Dartford Crossing

Sorry, I know this has been covered elsewhere (not sure where) and this might be the wrong place to post it.......but does anyone know, if you set up an account for the crossing (minimum £10 deposit I believe) does the money stay there until it's 'spent' (on crossings)?

Someone said that they thought it 'expired' after a year....but was that just a guess, because I can't find anything on the Gov.uk web site that says this.

Do you simply lose any money left in your acount?