PowerWall Proof of Concept

TheDagg

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Joined
Mar 2, 2019
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Going with a 4s packs.


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All cellstested with an Opus 3100. Only using cells that are 1.7Ah or above. Stripping off existing shrink wrap and replacing. After grouping the cells, complete batteries will have a working output voltage range of 16.8 to 11.2. Going with 5amp fuse wire and solid6ga copper for the bus bar. No balancing.

Proof of concept has been created as a single pack just to ensure programming compatibility with the inverter/charger. No high voltage cut-out, will invert above 16v and all the way down.All systems are go.

With a fully programmable inverter/charger that doesn't have an overvoltage disconnect until 17v, I don't understandwhy don't more people use 4s vs. 3s to optimize their battery capacity.
 
TheDagg said:
I don't understandwhy don't more people use 4s vs. 3s to optimize their battery capacity.

For one, it doesn't "really" fit a 12V setup. Also, it's hard to find inverters that can accept that higher voltage. And, you have to use much larger wire to carry the power any distance away from the pack to the inverter in the first place, if you want to use any decent amount of current. Plus the voltage drop of 12V setups is much higher than 48V. And to top it off, going 48V isn't really that much more expensive than 12V setup for the same capacity, as the inverters for 48V are about the same cost AND they are usually last longer and can handle more surge current.

3s/4s doesn't "optimize" capacity at all. In fact, it's the opposite. Regarding voltage drop, 12V (3s/4s) is much higher than that of 48V (14s) for the same distance for a given wire size. Sure you can go bigger on the wire, but that starts to get more costly and harder to install as the thicker wire is harder to bend and solder to, even using crimp connections is more expensive.

And finally, the FAQ talks about this all from an electrical stand point as well.
 
I do know about the efficiencies gained from a 24 volt system advoltage drop, but I'm referring to using 4s instead of 3sto maximize the usable capacity out of each 18650 in a 12v system. Let me explain. Since there are inverters I've tested that deal with the voltages produced from a 16.8 pack, that is not of concern. The inverter I tested has a high voltage cutoff at above 17.That means the inverter can use the full 4.2 to 2.8v per cell, meaning it drains the battery from 16.8 at full charge to11.2V when drained.Using a 3s configuration does not allow this, since it's voltage range is 12.6 to only 8.4. Inverters can't operate onless than 11 volts, so you are leaving 2.6 volts unused with this 3s configuration. That would be about .9 volts per cell, which is about 35% of the usable capacity. When I did tests on various Tesla packs (seen here) I found some interesting results. The voltage differential with using 18650s in a 24v setup is only made worse, since the total operational voltage range is even a larger. So with the inverter problem being moot, and using 12v as the baseline, why not go with4s and get all the usable capacity out of each cell?By using 3s, you are effectively dropping the usable capacity compared to 4s in this scenario, correct?

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Korishan said:
TheDagg said:
I don't understandwhy don't more people use 4s vs. 3s to optimize their battery capacity.

For one, it doesn't "really" fit a 12V setup. Also, it's hard to find inverters that can accept that higher voltage. And, you have to use much larger wire to carry the power any distance away from the pack to the inverter in the first place, if you want to use any decent amount of current. Plus the voltage drop of 12V setups is much higher than 48V. And to top it off, going 48V isn't really that much more expensive than 12V setup for the same capacity, as the inverters for 48V are about the same cost AND they are usually last longer and can handle more surge current.

3s/4s doesn't "optimize" capacity at all. In fact, it's the opposite. Regarding voltage drop, 12V (3s/4s) is much higher than that of 48V (14s) for the same distance for a given wire size. Sure you can go bigger on the wire, but that starts to get more costly and harder to install as the thicker wire is harder to bend and solder to, even using crimp connections is more expensive.

And finally, the FAQ talks about this all from an electrical stand point as well.
 
TheDagg said:
I don't understandwhy don't more people use 4s vs. 3s to optimize their battery capacity.

Folks here tend to be wanting to keep their spend as low as possible, inverters with a wide input window tend not to be of the cheap and cheerful variety seemingly favoured by many here - unless you've found a cost effective one ?

