Anyone interested in a bunch of Toshiba 2.9Ah SCiB LTO (lithium-titanate) cells?

In case this wasn't readily apparent from my initial post, I want to point out that the number of cells I have should be just enough for potentially two Insight IMA packs, the main reason it's probably not good to part with individual cells.

It should be possible to experiment with the tapped cells to make a 60-cell pack. And if you can find some way or someone to weld the untapped cells together, then you'd have enough for a second pack of 72 cells. Or, you could try to build most of the pack out of the tapped cells and use the untapped ones to supplement if some of them fail or if you want a bigger pack.

Point is, though, that there's just enough to experiment fairly freely for a good IMA pack. Once you get that built, then you'd have enough spares to build, say, some 12V batteries or have spares in case of any failures. Or sell them. Whatever...

Here's a couple more images.

The first is a model of an M4-tapped bore in the terminal, that I made. The taps as-is are M3 (or 3.5? can't remember - a size smaller than M4). This should give you an idea of the underlying terminal structure, clearances, etc.


This second image is a cell ripped open. The aluminum terminal 'nub' in the above pic fits into the holes in the busbars you see crimped to the cell tabs in this pic.



M4 taps or re-taps can work. I've done a few, by hand. There's barely enough material, though, as you can see in the diagram. They have to be near perfectly centered, and I'm pretty sure you'd have to use aluminum screws as the holding hardware, to help preserve the ampacity of the terminals. I bought some and have used them. I'm not positive though how that all works. The stock terminals are 'pure aluminum' with a low resistance; most aluminum screws, the ones I found, are a slightly different alloy, with slightly higher resistance. I'm not sure if current would flow through the lower resistance but smaller cross sections of the tapped terminals or through that as well as the higher-resistance aluminum screws... [I think the way it'd work is, current would flow through the lowest resistance parts first, and once those heated-up resistance would increase - at which point current would flow through the aluminum screws.]

You remove so much material from the terminal itself that there's not a ton left for the current to flow, it'd be a bottle neck, with M4 taps and screws. I calculated the ampacity of the stock terminals at one time to be about 100 amps - that is, they're big enough to be able to handle around 100 amps without much voltage drop.

Another issue is the holding strength of such taps: M4 taps with aluminum screws can work, but it's pretty weak, sketchy. It's hard to get a perfect tap made in the first place, but even if you do, the threads are soft. They only have to hold a ~3/16 or 1/8 busbar to the cell terminal, and it seems good enough for that. But you can't fiddle with them, you have to be really careful. You also need to make sure there's some other means to hold the cells together - that the taps and busbars aren't stressed, flexed, moved at all. That's not a big deal, but it has to be done, you can't just bolt the cells together and expect the busbars to serve double duty, holding the cells together AND holding the busbars to the cells.

My plan was to make 6-cell rigid modules, attaching modules together at ends. The modules would slide onto plastic (PVC) 90 degree angled tracks that fit in the stock battery case...


Here's an image* of a battery pack made from similar Toshiba cells, from an old Schwinn Tailwind, giving you an idea of how the terminal/busbar connection is/can/has been made. You can also look for images of the Fit LTO pack builds for ideas:

* image from this website: Schwinn Tailwind Battery Pack Teardown and Analysis

In this and other packs I've seen, the busbars have a hole in it, they fit around the little ledge on top of the terminal, and then they're laser welded - zapped. I take it the aluminum of the terminal and/or the busbar ends up melting together I guess around the inside edge of the hole, or perhaps underneath as well. I'm not sure why they need the hole**, why they don't just lay the bar on the terminal and zap... The busbars here in this pic look like aluminum; the Honda Fit LTO packs have I think nickel plated material, perhaps nickel-plated copper?

[** I think it's because the busbars would be too thick to easily, quickly weld with just a 'zap'. You can only do that zap type of welding on relatively thin metal.]

I always planned on using aluminum busbars, I figured it stays pretty dry in my IMA compartment, I didn't think corrosion would be much of a problem. Plus, I thought that all the material would be similar metals, reducing the chance of galvanic corrosion. I also planned on using an anti-oxidant paste, for the tapped cells...

For added strength, for the tapped cells, I also thought about using epoxy - attach busbars to terminals with screws, then spread some epoxy around the joint. The only problem with that was then it'd be permanent, and I had always wanted removable cells. But, done right, it'd probably be nearly equivalent in strength and ampacity to welds...

