system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do car air conditioners need to be recharged, but home units (window, central air, etc) can be used continuously?
gpt
Coolant molecules are very small, and coolant is under extremely high pressure, so it's prone to leaking. Cars use non-solid piping for parts of the A/C system because the pipes need to be flexible. These lines tend to leak, and their unions tend to use O-rings for seals which tend to leak as well.

Whereas the piping in home units tends to be solid copper or aluminum, and the unions are brazed. So everything which contains the coolant gas is made of solid metal. It's much less prone to leakage.