Need help with battery bank setup

[dawgleader1]

Hello, I'll start by saying I'm brand new to the forum and to solar in general. I have been doing research and even talked to a few online vendors about my battery bank setup and I'm very confused because I'm getting mixed information and some of it is running counter intuitive. I'm trying to figure out the following:

If I take (2) 24v/ 3kw LiFePO4 Powerwall all-in-one battery packs and connect them in series to get 48v and then connect that to another pair of the same battery pack in series what is my total battery power storage? I am thinking it's 12kw (4x 3kw/ battery pack). Please confirm that's correct, this is my main question. I'll elaborate my plan below to give some back story.


I'm wanting to build an off grid, back up, emergency solar system to run if the lights go out but won't be used every day as we are grid tied as our primary power source. As I said, I'm new to this so I'm trying to make it as simple as possible. So my plan is to get a 48v all-in-one inverter (https://www.ebay.com/itm/USA-Supt-48...gAAOSwlD5eVqbD) and then connect it to 4 re-purposed LiFePO4 Powerwall battery packs (https://bigbattery.com/product/24v-l...getvideosystem). Then I'll connect 10 or so 305w solar panels and I believe I should be good to go. I want to have 10-12kw of battery stored backup power so if I run a couple days without sun I can run the bare essentials until we get sun again and can recharge up. The way I figure it the plan looks good however a couple people have told me that if I connect 2 of those powerwalls in series I'll get my needed 48v for the inverter but I WON'T get my 12kw of battery storage I need. Their reasoning is that connecting 2 batteries in series increases the voltage but doesn't change the in-series battery capacity. But that doesn't make sense to me, it looks like as long as each battery had a 3kw beginning storage capacity then it shouldn't make a difference how I connect them together, either in series or parallel, I should be able to retain the total battery storage capacity of the 3kw x the number of batteries I'm using. I see it like this, W= V x A; a 24v 3kw battery would be at around 125Ah (3k/24). So if I take 2 of these and put them in series then that doubles my voltage to 48 but the Ah stays the same so I'm at 48v and 125Ah which is 6kw, the same as the 2 batteries added together. And then if I take 2 of these in-series pairs and run them parallel then I'll have 12kw (6kw + 6kw or 4 x 3kw).

my solar diagram2.PNG


But I'm being told that it would be 48v but only at 3kw per pair since they're in series for a 4 battery total of 6kw. I don't understand why I'm losing half of my battery storage capacity... Another reason this is important for me to understand is I'm considering doing a 3rd pair in series to up my total battery storage capacity to 18kw (6 batteries x 3kw/ batt) if my initial setup goes well.

Please help, I want to order my system immediately but don't want to get the wrong stuff and waste a hell of a lot of money by buying the incorrect batteries. Also any other advice on my layout or idea would be greatly appreciated, even if it's 'hey moron you're way off base here because....".

Thanks very much for the help and guidance.

[Mike90250]

Your sketch looks correct. 12kwh /48v = 250ah

What may not be valid, is the way the batteries can be wired with their internal BMS systems.

[Ampster]

I am not as much concerned with the internal BMS as I am concerned about the quality of those batteries. They are used and there is no guaranty of the capacity.
More importantly, I did some research on that company. Because at that price I was intrigued especially since I could pick them up in the SFV and save shipping. . I don't think I can post links but I discovered public information that the owner of the company spent some time in jail for copyright infringement. I know there are always two sides to a story but it might be worth your while to do some research to properly assess the risk. That price is still over $200 per kWhr plus shipping.and when I inquired there was no guaranty about declared capacity and you would have to rely on the integrity of the seller. These definitely did not come out of a Tesla Powerwall and the fact that they even used the term "powerwall" may be a copyright infringement or, if not, just a marketing attempt to mislead.
You can Google Eric Lundgren and form your own conclusion. Reasonable minds will differ.

[dawgleader1]

Hi Mike, I appreciate the comment and the confirmation that I'll be able to get at least close to my 12kw goal of battery storage with my 4 'powerwall' battery plan irregardless of how I configure them. That makes me feel better about my plan. I will definitely take your advice and check on the wiring of the batteries together in regard to the BMS, thanks for pointing that out. If you have any ideas or can weigh in on what Ampster said as well as my additional explanation below I'd greatly appreciate it.


