DIY 48v 280ah Build - Blowout/Fail

[Nobodybusiness]

It comes down to "Mis-Wired" BMS - Battery leads. And then overcharging and BMS unable to stop charging.

Here's a photo of the blownout cell from Lower Bank. Cell #9

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That’s what I was thinking when I first saw it.
Almost every time I have seen that type of expansion was from over voltage.

Well, expensive lesson but glad you got it figured out..

 

[Nobodybusiness]

It does not use pwm, but combines the output of four low frequency square wave inverters.
Each of these inverters has an output transformer that has a different secondary voltage. The four secondaries are connected in series to directly generate an analog ac sine wave. Its rather like direct digital to analog conversion at hundreds of volts and thousands of watts.
Each square wave inverter has potentially three output states +ve, zero, and -ve. There are four inverters (four bits).

As you will know, four bits of binary can only produce 2 x 2 x 2 x 2 = 16 output voltage levels.
Four bits of "trinary" can produce 3 x 3 x 3 x 3 = 81 output voltage levels.
That is an 81 step peak to peak sine wave, forty steps up, zero, and forty steps down.

I developed this gradually myself over many years. The original prototypes all used various microcontrollers to generate the required switching waveforms. But I gradually simplified it down to be an all hardware design (no software crashes !)
This makes it a lot easier to understand and repair for people that are not software geniuses.

At least two other software guys have come up with firmware that duplicates my hardware design using a Nano microcontroller.
There are now three different Warpverter driver boards available to do all of this from three different people. All are 100% compatible with identical plugs and identical functionality. Its all free and completely open source if you wish to build one yourself.

No data screens or firmware. Its a basic bare bones inverter, dc in, ac out. No problem adding a second microcontroller for purely monitoring, alarms, logging or control. That would be quite independent of the basic inverter function, so you can add whatever you want yourself.

As a retired professional power electronics design engineer, its been a bit of a part time hobby. Having designed mass produced commercial inverters and uninterruptible power supplies, overcoming the technical problems is what I do.

The only problem with it is its not commercially viable, its too expensive to build compared to pwm. So its only value is as a home project where you wind your own transformers and use recycled parts.
The biggest advantage is all the switching is done at at a very low frequency, so the circuit layout is far less critical than high frequency pwm, where it becomes increasingly difficult to successfully parallel multiple devices for very high power.

This is what the secondary voltages look like. All waveforms shown with the same oscilloscope settings.

Impressive.

Probably more than I can do based on my knowledge.

 

[Nobodybusiness]

Always good to learn from others’ mistakes for sure. Fortunately this one was all My Fault. No arcs or sparks, no fires. Nobody hurt.
My system will be better off because of me learning.
Now to find (4) additional 280ah cells.

It almost happened to me.
When I first started out I hooked up the BMS and all the wires. Connected a charger to the battery.

Notice I said battery not BMS. Fortunately, I was monitoring the BMS although it wasn’t controlling anything it was still reporting voltages.

Once cell was @ 3.7 when I looked and that led me to the “oh shit” moment and I disconnected the charger.

Happens to all of us at one time or another.

 

[cinergi]

It almost happened to me.
When I first started out I hooked up the BMS and all the wires. Connected a charger to the battery.

Notice I said battery not BMS. Fortunately, I was monitoring the BMS although it wasn’t controlling anything it was still reporting voltages.

Once cell was @ 3.7 when I looked and that led me to the “oh shit” moment and I disconnected the charger.

Happens to all of us at one time or another.

Yup, I did the same thing early in my build process.

 

[TimE]

With all respect to those who have far more experience than I, I am not a fan of "tight clamping".

My understanding is that cells expand when being charged. This is a physical manifestation of a chemical event. This expansion "could" be contained if the entire cell casing was constructed like a "pressure cooker", or similar device. A tight strap around the cells will only cause the inevitable swelling to occur in another path of least resistance. But it WILL occur. The "bus bars" are pretty much rigid, unless built of flexible material. They will maintain the distance between the terminals while the cell expands. This could put great force on the terminals.

I'll be happy to be shown to be wrong. But it's not easy to break the laws of chemistry and physics without unwanted consequences.

Those cells are not compressed adequately. Compressed cells are held between solid compression plates using threaded rod to evenly apply the end plate pressure. I am never seen 'compression" so poorly done. It looks like a disaster waiting to happen

 

[Warpspeed]

The idea is NOT TO COMPRESS the cells, but to prevent them from being able to grow any larger than they already are.
Its totally different.
There should be very little or zero initial force applied when assembling your rods and flat clamping plates.
Just enough to close up any small gaps between cells.

Perhaps a more descriptive name for this, might have been "anti expansion plates".

 

[upnorthandpersonal]

Perhaps a more descriptive name for this, might have been "anti expansion plates".

Or a 'fixture', as they mention in the datasheet.

 

[Warpspeed]

Or a 'fixture', as they mention in the datasheet.

Even better.
Clamping plates seems to lead some to the wrong conclusion.

 

[featherlite]

With an initial compression force of 300 kgf (±20kgf), as they mention in the datasheet.

 

[SilverbackMP]

It almost happened to me.
When I first started out I hooked up the BMS and all the wires. Connected a charger to the battery.

