You can add かい at the end of the word (sentence) in a daily casual conversation. It's the same meaning with "ですか？" ( ですか is formal as you know). 元気かい？大丈夫かい？お腹すいたかい？...etc Those sound like a tomboy or being a little silly for playing. We say that sometimes.

「かい」は疑問を表す終助詞です。話し言葉で使われます。 つまり、「大丈夫かい」は、「大丈夫？」と同じ意味なんです。

大丈夫かい。　→　大丈夫ですか。 聞こえているかい→聞こえていますか。 「かい」 isn't used in the situation between you and the senior person. It is used parent's speaking to the children, and boy friend's speaking to her, including the favor or kindness. But you had better not use to your boy friend.

ありがとう！

かい　is used casually. If you use it against your boy friend, it badly affects him. Even if you want speaking to him with love, he hears abnormally.