Correct me if I'm wrong, when you say 'bad' it's voiced; and the meaning is 'not well, not good'. If you say 'bat' (voiceless), you have an animal or a stick (so the meaning changes). That's a classic mistake Russians make. I remember asking one Russian guy to make his last sound voiced in the word 'as'; otherwise it sounded like in 'ass'. (Instead of 'as I know' it was 'ass I know' due to his phonetic mistakes).

I think Russian speakers should at first develop a habit of making all consonants voiced in such instances as 'bad, bed, hub, mug, etc."; it's just a most classic mistake due to our Russian rule of making all sounds at the end voiceless.

Another example I remember is when a Russian said 'room maid' with a voiceless 'd'. So everyone was thinking she meant 'roommate'.