Here's an image of a Fit pack busbar/terminal connection, from when Insightbuyer was experimenting with soldering to those stock busbars:


Here you can see that the busbar has a hole in it, and it looks like they zapped the terminal in the center, resulting in the aluminum of the terminal melting and adhering to the busbar around the inside edge of the hole in the busbar. Looking at other images, it doesn't look like the aluminum underneath the busbar melts, it's just around that inside edge of the hole in the busbar. The 'ledges' on top of the terminals are very small anyway; I think they're there just so you can easily place the busbar on the terminal and zap; they're not there for adhesion or extra surface area contact...

Here's another image of terminal-to-busbar connection, with 2.9Ah Toshiba SCiB-like cells. This is from the thread where dude was trying to figure out how to build a replacement pack, working with 'his suppliers', etc. KOK sent him an image of what could be done. This is the post where the original image is: Drop-In Lithium Replacement Pack, Design Discussion




Another plan I had was to install 'cell ports' - basically make sure the busbars had an extra hole in it where I could attach wires leading to a 6 port JST-style connector. That way I'd be able to pull each 6-cell module from the pack and test it with a hobby charger, looking at cell-level data...


Here's an image of my 'bastardized' LTO 12V experiment. I made this out of a hodge-podge of these LTO cells, the ones that had failed tapped terminals. You can see that some have aluminum M4 screws, some stainless steel smaller, M3.5, etc. I've also got the 'cell ports' attached. I meant to install this in the car but never got around to it.


It'd be a really compact, lightweight 12V battery for the car, though, and there's a perfect spot for it in this form-factor - right in front of the underhood fuse box. I needed to figure out some kind of box/case for it, that's where I petered-out. If I were into 3D printing it would have been easy enough to make a case, but I just ran out of energy for learning/acquiring/finding yet another technology (I actually have done 3D, for my OBDIIC&C box, just that I'm generally really tired of technology).

Oh, BTW, these cells in the image above have a clear heat shrink around them. You can't really tell in the pic. As far as I know the cells need to be isolated from one another - the cases seem to be in contact with the electrolyte. If you measure voltage between case and + terminal and case and - terminal, you measure half-cell voltages, i.e. potential between pos electrode and electrolyte and neg electrode and electrolyte, I gather...

Looks like the Alibaba price at >100 units is $7/ea so what you are offering here has a significant value. 2.9v*2.9 Ah*139 cells == 1.2kWh or 1000Ah worth of 12V packs == about 10 lead-acid batteries!

If I were still in the Insight game I'd consider your design concept and rack them in w/o mods to the NiMH system as you ideate here. Even without an Insight to use them in, they look like great source material for other DIY projects... I am sitting on my old Chevy Volt 16kWh pack I haven't put into service yet, so don't dare take on another project myself!

You have clearly put a lot of thought, work and $$ into this, so I very much hope someone bites on this. I saw your concept of tapping each cell for cell-balancing. With Lithium this seems like a very good idea, though more paralleling and staying well below 100% can help reduce the risks. There are some good cell monitoring circuits out there that make it easy to see if you have any cell(s) dangerously out of range under bulk charging.

AS with my last post on NiMH cells, you might find a MakerSpace locally who would be eager to get hold of them for hobby applications.

^ Yeah, so, it looks like a year has passed since I posted this idea - and they're still sitting in my closet. Since my NiMH pack has been holding up fine, more than fine, I have little incentive to put any effort into these cells, or lithium in general. Plus, just no time.

I'll have to check my LTO 12V battery soon, see how/if the cells are holding up just sitting there. Can't remember when I last checked them, probably back around when I posted this... Actually, I have a 'cell checker', I can at least check the voltages on them, right now... OK, so they're all around 2.417V (one at 2.42), that seems OK... At least they're not all dead or wacky imbalanced... Can't remember what I left them at, I'd have to look that up.

OK, I guess looking that up isn't so hard, these were my last notes:
"20201027
Checked cell voltages with cell checker and discharge-balanced them to 2.45V. One cell was a bit higher than others, but I didn't write it down. Cells were around 2.46V, high was around 2.48."

Looks like I last left them with a 'storage charge' of +1559mAh on 20200624 (about a year and 8 months ago), cell voltages were about 2.49V.

Example of the ones I buy...