The safest way (especially if you're not comfortable wiring your own battery, charger etc.)
is a "jump start" pack as mentioned, or even some smaller ones that are meant for non-boost
usage. I keep a small one (about 4Ah) and a "booster" (15Ah) for various purposes.
They can also have other features (built-in lamp, 3/6/9v regulated outputs etc.).
One comment on gel-cell charging from a wall-wart. It's better to use a regulated charger,
unless it's one built into one of these packs.
Dave