The nation's largest retail banks could lose $92 billion in deposits and $5.1 billion in revenues in the next year, as frustrated customers switch to smaller banks and credit unions.

Customer frustration has fallen in the past two years, but 9.7% of customers at the biggest banks are still likely to move their money to other companies in the next 12 months, according to a study released Thursday by the Connecticut consulting firm cg42. The study measured "bank vulnerability," or the risk of losing customers, by surveying 3,600 customers at the top 10 retail banks between February and March 2013.

Big banks still have not fully addressed customers' number one frustration since the first survey in 2011: incidental charges. These "egregious charges" include penalty fees for withdrawing money outside of a bank's ATM network and fees for reordering checks, says Steve Beck, founder of cg42.

"You're out there claiming relationships, Mr. Bank, yet whenever I turn the corner I hit a little fee here and a little fee there, and it doesn't make me feel good," Beck says, explaining many bank customers' reactions to unexpected charges.

Many retail banks have added fees or increased the pricing on once-free checking accounts in the past few years, as new regulations have cut into the overdraft and debit-card revenue that banks could once rely on. But the industry's efforts to recoup that lost checking-account revenue have often damaged its reputation. Bank of America's (BAC) aborted attempt in 2011 to start charging customers for using debit cards sparked a groundswell of customer anger and public fury over bank fees, and to community banks and credit unions.

The outcry over bank fees has cooled somewhat since then, but banks are still struggling to improve customer satisfaction. The study found that big banks have also failed to adequately respond to customer complaints over overdraft charges and mistakes on bank statements.

Big banks "promise relationships, they promise a lack of hassle, they promise to operate differently," says Beck. But "promises don't equal delivery."

Fewer customers are at risk of leaving the big banks than they were two years ago, Beck says; bank vulnerability has slightly improved since 2011, with the percentage of potential defectors falling from 33% to 26%.

But the problems remain the same, according to Beck. "Interaction [with banks] is not as bad as it was in 2011…[but] there has not been a demonstrable change that has led consumers to feel better."

The intensifying debate over the size and complexity of banks' operations has also affected their customers. The study found that 55% of customers are uncomfortable with how big banks have become, and 63% feel that banks only care about their own interests. That's up from two years ago, when 45% of customers were uncomfortable with banks' size.

Citigroup (NYSE:C) was ranked worst in terms of customer satisfaction according to the survey. Bank of America, which ranked most frustrating two years ago, has fallen to third-most frustrating in the new survey. Beck says that the Charlotte bank has improved its overall service experience, especially its online banking tools and hours, and it made its customer service more consistent across its different branches.

"B of A has improved from being the poster child for frustration with major banks [in 2011]," said Beck. "Good news for them. That means they listened."

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