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.

Balancing multiple battery packs at different wear levels is a huge nightmare. You have to run rebalancing operations all the time and on used packs it can be quite dangerous to trigger a thermal runaway.

If they do it with different types of batteries it is even more complicated, like you need to write some custom software to sync all that up. This is not a trivial project.