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I've seen lots of people jump start their cars with 18V lithium power tool batteries and I've also seen all the comments descend into arguments about it being too much voltage for the car and too much current for the power tool battery. I've never seen anyone actually measure them, they all just throw around imaginary numbers to suit them.

In this video, I measure the current through the power tool battery and the voltage of the car battery whilst performing a jump start on my car. The batteries used were Milwaukee M18 5ah and 12ah.

USE MY RESULTS AND CONCLUSIONS AT YOUR OWN RISK!
Different cars, car batteries, 18v batteries, and jumper leads will yield different results and may damage your car, battery, and yourself.

00:00 Abstract
00:13 Background
00:36 Setup type of car, conditions on the day, battery condition, batteries used
01:44 Results 5Ah jump
02:57 Results 12Ah jump
04:19 Results Trickle charge
05:15 Conclusions and all the graphs

References:
Lock your Hubs    • Tool Battery Car Jump Start?  
c01by25    • Life hack drill battery and two forks...  
Tales Taller    • Jumpstarting My Truck With A Milwauke...  
Screws Nuts And Bolts    • Jump Starting Your Car Using a Drill ...