having a charged batt for outages, etc, so you might want to take that into account too if that is one of your goals.

 

[WorldwideDave]

Comments about if your system is undersized seem basically right. But there is another aspect which will come into play, even before you expand the system. Making sure there is someplace for the PV energy to go when it is being generated.

So in answer to your #2, your will likely be better off running your loads during the day when the sun is out. This will ensure that there are loads to take in all the PV power available. If your loads are off during the day to charge the batt, then as it gets closer to 100% SOC, the CC will start feeding less power to the batt, as it should, wasting some potential output.

None of this will change if your system is undersized for your loads or not (although you might squeeze a few more W-H out of it by running loads during the sunny times). But to most efficiently use the system as it is now, this should help. Note though, this approach will effect other things, like having a charged batt for outages, etc, so you might want to take that into account too if that is one of your goals.

Thank you so much @Steve777 - yes I know it is undersized with MPPT and battery, but not panels Think 4800 is enough for goal of 4 hours per day to filter pool.
I did notice that as it approached 100% SOC it changed charge modes - maybe went to less amps output? - and as a result, was wasting the other 15 A of 20 A it has to output. But I did want to get to 100% SOC because it has not been there in 2 months because I was severely underpanelled before this Monday. I really don't know the pros and cons of going to 100% SOC other than seeing it show as a completed 'charge cycle' on the smart shunt, and have it go back to zero on the discharged amount in the history page.
So run loads at say 7:30 AM when sun is on the panels, but low SOC on the battery, and hope that the sun will catch up, or better to wait until higher SOC around 11:00 AM, and run for 4 hours then when sun is on full blast and have some fuel in the tank (volts in the battery) first?

I have run the pool 3 hours today with heat. I am running another 2 hours, and it will be done soon. SmartShunt says time since last full charge 3 hours and 3 minutes ago, 5 charge cycles, 4 syncronizations, and 6 full discharges - that sound right?
Also displays discharged energy at 35kWh and charged at 38kWh - suppose better to have more on charged side than discharge side?
Then on the discharge it says average discharge is -202 Ah, last discharge at -61 Ah. Think that last discharge is referring to right now?
Also says SOC is 69% - didn't expect so high - but current is 23 Amps right now, or about 300 watts. But the sun is charging at 290 watts/20 amps so i don't know. Does also say =61 Ah, which matches history screen's 'last discharge'. Victron very techinal!

Having reserve for when power out? Just runs pool. Sun's out, batteries charging, filter runs. If no sun or dead battery, I have a small 12V LiFePO4 battery charger I plug in that can run at 15 Amps for a few hours, then run the pool again for 2 hours to do a mini-filter/cleaning. Goal is to eliminate this need by changing the solar charge controller or adding a second.

 

[robbob2112]

Sorry, a novel

Convert to watts...

120vac * 5a = 300w, if on for 1 hour it takes 300Whrs to run it.
240vac * 2.5a = 300w

300w / 12.8v = 23.43a
2000w/ 12.8v = 156a

4800w / 14.2 = 338a... your battery won't charge that fast, your mppt isn't big enough.

Watts is a unit of power, the ability to do work. Horsepower is a unit of power. 1hp is about 745w.

So your 400w used to drive the pump is using it at a bit over 1/2hp. Because it is variable speed it is able to change how much work it does.

1 watt is one joule of energy per second...

It is all energy in different forms and you just have to convert between them in order to be able know how much work you can do with a given amount of sunshine.

For all of these solar things I find converting to watts is the simplest way to do the math.

So take it a step forward,

400w to run the pump for 24 hours = 9600Whrs.
Your bitty 12.8v battery at 100Ah = 1280Whr
9600Whr / 1280Whr = 7.5 batteries.

When you get to 48v the nominal voltage is 51.2v
51.2v * 100Ah = 5120Whr. This is the common size of a sever rack battery and you could run from 2 of them for around 24hrs...

