A balance transfer credit card is a popular type of card that lets you transfer high-interest debt from one card to another with a lower interest rate. With these cards, you can take advantage of a promotional 0% APR period to pay off credit card balances that might otherwise keep your debt growing with interest.

If you're struggling with credit card debt, a balance transfer card can be a lifesaver. But the savings are only sweet when you use them the right way. Otherwise, they can actually make it harder for you to erase that debt completely.

With that in mind, here are three of the worst mistakes you can make with balance transfer credit cards.

1. You overlook the balance transfer fee

Initiating a balance transfer isn't free. Nearly all balance transfer credit cards will charge a balance transfer fee, which is typically 3% to 5% of the transferred balance. For instance, if you want to transfer $10,000 to your new card, you can expect to pay about $300 to $500 in balance transfer fees.

When it comes to choosing between a balance transfer or paying off your current card, pick whichever is more economical. If paying the balance transfer fee is cheaper than racking up debt under your card's APR, obviously bite the bullet and pay the fee.

But run the numbers first, because a balance transfer card isn't always the way to go. This is easy to calculate when you know how much you can put toward paying down your debt each month.

For instance, if you pay $500 a month on a card with a $5,000 balance and 23% APR, you'd pay off the card in 11 months and incur about $600 in credit card interest. Transferring this balance to a card with a 3% balance transfer fee and 12-month 0% APR period would mean paying $150 in fees and zero in interest.

Convinced a balance transfer card is right for you? Click here to get our curated list of best balance transfer credit cards -- with intro 0% APR periods as long as 21 months.

2. You assume you can shift your credit card debt to a single card

Balance transfer credit cards only let you transfer as much as the new card's credit limits allow. This means that if your new card's credit limit isn't high enough, you might not be able to transfer the full balance from your old card.

Let's say you have a $8,000 credit card balance. If you open a new balance transfer credit card with a $5,000 credit limit, you wouldn't be able to transfer the full balance to the new card.

To be sure, shifting even some of your balance could save you money on interest over the long run. But you might need to factor in the possibility of accruing credit card interest depending on how much you currently owe.

3. You continue to rack up debt on your new or old card

While you're paying off credit card debt, try to avoid charging more to your credit cards -- whether it's your new balance transfer card or an old card.

This is important for two reasons. One, your balance transfer credit card may not have a promo 0% APR period for new purchases. Many balance transfer cards only give you a zero-interest period for balance transfers. That means if you're using your credit card for new purchases, you could run the risk of accumulating more debt.

The second reason is that a balance transfer will free up credit limits on your old card. For example, if you maxed out the $5,000 credit limit on an old card, a balance transfer of $5,000 would zero out your limit. Running up the credit limits on this card would leave you in the same place as before -- just with more debt.

Avoiding these three mistakes can help you get the most out of a balance transfer credit card. Click here to check out our favorite 0% intro APR credit cards, with introductory periods lasting as long as 21 months.