I'll plug my shop made auto and diesel truck emergency starter. From a different thread on the topic:
"The quality hand held boosters will start good size v-6 engines with the battery entirely removed from the car. Anti-Gravity units will. Especially the Heavy Duty unit. The purpose is not to partially recharge the low or dead car battery. They are made for immediate starting. Back in the Chevy V-8 days, starter motors required around 250 Amps to work. Todays starters in smaller engines require significantly less. The Lipo style battery in the small Anti-Gravity starter unit is only 3.6 Ah. The battery in the Heavy Duty unit is about 6.6 Ah. There's video of the HD unit providing 330 Amps to an electronic load. It maintains about 9.5 Volts or so for well over the time it requires an engine to start.
I just got through making my own unit using a LiFeP04 pack which provides a slightly higher Voltage than do the Lipo based starters and much safer to keep in the vehicle. Not to mention virtually no self-discharge."