My guess would be strict quality controls on each individual battery combined with a sophisticated battery management system. I've long thought that one of the

My guess would be strict quality controls on each individual battery combined with a sophisticated battery management system. I've long thought that one of the reasons Tesla built their own battery factory is so they can be absolutely assured of the consistency of the batteries. If the charge rate, discharge rate or internal resistance of an individual battery varies from the others over time, that's when battery packs tend to catch fire. Well, one of the reasons battery packs tend to catch fire, anyway.