+ 1.

The car battery chargers that work through a DC lighter socket take forever, and you'll do much better off with the portable units that jump the battery directly. Then let the alternator do the charging as God intended...

Also, batteries start to go bad once they leave the factory. That's why car and deep cycle batteries are dated and you shouldn't buy one older than 6 months. Even then, they are frequently on the way out. I bought a car battery with a CCA (cold crank amperage) of 800 at CostCo. Took it straight to one of those car care places that tested it at 500. Went straight back to CostCo. Either it was bad from the start, improperly stored, or they changed the date label. Take your pick.

Edit: Oh, you were talking about a portable jump starter -- my mistake. They've actually saved me a couple of times. Buy one with a built in trickle charger; leave it plugged in. If you're going on a long trip, throw it in the car. Replace the battery (if possible) or the unit every few years. I wouldn't go off the beaten path without one.