As soon as you start it it will drain more power from it. You could do with one of these
http://cgi.ebay.co.uk/12V-CAR-BOAT-B...item5885da87f5
Some diy stores sell them for about £25 but at least after you've shown the missus how to use it she would,nt get stuck anywhere if the battery did discharge. Only thing is once you've used it , it needs to be recharged for the next time.