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Why Charging Your Gadgets Over 80% Is Such a Bad Idea | iFixit News
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This is a most excellent place for technology news and articles.
Charge it from a smart power supply from battery at 1 to 100% then it can show you the number of mah/h it took to charge it.
I have this power supply which also has USB-C https://a.aliexpress.com/_mrChiQ6
Not sure how accurate this would be as charging is not 100% efficient. Also the amount of power the phone uses while charging would have to be taken into account as well.