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If you go mobile, use something like this one: https://www.conrad.ch/de/p/xtpower-xt-27000-dc-ao-pa-powerbank-26800-mah-li-ion-usb-usb-c-dc-buchse-3-5-mm-schwarz-2866160.html It has a special feature not to switch off automatically when power draw goes below 4W. Beware: manual says it switches off after 12hrs. I did not check this yet. Suitable barrel jack adapters are supplied. Verify voltage before use (long press on button, voltage blinks).