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Grid Tie, modular and flexible
#1
Smile 
Seriously, i hesitate to publish my starting project of a smaller type Powerwall.

Seeing the perfection and/or the size of some of the builds mine will be a shame.

So I will at least start to point out the different (to what i mostly see here ) concept i have.

I come from having a totally simple 600 Wp Grid Tie solution running since 3 months, still in experiment. Taking used panels and a cheap Inverter, i have a system providing power against basic continuous household  use, having a payback time below 3 years. The obvious problem is, if I increase the Solar Power, i have too much electricity during sunny day which i cannot use, during night (and muddy wheather) i have no electricity to input into my house to achieve saving.

Every standard Powerwall solution is able to fix that, with one "minor" disadvantage: the costs for the battery.
And now i have found secondlifestorage.com.

Basic idea is to feed power to the standard Solar MPPT Grid Tie Inverter at times, where solar panels don't deliver, or at any time i like.
I have a nights electricity usage of perhaps 1,5 kWh, that gives an idea about the size of my battery needed. Since i will use nothing additional eccept the inverter I have already, I am limited to 500 Watts max input. Thus, i can only supply general basic loads as refrigerators, no peak loads.

This has many advantages for the battery concept, because the max currents are not that high compared to what i have seen in other powerwall projects. Preferrable i would like max 1/10 C, short time 1/5 C discharge current, preferable less.

I can deal with all the technical problems of faking an solar like MPPT source, with the necessary measurement in the house and the regulation of the battery power feeding, using components freely available. But i want to have that all tested and brought to function, before I dive into the relative adventure of spending full  costs of a SL Battery of appropriate size.

As a resource I have a small stock of some 150pcs 18650 cells, and some Lipos, which have been "collected" since awhile for the latter reason, but without enough trust to start such a project solely on my own.

I made up my concept, as a flexible and modular one as follows:
- i chose 6S /24 Volts as a working voltage, because i can deal with any packs of that with my several B6ACs very easily.
- 6s4p build a basic pack. After combining them, balancing them and checking equal capacity i have a "submodule".
- i combine 2 submodules into a "module", putting them in a small wooden "rack".
- Every Rack gets a cheap 6S balancing BMS, not mainly because of the balancing aspect, but because of the cell-exact under- and Overvoltage cut off. This gives me a 250 Wh module of 6S/24 Volts.
- these racks shall be arranged in a cabinet. All outputs in parallel.

I start with 2-3 Racks, giving me about  6 Amps for quite awhile for testing all circuitry, and of course the modular concept as well.
I can then upgrade to many more racks, in parallel,  which will simply share the load. I can take out a rack at any time, even under load, for inspection or whatever. I lack an automatic surveillance of the cells, or if a rack has cut-off, i will deal with this later, if necessary at all.  
I can serial 2 Racks to a 12S8P, in  case I would later prefer to work with 48 Volts, without having to rework everything.  

That concept leaves a lot of loose ends, where i have made  no final decision until now. I am not sure about the BMS, i have no general control monitor, i am not sure about the concept at all. But it looks interesting as a start to get everything in order without having to spend too many bucks before i know everything works as planned, and especially how many control/surveillance/Info Board is necessary at all.

I know about no discussion here in this threads, but any comments are welcome.

Sorry for my bad english.
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#2
Why are you saying 6S for 24V and 12S for 48V? The traditional numbers used are 7S and 14S. Unless your equipment has a hard limit of ~25 or ~50 volts, 6S is going to be slightly under voltage for most of its operation. (3.9-4.2V is 10-20% of capacity for most Li-Ion / LiPo type batteries)
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#3
We all gotta start somewhere Wink Welcome to the power wall building team

(05-14-2018, 11:22 AM)Cherry67 Wrote: I made up my concept, as a flexible and modular one as follows:
- i chose 6S /24 Volts as a working voltage, because i can deal with any packs of that with my several B6ACs very easily.
- 6s4p build a basic pack. After combining them, balancing them and checking equal capacity i have a "submodule".
- i combine 2 submodules into a "module", putting them in a small wooden "rack".
- Every Rack gets a cheap 6S balancing BMS, not mainly because of the balancing aspect, but because of the cell-exact under- and Overvoltage cut off. This gives me a 250 Wh module of 6S/24 Volts.
- these racks shall be arranged in a cabinet. All outputs in parallel.

