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DIY Powerwall: A discussion and documentation of my build

 
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By popular demand this thread will document my journey of building a scalable Powerwall for my solar power setup. It's only getting started, but I thought it would be good to start documenting my reasoning for doing what I'm doing with my build. I feel like solar in general is a lot like RMH's in that they are going to be site specific and unique. Yes, there are common parts, but the mix of pieces and scaling of the system are all going to be different.

To start, for anyone who isn't familiar with a Powerwall, it's a term coined by Tesla when they released their own battery storage system for storing the energy you generate from solar panels. It's based off a number of lithium battery packs using the common 18650 battery. Rather than try cover it myself, the following article does a good job with an overview and links to other sources of information.

DIY Powerwall Builders Are Using Recycled Laptop Batteries to Power Their Homes


The Background

A few years ago I moved my family back to the area where my wife and I grew up. Specifically the farm that my grandparents bought in 1968. When they bought it they did a lot of updates like running water in the house, an indoor toilet, electricity, etc. It hasn't really had any updates since then, so for the first year after we moved back I set about major system renovations. The electrical system in the house was replaced from the weatherhead all the way in with up to, if not exceeding, current codes. The primary reason for this was safety as nothing was grounded and most of the house was being run off two fuses. No breakers. So out with the old, in with the new. We are skookum now.
So, now that the house was safe and livable I can start to look at peripherals like my shop. My shop is currently 15 feet of one end of a 40 foot High Cube shipping container. The container is next to a tractor shed that would be nice to have some electricity in, is 150' from the new main panel, and would require trenching through both gardens. Someday I would like to run a line out there, but that has not happened yet for many reasons.
Enter Solar. I already had a handful of small panels, and initially setup a 45 watt system to charge 2 Optima Blue Tops I had picked up from a storage yard cleanout. It appears that those batteries may have set for too long as I cannot get them to take a good charge. Bottom line is I needed to replace them. Lead acid batteries worried me. I didn't want to worry about cooking them or topping off water all the time if I wasn't going to be using the shop regularly. So I started having many, many conversations with myself along these lines;
Where do I store them?
Will they provide enough power?
How much will I really need?
Can I scale the system easily if I want to expand?
How many panels will I need for that?
Lifespan/cycles?
Should I just trench in the grid power and forget it? But the power goes out sometimes. It would be nice if I could backfeed to the house.
Grid tied system? No, no. That doesn't help me if the power goes out.
Inverter. Two Phase?
How many watts will I need if I want to back feed? Wow those are expensive.
What about two inverters? Yeah, one to run my shop needs now and one I can turn on for high load or back feed applications. Hmm... but what system voltage should I run? 12? 24? 48?...
What's the best battery type? AGM? Deep cycle Marine? Golf Cart?
Are there other types of batteries?
Edison Battery, what is that?
Interesting. Can I build that? I CAN build it, but my wife says it's not in the budget. She is right.
Tesla. They do solar, what's this Powerwall thing.




How much are they going to cos..




So it was back to lead acid. What's it going to take? At least 2 batteries, 6 volt, in series... but that life cycle and maintenance...

Wow, this guys got an awesome setup. Man it would take me forever to get there...

All this time I have been gathering parts that could be used in various configurations. I scored a deal on 3 - 100 watt Renogy panels during Prime Day last year. Then recently I caught those panels on sale for $109 each, shipped. So i bought 1 more to give me 400 watts in either 12, 24, or 48 volt configuration. My current solar controller is able to handle 12 or 24 volts.

I have an APC UPS that has a display on it. a few weeks ago I discovered the 2 SLA batteries had gone out. I started wondering if I could use the leads from that to connect to my Bluetops and use that instead of SLA's. A little googling and I ran across Jehu Garcia on YouTube. Specifically this video.





 
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Please be very careful. I've salvaged 18650s myself for use in flashlights. It... has gone well so far, but is not risk free.

I hope anyone choosing to engage in salvage and/or use of 18650s or other lithium cells is familiar with the risk of lithium poisoning.

The risk of problems increases as you add cells.

