Hello,

I just stumbled on your project, and it is very impressive! I'm very interested since I started looking at building a similar solution for my sailboat!

However, I just have a question regarding the capacity of your Phases. It seems to me that your calculations are wrong.

Since there is only 70 cells in parallel in a module, it seems to me that the capacity of the module is 10.44Wh*70 = 730.8Wh. Now, even if you were to put 100 of those modules in series, this would only serve to augment the voltage, not the capacity of the powerbank, that would stay at the nominal capacity of one module (730.8Wh). Now, if you were to connect your modules in parallel, then the capacity would be 10950Wh, but with a voltage of 3.7V.

Am I mistaken here? What am I missing?

For those eventually reading, I've answered my own questions.

I made a confusion between capacity (expressed in Amps-hour, Ah) and energy (in Watt-hour, Wh).

When you put the cells in series, the capacity doesn't change (thanks captain obvious) BUT the energy stored does. You calculate the energy by multiplying together capacity and voltage, so if the voltage goes up, so does the energy.

So, yeah. capacity, energy, two different things.

Thumbs up on your dedication and commitment. 

You are working very hard to hand build what will someday be available to the masses as prepackaged units.  You are a pioneer.

I have worked on some industrial lead acid ups systems and the challenges are great.  

what. a. savage.

Hi. Balancing is performed by the electronic cards that you can see on top of battery modules. Please see my project #A Lithium Ion Battery Management System  for a complete description of the BMS. 

Each group of cells wired in parallel has its own balancer.  I have 14 groups in series, so there are 14 balancers.

What do you exactly mean by P-L Layout ?

Hello,

I made some verifications about the parallelling of many cells. I guess you should check this video out : https://www.youtube.com/watch?v=NpSrHZnCi-A

You will learn that Tesla Model S batteries are made of 16 modules wired in series. Each module is a 76P6S pack. Finally this makes a large 76P96S battery containing 7104 cells.

Assuming that Tesla are competent regarding batteries, one can say that there is no issue wiring many cells in parallel. By the way, If you think about it, there is no other way to build a large pack.

Regarding the safety in case of a cell failure or even a hard short circuit, I made short circuit tests on "sacrifical" modules, and no fire occured at all.

You can also check this out: https://secondlifestorage.com/cellcount.php

These guys build there own battery packs, and many of them are much larger than mine. I hope they are all smart guys who want to keep their house safe !

My advice: get informed about the technology and run real tests by yourself.

Michel

Hi Michael,

I just now stared following your project.
Congratulations on the progress and results so far. 

I would like to know a couple more things about your project, if you dont mind.

How do you perform the thermal managment of the batteries ? One of the major concerns regarding lithium batteries is allways the safe temperature range.

I'm thinking since your load will not request huge amounts of power you never have te problem of the overheating . Am i right or do you have any kind of cooling system supporting the battery pack ?

Best Regards,

Bruno

Hi Bruno,

Thanks a lot for the follow and the encouragements.

I have added a project log entry dedicated to the thermal aspects of the Powerbuck.

https://hackaday.io/project/12296-a-110-kwh-powerbucket/log/53492-about-battery-temperature

As you mention, the power load is shared among the 1000 cells, so each cell is submitted to a low stress in my application. Imagine ants, each of them producing a small amount of the huge work produced by the anthill.

So no special thermal management has to be performed in my application.

Regards,

Michel

Hi Michel.

Can you tell me a rough idea of the total price of this battery pack (cells + battery holder + wires + fuses) ? (I've found 18650 cells for 2.5€ each over there : https://eu.nkon.nl/panasonic-ncr18650pf-3-7v-2900mah.html )

I've read that these battery were losing 20% of their capacity after only 200 cycles, and that its charge must be kept between a certain range. what range do you use to keep your battery at its best, and did you see any difference since the installation?

Hi Loïc,

You can get a better price (close to 2 €) if you purchase 1000 cells.

Battery holders: about 50 € - Copper bars: 100 € - Fuses: neglectible

About life: if you fully charge/discharge the cells (4.2 down to 3.0 V), you will have this life expectancy. If you stop charging at 4.1 V or less and stop discharging higher than 3.0V, the life will be a lot much longer.

