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The first EV with a lithium-free sodium battery hits the road in January
(www.engadget.com)
This is a most excellent place for technology news and articles.
So only charge it to 80% and pretend 80% is 100%, like iPhones do. Why is that a concern?
Or you could just use all of the space for a sodium battery and fully charge it as it won't need long term storage in that state.