I start with 2-3 Racks, giving me about  6 Amps for quite awhile for testing all circuitry, and of course the modular concept as well.
I can then upgrade to many more racks, in parallel,  which will simply share the load. I can take out a rack at any time, even under load, for inspection or whatever. I lack an automatic surveillance of the cells, or if a rack has cut-off, i will deal with this later, if necessary at all.  
I can serial 2 Racks to a 12S8P, in  case I would later prefer to work with 48 Volts, without having to rework everything.  

That concept leaves a lot of loose ends, where i have made no final decision until now. I am not sure about the BMS, i have no general control monitor, i am not sure about the concept at all. But it looks interesting as a start to get everything in order without having to spend too many bucks before i know everything works as planned, and especially how many control/surveillance/Info Board is necessary at all.

Sorry for my bad english.

Why 6s over 7s? What are the specs on your inverter? You state later on planning on 48V. 12s is not suitable for this, but 14s is. It would be better to go with 7s4p for your 24V to start off with, then serialize those to get 48V later on. Since you don't have the 48V inverter, the 14s is preferable so something you should look into if you want to keep them modular, going with 7s over 6s if you can.
I like the idea of modularizing your packs. Makes modify, adding, removing, maintenance a lot easier to work with.
At this stage, yes, each pack would need a BMS, mostly. Depends on the connection of the packs. If you plan on Paralleling the packs only at their absolute Pos/Neg, then each pack needs a BMS in this case. But, if you are connecting each 4p set of cells together (in other words, you have 7 connections for every 2 packs connected in parallel), you would need effectively have a 6s8p (for 2 packs; or 6s16p for 4 packs), then you could use 1 BMS to handle all of them. Though, you would need to get a beefy enough one that can handle higher currents.

Keep amp usage low is really nice. You've worked on that a lot I see, especially with the 1.5kWh/night usage. That's awesome!

Your english is just fine. Probably better than most American's english Tongue
Fdml and Gregor Scott like this post
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at https://discord.gg/c7gJ5uA
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#4
(05-14-2018, 12:23 PM)brwainer Wrote: Why are you saying 6S for 24V and 12S for 48V? The traditional numbers used are 7S and 14S.  Unless your equipment has a hard limit of ~25 or ~50 volts, 6S is going to be slightly under voltage for most of its operation. (3.9-4.2V is 10-20% of capacity for most Li-Ion / LiPo type batteries)

You are totally correct, and i am aware of all. This does just not apply to my planned hardware. My fault, i should have gven more details about that.
I'll explain together with the answer to Korishan.
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#5
(05-14-2018, 12:30 PM)Korishan Wrote: Why 6s over 7s? What are the specs on your inverter? You state later on planning on 48V. 12s is not suitable for this, but 14s is. It would be better to go with 7s4p for your 24V to start off with, then serialize those to get 48V later on. Since you don't have the 48V inverter, the 14s is preferable so something you should look into if you want to keep them modular, going with 7s over 6s if you can.
I like the idea of modularizing your packs. Makes modify, adding, removing, maintenance a lot easier to work with.
At this stage, yes, each pack would need a BMS, mostly. Depends on the connection of the packs. If you plan on Paralleling the packs only at their absolute Pos/Neg, then each pack needs a BMS in this case. But, if you are connecting each 4p set of cells together (in other words, you have 7 connections for every 2 packs connected in parallel), you would need effectively have a 6s8p (for 2 packs; or 6s16p for 4 packs), then you could use 1 BMS to handle all of them. Though, you would need to get a beefy enough one that can handle higher currents.