Balancing is important. I can't watch the linked videos, no bandwidth, so it's not clear what sort of aafety/balancing/monitoring is in use. Bare cells have NO safety mechanisms of any sort, except 'protected' cells which have a low voltage cutoff stuck on the end; salvaged cells are almost certainly not this type.

I used to follow the utube channel Hasta Alaska; he had a pack made from salvaged 18650s. Something went wrong and it burned his van out, fortunately while nobody was in it...

Personally, I run my 18650s in single cell lights. I will take the risk of salvaging them, but running a bunch stuck together is outside my risk tolerance unless there is cell-level monitoring/shutoff involved at a minimum.

Unfortunately this would be a huge hassle with so many cells needed for decent capacity.


Good luck, stay safe!
 
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Dillon Nichols wrote:..but running a bunch stuck together is outside my risk tolerance unless there is cell-level monitoring/shutoff involved at a minimum.


I'm sure this wasn't your intention, but this implies cell-level monitoring is optional when chaining LiIon cells together. I'm not an electrician, but I'm pretty sure it is mandatory.
 
Dillon Nichols
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Mike Lang wrote:

Dillon Nichols wrote:..but running a bunch stuck together is outside my risk tolerance unless there is cell-level monitoring/shutoff involved at a minimum.


I'm sure this wasn't your intention, but this implies cell-level monitoring is optional when chaining LiIon cells together. I'm not an electrician, but I'm pretty sure it is mandatory.



I consider it mandatory to keep the risk level tolerable.

I am not an electrician either, I am not even sure what code would apply. I suspect this sort of thing may not be well covered by codes yet.

A recommendation from knowledgeable folks on the flashlight forums is that you always supervise your cells while charging. This is cells being charged in dedicated chargers, NEVER in series/banks.

At the same time, there are plenty of multi-cell 18650 flashlights on the market, commercially built in China. I am not aware that *any* of them have cell level monitoring, and many use them with unprotected cells. I suspect there are many commercial products with lithium banks/packs where the safety mechanisms are rudimentary or nonexistent. I am uncomfortable with this.

The risk with a mismatched bank is inherently much higher, and one with so many cells, higher yet. Perhaps a statistician would be able to quantify it... to me it's not worth it. Even following SOP of mounting the bank in a dedicated shed, I don't want to have to clean up after a problem.
 
Caleb Mayfield
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Before this thread runs away I'd just like to ask not to get ahead of ourselves. I like to start the story from the beginning and bring it up to where I'm at instead of starting in the middle. I do appreciate the concerns with safety. Without going into all the details right now I can assure you safety is paramount when dealing with energy. Electrical or mechanical.
Please bear in mind that this story, as far as it's been documented here, is only up to the point of realizing that using 18650's are a viable option. Im actually at the point of starting to process reclaimed cells to check viability for powerwall usage. I have a lot to post still. It's coming, include a lot on safety concerns and hazard mitigation.

Also, do please ask questions if you have them. You don't know if you don't ask. Sometimes you have to ask why someone is doing something a certain way for them to realize it can be done better.
 
Caleb Mayfield
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Thank you for your concerns Dillon! They are not unwarranted.

Dangers do exist and must be addressed and mitigated. In my research I have come across a lot of various builders, some very sketchy, and others who are very thorough and safe. I feel that Jehu Garcia and Peter Mathews at HBPowerwalls are some of the best at addressing potential issues and discussing what you might encounter if you are reclaiming cells. I'm sure there are others out there, these are two who seem to be very prominent in this realm.

One thing I think bears specificity at this point is the distinction between types of Lithium based batteries. Lithium-ion, Lithium Polymer (LiPo), and Prismatic Lithium Iron Phosphate (LiFePO4). I am dealing specifically with Lithium-Ion 18650 battery cells.

