Toyota is recycling old EV batteries to help power Mazda's production line

Those who TL;DRd - it's for the factory, not the cars!

Old EV batteries are great for energy storage. A worse weight-to-capacity ratio doesn't matter for batteries sitting on the ground. A battery that holds only 70% of its original capacity is considered worn-out for EVs (and even replaced under warranty), but grid storage isn't driving anywhere, so any capacity left is still useful.

More often than not these bad cells can be removed, manufactured a new, and reinstalled. Not 100% but almost good as new.

Making your own cells is fun.

For Toyota, this is trivial and the energy storage these “left over” batteries provide, given a tinkering, is sufficient.

Your own cells? Like you put together a lasagna of metal sheets and chemical goops and roll it up?

Nah, you can buy 18650 cells, nickel strips, a spot welder, and make a BMS. Build your own battery pack.

If you don't know what you're doing, don't do this. Even if you know, probably don't. It works, and is a regular industrial process, sure, but you're trying to perform controlled melting of the protective housing of one of the many, tightly packed chemical energy bombs you're sticking together. Doesn't take too much of a mistake to go very wrong.

If you're building a battery pack in this day and age, use something like LiFePo prismatic cells and bolt-on busbars instead - way less dangerous chemistry, way less spicy process - but realistically speaking just buy the premade packs. For normal sizes, they're not more expensive (but don't buy the "too good to be true" ones) and means not having to deal with entirely unprotected battery terminals eager to give you a Very Bad Time.

Generally speaking the cells that are welded on are designed to be welded on in the areas were you do the welding. Doing something other then welding on them properly is going to be more unsafe then welding.

The proper tools to do this are not that expensive anymore in the greater scheme of things. It is just a question of whether or not it is worth to do it at the scale you are doing it or pay somebody else to do it.

Of course if you buy cells that are designed to be bolted together then bolt them together.

Of course the bolts, or whatever else provides the threads, on those cells are welded on.