Without being a battery chemistry expert, why do these battery packs become not useful for an EV yet could still be useful for energy storage. They keep saying that 80% of life becomes unusable for EV, but that's still a lot of life. Is it that grid energy is more of a constant drain while the EV is lots of hard pulls (for lack of better wording)? In an EV, the battery cannot provide the higher volts being requested within rating, but a grid is never demanding peak performance?
It's not just about capacity (80% is still a lot), it's that degraded batteries lose their ability to deliver high current under load—so acceleration suffers and voltage sags under hard pulls. For grid storage, you're doing slow, steady charge/discharge cycles over hours, so the same battery that can't handle aggressive driving anymore works perfectly fine. Plus, grid storage has virtually unlimited space and no range anxiety, so if you need 25% more packs to hit your capacity target, you just stack them in a warehouse where real estate is cheap.
Also, batteries will degrade faster over time when they start to degrade, because they need more frequent charging. Their internal resistance increase and that promotes heat buildup during fast charging/discharging, another thing that promotes degradation. Slow charge/discharge cycles also help with heat management.
They claim to have taken the Moss Landing fire into account with how they are placing their batteries. We won't know if they've really solved the problem or not until their first battery pack experiences a runaway thermal event.
Space and weight are serious constraints in the car space, but not such a big deal on the side of a house. That’s how they retain their usefulness.
80% could indeed be plenty of usable life for your EV use cases, but it strongly depends on usage patterns. More degradation means more trips to the charger on a road trip. It means trips that you’d regularly make just charging at home at the end of day now require you to plug in at the destination too. It means more range anxiety as a whole.
The load in an EV is very different than in storage. Basically the charge and discharge rate is what deteriorates the battery. EVs need a lot of power delivered quickly in bursts when you accelerate (tens/hundreds of kw). And then fast charging when the driver is in a hurry also puts a lot of stress on the battery. With storage solutions, the power requirements are much less intense. These high bursts of energy would actually blow the fuse in your house. They are simply not needed. And there is no need for fast charging them either. Instead they get charged over many hours when there is cheap power available.
Companies like Redwood are good at assessing the state of the battery and then managing it such that it is run optimally. Usually, it's just a few cells that are no longer working; the rest of the pack is still be fine. So that just means the max output of the pack drops a bit. But that's still more than fine for storage if all you need is a few kw of output.
Running the battery optimally also extends the useful life of the remaining cells.
