Past consideration is not valid consideration. Past consideration is defined as something already done at the time the agreement was made. Therefore, where the consideration was already made there will not be a valid contract. E.G case of Re McArdle (1951. In the case, Majorie McArdle carried out certain improvements and repairs on a bungalow. The bungalow formed part of the estate of her husband's father who had died leaving the property to his wife for life and then on trust for Majorie's husband and his four siblings. After the work had been carried out the brothers and sisters signed a document stating in consideration of you carrying out the repairs we agree that the executors pay you £480 from the proceeds of sale. The court held that the promise of payment did not create a contract as the promise was made after the alteration were already done before the promise. Thus the consideration was past. However, past consideration may be valid where a request is followed as in the case of Lampleigh v Braithwaite (1615). In this case, the defendant had killed a man and was due to be hung for murder. He asked the claimant to do everything in his power to obtain a pardon from the King. The claimant went to great efforts and managed to get the pardon requested. The defendant then promised to pay him £100 for his efforts but never paid up. The courts decided that although L's consideration was past he had already gotten the pardon therefore B' promise could be linked to his earlier promise and is therefore valid.
I'm not too sure about the exceptions. Hope this helps though...