So how would one go about creating a battery pack with 18650 batteries and getting a bms to manage the pack, then installing it into the vehicle? Because this seems like the way to go. They seem cheap enough. Cheap ish.. and the car is old and needs a new pack. Where can I get a bms that can manage that many cells and 360v? And at least 24kwh? And how the f*** do I get it into the case that's there. Seems heavy and like I'll need some jacks and something with wheels to get the thing out after pulling the fuse in the back seats and undoing the wiring connectors.
This is a bad idea. But it's not super obvious why, so I'll give some insight.
First of all, 18650s are really small, so you need lots and lots of them to make a pack. Each cell has two connections that you'll be hand-soldering or spot welding. Even if you're really good and only 0.1% of all the welds you make are in some way defective (over the lifespan of the pack), that still means a handful of cells will eventually become loose or have an intermittent connection. Even if that doesn't lead to some catastrophic failure, that still means one or more of your 96-series packs is going to have drastically lower capacity suddenly, which means your entire pack will have less capacity and the balancing solution you use will quickly not be able to cope with the energy imbalance. That in turn accelerates wear on the remaining parallel cells and trashes your pack relatively quickly. Unless you have access to really good quality spot welding equipment and know what you're doing, building a large 18650 pack is not a good idea.
Second, the pack is mechanically complex. You need to mechanically immobilize all the cells, and make sure that even as cells slightly expand and contract, heat up and cool down, and as the car shakes around, they stay put. This almost inevitably means having some sort of compliant foam or plastic support, and that makes cooling the pack really hard.
Third, 18650s aren't all created equal. Because of internal resistance differences, some cells in a parallel pack are going to for a short time deliver more current than others. If there's 30 cells in parallel and you're pulling 300A from the pack, not every cell is going to deliver an equal 10A. Statistically, in the worst case, current sharing scales as I*log
/n, so with this example, you'd expect 300*log(30)/30~14.77A. And importantly: your current capacity drops as a cell ages, for some chemistries the internal resistance goes up to 6x original at 60% SOH and the current delivery capacity goes down about a factor of 2.5. This is often the limiting factor on how you have to size the pack. For instance, if you'd use the venerable NCR18650GA rated at 10A peak, at EOL that will be 4A peak, which means in a gen4 with 300A peak current you need like... 100 cells in parallel. That in turn means you'll end up with a 100kWh pack and more cells than you can fit.
Properly designing a pack for a good service life is pretty hard with 18650s. That's why it's much easier to use purpose-designed automotive cells. They're larger so you can't make as many mistakes and their performance matches requirements for electric cars.