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PowerWall Proof of Concept
I started by replacing the dead "12v battery" in my Duracell 600 powerpack with 3s at 4.2v. It worked! but cut-off was at 10.9v (3.63v/cell) giving me about 50% DOD - leaving a bad feeling that power was squandered and not accessible. I bought an external inverter to try to lower the range but as stated, that direction lies unnecessary expense and unsatifactory experience. IF you want to do 12v its best to go 4s with LifePo4 at 3.2v nominal per cell. I simply did not understand the pros and cons as I do now with much more experience under my belt.

Next I got a couple of panels (3x 300w - wo ho!!) and went 24v (7s at 3.7v nominal). This is perfectly fine voltage range for standard controllers and inverters. All worked just fine but then I began to understand how 'little power' that is. This lead me to 3 x 300w panels and....

The jump to 48v (14s) was born. As in 7s, this voltage range fits right in with ... wide .... range of options for controllers and inverters. Also, as I began to understand what it takes to deliver home scale power (e.g. 2000w sustained with spikes to 6000w) I can no longer imaging 12v or fooling with cables thick enough etc etc. Even at 48v @ 12,000 watt inverter its 250amp circuit breaker!!!

So I would think about where you want to be in the future and if it includes significant power - e.g. more than 1,000 watts in panels and to power 2,000 watt inverters - then I would just go 48v right away.
Ibiza likes this post
(03-29-2019, 09:56 AM)daromer I completely understand the viewpoint that 4s is expensive because the cost of getting an inverter that works across the 16 to 11 volt spectrum is hard to find. However using a 4s configuration uses near 100% of the battery capacity, rather than just 50% as you get in 3s configurations. Let me explain.  A 4s pack has a voltage of 16.8 to 11. This uses the full capacity of each cell from 4.2v at a full charge to 2.75v when drained. This allows the pack to have a working voltage over nearly it\s entire SOC, allowing a near full depth of discharge. A 3s pack has a voltage of 12.6 to 8.1v. This uses only a partial capacity of each cell from 4.2v at a full charge to 3.6v - It can only be drained to 3.6v per cell, or 10.8v on the pack, because an inverter will cut out at 11v.   This leaves 2.9v unusable in a 3s bank. That is almost a full volt per 18650 cell that cannot be utilized. Since each 18650 cell has 1.5v of capacity variance, this means you are only using about a third of the available storage of each 18650 in a 3s configuration. Using a 4s configuration allows me to purchase a third of the cells and get the same working capacity. Wrote: 4s is only useable on a very very few inverters. 12V systems with lithium cells having a nominal voltage of 3.7V is not optimal in any way unless you have money to spend. But at that stage its often easier and more cost effective to long term go higher voltage or even LiFe.

(03-31-2019, 08:40 PM)Crimp Daddy I completely understand the viewpoint that 4s is expensive because the cost of getting an inverter that works across the 16 to 11 volt spectrum is hard to find. But the inverter is already purchased, so lets focus on the efficiency to ensure my math is right. Using a 4s configuration uses near 100% of the battery capacity, rather than just 33% as you get in 3s configurations. Let me explain.  A 4s pack has a voltage of 16.8 to 11. This uses the full capacity of each cell from 4.2v at a full charge to 2.75v when drained. This allows the pack to have a working voltage over nearly it\s entire SOC, allowing a near full depth of discharge. A 3s pack has a voltage of 12.6 to 8.1v. This uses only a partial capacity of each cell from 4.2v at a full charge to 3.6v - It can only be drained to 3.6v per cell, or 10.8v on the pack, because an inverter will cut out at 11v.   This leaves 2.9v unusable in a 3s bank. That is almost a full volt per 18650 cell that cannot be utilized. Since each 18650 cell has 1.5v of capacity variance, this means you are only using about a third of the available storage of each 18650 in a 3s configuration. Using a 4s configuration allows me to purchase a third of the cells and get the same working capacity. Wrote: That line of thinking IMO is an error, because I once came from there too...  There is no reason to try and accommodate a 12v mindset with lithium cells...  while I made a recommendation for that Victron 12v inverter, its probably my last voltage pick when working with lithium cells.

Plus when working with larger loads, it makes loads more sense to increase the voltage... not only it is easier to find better equipment, your cable sizes requirements become unnecessarily large.

If you want 12v native, work with LiFePO4 cells... its what boating and RV industry does as "industry convention"
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Sorry about the incorrect reposting/correcting. Just getting started here.

So in summary - With the voltage being a fixed variable at 12v and all else being constant. You do indeed get the most usable energy with a 4s configuration rather than 3s. This is assuming you have an inverter than can invert up to an incoming 17vDC , and can invert down to 9.8vDC. A 4s configuration can drain each cell to 2.7, while the 3s configuration can't drain each cell past 3.26v.

That being said, most inverters have a high voltage cut-out at 16 and will resume inverting at 15v to 15.5v. Since most inverters will cap each cell with a high voltage limit of 3.75v, this makes the 4s configuration a poor choice for the vast majority of available inverters.

Increasing your battery bank from 12v to 24v or 48v will greatly increase your efficiency.

Thanks all!

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