I'm not an expert, but my understanding is that reduced capacity of old batteries is a symptom, not the problem. If reduced capacity was the only issue, then ye

I'm not an expert, but my understanding is that reduced capacity of old batteries is a symptom, not the problem. If reduced capacity was the only issue, then yes, it would make total sense to run old batteries into the ground. But the issue is that the reduced capacity is a symptom that the battery is decaying internally, through a variety of mechanisms (cathode/anode micro-cracking, dendrite growth, physical leakage, who knows). Old batteries are dangerous fire risks, exactly what you don't want in a warehouse full of flammable batteries. Even a 1 in 100,000 risk of fire is unacceptably high if it causes a warehouse to explode. For this reason, it's risky to run old batteries into the ground. This is why there isn't a market to turn old electric car batteries into utility-scale storage. Then again, this doesn't seem to be an issue for consumer electronics like laptops and phones and electric toothbrushes, so I wonder what's different, if anything.