The power supply is a motorcycle 12V lead acid battery. Running in a racing sidecar. When you turn the "ignition" off, the power will be lost to the arduino.
I suppose it is possible I could take a feed directly from the battery, and use this when the ignition is off, but I don't want it to be powered all the time.
I will do some testing once my SD card module arrives. Once I have a time it should take to finish writing data to the card, how would I work out the value of capacitor that I need?