As straightforward as the concept of taking battery packs out of an old electric or hybrid car and reusing them for home power storage sounds, this thought process skips a few essential steps. As argued by [Ed] in a recent video based on his own experiences with high-voltage Nissan Leaf batteries in a home PV system, the main problem is that you’re taking a battery out of a larger system including a lot of the management hardware and software.

The referenced Battery Emulator project is an open source effort to create a suitable interface between these EV batteries, with the mentioned Nissan Leaf being just one example in the project Wiki, with the connection scheme shown in the top image. It’s also noted that the Leaf battery BMS is not designed to operate continuously, so they need to be restarted every day or so lest they become too inaccurate.

These and other things are all solid reasons why you have to be absolutely certain that you want to integrate these high-voltage battery packs into your 12 – 48V low-voltage DC system. You’re after all assuming all the responsibility of setting up a system that’s both safe and reliable, so having a good read through something like the Battery Emulator Wiki and sourcing first-hand experiences from the folk in this community would be a very wise first step.