Interesting cells on Battery Hookup

[harrisonpatm]

BH posted today a couple interesting options for large-format LiFePO4 cells that are a great deal, especially if you're planning a small car EV or larger E-moto.

3.2v 105ah Lifepo4 Prismatic Battery

These batteries were recovered from modules that had a bad laser weld. We tested 40 different batteries and they all tested above 95% capacity. It is likely they were never fully cycled since the laser welds were not done properly. They have aluminum tabs on them. If you are comfortable welding...

batteryhookup.com

12.8v 105ah 1.34kWh Lifepo4 Battery - Needs BMS - $63/kWh

These modules are pretty awesome and unique. They are 12.8v 105ah lifepo4 in 4s1p configuration. They also have compression. You can buy 4 of these, top balance, series connect, then add a 16s BMS to make 51.2v 105ah batteries. We are not sure what the amp rating is on the laser welded flexible...

batteryhookup.com

Problem is, these cells don't have studs, bolt holes, or thin tabs for assembling a pack. BH recommends in the description that they're good for people who can aluminum weld or have a laser welder, which I'm sure some people have, but not most.

Anybody have any other ideas on how the average joe can use these cells? I thought that it'd probably be possible to solder to the contact surfaces, but not only would it take a ton of heat and solder, it's not the best idea for EV's that experience a lot of vibration. So maybe that would work if someone would use the cells for a stationary powerwall application.

 

[amberwolf]

Given that soldering will pass that (probably long-duration high temperature) heat directly into the cell itself, into it's electrolyte and layers (the "jelly roll"), soldering is probably not a great idea. To be sure of what it will do to the cell, you'd have to actually do it and test what it does to the characteristics of one set of cells vs another set that is not soldered, over time. And/or do it to half of a set of cells and cut them open and compare them to the other half, for a visual result.

As long as you can use some method that keeps the total energy over time low enough, it'd probably be ok (like laser, ultrasonic, stir, or spot welding).

If the connection pads were thick enough, you could drill and tap threaded holes for bolts to secure contacts to them, but I doubt they're any more than the 6mm or so the drawing indicates, maybe not even that if there's just a cap over a hollow terminal space.


You could build a clamping system that encloses the pack to secure busbars between cells, but I don't know what pressure would be required for contact, or how to keep the contact areas all level with each cell's terminal pads, and what pressure the cell itself can handle structurally without deformation (which would reduce pressure on the terminal and increase contact resistance).

Because cells like this require clamping the casings across their surfaces to prevent swelling and keep internal resistance/etc correct, I highly recommend buying the modules, or making sure BH sends you the module hardware to put them back together if you buy sets of four of the cells. Otherwise you have to come up with your own structure to clamp the cells together and get the required pressure (which you would probably not find a spec for anywhere, so you have to guess....)

 

[amberwolf]

Here's an image of a similar cell where they seem to have welded on threaded terminals

https://endless-sphere.com/sphere/attachments/img_1254-jpeg.333764/

 

[harrisonpatm]

Just saw those on a separate post as well. Probably laser welded at the factory, to reduce heat transfer into the cell body.

Anybody got links to a diy laser welder?!? Lol

 

[amberwolf]

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[From-A-To-B]

I was researching other prismatic cells — similar form factor, different cell, YMMV — and read that the terminals can be quite beefy. Thus, with lots of care, you may be able to drill and tap holes into the terminals. It’ll be just a few mm in depth— just enough to get a bolt down there and secure a bus bar.

Bit of an involved process, but way better outcomes than soldering. Those cells w/o easily used terminals are door stops until you find a way to connect them.

 

[js1pxl11]

I bet you cld tap those blue ones.