Used or Reclaimed Cells
Why? They can be cheaper than new cells. They recycle a waste stream that otherwise may end up in the landfill. I say they can be cheaper because in reusing the cells you must check each one individually for viability in your system. You could take a used laptop battery that may no longer work for a laptop and recover 8 or 10 cells from it. Through testing you might find out that only a few of those cells are good enough to use. If you paid $10 for the battery, and say it had 10 cells in it, but only 2 were usable you paid $5/cell. That's about what new cells run. Good, reliable, bulk purchased cells. On the other hand if you crack open that battery pack and the BMS went bad, and all the batteries test out you could end up with a really good deal at $1/cell. This video is a good run down on the topic, and this video is a good overview on extracting cells. Both guys have much more in depth videos on the topics.
Now, if you are asking what the BMS is, it's a Battery Management System. This device monitors each cell and manages it depending on the type of BMS you get. This is one of those safety features you should not skimp on. Some DIYers do not use them. Some do. There are reasonable arguments on both sides, and unreasonable arguments. The thing is it depends on the application and the usage. I plan on using a BMS and will expand on that when I get to that part of the build.
18650 Lithium cells are in a lot of places aside from laptop batteries. There are now vendors who specialize in collecting, testing, and selling them. alarmhookup on eBay is one that I have had excellent results with when sourcing used 18650's.

New Cells
What about new cells? Well, if you want to maximize the life of your pack, and reduce the risk of having a battery cell or pack failure, and you can invest the extra up front cost in them, do it. Find a group buy on a large lot of name brand cells(LG, Samsung, Sony, Sanyo, etc) from a reputable vendor and go for it. I'd love to, but we are not going full scale solar at this point. I'm trying to get my shop solar powered as a proof of concept. If we are able to build the home I've always dreamt of building it will be solar powered, it will have powerwalls I built, and I will use new cells.

So what risks are there? As was mentioned previously, the worst case would be having a melt down. To be fair though, you can have a melt down with lead acid batteries as well. I've seen that happen. With lead acid batteries you generally do not have a BMS that manages each cell or bank to ensure it is charged correctly and disconnected if there is a problem.
To mitigate this there are a number of things you can do. The first is to get all the same cells, or cells that are identically rated. Although the cell is an 18650 format, there are differences in capacity (mAh), voltage (V), charge rate, discharge rate, max discharge rate, and so on. How do you determine what is what? Data sheets. They can be found a lot of places, but the fine folks at DIYPowerwalls, aka http://secondlifestorage.com, have created a database to log and track the various cells on the market. So we need to get an idea on how many battery cells we are going to need.

Lithium-ion cell packs have 3 voltages that you need to get initially. Nominal, minimum, and maximum. Generally, all lithium-ion cells will have a nominal voltage of 3.6 or 3.7 volts and a max voltage of 4.2 volts. The minimum varies the most from what I have seen, but most builders will say you should not discharge below 3.0 volts as a general rule. Your needs and spec sheet can dictate how far you go, but the more you deep discharge them the less life you will get out of them.
On the spec sheet you will also find the capacity of the cell measured in milli-Amp-hour (mAh), and sometimes they are kind enough to give you Watt-hour (Wh). What we need to determine is the Wh/cell. For system sizing use the Nominal voltage. The batteries I will be using are rated at 2600mAh.

3.7V x 2600mAh = 9620 mWh / 1000mA = 9.62 Wh per cell. My initial target is to provide 1000 Wh, or 1 kWh, of power to my shop. My lighting, all on, draws 180 watts. I typically go out there for a few hours at a time, and only use half the lights. Rounding up, in theory and without inefficiencies, I could run my shop lights for 5 - 10 hours depending on how many lights I have on. Long term I'd love to expand to 3-5 kWh of available electricity to run my bench lathe or mini mill, and be able to back feed to the house in the event of an extended power outage.

So, 1000 Wh / 9.62 Wh = 103.95 or 104 cells. I came across a bulk listing of battery packs that have 3 cells in them, in boxes of 33, so you get 99 cells if you want to disassemble them. I ordered one box and should have gotten 2. In communicating with alarmhookup they are getting more of these in all the time and will run another auction when they get enough tested.

Now that I have a cell count, I need to determine how these cells will be arranged, and that's going to have to wait for the next post.

 
Dillon Nichols
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Looking forward to watching the project progress!

I promise I will shut up about safety concerns after this for at least a few minutes!

A meltdown/explosion is indeed the Bad Thing to be scared of.

