Running NodeMCU ESP8266 on 3s 18650 battery pack powered by solar panel
Get a proper DC-DC converter that accepts the full battery voltage input and outputs 5V. With a suitable &quot;buck&quot; converter this can be very efficient.
(Possibly 3.3V directly? It's not entirely clear that the board needs 5V and if you can just take it directly to 3.3V that would be more efficient.)
Or alternatively just use the two batteries and remove the third entirely. Having it charged but never discharged will cause problems as the pack becomes unbalanced.