I have an old MacBook with an OK battery. When I go out sailing or traveling I have a lead battery with me to be able to charge USB-things. iPhone, portable speakers, etc.
The old MacBooks battery is a lithium battery with excellent power density. I would like to use that instead. Have you heard about an adapter or a case that lets you take a MackBook battery and get one or several USB outlets?
Charging of the MacBook battery is not needed since I can do that in my old Mac.
Think the MacBook is 2007-ish. It's from when the batteries were easily removable with a coin anyways.