Central Charge Service Inc., with approximately 500,000 credit card customers in the Washington area, announced a series of changes yesterday that will make using the card more expensive for Maryland and Virginia residents and will speed up repayment of large bills.

The company said in the future that finance charges will be governed by state laws that allow rates to rise higher than rates now in effect.

In Maryland, customers will be charged 18 percent on the first $700 of the average daily balance and 12 percent on amounts above that. Central Charge now charges 18 percent on the first $500 and 12 percent above $500.

In Virginia, customers will be charged 18 percent on the average daily balance. Rates in the District will remain unchanged.

The changes are effective August 1.

Harry M. Schoenly, president of Central Charge, a subsidiary of the Riggs National Bank, said the changes were triggered by present economic conditions and because of the increased cost of doing business.

Central Charge also announced a change in the manner in which the average daily balance is computed, similar to actions taken earlier this year by Sears and other retailers.

In the future, if a customer carries a balance from the previous month, current purchases will be included in the computation. If a customer's account is clear from the previous billing period, however, interim purchases will not be included in computing a balance during the month.

In another change, Central Charge will alter repayment schedules on balances of more than $200. The minimum monthly payment will be 5 percent of the highest new balance on the account since the balance exceeded $200, and payments will stay at that level until the balance reached $200 or less again.

On balances of less than $200, the minimum repayment will remain 5 percent or $10.