Michel

Thanks for your feedback! 

I have another question regarding your battery pack that you want to be 110kWh.

I've seen that your house's av erage consumption is 10kWh per day. I'd understand that you want to have 40, or 50kWh battery pack, in order to cover the peak consumptions, but why choosing 110kWh? it seems much more than your house's need. Is it in order to get a charge when you have some days without any sun?

If this is the case, ins't the cost prohibitive (around 12000€ for this extra 60kWh) regarding the number of time it would be used? Wouldn't it be more interesting to buy a power generator (around 3 to 4000€ for a good one, that could be ran automatically) for the very few times it would happen during the year?

Or is there another reason for that 110kWh?

Loïc

EDIT : Oops, sorry, I've just seen that Kent asked the same question. But if this is just for cloudy days, what do you think about the power generator solution then?

110 kWh is the max capacity of my "Bucket", filled up with Li Ion cellls.  This has to be compared with the 10 kWh I had with the 500 Kg lead acid batteries. 11 times more energy with the same weight and volume. 

110 kWh is a good thing at the start and at the end of winter. In autumn it can extend the period of autonomy for maybe 2 weeks. In the same way it allows you to become autonomous again a bit earlier by the end of the winter. But it will certainly NOT increase dramatically the autonomy ratio. To become 100 % autonomous, I would need a 1 MWh (1000 kWh) storage to use the surplus of energy received during summer. But this is really not affordable and it would weigh 5 tons, even with lightweight Li Ion cells ! 

In the future, to store a large amount of energy for a long time (several months), the solution will probably be to produce hydrogen from water during summer, to capture it on nickel foam, and then to burn it again in a fuel cell during the winter. But this is very complicated and also unaffordable for an individual... Currently - Maybe in 10 years, it will be as easy as to buy a smartphone...

Check out Macphy's technology: http://www.mcphy.com/fr/

The efficiency of the hydrogen production X the fuel cell will not be very good (less than 50 %, I guess), but there is so much available free power during summer, that even if you loose 60 % of it, what remains will be enough to cover your needs during the winter !

Nature is so: either abundance in summer or shortage in winter !

New Lithium Ion batteries cost about 200 € / kWh currently, so at this price, I will never fill-up the bucket. If I find a way to get them at 50 € / kWh or so, it would change everything. 

I have bought new cells for the first 10 kWh string to validate the concept and be sure that in case of a failure, it was not due to the fact I used "second hand cells". I have heard of batteries from wrecked Tesla model S cars. This could be a way to get very good cells for cheap. I will start digging into this.

Power generator: you mean a genset with a diesel or gas engine ? I guess I will not do that. Burning fossile fuel to get electricity is not a good solution in my case. I think that would be much worse for the environment and much more expensive than nuclear power from the grid.

I hope I have answered your questions.

Michel

I have been so focused on lead acid batteries that it never occurred to me to string together thousands of rechargeable 'torch' batteries !! 

Are you at any point going to release the board layouts and source code for the electronics ?.....

I was also interested in the construction of the On-Line Source Selector, are you going to make a hackaday project of that too?

Hi Brian,

I hope you are doing well and I thank you so much for following several of my modest projects... 

Regarding the documentation, source code and all the related stuff, I will do my best to push them in the different HAD projects entries before the end of this year. Everything is (more or less) up to date, it will cost a little effort to gather the files and to click the "upload" button.

I am rather proud of the On-line Source selector and, as you mention, it is probably worth a separate project. I must admit that It works really well and it never let my house in the dark ! The documentation of this project is ready and up to date, so creating an HAD project entry from scratch is not a tremendous effort.

About batteries: I did not make a good experience with lead-acid batteries: the first set died sulfated due to deep discharges in the beginning and the second set has been overcharged during months (bad charger settings partially due to lack of documentation from the manufacturer). 

I must admit that I was a bit frightened when I turned the main power switch on the Lithium batteries. After almost a week, I must say that the result is beyond my dreams. The efficiency of these little things is overhelming. Nearly every single Wh pushed into them can be retrieved.  So, no regrets. :-)

Best regards,
Michel

Hi Michel

I'm very well thanks.

