If you live in the North you probably have jumper cables in your trunk. I do and last Winter I had to jump vehicles for others four times.  3 times after bad storms and I could not get close enough to use cables. For those times I used a portable power supply that I bought at Home Hardware many years ago. It is heavy, awkward and getting old but it will still work.

I happened to notice a wee little power pack at local bike dealer and it reminded me of the Antigravity product I had seen a few years ago.  They are expensive and I never bothered.  I tip toed off to a favourite dark site maintained by Fung Su and found the product at a 75% discount.  Bought with PayPal and 58 days later it arrived.

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I had watched numerous YouTube videos which seemed impossible but at a deep discount what the heck.

Today I was servicing my diesel tractor and had battery disconnected as I was doing some wiring.  It was a good opportunity to try the little battery.  I clamped the battery on and started the tractor 4 times and the little display on the battery indicated it still had a full charge.  What a cool little device. It will now stay in my car as another backup.