How to Build a Safer Battery - Looking to Collect Knowledge

[docw009]

Questions for the DIY battery builder

-Is it worth running LiFePO4?

You're weighing different values against each other. Longevity? Performance? Weight? These things trade off differently for different applications and different desires.

Rhetorical questions, although some EV's are LiFe, aren't they?. It seems like that safety requires a weight penalty most ebikers don't want,

 

[Chalo]

Rhetorical questions, although some EV's are LiFe, aren't they?. It seems like that safety requires a weight penalty most ebikers don't want,

I think cycle life and materials costs/price volatility factor into those design decisions at least as much as safety.

 

[LewTwo]

I primarily run LiFePO4. They have a very flat power/discharge curve, long life and I do not have to worry about them blowing up. However they are a lot heavier and require more volume. Depends on what your priorities are.

 

[99t4]

Some try to put a fuse at every cell.

Tesla did that in their packs of 18650's. (or was that the larger cells?)

Wasn't there some controversy whether that was really fuses or simply a byproduct of a new assembly method?

 

[SafeDiscDancing]

Wasn't there some controversy whether that was really fuses or simply a byproduct of a new assembly method?

It was a few years ago by now but I'm pretty sure I saw Elon Musk talking about those wires and that they tested them to make them the precise thickness so they would act as a fuse.

But they also did all the connections by robotics so there was a speed of assembly aspect of it too.

However... it might have been some random video and I might be wrong so we are on the internet and you need to doubt everything all the time.

Critical Thinking... Grammar (evidence), Logic (individual thinking), Rhetoric (free speech).

So my Rhetoric and Logic is not linked by Grammer so you ideally need to dig, dig, dig up some old stuff.

Tesla has moved beyond that technology and now they literally "glue" their packs together. And I think they may have dropped the fuse idea but not sure.

 

[Braddudya]

Any feedback about having the batteries in a triangle configuration and spread out?

I don't mean like a traditional triangle battery, I mean a triangle arrangement that follows the frame and is open in the middle so that the battery density is actually quite low. It seems to my inexperienced mind that it would help spread and dissipate heat better. It would also spread the mass and allow for a very secure mounting of the battery (lots of mounting points).

 

[99t4]

Any feedback about having the batteries in a triangle configuration and spread out?

I don't mean like a traditional triangle battery, I mean a triangle arrangement that follows the frame and is open in the middle so that the battery density is actually quite low. It seems to my inexperienced mind that it would help spread and dissipate heat better. It would also spread the mass and allow for a very secure mounting of the battery (lots of mounting points).

Can you present a rough sketch so we can get a better idea of what you are talking about?

Critical Thinking... Grammar (evidence), Logic (individual thinking), Rhetoric (free speech).

Nice try but better study up on your trivium before "defining" it to us non-classical educated types.

 

[SafeDiscDancing]

Nice try but better study up on your trivium before "defining" it to us non-classical educated types.

Critical Thinking is the same as Christian "Logos".

Grammar, Logic, Rhetoric

- Grammar is the raw data unfiltered by some tyranny that tries to prevent you from knowing what is going on. (a big problem now)

- Logic is the full spectrum of individual free thinking that encompasses all prior knowledge and even intuition.

- Rhetoric is simply your arguments to support your own Logic.

Are you seriously questioning this?

 

[99t4]

Are you seriously questioning this?

Yes but this thread is not the place.

OP is looking for some answers to specific battery arrangement questions.

 

[SafeDiscDancing]

OP is looking for some answers to specific battery arrangement questions.

The context was about whether I could say as a "fact" that Telsa welded those small wires as fuses and I was drawing on older memories that were not that certain.

This idea of wires as fuses is now pretty old... when did they first do it?

Five years ago?

All I was saying is always assume that if you want to know things and it matters that you are correct then:

"Trust no one... not even me."

 

[SafeDiscDancing]

https://circuitdigest.com/article/tesla-model-s-battery-system-an-engineers-perspective

Tesla uses a wire bonding technique to connect each cell with the battery pack. The wire also acts as a safety fuse improving the safety of the overall system in case of a cell failure. The red circle in the below image shows how cells are connected using wire bonding.

-------------------------

That article seems credible. At least that adds evidence towards the thing in question.

First introduced in the 2013 Tesla Model S which means:

2022 - 2013 = 9. Nine years ago. How time flies.

That was worth the effort to dig just for the knowledge that nine years have passed.

 

[Braddudya]

Can you present a rough sketch so we can get a better idea of what you are talking about?

This is a very rough sketch of the area that the pack would take up.

My thinking is I could probably get away with just 2 rows of staggered cells running the perimeter of that triangle which means there is no hot cells buried inside a pack of other hot cells.

 

[amberwolf]

Did you ever test out this idea?

 

[spinningmagnets]

Bottom balancing is a strategy that seems to have some smart and passionate followers when using LiFePO4.

Vehicles generally continue to use NCA/NCM lithium-Ion chemistries due to them providing the most power and range in the smallest package.

LiFePO4 is very fire-safe, so I've seen it used on small airplanes to start thr engine, and RV 48V power systems.

If the only BMS failure mode was to kill a cell or pack, it would be less of an issue. However a BMS is also supposed to prevent a cell overcharge, which is a major cause of pack fires.

A bulk charger could charge a 2S pack to 8.4V, with one P-group at 4.0V and the other at 4.4V. Of course if all the cells are of a consistent quality and internal resistance, this wouldn't happen, regardless of the BMS.

Storing a pack at a full 4.2V per cell will erode its capacity.

For pack life and fire safety, charge to 90%. Nothing wrong with charging the pack to a full charge just before a ride, just don't store it fo a week at a full charge..