Purchases on credit and charge cards picked up in December thanks to Christmas shopping but, with households keen to keep debt in check, the value of balances accruing interest fell.Consumers made 228.1 million purchases worth A$27.8 billion on credit and charge cards in December, according to the latest Reserve Bank payment data.This was the highest number of purchases recorded in the data series (which goes back to 1994) and also the greatest value recorded. But an indication of how much more disciplined consumers are in the use of their credit cards is that balances accruing interest fell to one of the lowest levels recorded in recent years - $32.1 billion, or $1970 per account.On their debit cards, consumers made 421.5 million purchases worth $23.1 billion in December.