What may not be known to all, is that the danger from released toxicity is a potentially big deal.

Linked below is a thread detailing the experience of a very unlucky person who had a pair of CR123A flashlight batteries blow up in the same room as him. These are 1/2 the size of an 18650; while the chemistry differs, both have the potential for hydrofluoric acid poisoning as described in the linked thread.

http://www.candlepowerforums.com/vb/showthread.php?340028-Flashlight-Explosion
 
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This computer is powered by an Tesla EV battery run off solar most of the time ( except for cloudy days ).

I run a group on MeWe for lithium battery builds. ->

https://mewe.com/join/lithiumbatteries18650diyevpowerwalls

Be sure to not charge the lithium batteries below freezing, this can kill the batteries.

Power to the cause!  :-)

 
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I just saw this article about Bird app-driven rental electric scooters accumulating in impound yards in cities (most cities) where Bird has not taken care to comply with local permitting and parking laws.  They are expected to become available in bulk at impound auctions very cheaply.  There are a lot of interesting components in these $300 units, but germane to this discussion is that they are full of 18650-based batteries:

Right now, scooter hacking is becoming one of the most interesting adventures in modern-day hacking. You’ve got batteries and electronics and motors just sitting there, ready for the taking (and yes, through these auctions you can do this legally). We’re looking at a future filled with 18650-based Powerwalls from discarded electric scooters and quadcopters built around scooter motors filling the skies.



(The article also explains that if you these scooters at impound, you can take out the Bird rental circuit board (totally legally, it's your scooter now) and replace it with a $30 board from China to make it like new again.)
 
Caleb Mayfield
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Dillon Nichols wrote:Linked below is a thread detailing the experience of a very unlucky person who had a pair of CR123A flashlight batteries blow up in the same room as him. These are 1/2 the size of an 18650; while the chemistry differs, both have the potential for hydrofluoric acid poisoning as described in the linked thread.

http://www.candlepowerforums.com/vb/showthread.php?340028-Flashlight-Explosion



Dillon, That is a scary story indeed and worth reading. Particularly SilverFox's comments. One thing to note is that both types of batteries have a different chemistry and only Primary (non-rechargeable) cells based on the Li-(CF)x chemistry can produce Hydrofluoric Acid as that is the only current Lithium based chemistry with Fluorine atoms. Batteries like CR123 batteries with a nominal 3 volts have this. Page 3 of that thread starts to discuss it. Li-ion rechargeable 18650's do not. They do have their own hazards though, HF is not one of them thankfully.
 
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Caleb Mayfield wrote: ...and only Primary (non-rechargeable) cells based on the Li-(CF)x chemistry can produce Hydrofluoric Acid as that is the only current Lithium based chemistry with Fluorine atoms.



Everyone:  I get that everyone is trying to help, and making suggestions, and you think that reading stuff on the internet that hackers and DIYers have done somehow makes it true, but the amount of incorrect, missing, and generally misguided statements which are unsafe in this thread is concerning.  

First off, CFx is absolutely not the only type of Li-based battery that forms HF in during combustion or venting.  All of the standard Li-Ion cylindrical cells you are proposing have a liquid electrolyte which is an extremely complex mix of usually more than 15 different chemicals and additives; things like EMC, DMC, and, of course, LiPF6 (lithium hexafluorophosphate).   The "F" in LiPF6 readily forms HF when exposed to water, including moisture in the atmosphere.  I am betting you don't have a calgonate kit on hand in case you get some electrolyte on your skin.  I'm betting you don't know how HF goes into your skin, and then effectively continuously leaches calcium out of your bones which if it happens to a large enough extent is painful, debilitating, and completely untreatable once it happens (unless you treat with calgonate quickly).   Which is why no one in our cell chemistry lab is allowed to work with or open cells until they've taken HF training along with specific cell safety training.

