The battery, the charger and the switch

What the base is for

The C3 board runs off USB and nothing else. A base is what makes it a thing you can carry, and all three bases carry the same circuit: a JST PH 2.0 socket for a lithium cell, a charger that tops the cell up when USB is plugged in and the switch is ON, a slide switch between the cell and the board, and a pad you can bridge to let the board measure its own battery.

The battery

Use a 3.7 V lithium polymer cell with a protection circuit on it — a PCM or BMS board, which almost every pouch cell sold with a JST lead already has. The charger on the base takes the cell up to 4.2 V and stops; it does not protect against over-discharge, and a lithium cell taken below about 3 V is damaged rather than flat. The protection circuit is what handles that, and it is not optional.

The connector is JST PH, 2 mm pitch, two pins. The polarity is printed on the board: there is a + on the silkscreen beside the positive pin. Trust that rather than a description of which side it is on, because which side it is on depends on which way up you are holding the base.

Cell leads are not standardised — the same red-and-black pair comes wired both ways round depending on who assembled it — and a reversed cell is the second way to lose the board. If the lead did not come with the base, put a meter across it and check before you plug it in.

Charging needs the switch on

Every base has its own charge indicator: LED1 in the charging-circuit notes, connected to the charger's CHRG pin. This is separate from the power and IO8 LEDs on the C3 board. The base's slide switch, SW2, also controls whether the battery is connected to the charging and battery-detection circuit. Plugging in USB is not enough: the switch must be ON to charge the cell.

1. Fit the board in the correct orientation and connect a suitable single-cell battery with the correct polarity.
2. Move SW2 to ON, then connect USB/5 V power. The charge LED stays on steadily while the battery is charging.
3. When charging finishes, the charge LED goes out as CHRG becomes high-impedance.
4. To disconnect the battery for transport or storage, move SW2 to OFF. The battery will not charge in this position, even with USB connected.

With USB/5 V supplied, the indicator behaves as follows:

The supplied charging-circuit notes describe the OFF-state glow as rapid switching of CHRG between low and high-impedance when the charger cannot detect the battery. Your eyes blend the flashes into a dimmer light. On the documented circuit, this behaviour alone is normal and does not indicate a fault; it is not a reduced-current charging mode. An unlit LED by itself is not proof of a full battery either — check that power and the battery are connected and the switch is ON.

Inside the charging circuit

The following component references come from the supplied Charging Circuit Instructions (充电电路说明书), based on schematic Schematic2_9, P1, ESP32-C3, V1.0, dated 2025-10-06.

In the notes' description of the switched battery path, Q1's drain connects to battery positive through F2, and its source leads towards SW2. With SW2 ON, contacts 2–3 connect the battery to U13's BAT network and the R10/R11 sensing circuit. With SW2 OFF, the contact moves to pin 1, which is unconnected, opening that path. Pins 4–6 are marked unused.

The two equal divider resistors halve the sensed voltage before it reaches the ADC. The base's BATTERY MONITOR solder jumper must be bridged to read that voltage on GPIO 2; the battery-level article explains the conversion and calibration.

For a custom hardware modification, the notes also suggest monitoring CHRG with an ESP32 GPIO and a 4.7–10 kΩ pull-up to 3.3 V, using its level changes to help identify battery connection status. That needs an added connection and a check of the existing LED circuit's voltage levels before wiring it to a GPIO; it is not a signal already available to a stock sketch.

The battery monitor pad

Beside the connector there is a small block of silkscreen marked BATTERY MONITOR and IO2, with two resistors and an open solder jumper. The two resistors are a divider across the battery; the jumper connects its midpoint to GPIO 2.

It ships open, and that is deliberate — a divider bridged across a cell draws from it continuously, and a pin is worth more than a reading you were not going to take. Bridge it only if you are going to use it, and know that GPIO 2 stops being a free pin the moment you do. Reading it is a later article.