U.S. banks have collected a record $7.5 trillion in deposits as customers feel reassured by government support. But the biggest banks are also watching in frustration as longtime clients spread their business, and their deposits, among multiple other banks.

Consider Ted Feight, who plans to move most of his business away from what was once National City Corp. after doing business with the bank for 25 years.

The president of Creative Financial Design in Lansing, Mich. has told some financial planning clients to do the same - especially those who hold more than $250,000, the limit for government insurance. He watched the big Cleveland bank crumble last year under bad loans and has now grown nervous about its sale to Pitsburgh-based PNC Financial Services Group Inc. (PNC)

"I used to take clients there," Feight says, "whereas now I'm taking clients away from them."

PNC spokesman Fred Solomon said the bank has "gained back more than $2 billion in deposits from National City customers since January."

"This is our eighth acquisition in the past five years," he said, "and we have a successful track record of retaining more than 90% of the customers."

A year ago, consumers placed 44% of their financial business with their primary bank, according to consulting firm Mercatus. They now place only about a third of their business at their main bank and report being frustrated by failed banks, bonuses for bailed-out bankers and rising account fees at government-supported banks.

The awkward coincidence is that customers are pouring record levels of cash into reserve accounts at the biggest banks - normally a good sign that business is booming. Domestic U.S. deposits grew nearly $500 billion to a record $7.5 trillion during the year ended in March, according to the Federal Deposit Insurance Corp. And they appear to have kept growing since.

Crowds of investors sold assets for cash last year as markets tumbled. And more recently, a renewed focus on savings has helped swell deposits further.

But overflowing deposits don't necessarily lead to big profits, since big banks have to cover hefty fixed costs for buildings, computers and layers of full-time staff.

In fact, grabbing "wallet share" - or bankers' speak for winning more of a customer's business - is so important to profits that banks actually track their progress through various gauges.

Wells Fargo & Co. (WFC), for example, has for years tallied a "cross-sell ratio" every quarter to track how many products it sells to its average customer.

But even when published, "cross-sell" ratios and other such metrics don't necessarily reveal how many customers are actually using those products, or by how much. Customers are famously known to open a variety of bank accounts and credit lines, only to let many sit dormant.

In fact, the broad surge in deposits and new accounts can disguise an underlying dynamic where big banks merely trade disaffected customers.

"All those new accounts have to come from somewhere," says John Philpott, general manager of S1 Enterprise, which works with banks.

Higher customer turnover is a curve ball for investors and analysts trying to estimate banks' future "normalized earnings" - or what kind of profits they're likely to generate once the U.S. economy emerges from its recession.

Just how high those earnings will be is becoming a more crucial question, since financial stocks have more than doubled since early March on hopes that banks will soon start turning post-crisis profits.

Whether actual future earnings will justify yet more buying is very much open to debate.

Analysts' estimates for Wells Fargo's earnings in 2011, for example, range from $2.60 to $4.38 a share, according to FactSet.

For Bank of America Corp. (BAC), which is struggling with high loan losses and its imposing purchase of Merrill Lynch, estimates range from $1.68 to $2.91 a share.

How those earnings end up panning out will depend heavily on which customers stick around that long.

-By Marshall Eckblad, Dow Jones Newswires; 212-416-2156; marshall.eckblad@dowjones.com