Apple’s lithium-ion batteries charge to 100% in two phases to extend battery lifespan. This process is called Optimized Battery Charging. In Stage 1, batteries fast charge to 80% capacity. In Stage 2, batteries ease into a slow charging or “trickle charge” state until they reach 100% capacity. In rare cases, your Mac may need to cool down before it can charge past 80% capacity. Thankfully, Apple provides recommended ambient temperature guidance for all MacBooks on its battery support website.