From the cell to 3.3 V
The regulator makes a steady 3.3 V from the cell for as long as the cell stays a few tenths of a volt above that. Below about 3.5 V the rail starts to follow the cell down on every Wi-Fi burst, which is why a lithium cell's last few percent are not worth chasing.
A cell is not 3.3 V
A lithium cell is 4.2 V full, spends most of its charge near 3.7 V, and is empty at 3.0 V. The ESP32 and the flash chip beside it want 3.3 V, steady. The regulator on the board sits between them and turns whatever the cell is into 3.3 V — by dropping the difference.
A regulator like this needs a little room to work: its input has to stay a few tenths of a volt above its output. While it has that room, the output does not move. When it runs out, the output follows the input down.
Watch the cell run down
The drop drawn is a typical figure for a small regulator at the 240 mA of a Wi-Fi burst, not a measurement of this one — the part is not documented. The shape is what matters: flat until about 3.5 V, then following the cell.
What the ESP32 does at each stage
| Cell | Rail in a Wi-Fi burst | What happens |
|---|---|---|
| 4.2–3.6 V | 3.3 V | Nothing. Most of the cell's charge is here |
| 3.5–3.4 V | Dips a little below 3.3 V | Still works. The start of the end |
| 3.4–3.0 V | Dips further | Works, with little charge left; resets if the cell is small or cold |
| Below 3.0 V | — | The cell is being damaged. Its protection cuts it at 2.5–3.0 V |
Where to stop
Stop at about 3.4 V, in software: read the cell, and when it gets there, finish what you are doing and go to sleep for good or switch the radio off. Between 3.4 V and empty there is very little charge left, so stopping costs almost nothing and saves the cell being run down to its cut-off.
The protection circuit on the cell is the last resort, not the plan. It cuts the cell off when it gets there, which is abrupt — no chance to save anything — and a cell taken there often wears faster than one that is not. Reading the cell is the sketch that knows when to stop.
When it does not work
The rail is sagging when Wi-Fi transmits. Charge the cell and try again; if it still resets, fit a 100 µF capacitor across 3V and GND on the shield, and turn the radio down with WiFi.setTxPower(WIFI_POWER_8_5dBm) after WiFi.begin().
The cell cannot deliver the burst current without its voltage collapsing. Small, old and cold cells all have more internal resistance than the voltage suggests. Try a larger or newer cell, or a capacitor across 3V and GND.
Yes: 3V is the regulator's output whichever source is feeding it. Every milliamp a sensor draws comes out of the cell too, so it shortens the run time, and anything that draws in bursts — a servo, a motor — needs its own supply.
Capacity over the average current, and what decides the average.
How long a cell lasts →Edit this page — content/books/esplipo/from-cell-to-3v3.mdx
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ESP32 LiPo Dev Kit: 3 Boards with LiPo Charger + 3 Shields
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