Banks are urging some of their largest customers to take their cash elsewhere or be slapped with fees, citing new regulations that make it onerous for them to hold large deposits in accounts that were previously free. The new measure requires the banks to maintain enough high-quality assets that could be converted into cash during a crisis to cover a projected flight of deposits over 30 days. Because large, uninsured deposits would be expected to leave most quickly, the rule requires that banks maintain reserves that they cannot use for profitable activities like making loans. Some bankers said they are advising corporate clients to break up large deposits across several banks, including smaller ones not affected by all of the new rules.