Partial charging is just fine for lithium-ion batteries and even has some positive benefits.

Notably, operating at a low voltage is good for a battery’s lifespan, increasing the number of available charging cycles before you’ll start to see a significant reduction in capacity. Roughly speaking, every 0.1V decrease in cell voltage doubles the cycle life, according to Battery University. Therefore, charging up your phone in that 30% to 80% range keeps the voltage lower and might slightly prolong the battery’s lifespan.