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PowerWall Proof of Concept
#1
Going with a 4s packs.



All cells tested 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 solid 6ga 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 understand why don't more people use 4s vs. 3s to optimize their battery capacity.
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#2
(03-28-2019, 11:19 PM)TheDagg Wrote: I don't understand why 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.
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#3
I do know about the efficiencies gained from a 24 volt system ad voltage drop, but I'm referring to using 4s instead of 3s to 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 to 11.2V 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 on less 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 with 4s 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?


Korishan pid=' dateline= Wrote:
(03-28-2019, 11:19 PM)TheDagg Wrote: I don't understand why 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.
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#4
(03-28-2019, 11:19 PM)TheDagg Wrote: I don't understand why 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 serious project 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.
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#5
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.
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#6
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.
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#7
(03-29-2019, 01:06 PM)Korishan .  Makes complete sense, I fully understand that part of the entire point of the build is repurposing or reusing existing equipment. Finding an old inverter that doesn\t have a low cutoff until 17v is near impossible, so a 4s bank wouldn't make sense for most in that regard. But the Victron Multiplus is only just over $1000, which is reasonablly priced considering how much more usable capacity I'm getting out of my 4s bank. Many Tesla users go with the Multiplus 3000 for this exact ability to fully customize programming (See "mortons on the move" on youtube). But without going off on a tangent - Using this inverter, I'm now able to use all of my 18650 capacity per cell. If I went with a 3s configuration, i would be forced to almost double my number of 18650s to get the same usable capacity. That more than justifies the Multiplus and 4s in the long run.The Victron Multiplus is not cheap, but it's damn good at what it does and is one of the only inverters that allows full customizable programming with a computer and a 17v high cutout.I suppose I'm in a good spot being able to get a used Victron unit, but it seems that being able to use almost half the cells for the same usable capacity justifies 4s.Thank you for the input, and please keep it coming as I progress. I'll need it! Wrote: 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.
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#8
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/upload/doc...0VA-EN.pdf

https://www.victronenergy.com/inverters/...50va-800va
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#9
(03-30-2019, 12:33 AM)Crimp Daddy Wrote: 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/upload/doc...0VA-EN.pdf

https://www.victronenergy.com/inverters/...50va-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.

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.

(03-29-2019, 01:06 PM)Korishan That\s why I'm reaching out for advice - I'm saying in my case I do have an unlimited supply of 12/24v inverters due to my involvement with the solar industry. So with that consideration in mind, do you see a reason for me to not go 4s for 12v? It will allow me to easily add more rows to convert it to 24v in the future if I want to increase voltage, and from my testing thus far, allows me to utilize the full voltage range of 2.8 to 4.2v of each cell. Am I missing something about 3s or not considering a key component, since everyone seems to do 3s and not mind sacrificing over a third of their storage capacity? Wrote: 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.
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#10
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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