Other things of note: someone mentioned that if you charge cells below freezing it can kill the battery.  True.  But it's not that simple; you need to understand WHY that happens, because there is a direct safety implication there too.  When you charge a cell either too quickly, or below a certain temperature, you can't intercalate Li ions into the anode at the same rate you're pushing them across the separator boundary, and you end up plating out metallic lithium (which is then irreversible by the way).  That plated lithium forms dendrites which then eventually grow to puncture the separator, and short the cell internally.  If it's a small dendrite, it can form a "soft short" which keeps discharging your battery.  But if that grows, or forms a larger short as it tends to over time by melting back the separator in the surrounding area, and if you have no dV/dt monitoring to catch it and stop using that battery, there is a reasonable probability that you WILL start a fire.   So what's the magic temperature?  There is none, because it's a relationship between temperature and charge speed that is specific not only to a particular capacity or chemistry of a cell, but how cell is rolled, the porosity of the separator and any protective coatings on it, the electrolyte used, the level the anode was calendared (compressed), the ratio of natural to synthetic graphite used in the anode, blah blah blah.

I applaud anyone's efforts to do home-builds as I love the can-do attitude, but from what I'm reading here, just don't think for a second that your build is going to be 'safe.'  I'm not saying that it's not going to work and possibly work well for years, but that doesn't mean it's inherently safe.
 
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Ken Zemach wrote:
I applaud anyone's efforts to do home-builds as I love the can-do attitude, but from what I'm reading here, just don't think for a second that your build is going to be 'safe.'  I'm not saying that it's not going to work and possibly work well for years, but that doesn't mean it's inherently safe.


What WOULD be safe then? I can't afford (and don't want) a premade powerwall, I will need something of this ilk at some point, what IS safe? I read your post and all the unsafe things, what is left? What is a good answer? You have a LOT more experience than me, what do I need to look at?

 
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Pearl Sutton wrote:What WOULD be safe then? I can't afford (and don't want) a premade powerwall, I will need something of this ilk at some point, what IS safe? I read your post and all the unsafe things, what is left? What is a good answer? You have a LOT more experience than me, what do I need to look at?




So perhaps I'm a bit more safety oriented, but I work in the field (obviously), so I'm going to lean on only releasing things which are safe.  There are generally three types of controls one considers when putting a complex LiIon system together:

1. Ensure the incoming cells are "as safe as can be reasonably assured."   This is done by a) only purchasing from a reputable manufacturer and b) assessing the health of each and every cell, even though they're brand new, before they go in a product.  There are a couple of ways this can be done, such as AC impedance checks, self-discharge rate, etc., but all require you to know what the results for that cell are supposed to be.  

2. Ensuring your assembly is well protected, monitored, and features various safety checks and disconnects.  Things like low voltage cut off (that's more to protect the cells; not so much a safety thing), temperature monitoring (and possibly control; note that Tesla and Chevy and basically almost every EV manufacturer out there save Nissan has liquid cooled/heated battery packs), current interrupters (usually an integral part of the cell design), PTC (again, usually designed into the cell already), brick level voltage monitoring (meaning monitoring the voltage and as well as charge and self-discharge rate of each parallel string of cells), and I'm probably forgetting a few, other than having your charge and discharge controller (part of your BMS) take all this into account.

3. A system design that accounts for what will happen WHEN a cell goes into thermal runaway.  Because it happens, even if you do everything right, there is a very very small, but not zero chance that any given cell may have an internal defect that was not caught in cell formation, incoming checks, and even the first few years of life.  So if that cell DOES go into thermal runaway, what is your propagation protection to keep it from setting off its neighbor, and then that one setting off its neighbor, and getting a chain reaction.   Sometimes it happens, but you'd be surprised at how a well-designed module or pack can have multiple cells be thermally initiated and NOT translate into the entire battery catching fire.    Think of this as planning for the worst case scenario.  Go search the internet for a video of a teardown to the cell level of a Tesla Powerwall, or Model 3. [Edit:  forget the powerwall videos, all the ones I just checked are from older Gen1; just go straight to the Munro work on the Model 3.] Then ask yourself "what the heck is all that stuff in between all the cells?"  Answer:  protection.