These examples use 100% of the battery capacity. In theory you should not go below 20%...
5120 * 0.8 = 4096Whr

The same sort of thing applies to the heater, but pretty much all things that heat or cool have a duty cycle of on time to off time.

So a 5000w heater with a 50% duty cycle uses 2500Whrs per hour. In 24hrs that is 60kWhr... what also factors in is how fast your pool looses heat... that is meassure in something called specific heat... all materials have a specific heat which determines how fast or slow it can gain or loose energy. Going to stop there, this is all chemistry and physics and it has been 3 decades since I had classes on it.

Heat pumps exchange heat between the liquid they contain and the air. Air has a much lower specific heat than a liquid so you have a fan that blows it across the heat exchanger fins and you pump the fluid around in circles. This is far more efficient to move heat than it is to create it through resistive wires, so a heat pump is more efficient to heat something (transfer heat) than a 'heater'. So if you have a heat pump with the exchanger in the water it costs less energy to move it around than to heat it. This is how water exchange heat pumps work. So if you want to heat a pool with the sun the cheapest way will be black tubes filled with water and a pump to cycle it. Next would be burning gas to create heat and in turn circulate that water. After that you use electricity to heat a resistive wire and flow water over it.

Moving heat is efficient, creating heat is not...

Heat is measured in calories and BTUs. 1 joule of energy will raise 1 cubic centimeter of water by 1 degree celcius. 1 calorie is roughly 4 BTUs...

This is all basically how matter and energy are exchanged from one to the other using e=mc^2.... energy = mass * the speed of light squared.

Energy causes matter to change states in the battery...put energy in and it goes one way, take it out and it goes another way.. just adding or removing electrons... that is how batteries work

Now you know how star trek replicator work.

 

[WorldwideDave]

That was a huge response @robbob2112 thank you.
I get confused.
Leave 240V out of this for now, please.
Also please leave out 48V for a second, as it will confuse me.

Here's some numbers for you:

The 12V battery is actually 200Ah. It is small, yes. Going to 48V someday, but not today. By year end.
I will have 12 panels mounted by Saturday. Just 9 up now, but no SCC to control them all yet. Running in 2s2p right now, for 800 watts, on victron 100/20. Trying to figure out which MPPT on another thread.

Also the magic number is 8 hours of running pool a day. 4 hours minimum is good, too.

Once battery reached 100% SOC, I ran pool for 5 hours today. Finished 30 minutes ago, and says SOC is 33%.

My hope is to run it for 4 hours tomorrow, starting at 10 AM, ending at 2 PM, when we are making peak energy. Not sure that will work. Want to figure out how long it will run for, consistently, if:

Scenario 1.) everything that is installed stays the same,
Scenario 2.) a newer, bigger MPPT or a second MPPT were added to take advantage of all 12 panels at 200 watts

I will ask 48V questions when I get there. For now, I know I need a bigger MPPT whether I go with 12V or 48V, so I'm focusing on that. While I wait to get that figured out, I wanted to know what I could run until I have the panels up and new MPPT. Might take a month to get to it all.

 

[robbob2112]

Ok, unified field theory was probably a bit much

12.8v * 200ah = 2560Whrs
2p2s = 8 panels not 9...

800w x 5hrs a day = 4000Whrs if your mppt can get all that power in the battery.

But the mppt puts out 20amps at 14.2v = 284w actually hitting the battery.

2560Whrs / 284w = 9.0hrs. This is how many hours of direct sun you need to fully charge the battery from empty..that doesn't account for the mounting, if they are flat you will get 60% of that

9hrs / 0.6 = 15hrs. .. way underpaneled/under-mpptd for the battery, but you knew that...

So what this shows is your pool should run fine when the sun is fully out. When a cloud passes the battery will keep it running until it is sunny again. But because of the slow rate of charge I wouldn't expect it to last more than 30~60 minutes after you loose direct sun.

Where you might be better off is charge the battery in the middle of the night without the pool running... Run the pool in the day once you have full sun with the battery connected, then it will run for about 3hrs from battery once you don't have full sun.