I've already investigated how to power the RPi with batteries (I did some tests with AA batteries and with 18650 batteries) using a 5V "boost" step-up DC-DC converter, in some other questions here.
But now I'd like to test directly with
18650 3.7V battery ========> 3.3V of the Pi
Are there special thinks to know before trying? Should I put a 3.3V regulator? A DC-DC step down convertor? (I'd like to avoid regulators that dissipate the over-voltage into heat...)
Or can I plug it directly ? Will 3.7V harm the RPi's 3.3V input ? Does it need exactly 3.3V?
Why do I want this? I'd like to avoid
3.7 battery === DC/DC converter ===> 5V ===> Raspberry Pi (converted in 3.3V again !)