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One other thing if you're using a Dewalt is the low voltage shut off is on the tool and NOT on the battery. So it's easy to kill the battery. The adapter I have has a low volt shut off to prevent that. Not sure about other batteries if the shut off is on the battery or the tool.
Here is the one I bought, they have other battery adapters also and the cable, make sure you buy the cover!:
https://www.partsbuilt.com/battery-adapt...e-milwalt/