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Reduce battery-reading noise with a trimmed-mean ADC sample

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Sometimes a single reading came in noticeably off from the real trend
(a regulator/RF transient during sampling), and the next normal reading
would then look like a big jump relative to that bad one -- server-side,
enough to misfire the recharge-cycle heuristic (see the paired server
commit). Went from 8 raw-averaged samples to 16, sorted, with the 3
extreme samples on each end dropped before averaging the remaining 10 --
a handful of outliers can no longer skew the result the way a plain
average let them.
This commit is contained in:
tfaour
2026-07-22 16:51:28 -04:00
parent 02934b1d10
commit 845e4f9509
1 changed files with 30 additions and 7 deletions
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firmware/main/battery.c
+30 -7
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@@ -1,3 +1,5 @@
#include <stdlib.h>
#include "driver/gpio.h"
#include "esp_adc/adc_cali_scheme.h"
#include "esp_adc/adc_oneshot.h"
@@ -11,7 +13,13 @@ static const char *TAG = "battery";
#if CONFIG_FRAME_BATTERY_ADC_GPIO >= 0
#define BATTERY_ADC_GPIO CONFIG_FRAME_BATTERY_ADC_GPIO
#define BATTERY_SAMPLES 8
#define BATTERY_SAMPLES 16
/* Trimmed mean: the extreme BATTERY_TRIM samples on each end (regulator/
* RF transients, not the true resting voltage) are dropped before
* averaging the rest -- a plain average lets even one or two of those
* skew the result enough to read as a real percent change downstream
* (see the recharge-jump handling in routers/device.py). */
#define BATTERY_TRIM 3
/* The external divider halves the battery voltage (2x200k, per the
* Seeed-documented XIAO wiring) so a full 4.2V cell reads ~2.1V at the
* pin, inside the 12dB-attenuation ADC range. */
@@ -34,6 +42,11 @@ static const struct {
{ 3300, 5 }, { 3000, 0 },
};
static int int_cmp(const void *a, const void *b)
{
return *(const int *)a - *(const int *)b;
}
static int mv_to_percent(int mv)
{
int n = sizeof(LIPO_CURVE) / sizeof(LIPO_CURVE[0]);
@@ -139,19 +152,17 @@ int battery_read_percent(void)
ESP_LOGW(TAG, "ADC calibration unavailable, using nominal scaling");
}
int mv_sum = 0;
int mv_samples[BATTERY_SAMPLES];
int samples = 0;
for (int i = 0; i < BATTERY_SAMPLES; i++) {
int value;
if (calibrated) {
if (adc_oneshot_get_calibrated_result(adc, cali, channel, &value) == ESP_OK) {
mv_sum += value;
samples++;
mv_samples[samples++] = value;
}
} else {
if (adc_oneshot_read(adc, channel, &value) == ESP_OK) {
mv_sum += value * 3300 / 4095; /* nominal 12-bit full scale at 12dB */
samples++;
mv_samples[samples++] = value * 3300 / 4095; /* nominal 12-bit full scale at 12dB */
}
}
}
@@ -167,7 +178,19 @@ int battery_read_percent(void)
return -1;
}
int battery_mv = (mv_sum / samples) * BATTERY_DIVIDER_RATIO;
/* Only trim if there's enough left afterward to still be a
* meaningful average -- falls back to a plain average of whatever
* came in on a wake where most reads failed. */
qsort(mv_samples, samples, sizeof(int), int_cmp);
int trim = (samples > 2 * BATTERY_TRIM) ? BATTERY_TRIM : 0;
int mv_sum = 0;
int kept = 0;
for (int i = trim; i < samples - trim; i++) {
mv_sum += mv_samples[i];
kept++;
}
int battery_mv = (mv_sum / kept) * BATTERY_DIVIDER_RATIO;
if (battery_mv < BATTERY_MV_MIN || battery_mv > BATTERY_MV_MAX) {
ESP_LOGI(TAG, "Reading %dmV outside plausible battery range, ignoring", battery_mv);
return -1;