Keep amp usage low is really nice. You've worked on that a lot I see, especially with the 1.5kWh/night usage. That's awesome!
My inverter is, as i mentioned, the one and only grid tie solar inverter with one MPPT input i have. No external control at all. If a battery is connected, it runs up to its (power or current-) limit, which is more than i need (in terms of Power) and less than i need (in terms of analog control).
Using this very inverter was the only choice of getting the project to go, since i do not know any cheap (!) battery operating inverter which has my requirements.
In trying to fake the battery as MPPT supplier it is obvious that this is more or less a current limiting function with a nice smoothe shape. There is little chance to get any shunt principle going (because of power loss) and so any type of dc dc inverter has to do the trick.
That means, a step up or a step down will have to provide the current shape, and thta defines the input voltage range.
6S 22 Volt is a good start for a step up giving 36 volt MPPT.
Thus, 6s is no bug, its a feature ! Smile 
The 2 sub packs are completely connected, 7 rails. And one BMS.
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#6
An MPPT doesn't care about the powers of 2. It will take what ever voltage input and convert it to what ever the output needs to be. This is done using pulses through an inductor and a capacitor and diodes on the output to make sure the power is smooth and doesn't feed back.
So, going with 6s or 7s to fake the input, makes no different to the MPPT. It's just the very nature of an MPPT makes this irrelevant. Now, if you were using a device that used a transformer to step up or step down, then yes, that makes a difference. But transformers are only used with AC, not DC.

The next question is, what MPPT are you looking at getting. And what inverter do you currently have? Make/Model would help out a lot here.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at https://discord.gg/c7gJ5uA
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#7
(05-14-2018, 09:10 PM)Korishan Wrote: An MPPT doesn't care about the powers of 2. It will take what ever voltage input and convert it to what ever the output needs to be. This is done using pulses through an inductor and a capacitor and diodes on the output to make sure the power is smooth and doesn't feed back.
So, going with 6s or 7s to fake the input, makes no different to the MPPT. It's just the very nature of an MPPT makes this irrelevant. Now, if you were using a device that used a transformer to step up or step down, then yes, that makes a difference. But transformers are only used with AC, not DC.

The next question is, what MPPT are you looking at getting. And what inverter do you currently have? Make/Model would help out a lot here.

I  am not sure how to explain.

The Inverter is a chinese brand, Input range 22-60 V, Mppt Range 24-48V.
The typical 72 cell panel provides about 41 V unloaded. The Tracker starts to load it more and more, in 0,2A steps,a few 100ms per step, until the panel voltage starts to drop. at abot the Panels Mppt Voltage, nominal 33V for mine, the product voltage times amps (the el. power) reaches a maximun. The tracker loads even  more current, afew steps, the voltage drops more, the procuct Volts Times Amps now drops, so the tracker unloads again, Voltage rises. This is an iterative process done by algorithm in the MPPT Trackers firmware. after a few cautious up-down-up-down moves the tracker locks in and stops changing the current. this stays stable until the Panels procuction changes, means Voltage rises (by same current) or Voltages drops, the Software starts again to find the V/A point of max power. Its a try and error process.

(This is done to squeeze the max power out of the panels, independently of the incoming radiation level. and with fading radiation the max power Voltage changes/drops a bit)

Now, if you simply connect a battery, the Tracker loads current endlessly higher, but the voltage never drops steep enough to reduce the V*A product, so the current finally reaches max limit of the inverter. To get the MPPT algorithm find a voltage drop, you need a "current limiting curve", like any protected power supply has. An i use a step up converter, wher i have added, by hardware, this current limiting function.

I use a step up converter for this task. The current limiter is added. The disadvantage of a step up is usually, that it can provide no voltage lower than the input. this is inherent in the function of a step up. It just feeds through then, completely out of control, the current limiter does not work anymore. If the mppt tracker would find that working point, unlucily, it would not find a voltage drop anymore, ending up in loading its maximum current.
But, with a 6s, 23 Volt, minus voltage losses, i am just out of the mppt range.... just a little bit, and the tracker will not reach current lock in at that voltage. It unloads, the voltage rises (to the level defined by the step up voltage adjustment- and just in the middle of the mppt range, say 32 volt as my panels have) ), and the tracker "seeks" there again.

