I am not sure how often this happens and even though charging at some places can be free, which feels very nice – free energy – you are not really saving a great deal of cash.
But where are these free chargers?
Near where I live there is a free charger in a pay-car-park. BUT the parking for a charging electric car is free as well. It is next to a supermarket that we use. Park up, plug in and go and do your one hour shop.
Another supermarket has free chargers while you shop for as long as you like.
Many chargers get put of free charge for a while after they have been maintained. I guess they do this to check the charger is now working properly.
Social media is good as folk will note when a charger is set to free vend. A very useful app is ZapMap which shows the location of public chargers and if you click on a location it tells you whether the charger is working, what rate(s) of charge it provides, when it was last used successfully and if it si free.