diff --git a/mos.yml b/mos.yml
index d15c473..3e34480 100644
--- a/mos.yml
+++ b/mos.yml
@@ -38,6 +38,8 @@ config_schema:
   - ["app.backlight_pin", 22]
   - ["app.inactivity_timeout", "i", {title: "Inactivity timeout in seconds"}]
   - ["app.inactivity_timeout", 10]
+  - ["app.battery_calibration", i, {title: "Battery ADC value at 4000mV"}]
+  - ["app.battery_calibration", 2360]
 
 
 # List of libraries used by this app, in order of initialisation
diff --git a/src/widget_battery.c b/src/widget_battery.c
index b95f943..94b5222 100644
--- a/src/widget_battery.c
+++ b/src/widget_battery.c
@@ -1,5 +1,6 @@
 #include "mgos.h"
 #include "mgos_adc.h"
+#include "mgos_config.h"
 #include "tft.h"
 #include "mongoose-touch.h"
 #include "mgos_prometheus_metrics.h"
@@ -8,11 +9,15 @@
 #define WIDGET_BATTERY_ADC_PIN 35
 
 // Returns millivolts - Huzzah32 has a 1:1 voltage divider on I35 (A13)
-// so it returns half of the battery voltage. LiPo's are full at 4200mV
-// and critically empty at 3000mV.
+// so it returns half of the battery voltage. Calibrate the function by
+// measuring the ADC output at 4000mV.
 static int widget_battery_getvoltage() {
-  // Value is between 0..4096 which maps to 0..6600mV
-  return (6600*mgos_adc_read(WIDGET_BATTERY_ADC_PIN))/4096;
+  int val, mvolts;
+
+  val = mgos_adc_read(WIDGET_BATTERY_ADC_PIN);
+  mvolts=(4000*val)/mgos_sys_config_get_app_battery_calibration();
+  LOG(LL_DEBUG, ("ADC value %d, voltage %d mV", val, mvolts));
+  return mvolts;
 }
 
 static void widget_battery_render(struct widget_t *w, void *ev_data) {