I hope this got more clear now. If not, i will have to add a few current sheets. Sorry little time at moment.
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#8
I know how an MPPT works. That's not what I was asking about.

Just because it says 24V - 48V doesn't mean it's exactly those 2 settings (or 3 if it includes 36V). There's a range at each setting. 24V would be something like 22V - 29V, 48V would have a range of 42V - 58V. The range is the amount of voltage in the battery from discharged to fully charged. That's what I was asking about, not what the settings are.

Plus, you still didn't tell us which MPPT you have. You say a "chinese brand". Most are chinese brand and there loads of good ones and crap ones that are chinese brands. Calling it chinese brand doens't tell us anything. Make/Model and/or link to the product will give us loads more information. If we don't have the datasheet to look at, we can't give you solid answers or reliable suggestions on what/how to build.

If I'm building a house of "some kind of wood" and would like suggestions on support structure. This doesn't tell anything as the wood could be oak, cedar, pine, fir, or balsa. Each has a completely different structural ability and if I used balsa wood, would be better off building out of straw.
Get my point? It makes a difference as to what the device is you are building with to get the correct responses.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at https://discord.gg/c7gJ5uA
(this chat is not directly affiliated with SecondLifeStorage)
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#9
(05-15-2018, 05:38 AM)Korishan Wrote: I know how an MPPT works. That's not what I was asking about.

Just because it says 24V - 48V doesn't mean it's exactly those 2 settings (or 3 if it includes 36V). There's a range at each setting. 24V would be something like 22V - 29V, 48V would have a range of 42V - 58V.  The range is the amount of voltage in the battery from discharged to fully charged. That's what I was asking about, not what the settings are.

Plus, you still didn't tell us which MPPT you have. You say a "chinese brand". Most are chinese brand and there loads of good ones and crap ones that are chinese brands. Calling it chinese brand doens't tell us anything. Make/Model and/or link to the product will give us loads more information. If we don't have the datasheet to look at, we can't give you solid answers or reliable suggestions on what/how to build.

If I'm building a house of "some kind of wood" and would like suggestions on support structure. This doesn't tell anything as the wood could be oak, cedar, pine, fir, or balsa. Each has a completely different structural ability and if I used balsa wood, would be better off building out of straw.
Get my point? It makes a difference as to what the device is you are building with to get the correct responses.
Korishan, I'm sorry about the unnecessary MPPT explanation. I just didnt know what to clarify.
I agree to the range explanation you give. 

The inverter i bought is here (at moment, its not the same but a lookalike) as 600 Watts
https://www.ebay.de/itm/500-600-1000W-So...2906322941
I see it in the "crap" region Smile , but it does properly work until now.



With every respect to you any any others, the information I gave where intended to the ones interested what i do, how i do it and why i do it. I am not fishing for advice, for the moment i know what i want and how to do it. That may change, of course Smile Smile

(But, ideas of support  and improvement are never rejected Smile, or if others want to pick up the idea, thats why i am here )

If i am allowed to ask (here in this thread) then it would be the questions:
- how and where to buy used 18650 in Germany

- what cheap ( <200$) inverters ( about 500 Wp) are known to be able to work with panels and batteries and have an external  control input to switch load/supply of the battery and allow to adjust the power of both from external control. That would be a replacement to what i build by my own, because i believe there is nothing of that in the market. Sure I would prefer to buy if i can.

- is there an "Batrium-alike" BMS system below, say, 200 $ (preferable less) for a full system of Hardware and Software for an 2kWh LI-Ion size. This would change my approach.

Thanks to all.
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#10
So, if I understand correctly, you are going to use a controllable DC/DC converter to throttle the power from the battery to the Grid-Tie inverter as a way of regulating the power that it generates and feeds into your house grid? Neat idea!
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