Hello Ampster, I appreciate your comments and your research. I completely understand your hesitation and concern, I don't trust anyone as a general rule, there are just too many people out there trying to get over on others and the internet makes that easy sometimes. That's one reason I'm on here, to confirm my research and gather other people's opinions and ideas to make the best decision for me. I'll try and explain my reasoning and thinking that this is the best route for me and then hopefully you'll weigh in a little more and offer a little more help to this ignorant newbie, lol.

The company is upfront about the batter packs/ 'powerwall' being used/ re-purposed which I have no problem with as long as they work; I always buy used cars a couple years old to avoid deprecation. If they work then I feel I'm getting a great deal and also saving some of the environmental affects of old electronics/ batteries. These battery packs are originally set and rated at 4kw but the company is listing them as a 3kw on their site to take into account their 'used' state and in an attempt to be conservative they are only claiming 75% of the new rating. They claim that the batteries are 3-5 years old and from my research most of these type lithium batteries should have a 15 year or so life expectancy, of course based on usage and how they are cycled. Of course I have no idea how they were used or how they were cared for but, while I'm new at this, that 3kw sounds reasonable but I may be wrong on batteries, maybe they have lost more of their life than that. Also, since they source these by the hundreds from large companies I would think that most of the large companies would care for their batteries pretty well to avoid issues and failures. From my understanding these 'powerwalls' are basically these used batteries here ---> https://www.batteryclearinghouse.com...lar-power-pack, that the company then builds up with the BMS into a self enclosed case. Those used batteries can be found at numerous online sources and most other sites claim more than the 3kw that the company I'm considering does so I'd consider them being conservative a plus in their favor. As you suggested I did Google Eric Lungren and while you are correct that he did go to jail, if the information is correct that I read, I have no problem with his 'crime'. Firstly, what I see is that he has a long history in the electronic recycling/ re purposing world which, in my opinion, experience is a good thing compared to many other online companies that just jump into something for the money. Secondly, his 'crime' was that he created Microsoft restore disks to give away with recycled computers he was selling. While I'm a big fan of Microsoft's systems because they make our lives better and easier, I am not a fan of how Microsoft and many other companies use their size and influence to control the little guy so in not so polite words, screw Microsoft, lol. Thirdly, from what I read he didn't do the copyright infringement in an attempt to defraud his customers and this 'pirating' didn't harm any of his customer that I can tell. As far as the company using the term 'Powerwall' on their website, yea you're right that's not correct or proper but I think most likely it's an honest mistake and on top of that the term is probably going to become a pretty popular word, just like many people call all soda Coke whether it's RC, Pepsi, or another brand so again not a biggie for me. The initial and primary reason I ended up liking this battery system is because I came across it from Will Prose, a popular utuber on solar, who suggested it.

Please don't get me wrong, I am trying to get your opinion, not argue that you're wrong. I honestly want to know what you think so that I can make the best, educated decision for me. However, if I decide to not purchase these battery packs, do you have any other suggestion? Would I love to buy a bunch of new BattleBorn batteries, hell yea, but it's not in my budget. To get those or a comparable new Lithium battery bank for 12kw would cost me more than quadruple of what I can get these used battery packs for. BattleBorn would be $1k each for a 24v50Ah battery at 1200w so I'd need 12 of them for over $12000 plus tax. That would be my dream battery but I can't afford $12k plus the panels and inverter and, etc... However if you know of a different route or source then please, please let me know before I make a mistake. Once again, I promise I'm not being argumentative, I'm honestly looking for help and guidance for people more knowledgeable than me on solar and electronics.

Thank you very much for your time and willingness to help, I'm sure you've very busy with life and I appreciate your feedback and input very much!

Sincerely, Scott

[bcroe]

Hi Mike, I appreciate the comment and the confirmation that I'll be able to get at least close to my 12kw goal of battery storage with my 4 'powerwall' battery plan irregardless of how I configure them.

