Is this how recharging works?

12 volt panels are about 21Voc and 18Vmp
Volts x Amps = Watts
It is easier to figure it out using watt hours.
100 watt panel will put out about 80 watts.
80 watts x 5 panels = 400 watts x 4 hours = 1600 watt hours.
400Ah battery at nominal 12 volts is 4800 watt hours.

MBR said:

12 volt panels are about 21Voc and 18Vmp
Volts x Amps = Watts
It is easier to figure it out using watt hours.
100 watt panel will put out about 80 watts.
80 watts x 5 panels = 400 watts x 4 hours = 1600 watt hours.
400Ah battery at nominal 12 volts is 4800 watt hours.

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So if volts (12v) x Amps (108a) = watts (1296w) used in those 4 hours then it seems that 80 watts x 5 panels = 400 watts x 4 hours = 1600 watt hours would recharge what I used. Is that correct? I would be around 25% DOD?

Sage said:

It would be around 25% DOD?

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Is your 400Ah battery lead acid and what is the max DOD for its chemistry? AGM, gel. flooded?
At 50% DOD a 400Ah battery has only 200Ah useable.
So 100Ah used would be 50%
That is one of the many reasons I like LiFePO4 batteries with up to 100% DOD if you need it.

Sage said:

Here's the set up. 5 solar panels, 12 volt, 100w each, producing 5.40 amps maximum each. 5.40 amps x 5 panels =27 amps. Does this mean that if I use 108 Amps of a 400 Ah battery in 4 hours, and run the 5 panels for 4 hours in direct sunlight using a 40 amp MPPT controller, (5.40 amps x 5 panels x 4 hours = 108 amps), that these 5 panels will fully recharge in 4 hours, the 108 amps that I used?

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a qualified 'yes'. The charge generated by the panels in your example equals what you took out. However, the charging process is not 100% efficient. So, it'll take a little more than 4 hrs for the panels to fully recharge the batteries.

I have edited (in red) your writing to clarify things:

Here's the set up. 5 solar panels, 12 volt, 100w each, producing 5.40 amps maximum each. 5.40 amps x 5 panels =27 amps. Does this mean that if I use 108 Amps-hrs of a 400 Ah battery in 4 hours, and run the 5 panels for 4 hours in direct sunlight using a 40 amp MPPT controller, (5.40 amps x 5 panels x 4 hours = 108 amps-hrs), that these 5 panels will fully recharge in 4 hours, the 108 amps-hrs that I used?

MBR said:

Is your 400Ah battery lead acid and what is the max DOD for its chemistry? AGM, gel. flooded?
At 50% DOD a 400Ah battery has only 200Ah useable. So 100Ah used would be 50%

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It's AGM. Max DOD is 50%. So if 1296 watts are used in those 4 hours then does 1600 watt hours recharge it?

MBR said:

Is your 400Ah battery lead acid and what is the max DOD for its chemistry? AGM, gel. flooded?
At 50% DOD a 400Ah battery has only 200Ah useable.
So 100Ah used would be 50%

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this is wrong in so many ways!

A 400 Ah battery has 400 Ah of usable charge. Most lead acid batteries will give 300-500 cycles of full (100%) discharge before they are dead. At 50% DOD, one can expect a little more than 2X cycles (800-1100); at 25% DOD, batteries will last 4X+ cycles.

In the OP's example, drawing 108 Ah from a 400 AH bank works out to 108/400 or 27% DOD.

I've stated this in other threads - what kills lead acid batteries is not how deep they are discharged, but how long they stay discharged.

edited to remove reference to LFP

The answer most likely is sort of. You can't expect your panels to be at full output for many hours over the day unless you have them on a tracker, and even then, power levels will still be lower in the early morning and late afternoon. A good rule of thumb for stationary panels I like to use is to multiply your output by the following conversion factors, with X being your panel rating.
8am = 0.25X
9am = 0.40X
10am = 0.6X
11am = 0.75X
12noon =0.85X
afternoon the reverse of above

So, with 500W of panels, expect to see ~300W at 10am. (500W x 0.6 = 300)

Let's say the clouds dissipate at 10am and your panels start producing power. From 10am till 2pm you can expect 300 + 375+ 425 + 375 = 1475W. Keep in mind that there's always some lose to heat. Shave 10% of what the controller gets to what it outputs. Shave another 10% from what the batteries receive compared to what they store. So, if you use 1296W, you probubly need to feed the batteries 1550W. Four hours appears a little low, but most likely yes, assuming the panels get sun for 5 hours per day.

MBR said:

That is one of the many reasons I like LiFePO4 batteries with up to 100% DOD if you need it.

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@Sage
The only one that gave you that answer on the other forum was me and that started a flame war. If you want good info on AGM that is still a useful forum. This forum is a great place for help on how to save money using LFP.

Since I commented on @MBR's post, I want to add something about lead acid batteries.

What a LA battery manufacturer guarantees is the lifetime amp-hrs one can expect from the battery. If a hypothetical battery is rated at 100 Ah & 500 charge cycles, you can expect to get 50000 Amp-hrs from it before the electrodes completely disintegrate. This 50k Ah can be taken in big bills or small bills. Taking out the charge in small bills extends the cycle life slightly beyond what you'd expect from a simple math. There is a price for limiting DOD to 50% (or 25%) - one needs 2x (or 4x) more batteries to meet a given daily need.

The best way to get the most out of lead acid batteries is to promptly recharge them fully (& some) pef mfg's recommendations.

Sage said:

So if 1296 watts are used in those 4 hours then does 1600 watt hours recharge it?

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That sounds about right for Pb technology. I have heard it is 75% efficient.

fat_old_sun said:

I've stated this in other threads - what kills lead acid batteries is not how deep they are discharged, but how long they stay discharged.

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Depth of discharge absolutely kills batteries. Starting batteries are designed with lots of thin plates for maximum surface area, to give the biggest surge of starting current to start a vehicle. These thin plates are fragile, and deteriorate quickly if used as a deep cycle battery, regardless of how quickly it is recharged. A real deep cycle battery has fewer, but thicker plates that won't give the big surge of current a starting battery will, but the thick plates hold up much better in the deep cycle application.

My AGM batteries were discharged to 12.57V which is a little over 50% (according to a site I visited) when the load was removed they came back to 12.8V They won't be charged until sunrise tomorrow. So my logic tells me that they are still depleted, yet showing a full charge now, so should be ok to sit overnight without damage ?