The expansion board, and which way round it goes

What the base is for

The C3 board runs off USB and nothing else. The expansion base is what makes it a thing you can carry: 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 switch also disconnects the battery from the charger when OFF. In that position, USB power can make the charge LED flicker or look half-lit without charging the cell. The charging instructions and LED states explain what to expect.

The two rows of holes on the base are the same footprint as the two rows on the C3 board, in the same order, so the C3 board drops straight onto it.

It drops on just as neatly the other way round.

Which pad meets which

The header is not symmetrical. One row is eight GPIOs; the other ends in 5 V, ground and 3.3 V. Rotate the board through 180° and those three land on the pins marked 3, 4 and 5 — so the battery rail goes into three chip inputs with nothing between them, and one of the three is half of the display's bus.

The rule printed on the box is antenna at the power-switch end, USB-C at the battery-connector end. That is correct, and it is also a rule you have to remember. The check that does not need remembering is to read one label on each board: find V5 on the C3 board and 5V on the base, and make sure they are at the same end. If they are, all sixteen are.

Do that with the switch off and the USB cable out.

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.

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.