Battery props

A prop in the middle of the lawn with no extension cord is possible with a board on a battery. The board sleeps when the show is off and wakes when it is needed.

The board

The Adafruit HUZZAH32 Feather has a LiPo battery connector and charger built in, and it reports its battery voltage. The ESP32-C6 SuperMini and ESP32-C3 SuperMini sleep on very little power but need an external charger and regulator (some C6 versions have charger pads; see the board page).

How far this has been tested

Battery sleep has run on the project’s Feather on the bench: deep sleep, timer wake, and cues reaching a board that listens in windows. A full night in the yard on a battery hasn’t happened yet, and neither SuperMini has been flashed at all. Expect to tune the numbers below.

Turning it on

Open the board in Nodes & Channels, switch on On battery, and save. From then on the board:

• becomes a leaf. It never passes messages along for other boards, because they can’t count on it being awake.
• listens in short windows while the show runs. By default it listens for 100 ms out of every second. The other boards know this and send cues early enough to land in a window, so a battery board fires together with everyone else.
• deep-sleeps when the show is off. It checks in every five minutes by default, and wakes on its own in time for the next schedule window. Check-ins happen on the clock, at :00, :05, :10 and so on, so every battery board with the same setting wakes at the same moment. A check-in lasts only a few seconds: the board says hello, hears the bridge, and goes back to sleep. The console shows when a sleeping board will next check in.

The SLEEP settings on the board’s page let you change those numbers, and show roughly what each choice costs in average current.

Sensors on a battery board

A sleeping board wakes the moment its own sensor fires, but only from certain pins. The board pages mark them as wakes from sleep, and the console warns you if a sensor is on a pin that can’t wake the board.

What a battery board is bad at

Anything that needs it awake. While the show runs, a scare that involves a battery board starts about a second after the sensor fires instead of almost at once, because the cue has to wait for the board’s next listen window. While the show is off, it only wakes for its own sensor. Use it for props that do their own thing, such as a sensor and a light on the same board, and keep props with tight timing on mains power.

Reading the battery on a board without a sense pin

The Feather reports its battery voltage on its own. The SuperMinis and the DevKit don’t have a sense pin, but two resistors give them one.

Wire two equal resistors in series from the battery’s + to ground, 100 kΩ each, and connect their midpoint to an analog pin. Equal resistors halve the voltage, which brings a full 4.2 V cell down to 2.1 V, inside what the pin can read. Values of 47 kΩ to 220 kΩ all work; higher values waste less battery.

In the console, assign that pin to Battery voltage (divider) and set divider to the ratio: (top resistor + bottom resistor) ÷ bottom resistor, so two equal resistors are 2.00. The formula is shown next to the field. Resistors and the chip’s analog input both have tolerances, so after saving, measure the battery with a meter and type the figure into meter reads: the console works out the ratio that makes its reading agree, and Use applies it. The TEST column shows the voltage and a percentage, for example 3.87 V · 62 %, and the same figure appears beside the board in the roster and on the Yard.

About the percentage

The percentage comes from a typical resting curve for a single lithium cell. A cell’s voltage sags while it works, so a board that has just pulsed a coil or lit a strip reads a little low, and recovers when it rests. Treat the percentage as a guide, not a gauge. Below about 3.5 V the reading turns amber.

Analog-capable pins on each board are marked analog in its pin table. On the C6 SuperMini they are GPIO 0 to 6; on the C3 SuperMini, GPIO 0 to 4.

A battery board as a repeater

Sometimes the far corner of the yard is out of radio range of everything else, and there’s no outlet to power a board in between. A battery board can bridge that gap by passing messages along, which the mesh calls relaying. A relay can’t doze, though. Anything sent while it sleeps is simply lost, so a relay has to keep its radio on, and that costs about 100 mA. There are two ways to set one up.

For a short run on a big battery, leave On battery switched off. The board then behaves like any plugged-in board: it relays all the time, fires its own sensor instantly, and still reports its battery voltage. Runtime depends on the battery:

Battery	Runtime, always on
10,000 mAh	about 4 days
20,000 mAh	about a week

A USB power bank plugged into the board’s USB port works well here.

For weeks on a battery, switch On battery on and turn on Relay for other boards while the show runs in its SLEEP settings. The board stays awake and relays during the show, then deep-sleeps when the show is off. With a 5-hour show each night, a 10,000 mAh battery lasts roughly two weeks. Most of that goes to the show itself; the rest is the check-ins, about 14 seconds awake every 5 minutes. A longer check-in setting stretches it. Use a LiPo on the battery connector for this one: many power banks switch themselves off when the board’s draw drops in deep sleep.

What the boards behind it lose

While the repeater sleeps, any board that reaches the bridge only through it is cut off from the console. Those boards keep running their own schedule and triggers, and any edits you save reach them when the repeater wakes for the next show. The Yard marks these boards ☾ via battery relay. If you need to reach one after hours, wait for the repeater’s next check-in, five minutes by default.

Not yet tested on real boards

The repeater setting is new and has only run in the console’s simulator. The runtimes above are worked out from published figures, not measured.

On the Yard

In the Yard, a sleeping board shows ☾ asleep and its battery voltage. The voltage turns amber when it is getting low (under 3.5 V).