Years ago I bought a couple battery jump packs. They worked great but were big and bulky and only lasted a few years. Last April we stopped at our buddies shop and we were trying to start the 572 in Norm's '55 Chevy. Unfortunately the battery was dead. Dave had just purchased a small jump pack off the Mac truck so we gave it a try. It started the engine several times. Dave had told us that the jump pack was about $150.
A couple weeks ago I was wandering through the automotive section at Walmart when I found a similar jump pack and it was only $60. I picked one up and just had to use in to start my Cadillac 500. It started it three or four times and still had a charge. I was having a carburetor problem. It will hold a charge for over a year and it has a usb port to charge anything that uses one.