I used to have a Black and Decker portable battery charger. The one time I needed it, it didn't do anything. I'm not all that handy, so in that instance, we'll allow for a 10% chance of user error. Pretty sure it was the charger, not me.
Last year my battery died, and I called roadside assistance. The mobile mechanic rolled up with a portable battery charger and zapped my car in an instant. I told him about my negative experience with portable chargers. He said the one he had - I think it was a Schumann or something - made his day pretty easy, but not to tell his bosses. (It was a good line...but I think he was the boss.)
I looked high and low (ok, 20 minutes online) for that model, and I thought I'd tracked one down. But online results don't always match actual store results. The store didn't have that model. The sales representative talked me into some lithium battery space age charger.
I have the opportunity to test out its effectiveness today.