never had a problem with my Sealey one, and it's a few years old.

Although sometimes it has been a struggle on totally flat batteries, but for it's size it's been great. And even a few years old, still working great.

Plus a proper booster pack will be safer than a big battery and leads. Dont forget modern car electronics are easily fried using jump leads. If you do go down that route, get a surge protector.