I have a lithium Jump Starter pack and find it works really well.
Initially used it on my LT with a knackered
battery (and it was REALLY knackered, but I wanted to wait until spring to replace). Got a Video below of how it starts the 2.8L diesel engine with this
battery
You could use the various
battery to
battery trickle leads to transfer some energy to a flat
battery, but if you get in, find you have a flat
battery, you may not want to wait the time that would take and a jump pack is a lot quicker.
Also handy compared to a Starter-Leisure connection as youy can use it on multiple vehicles (used mine on the car when it had a flat
battery for some unknown reason, and also to start someones van in Durrige Bay at a Wild Camping meet a few years ago).
I used to have a couple of the Ring Power Packs like in mfw's post but found they were a ) heavy and awkward compared to the Lithium packs and b ) If you didn't keep it charged regularly it would die prematurely and be useless and if you left it plugged in to charge it would damage the
battery due to the very basic charger built into it.
Old fashioned dated technology, long superceeded by the newer options.