Good to hear that everything is working really well and that you'll be uploading the files, and making a new HAD project for the On-line Source selector, I'll look forward to reading them.

A question on the battery welding - Did you have any concerns regarding the welding of the batteries together? in case the electrical surge damaged the batteries in anyway?

Regards

brian

Hi Brian,

Regarding battery welding:

In fact the welding current crosses only one terminal of the battery, so no surge can occur at the battery level. It is totally safe for it.

Imagine a bird on a wire - no danger for him as long as he does not get in contact with the other wire.

Further on, the welding open circuit voltage is very low (less than 2 volts), so there no danger for the guy who welds neither :-)

Nevertheless I had some issues welding the fuse wires on the negative side of some batteries. Sometimes the battery got perforated due to the very localized heat density at the soldering point. Panasonic NCR18650PF were very sensitive to this. Next time I will try soldering the fuse on the positive side. it seems to be made of thicker metal.

I think it was a very good idea to build this spot welder. Thanks to this tool, assembling the packs becomes a fast and reliable process, to my point of view.

CU,
Michel

Hi Brian,

The project entry for the #An On-Line AC power source selector exists now. Check it out, please,

Regards,

Michel

I'm really curious just _why_ you want to make such a massice power bank ? Why do you need to run the house for 11 days at normal consumption on battery ? A wide area power outage of that magnitude in central europe  is unheard of .

Hi Kent,

I hope you apologize my very late replay.

Regarding the huge size of this energy storage, my main motivation is not related to the fear of a long lasting mains power outage, but rather to a "sun outage".

Keep in mind that my goal is to use as much as possible the solar energy rather than the power from the grid. 

It happens often that in autumn and in winter we have several very sunny days followed by a few dull days (few meaning 5 or so). In this perspective, a big storage could make the bridge between the sunny days, and so avoiding the use of grid energy as much as possible.

The main obstacle to this approach is financial. Currently, the cost of the batteries I am using is about 200 €/KWh. A 110 KWh storage would cost about 20 K€. That's a huge amount of money. At this cost level, economical amortization is really impossible. Maybe the prices will drop and it will become possible to have big storages at home.

I hope that I have answered your questions.

Regards,
Michel

Oh wow! I'll be following this project, very cool. My first thought though was a nightmare case of just a single cell (perhaps with a manufacturing defect, what's the chances) developing a short and reaching the point of no return only to have the whole 11,550 of them burn up like a bomb!

Reminds me of the technique Tesla and others have been using in their ev cars and home power walls to isolate a single misbehaving cell from the rest, using essentially a piece of wire rated to melt and disconnect when the cell experiences too much power flow. Check this: (at about the 5 minute mark)

Still though - 11,550 cells to wire individually!! I hope your method of closely monitoring the temperature and other vitals using wireless tech will be as safe..

Hello Kristan,

Thanks for the compliments. 

The safety of the stack is, of course, the most important point to consider. First, one must take in account that the cell manufacturers make a lot of efforts to make the cell as safe as possible, by design. The cells comply with strict safety standards. There are billions of cells in service on earth, and accidents are very rare.

So, I keep thinking that the catastrophic "chain reaction" you describe is very unlikely.

Nevertheless, better be safe than sorry, and I have planned to add a "Tesla Style Fuse" on every single cell since the very beginning of this project. I have watched the videos you mention and they have inspired me a lot.

I have purchased a roll of 0.5 mm tinned copper wire for this purpose:

https://hackaday.io/project/12296-a-110-kwh-powerbucket/log/42686-fuse-wire

Monitoring: the temperature of each module will be monitored with a 0.1 °C resolution. Any deviation with respect to the ambient temperature will be detected. There will be a warning threshold and an alarm threshold. When the alarm threshold is reached, the BMS will disconnect the charger and the load.

BTW thanks a lot for the skull.

I don't understand the function of the fuse. How would one cell be shorted? The only thing I can think of is that the failed cell may short the other cells in the circuit. Is that the failure mechanism?