Obviously, 12v inverters will always be of limited use, anyone with a seriousproject will be using a nominally 48v system, and they'll hopefully be aiming to maximise the longevity of their battery,using it at both extremes of its voltage range isnt conducive to that.
 
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.
 
As Sean and Daromer mentioned, 4s voltage range is on a "few" inverters. The majority of them do not go above 16V. So it's not that people aren't wanting to use the full voltage range with a 12V inverter, it's usually because the inverters are available to use the full range.

I understand the math you posted, it's listed in the FAQ.

If you have an inverter that "can" use the full range, then by all means use it. No one here is stopping you from doing it. Just keep in the mind the limitations of such a system. You won't be able to scale up easily/cheaply in the end, for one. Unless where you got the inverter is from a buddy down the street that's smuggling them out of the manufacturer/warehouse, or, more likely, they are refurbished or reject units that "really" don't meet the surge requirements to sell at full price.
 
Korishan . Makes complete sense said:
As Sean and Daromer mentioned, 4s voltage range is on a "few" inverters. The majority of them do not go above 16V. So it's not that people aren't wanting to use the full voltage range with a 12V inverter, it's usually because the inverters are available to use the full range.

I understand the math you posted, it's listed in the FAQ.

If you have an inverter that "can" use the full range, then by all means use it. No one here is stopping you from doing it. Just keep in the mind the limitations of such a system. You won't be able to scale up easily/cheaply in the end, for one. Unless where you got the inverter is from a buddy down the street that's smuggling them out of the manufacturer/warehouse, or, more likely, they are refurbished or reject units that "really" don't meet the surge requirements to sell at full price.
 
CrimpDaddy said:
I am still a bigger fan / proponent of 24 or 48v systems... I really like 15s on 48v because it keeps me towards the top end of the efficency curve, and plenty of 48v inverters can handle the 15s input voltage.

Victron has always been good about high input voltages... 4s would work well on their 12v platform as it supports 17v max.

https://www.victronenergy.com/uploa...hoenix-Inverter-VE.Direct-250VA-1200VA-EN.pdf

https://www.victronenergy.com/inverters/phoenix-inverter-vedirect-250va-800va

I'm a fan of larger voltage systems as well, but having worked in the moving solar industry, I have an abundance of 12v equipment. That being said, bucking it down for any 12v appliance is actually rather efficient. So I suppose industry convention is keeping me on the 12v line of thinking, maybe that is an error.


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I'm disappointed Victon's BuckBoost isn't a 3 stage charger, but the 100A model is beautiful and runs at about 95% efficiency. Maybe I should consider moving off 12v and always having a buck on hand for any 12v applications.

Victron was what I was running tests on today, in fact!! It ran my pack above 16v just fine.It's a beautiful machine, even though the MK3 can be a hell to program with some PCs.


Korishan That\ said:
As Sean and Daromer mentioned, 4s voltage range is on a "few" inverters. The majority of them do not go above 16V. So it's not that people aren't wanting to use the full voltage range with a 12V inverter, it's usually because the inverters are available to use the full range.

I understand the math you posted, it's listed in the FAQ.

If you have an inverter that "can" use the full range, then by all means use it. No one here is stopping you from doing it. Just keep in the mind the limitations of such a system. You won't be able to scale up easily/cheaply in the end, for one. Unless where you got the inverter is from a buddy down the street that's smuggling them out of the manufacturer/warehouse, or, more likely, they are refurbished or reject units that "really" don't meet the surge requirements to sell at full price.
 
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"
 
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.
 
daromerIcompletelyunderstandtheviewpointthat4sisexpensivebecausethecostofgettinganinverterthatworksacrossthe16to11voltspectrumishardtofind.Howeverusinga4sconfigurationusesnear100%ofthebatterycapacity said:
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.


CrimpDaddyIcompletelyunderstandtheviewpointthat4sisexpensivebecausethecostofgettinganinverterthatworksacrossthe16to11voltspectrumishardtofind. But the inverter is already purchased said:
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"
 
TheDagg: Please make your comments before or after the [ quote ][ /quote ] blocks, not inside, especially the header section. Also, please preview your replies before posting as you see the error of doing such before posting.
 
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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