I will share a story I heard from the dealer this summer when exchanging my CNG tank. I asked why it has become so difficult to find CNG exchange. She explained that CNG is plentiful, of course. The problem is the tanks we use are hard to come by. As they go through the exchange process the tanks are inspected, tested and recertified. Inevitably, some tanks fail the testing and are taken out of service, with no ready replacements available. Hence, a steadily decreasing tank supply. The point of the story is that these tanks do not last forever. The tank exchange process ensures in-use tanks are regularly inspected and retired when no longer safe. When refilling tanks independently this reinspection does not automatically occur and over time unsafe tanks may remain in use. One would be wise to consider the risks if refilling uncertified, potentially unsafe CNG tanks.