help with understanding battery bank/charging

[Hue001t]

Hello all,
I have a 24 volt system with 8s 3.2volt lifepo4 cells. 280AH
12 250watt panels (2p6s)
1x victron SCC 250/100
I know this battery bank is small but I thought once the sun went down it would last for a few hours but it doesn't last at all once the sun goes down.
I use this to run 2 window unit ACs mostly which use about 1100Watts.
So I thought while the sun is out the system should be able to power the 1100 watts and still be able to keep the battery up so that when the sun went down it would run for maybe 5 hours. (24v x 280 AH)=6720 Watt hours and so 1100watt load/6720 watt hours =6 hours
Does that sound right? Im sure there is plenty I am missing but I want to make sure I understand.
Thanks for any help.

 

[rmaddy]

You're right in theory but reality is different unfortunately.

You have 3000W of solar panels. The 100A SCC supports at most about 2840W (28.4V battery charge voltage x 100A). Since you'll never get the full 3000W out of the panels that's actually a perfect sized SCC for your panels.

If your 2 A/C units use 1100W combined then that actually pulls about 1290W from the batteries since the inverter isn't 100% efficient.

Your 8 3.2V 280Ah cells = 7168Wh. If you use 85% of the battery then you have 6093Wh of usable power in the batteries.

If your batteries were at 100% full charge when the sun sets (more on that shortly). then your batteries will last 6093Wh / 1290W = 4.72 hours.

Let's say you start with full batteries. The sun is out. You turn on the 2 A/C units. As long as your solar panels are able to produce 1290W or more then the two A/C units won't use any battery power at all. As it gets later in the day the sun gets lower and you get less and less solar. At some point your panels can't produce 1290W any more and now your A/C units are using a little battery and lots of solar. As it gets later in the day the panels are generating less and less and the batteries are providing more and more power. So by sunset your batteries will probably be 20%-40% depleted (maybe more) already.

So now your batteries can only run the A/C for an hour or two after sunset. Come morning you have two nearly dead batteries that need solar to recharge. Your batteries might not be fully recharged until noon if you get sun all morning and you do not have any loads at all (especially no A/C running). If you try to run the A/C while the batteries are trying to charge then they will take all day to fully charge, if they can. And the late afternoon A/C use will be re-draining the batteries all over again.

If you have any way to monitor the state of charge on the batteries and how much solar you are getting during the course of the day you can see what is actually happening.

And if you have any other loads besides the 2 A/C units then that will just compound what I described.

 

[Hue001t]

You're right in theory but reality is different unfortunately.

You have 3000W of solar panels. The 100A SCC supports at most about 2840W (28.4V battery charge voltage x 100A). Since you'll never get the full 3000W out of the panels that's actually a perfect sized SCC for your panels.

If your 2 A/C units use 1100W combined then that actually pulls about 1290W from the batteries since the inverter isn't 100% efficient.

Your 8 3.2V 280Ah cells = 7168Wh. If you use 85% of the battery then you have 6093Wh of usable power in the batteries.

If your batteries were at 100% full charge when the sun sets (more on that shortly). then your batteries will last 6093Wh / 1290W = 4.72 hours.

Let's say you start with full batteries. The sun is out. You turn on the 2 A/C units. As long as your solar panels are able to produce 1290W or more then the two A/C units won't use any battery power at all. As it gets later in the day the sun gets lower and you get less and less solar. At some point your panels can't produce 1290W any more and now your A/C units are using a little battery and lots of solar. As it gets later in the day the panels are generating less and less and the batteries are providing more and more power. So by sunset your batteries will probably be 20%-40% depleted (maybe more) already.

So now your batteries can only run the A/C for an hour or two after sunset. Come morning you have two nearly dead batteries that need solar to recharge. Your batteries might not be fully recharged until noon if you get sun all morning and you do not have any loads at all (especially no A/C running). If you try to run the A/C while the batteries are trying to charge then they will take all day to fully charge, if they can. And the late afternoon A/C use will be re-draining the batteries all over again.

If you have any way to monitor the state of charge on the batteries and how much solar you are getting during the course of the day you can see what is actually happening.

And if you have any other loads besides the 2 A/C units then that will just compound what I described.

Hello again. TY for the reply. I do have the victron smart shunt on it to monitor but normally I just open the scc app and look at that. While I am looking at the app (victron app) at the top it shows solar volts and current and I watch it a lot to try and understand better and at like 530pm where I am that section shows its at like 1200 watts then the sun goes behind a tree and is completly done for the day at that point. Right when it hits that tree I lose all power like the battery did not have any charge. I see what you're saying that I should definetly expect much less then the 6 hours especially because yea i'm sure the battery is low at that point (right now its 749 pm here battery is showing 24.1 volts. I guess I just thought it would last any amount of time but I guess with it at 24 volts and then I add the ac load to it it drops the voltage obviously immediatly and then is done (my inverter sends an alarm at 20 volts)

 

[rmaddy]

24.0V for a LiFePO₄ 24V battery is under 10% charged. What SOC does the SmartShunt show when you see 24.1V?

You need to setup the inverter to give an alarm at 24.0V. 20.0V is 0% and way too low of an alarm setting to be useful. Your battery is already dead at that point.

You need to let the battery properly charge to 100% with no loads and then see how it goes running the A/C.

 

[Hue001t]

24.0V for a LiFePO₄ 24V battery is under 10% charged. What SOC does the SmartShunt show when you see 24.1V?

You need to setup the inverter to give an alarm at 24.0V. 20.0V is 0% and way too low of an alarm setting to be useful. Your battery is already dead at that point.

You need to let the battery properly charge to 100% with no loads and then see how it goes running the A/C.

ok I will let it charge to 100% with no loads and then report back.