DIY- 10+ amp Cordless car battery charger- Dewalt 20v AND million volt DC step-down in China

My car battery is 6 yrs old, so i made up a charger in case it gets too weak to crank while i am out.
Since i have two 20v 4ah Dewalt tool packs from my ebike, i hooked 1 to this 300w $6 module
https://www.ebay.com/itm/DC-DC-Converter-10-12-15-20A-150-250-300-400-1200W-Step-up-Step-down-Buck-Boosts/293282788628?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160811114145%26meid%3Dfa8a241cdc4a4787b037852ad638e49b%26pid%3D100667%26rk%3D7%26rkt%3D8%26mehot%3Dpp%26sd%3D324282871316%26itm%3D293282788628%26pmt%3D1%26noa%3D1%26pg%3D2334524%26brand%3DUnbranded&_trksid=p2334524.c100667.m2042
anybody want my test results?

imagine the size of this converter
https://www.youtube.com/watch?v=rThkjp-bp8M&ab_channel=JustHaveaThink