If your battery is dead flat ie close to zero volts the Li-on will not start the engine. (Don't know about the lead acid ones.) This has been proven. What they rely on is some charge remaining.
eg you cannot disconnect the battery and connect the Li-on unit in its place.
Having said that I swear by my Powerall. Have started my own vehicle, my small diesel tractor, my ride on (not really a test) with my no probs. Have actually started more other people's vehicles than my own and that includes older 3l diesel utes.
However, if your battery is getting tired there is no substitute for replacing with a new one. Buy a jump start unit later on as a back up.
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