I use these:
https://www.northerntool.com/shop/to...2201_200332201
If your need isn't immediate, they go on sale every so often for $20. Might be worth waiting until Black Friday and see if they drop the price.
One of mine is several years old and used on a motorcycle. Picked up two more a couple years ago when I moved and needed to keep a tractor and ATV charged up.