Honestly, what _I_ would do if I were going to do a home build is either salvage an entire pack from some EV that has hit end of life, or use deep cycle lead acid.   Seriously.  I would build a small module (say, maybe 9 cells in a 3x3 config max), but after that, you would not catch me trying to assemble a larger home build out of Li-Ion cells.   Ask yourself why the TSA won't let you check batteries.  Terrorists?  Riiiight.   If the general public knew the number of Li-Ion fires that occur on planes each year, they wouldn't complain about the packing rules so much.  Yeah, it's usually less than 10, but it's always more than 1. [Edit:  it's worse than I recall...  "According to FAA records, there were 46 incidents with lithium-ion batteries on aircraft in 2017 -- up from 31 in 2016."  Remember these are commercial batteries made by real companies.]

If after all that you still REALLY want to do it, and you don't want to buy something already assembled at the base level from a place like BatterySpace.com (which is still very ghetto, but better than a home-build), then just understand the level of risk.   I'm not saying don't do it, but if you're going to do it, figure out what your #3 is.  If/when something goes wrong, ensure it won't harm a person, or cause significant financial harm.   Then have at it.
 
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Ken Zemach:
Thank you! That makes sense :D
 
Caleb Mayfield
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Wow. Gone for a few days and I feel like the train has gone off the rails.

Ken, thank you for pointing out my error. The way I stated that was poor.

Dillon, my apologies. Ken has correctly stated that there are other chemicals in 18650 batteries that contain fluorine. In the case of my batteries it is a constituent of the electrolyte that represents a maximum of 3% of the net contents of the battery. So technically there is still a risk of HF exposure in the event of a rupture.

Ken, thank you for taking the time to explain what you have. It is very valid and well written, and I appreciate that. What I have issue with are statements like

Ken Zemach wrote:
I am betting you don't have a calgonate kit on hand in case you get some electrolyte on your skin.  I'm betting you don't know how HF goes into your skin, and then effectively continuously leaches calcium out of your bones which if it happens to a large enough extent is painful, debilitating, and completely untreatable once it happens (unless you treat with calgonate quickly). Which is why no one in our cell chemistry lab is allowed to work with or open cells until they've taken HF training along with specific cell safety training.  


and

Ken Zemach wrote:
... but from what I'm reading here, just don't think for a second that your build is going to be 'safe.'  I'm not saying that it's not going to work and possibly work well for years, but that doesn't mean it's inherently safe.



I have issue with that because as far as I am aware you don't know who I am, and have a very limited view based on what I have had time to document here. Which is very little, relatively. Yes, I made an error and I appreciate you identifying it. I don't mind owning my mistakes and learning from them, but I hope you are not really a gambling man as you would have lost both bets. If you were directing that to any reader in general then you are most likely correct. Most people are not going to have a kit and know much about HF. Then again most aren't going to be working with large numbers of lithium cells. building their own powerwall, or working in a Cell Chemistry Lab with a much greater chance of exposure to larger quantities of dangerous chemicals.
I think a recommendation on having a calgonate kit on hand, a link to where to get one, and a description on how HF goes into your skin would be a great addition to the Safety aspect of this thread.

As far as this build being 'safe', can I ask you to you withhold judgement until I've actually had a chance to explain how I'm building it? Because I would greatly appreciate your input, if you're willing, as this build continues. Your response to Pearl's question is spot on. The 3 points you layed out have me asking if you're telepathic.
 
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Pearl, I would second Ken's recommendation on how to assemble your own "Powerwall", be it lithium or lead-acid based, although deep cycle lead acid batteries are hard to wall mount. (Yes, my sense of humor is a touch dry). There are a few people starting to get their hands on reclaimed Tesla battery modules from cars and using them more or less as is to create a powerwall. This video is the first one that comes to mind. It can be done.

Personally I think it's difficult to put a hard definition on what system is 'safe'. I believe a lot of 'safe' depends on the user and the application, and on hazard identification and mitigation.
Lead acid batteries can be extremely dangerous. You still hear of incidents where a battery melts down or blows up. Lead acid batteries also the require routine maintenance of adding pure water to a cell. That means a routine exposure to acid. The reason you add water is the charge and discharge cycle generates hydrogen gas. So you have to ensure adequate ventilation to make sure you don't end up with a hydrogen explosion. There are risks. They can be mitigated.
Ken has given a good overview on the lithium hazards and what should be incorporated into a system. Again, there are risks and I believe they can be mitigated if you take the time to research and understand the systems and operate within the design criteria of your components. Even then, I believe the design must incorporate critical failure safe guards, regardless of which one you decide to use because there are aspects of ANY system that are beyond your control.

