found this jump starter of 400A , 12000mAh for $35 after coupon AUKEYBC1 @ Amazon.
hope this will help your backyard car to start.
also helps boat, bike, phone jump start.
AUKEY Jump Starter with 400A Peak Current & 12000mAh Portable Charger for Car Battery, Boat, Motorcycles, Lawnmowers and More
https://www.amazon.com/dp/B01EKPZ…FAbR3
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