Same happened to me a few weeks ago, so I paid a visit to my local motor factors and bought a cigar lighter plug (not the actual lighter but a plastic accessory thingy), length of twin cable, in-line fuse holder and a pair of croc clips, wired them all together. Now if my battery dies, I can just plug into lighter socket and attach croc clips to flat battery, drawing power from van battery. Very easy to do, cost about 15euro max.