I believe that at the end of the day each individual is going to have to assess their own application, evaluate the options, and decide on which set of risks and hazards they want to live with and mitigate.

Obviously in my case I have decided I do not want to mitigate the lead acid risks. I believe that in my application, the lithium battery bank will be safer and more manageable power supply.
 
Caleb Mayfield
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Battery Testing and Sorting

Currently I have a baseline target of 104 batteries at 2600mAh. That will provide the total capacity that I am targeting, but I also need to evaluate actual power draw and cell load. I plan to cover that later, just know what it's something that needs to be factored in. Right now though I am collecting cells, testing them, and sorting them. So what do we test for? In general terms we are looking for the battery capacity, ability to hold a charge, and internal resistance as measured by AC Impedance. If you start digging into this there seems to be a lot of opinions on how to test and sort batteries, what is most important, and the order you should do them. I'm of the opinion the AC Impedance test should be done first. As Ken pointed out though, to make any meaningful determinations from this test you need to know what the AC Impedance is supposed to be to begin with. So far LG, Samsung, and Sony have the best Product Data sheets that I have seen with LG providing really thorough data for the batteries I have looked into. A good place to start is the Cell Database over at Second Life Storage. If they do not have your battery listed, and there is a number down the side of your battery type that into Google along with Product Data. If that doesn't turn up any results, I recommend looking for a battery you can test.

So, in my case I'm starting this build using LGABB41865 batteries. The LG Model number is ICR18650B4. AC Impedance is 70 mOhms or less for my cells.
To test the AC Impedance, you really need to invest in a unit that uses 4 wire testing at 1kHz AC. We could start a whole new thread on this topic alone, but I'm going to leave it at this more or less. The YR1030 units seem to be the best bang for your buck in terms of cost vs decent measurement. For $50-$75 it's a relatively cheap way to add a degree of safety to this project. Having a unit to take measurements with can also help you down the road in troubleshooting if you start having a pack that is giving you fits.

After collecting battery packs, I need to remove the individual cells, clean the nickel strips from the ends, and inspect for any physical damage. WEAR GLOVES AND SAFETY GLASSES. Set aside any that have dents or dings. Those go to the recycler.(Overview video. I advise against the "whack it on the ground" method) Now that I have a pile of cells, they need to be tested. By checking the AC Impedance now with the YR1030 I get cell voltage and a PRELIMINARY impedance value. I say preliminary because the product sheet specifies testing at full charge. Why not just charge the cells? Time. It can take a while to charge a cell, so I like to sort out anything I don't want to waste time on now. Yes, there will be more getting discarded at each level of testing, but his get's the bulk of them for me. In my first batch of 99 cells, 2 were DOA. Totally dead. 7 of them tested over 70 mOhms(over 85 mOhms at full charge). Of the remaining 90, they tested between 40.8 and 60.5 mOhms. I use a sharpie and write this preliminary value on the side of the battery.

Now that I have sorted out the batteries I'm mostly interested in, I need to charge them and test them for capacity. I use an Opus BT-C3100 v2.2. There are others out there that in some respects are better, but I like the features of this one and it does well. There is a guy in the forums on Second Life Storage who has run numerous tests on multiple units of different makes and models. The Opus measures up well. I believe his review is in the FAQ now.
Four at a time, the batteries are loaded into the Opus and run through a test cycle. The cycle charges the battery up to full capacity, rests, then discharges it at a set rate until the voltage drops to a set voltage. It then calculates the capacity of the battery in mAh based on how long it took to discharge to said voltage. After it has completed the discharge test, it then charges the battery back to 4.2 volts and blinks between FULL and the mAh rating. I use a sharpie to write that value on the side of the battery below the mOhm value. The cells get set into a tray and the next 4 go in. I'm doing blocks of 36. When I get 36 batteries capacity tested and recharged I let them sit a few days, then run back through the AC Impedance test. If the value is +/-0.3 mOhms I call it good. If there is more that that I will remark the battery. I also check the voltage as the YR1030 provides that along with the mOhms. If any battery has dropped more than 0.5 volts since charging I pull it from the block and monitor it over the next few weeks to see if it really is self discharging or not. Anything that is self discharging gets pulled. Anything that tested over 65 mOhms during the full charge test may be usable in some projects, but I don't want to build it into the powerwall. Particularly when I have 70 battery cells testing 45.0 +/- 5.0 mOhms with capacities between 2650 and 2780 mAh.