Notice I said battery not BMS. Fortunately, I was monitoring the BMS although it wasn’t controlling anything it was still reporting voltages.

Once cell was @ 3.7 when I looked and that led me to the “oh shit” moment and I disconnected the charger.

Happens to all of us at one time or another.

I realized that I had wired battery #5 (out of eight) completely backwards on a REC BMS. I caught it thankfully. I mostly successfully pulled the ends out of the wire loom and reordered them. I say mostly successfully as I did have one contact the aluminum surface I was working on and I was welding for a second or two. I replaced that wire.

 

[Checkthisout]

Even better.
Clamping plates seems to lead some to the wrong conclusion.

I've read they are supposed to be clamped with significant force so mine are.

However, I don't think we are achieving EV chargers and discharge rates so it probably doesn't matter.

If one were charging at 3C, it's probably a good idea to follow the manufacturers instructions and clamp.

 

[Checkthisout]

What impact would creating a paralell path around the BMS have had on balancing?

 

[upnorthandpersonal]

I've read they are supposed to be clamped with significant force so mine are.

With an initial compression force of 300 kgf (±20kgf), as they mention in the datasheet.

Keep in mind that if this is done with 4 threaded rods an two plated on each side of the pack, this amounts to 'hand tightening' the nuts on the rods. In other words, don't torque the heck out of those nuts.

 

[Q-Dog]

I have yet to see a commercial battery where the cells are compressed with rods and/or springs. I have seen plenty that are clamped or glued or taped or use foam pads inside the case to prevent cell movement.

 

[Checkthisout]

Keep in mind that if this is done with 4 threaded rods an two plated on each side of the pack, this amounts to 'hand tightening' the nuts on the rods. In other words, don't torque the heck out of those nuts.

300 kgf looks to me to be 660 lbs of clamp load. 25 in/lbs of torque on 5/16-18 threads.

So yeah, not much actual torque on the fasteners themselves.

 

[Hedges]

We went through torque vs. compression in threads on the topic.

I like the idea of springs rather than rigid clamp.
Some members used spring displacement to determine force, rather than torque.

 

[upnorthandpersonal]

I have yet to see a commercial battery where the cells are compressed with rods and/or springs. I have seen plenty that are clamped or glued or taped or use foam pads inside the case to prevent cell movement.

No, but the principle is the same. One of Andy's (off grid garage) off-the-shelf boxes to mount cells in (don't remember the manufacturer) has compression plates that operate in a similar manner. To replicate that in a DIY fashion, some threaded rods and two end plates works fine, just know not to use an impact driver to tighten the thing...

 

[Checkthisout]

We went through torque vs. compression in threads on the topic.

I like the idea of springs rather than rigid clamp.
Some members used spring displacement to determine force, rather than torque.

Understood but what is the benefit of allowing the cells "room to swell"?

 

[SilverbackMP]

We went through torque vs. compression in threads on the topic.

I like the idea of springs rather than rigid clamp.
Some members used spring displacement to determine force, rather than torque.

Seems like it’s a bit of an over engineered mouse trap IMHO.

 

[stienman]

Given enough time every diysolarforum thread turns into a compression thread.

Please consider taking compression discussion to one of the many, many, many threads dedicated to various aspects and assertions regarding compression.

Search results for query: compression

diysolarforum.com

And, of course, in my completely unbiased* opinion here are the two best posts about compression - mostly because they link to research papers about the topic:

The secret to compression

Compression affects lithium based cells. There are a number of mechanisms associated with compression, as each component of the cell is affected and must be taken into account separately and together. This forum has many, many, many discussions about compression, and this isn't the first...

diysolarforum.com

https://diysolarforum.com/threads/the-secret-to-compression.26643/page-4#post-317903 <--Note the paper linked in this second post isn't really applicable to our use, however it contains an excellent bibliography and citation index of papers which are applicable

*If your sarcasm detector didn't go off here, please perform a calibration.

 

[Hedges]

Understood but what is the benefit of allowing the cells "room to swell"?

Avoiding excess pressure.
Ever take a bite out of a sandwich and get mayo and mustard on your clothing?

Also would accommodate any point in the SoC/expansion range during assembly.

 

[Checkthisout]

Avoiding excess pressure.
Ever take a bite out of a sandwich and get mayo and mustard on your clothing?

Also would accommodate any point in the SoC/expansion range during assembly.

I've been instructed not to discuss the topic on this thread anymore but,

What does the compression/fixture look like in an actual electric car that utilizes these types of cells?

 

[Hedges]

Like this:

New to solar, just bought 71 kWh of batteries for DIY some help please :)

I am going to be setting up solar in my new home. I plan to run off solar/batteries and will still be connected to the grid in case no sun and batteries are dead. I also have a whole house generator that can be incorporated into this as well. These batteries are the first thing I have bought...

diysolarforum.com

So far as I know, always just a box with insulating spacers.

 

[Quattrohead]

It is not about squeezing your cells, it is about not allowing them to expand.

 

[Nobodybusiness]

It is not about squeezing your cells, it is about not allowing them to expand.

If that’s the case just a few loops of Kapton tape would work..