You will please note Mike said KWHour, your KW do not represent energy storage. Bruce Roe

[Ampster]

I don't disagree with your analysis. I also read about the heavy handed Microsoft tactics. Keep us informed about your progress.

As far as hook up strategies a lot depends on how you plan to monitor them. If you have the skills to get the internal BMSs working or at least reporting that would be good. To me the most important use of a BMS is the reporting ability, especially with used batteries. If there is some imbalance it will show up near the top of a charge cycle or near the bottom of a discharge cycle.

If you can't get the internal BMS working then the least expensive solution is a voltage meter and some graph paper. I use a logging BMS that let's me see the trends and set my overall high voltage disconnect (HVD )and low voltage disconnect (LVD) accordingly to keep the pack healthy.
I don't want to make this a long post but there is some more knowledge to share. Don't hesitate to ask questions and do visit the Lithium battery stickies..

[dawgleader1]

Hey Bcroe. Thanks for chiming in. Please clarify/ dumb down your comment for me. If I understand what you're saying is that 12kw is different than 12kwh? Am I correct in what you mean is 12kw refers to straight energy as it just means kilowatt and 12kwh with the added H refers to battery energy storage? So I'm just using incorrect nomenclature and that needs to be corrected to be properly understood. Thanks again for stepping in to help.

WB Ampster. I think I've decided to go this route with your help and some other discussions I've had with others. I realize it's somewhat a gamble but hell, I did live in Vegas for awhile so what the heck. Unless I blow myself up or burn down my house I'll try to update some info on here as I get into it. I'm just trying to make my decisions and get an order done forwhat I need now before the whole world shuts down and I can't get anything delivered. I will do some more digging and reading on the BMS issues you've mentioned as well as other hook up strategies. If you have anything else I should know or any red flags I should be aware of please let me know. Links and other source suggestions are also great. I'm trying to make this as simple and straight forward as possible for myself since I'm not an engineer or electrically inclined but I want to get a system up and running as soon as I possibly can, this seems the easiest way right now. As I get into solar more in the future I'll probably be more like you guys and want to build my own whole house 30kwh, off grid system by starting with a couple AAA batteries, some duct tape, and a roach clip.

I'm probably going to place an order in a couple hours. Thanks very much!!

[Ampster]

.....

The initial and primary reason I ended up liking this battery system is because I came across it from Will Prose, a popular utuber on solar, who suggested it.

Please don't get me wrong, I am trying to get your opinion, not argue that you're wrong. I honestly want to know what you think so that I can make the best, educated decision for me. However, if I decide to not purchase these battery packs, do you have any other suggestion? Would I love to buy a bunch of new BattleBorn batteries, hell yea, but it's not in my budget. To get those or a comparable new Lithium battery bank for 12kw would cost me more than quadruple of what I can get these used battery packs for. BattleBorn would be $1k each for a 24v50Ah battery at 1200w so I'd need 12 of them for over $12000 plus tax. That would be my dream battery but I can't afford $12k plus the panels and inverter and, etc... However if you know of a different route or source then please, please let me know before I make a mistake. Once again, I promise I'm not being argumentative, I'm honestly looking for help and guidance for people more knowledgeable than me on solar and electronics......

Sincerely, Scott

I realize I didn't answer you last question about alternatives. I saw that video and another that he did on Tesla modules Big Battery did have 5kWhr Tesla modules for $1200 which is $240 a kWhr vs $233 for the powerwall batteries. They apparently don't have any more but I saw some on Ebay for similar prices. The trick with Tesla modules is to try and get them all from the same car which could make the process more expensive. . LFP chemistry is much safer than the Tesla modules. It is also some comfort that those LFP modules came out of a medical or other controlled environment which is less harsh on batteries than a motive application. So it sounds like those Big Battery LFPs would work for you. I can promise you won't hear a. "I told you so" from me/

Once you get them and find out about the BMS give us a shoutout because there are a number of alternatives. Depending on your environment, temperature could also be an issue. At the most you would need four 8 channel BMSs or two 16 channel BMSs. Some of the higher end ones have temperature sensors.