So currently I have 70-75 batteries that are excellent and another 15-20 that are good to very good. If I can get another case or two in the few weeks to month I'll have enough to hit my target capacity.
 
Caleb Mayfield
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Quick update; I have 70 good cells from an eBay auction and have ordered another 80 from an online source that should match the ones I have. However, online sources can be total scams, and due diligence says to treat them as used cells even though they are allegedly new. Once those arrive and have been fully tested and sorted, I will use the best 70 from each set and begin to assemble 20P cells of 10 each from the two types. Since these are not from the same source and manufacturer I am mixing them in each block with the goal being to keep the entire system in balance. The 20 cells in Parallel will act as one large cell and be connected in series with other blocks. I need 7 blocks in Series to operate in the 24 volt range.

My initial plan was to harvest parts from the UPS to provide the inverter and possible grid-tie/charging to the battery bank. In the past few weeks though I've realized that I need to make some changes in the shop and that the UPS is not going to provide what I'm looking for. So, I ordered a Pure Sine Wave 24V 800/1600 watt inverter to provide power to the shop from the batteries and the solar array. This should provide all the power I will need in there and then some and room to grow.
My startup limit is 7S20P. If I limit draw on the batteries to 500 mA/cell I can use roughly 200 watts with inefficiencies in the inverter. That will be plenty to get the lights running and at 140 cells at 2600mAh that's 1.3ish kWh of power.

Long term goal is to continue collecting cells and expanding the battery bank out to where if needed, I can draw the full 800 watts on the inverter, if you could actually sustain 800 on an 800 at 500mA/cell.

One thing that is needed is a BMS, Battery Management/Monitoring System. That is a bit of an umbrella term as the cheap end of it is really a battery protection board that disconnects the battery from the system in the event of an over or under voltage on a cell, and the upper end is a full on management and balancing system with displays, graphs and interactive tools. I am opting for the full DIY experience and building my own BMS based on the work being done by Stuart Pittaway and Colin Hickey. In terms of an off the shelf option, Batrium seems to be the leader in this area with the Electrodacus gaining ground for dedicated solar applications.

I'm opting for the diyBMS for many reasons. The biggest one being the ability to tailor it to my system and integrate the level of monitoring and safety disconnects that I want in my system. That could be another thread in and of itself, but the short of it is I think fuses and breakers should be the secondary shutoffs and that a monitoring system with redundant sensors should provide the first level of protection.
 
Mart Hale
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I have been testing medical cart batteries from hospitals.      They are required to change them out every 2 years, so the supply is good.

I am pleased so far with the solar charging I have done with these, they come in 3s config so very handy.      The trouble with them is they will cut an inverter out before they are at min capacity,    I have been using the with direct DC with USB devices and I am pleased with them.
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Caleb Mayfield
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You are fortunate if you have a steady supply of those. I'm trying to locate one where I am at. Those packs are generally some of the best second hand batteries you can get as they either see few cycles or light loads.

 
Mart Hale
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Caleb Mayfield wrote:You are fortunate if you have a steady supply of those. I'm trying to locate one where I am at. Those packs are generally some of the best second hand batteries you can get as they either see few cycles or light loads.



Ebay is where I got mine.     I will know better soon after my load tests.     But for direct DC power supply no inverter this really is nice.     I was able to get a bench grinder running off just one pack of these.     Appears to be fused for 60 amps on the BMS, but I am bypassing the BMS and will use my own.     Batteries were perfectly balanced when I checked.
 
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