Hello,

Q: How would one cell be shorted ?

A: during the manufacturing process of the cells, particles of metal can enter the battery and create a short when they migrate. The risk is low, especially with famous brands, but is not null.

See some interesting background information here:

http://batteryuniversity.com/learn/archive/lithium_ion_safety_concerns

Q: Is that the failure mechanism?

A: Whenever a cell gets shorted internally, the current flowing through it is mainly limited by its own internal resistance, because the 69 other cells of the module (my modules comprise 70 cells in //)  have a very low equivalent resistance (less than 1 milli Ohm) and push hard into the faulty cell. Without a fuse, the current and therefore the power dissipation inside the faulty cell can become huge (more than 100 A, hundreds of watts) and may lead to a fire. With a fuse the current may reach maybe 20 or 30 amps, but only a during a few seconds,  mitigating a lot the fire hazard.

Without a fuse, the duration of the high current phase is impossible to predict. With a fuse (even a very basic one), one has a much better "control" of its duration.

I hope my explanation was clear.

Edit - 2016/08/13 - A few thoughts about an external short circuit.

Dustin,

An external short cannot be excluded and I imagine four scenarios for it:

First scenario: a piece of light gauge wire (cross section of 1 mm² or less) gets connected across the terminals of a module. Assuming a resistance of 20 milli ohms for the wire and a voltage of 3.7 Volts for the module, the current would reach 3.7 / 0.02 = 185 Amps. This current would make the wire burn and melt within a few seconds. The individual cell current would be 185/70 = 2.6 Amp, well below the "Tesla Style Fuses" rating. No damage would occur to the module.

Second scenario:  the same wire shorts the whole stack of 15 modules in series, the new current is: 15 * 3.7 / 0.02 = 2775 Amps. The real current would probably lower than this, but this calculation gives the order of magnitude of the surge current.

This current would make the wire burn instantly with an explosion noise, creating a very hot plasma and a dangerous flash of ultraviolet light. No damage to the modules making up the stack neither.

Third scenario: I drop accidentaly a piece of heavy gauge copper bar (section of 100 mm²)  accross the terminals of a module. The resistance of the copper bar is about 1 milli ohm. the current reaches 3.7 / 1E-3 = 3,700 Amps. (Theoretical calculation). This current would make the bar get very hot but it would not melt. The individual cell current would reach about 50 amps (good cells are able to deliver this current), well above the "Tesla Style Fuses" rating. The weakest fuse would open first after a few seconds. The 69 remaining cells would share the current, with a corresponing increase of the current in each fuse. The "domino effect" would make all the fuses melt one after the other within a few seconds. After the last fuse opens, the short circuit condition no more exists.

Fourth scenario: the same kind of copper bar shorts the whole stack, the current would be limited by the current capability of each cell to about the same current as the short of a single module. The behavior of the cells and the fuses would be the same.

After such a catastrophic event, the cells of the module would certainly be severly damaged, but I guess that the risk of cells "venting with a flame" is low.

I did never test the Third scenario neither the Fourth in "real life" and I cannot be sure that everything occurs like I describe it above.

I have purchased several spare cells for my stack and I will consider testing the Third scenario with a small module of 5 cells or so. I will keep you updated, with a video of the experiment.

Regards, Michel

Thanks Michel. I wasn't clear on whether the protection was for an internal or external short. Internal is the only thing that made sense, but I wasn't aware of the cause.

I will catch up this weekend...  I just got really good pricing on LifePo 16340 cells, and am probably going to update for that...   'til then, happy hacking...

OK - looking forward hearing from you.

BTW I have ordered 16 sets of 5 * 14 battery holders here: http://fr.aliexpress.com/item/5P14S-18650-li-ion-battery-holder-Used-for-14S-51-8V-10ah-12Ah-15Ah-lithium-battery/32448556875.html

Hello Thomas,

Thanks a lot for your proposal. Of course I am interested. Send me a private  message so we can exchange our email addresses.

Regards,

Michel

I have a 12V power module design for using 18650 cells.   Designed to be cheap and easy to slap together/replace.   Let me know if you are interested.