Some millennials still have a lot to learn about using plastic, according to a LendEdu survey.

The survey, of 500 respondents, found that 45 percent of millennials didn't know their credit-card interest rate.

About half said they carry a balance from month to month. 36 percent said they had maxed out a credit card, 34 percent said they were not aware of how much the late fee would be if they missed a payment and 17 percent said they don't know their current card limit, the survey shows.

For the 29 percent of respondents who had missed a payment, 17 percent thought their credit score would remain unchanged afterwards and almost 6 percent believed a missed payment made their credit score go up.

Also "the majority of millennial credit card holders are unaware of a rule that allows them to protest an interest rate hike from their credit card company," the survey says. When asked, "If your credit card company increases your interest rate, can you do anything about it?" 68 percent of respondents wrongly believed they would have to accept the increase.

Under the Credit Card Accountability and Disclosure Act, cardholders actually have the right to refuse to pay a higher annual percentage rate set by the credit card company if they meet certain requirements.