A method for power management of a wireless device comprising a processor and a volatile storage device which is connected to and used by the processor, the method comprising: supplying a first voltage level to the volatile storage device during a period when the processor is in an operating state; supplying a second voltage level to the volatile storage device for retaining data during a period when the processor is in a reduced power state, wherein the second voltage level is lower than the first voltage level; and determining a value of the second voltage level by: (i) supplying an initial value of the second voltage level to the volatile storage device; (ii) performing a self-test of the volatile storage device while the second voltage level is supplied to the volatile storage device to provide a result indicating a number of data errors in the volatile storage device; (iii) comparing the result of the self-test with a threshold number of data errors; (iv) varying the second voltage level to a new value and repeating steps (i)-(iii) with the new value of the second voltage level to find a minimum value of the second voltage level at which the result of the test gives a number of data errors less than the threshold number of data errors, characterised in that the volatile storage device comprises a plurality of storage banks and the step of determining a value of the second voltage level is performed for at least two of the storage banks to find a minimum value for each of the at least two storage banks. supplying a first voltage level to the volatile storage device during a period when the processor is in an operating state; supplying a second voltage level to the volatile storage device for retaining data during a period when the processor is in a reduced power state, wherein the second voltage level is lower than the first voltage level; and determining a value of the second voltage level by: (i) supplying an initial value of the second voltage level to the volatile storage device; (ii) performing a self-test of the volatile storage device while the second voltage level is supplied to the volatile storage device to provide a result indicating a number of data errors in the volatile storage device; (iii) comparing the result of the self-test with a threshold number of data errors; (iv) varying the second voltage level to a new value and repeating steps (i)-(iii) with the new value of the second voltage level to find a minimum value of the second voltage level at which the result of the test gives a number of data errors less than the threshold number of data errors, (i) supplying an initial value of the second voltage level to the volatile storage device; (ii) performing a self-test of the volatile storage device while the second voltage level is supplied to the volatile storage device to provide a result indicating a number of data errors in the volatile storage device; (iii) comparing the result of the self-test with a threshold number of data errors; (iv) varying the second voltage level to a new value and repeating steps (i)-(iii) with the new value of the second voltage level to find a minimum value of the second voltage level at which the result of the test gives a number of data errors less than the threshold number of data errors, characterised in that the volatile storage device comprises a plurality of storage banks and the step of determining a value of the second voltage level is performed for at least two of the storage banks to find a minimum value for each of the at least two storage banks.