diff --git a/include/config.h b/include/config.h
index 3df1b16..5a30371 100644
--- a/include/config.h
+++ b/include/config.h
@@ -73,6 +73,7 @@ constexpr uint8_t BATCH_QUEUE_DEPTH = 10;
 constexpr BatchRetryPolicy BATCH_RETRY_POLICY = BatchRetryPolicy::Keep;
 constexpr uint32_t WATCHDOG_TIMEOUT_SEC = 120;
 constexpr bool ENABLE_HA_DISCOVERY = true;
+constexpr bool SERIAL_DEBUG_MODE = true;
 
 constexpr uint8_t NUM_SENDERS = 1;
 inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = {
diff --git a/src/main.cpp b/src/main.cpp
index 92702eb..25f440e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -13,8 +13,10 @@
 #include "display_ui.h"
 #include "test_mode.h"
 #include <ArduinoJson.h>
+#include <stdarg.h>
 #ifdef ARDUINO_ARCH_ESP32
 #include <esp_task_wdt.h>
+#include <esp_system.h>
 #endif
 
 static DeviceRole g_role = DeviceRole::Sender;
@@ -83,6 +85,19 @@ static MeterData g_inflight_samples[METER_BATCH_MAX_SAMPLES];
 static uint8_t g_inflight_count = 0;
 static uint16_t g_inflight_batch_id = 0;
 static bool g_inflight_active = false;
+static uint32_t g_last_debug_log_ms = 0;
+
+static void serial_debug_printf(const char *fmt, ...) {
+  if (!SERIAL_DEBUG_MODE) {
+    return;
+  }
+  char buf[256];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(buf, sizeof(buf), fmt, args);
+  va_end(args);
+  Serial.println(buf);
+}
 
 static uint16_t g_last_batch_id_rx[NUM_SENDERS] = {};
 
@@ -359,6 +374,10 @@ static bool send_inflight_batch(uint32_t ts_for_display) {
   if (!meterBatchToJson(g_inflight_samples, g_inflight_count, g_inflight_batch_id, json, &g_sender_faults, g_sender_last_error)) {
     return false;
   }
+  if (SERIAL_DEBUG_MODE) {
+    serial_debug_printf("tx: batch_id=%u count=%u json_len=%u", g_inflight_batch_id, g_inflight_count, static_cast<unsigned>(json.length()));
+    Serial.println(json);
+  }
 
   static uint8_t compressed[BATCH_MAX_COMPRESSED];
   size_t compressed_len = 0;
@@ -369,6 +388,9 @@ static bool send_inflight_batch(uint32_t ts_for_display) {
   bool ok = send_batch_payload(compressed, compressed_len, ts_for_display, g_inflight_batch_id);
   if (ok) {
     g_last_batch_send_ms = millis();
+    serial_debug_printf("tx: sent batch_id=%u len=%u", g_inflight_batch_id, static_cast<unsigned>(compressed_len));
+  } else {
+    serial_debug_printf("tx: send failed batch_id=%u", g_inflight_batch_id);
   }
   return ok;
 }
@@ -496,6 +518,13 @@ void setup() {
   g_boot_ms = millis();
   g_role = detect_role();
   init_device_ids(g_short_id, g_device_id, sizeof(g_device_id));
+  if (SERIAL_DEBUG_MODE) {
+#ifdef ARDUINO_ARCH_ESP32
+    serial_debug_printf("boot: reset_reason=%d", static_cast<int>(esp_reset_reason()));
+#endif
+    serial_debug_printf("boot: role=%s short_id=%04X dev=%s", g_role == DeviceRole::Sender ? "sender" : "receiver",
+                        g_short_id, g_device_id);
+  }
 
   lora_init();
   display_init();
@@ -545,6 +574,17 @@ static void sender_loop() {
   uint32_t now_ms = millis();
   display_set_sender_queue(g_batch_count, g_build_count > 0);
   display_set_sender_batches(g_last_acked_batch_id, g_batch_id);
+  if (SERIAL_DEBUG_MODE && now_ms - g_last_debug_log_ms >= 5000) {
+    g_last_debug_log_ms = now_ms;
+    serial_debug_printf("state: Q=%u%s A=%u C=%u inflight=%u ack_pending=%u retries=%u",
+                        g_batch_count,
+                        g_build_count > 0 ? "+" : "",
+                        g_last_acked_batch_id,
+                        g_batch_id,
+                        g_inflight_count,
+                        g_batch_ack_pending ? 1 : 0,
+                        g_batch_retry_count);
+  }
 
   if (now_ms - g_last_sample_ms >= METER_SAMPLE_INTERVAL_MS) {
     g_last_sample_ms = now_ms;
@@ -593,6 +633,7 @@ static void sender_loop() {
       if (ack_sender == g_short_id && ack_receiver == rx.device_id_short &&
           g_batch_ack_pending && ack_id == g_last_sent_batch_id) {
         g_last_acked_batch_id = ack_id;
+        serial_debug_printf("ack: ok batch_id=%u", ack_id);
         finish_inflight_batch();
       }
     }
@@ -601,8 +642,11 @@ static void sender_loop() {
   if (g_batch_ack_pending && (now_ms - g_last_batch_send_ms >= BATCH_ACK_TIMEOUT_MS)) {
     if (g_batch_retry_count < BATCH_MAX_RETRIES) {
       g_batch_retry_count++;
+      serial_debug_printf("ack: timeout batch_id=%u retry=%u", g_inflight_batch_id, g_batch_retry_count);
       resend_inflight_batch(last_sample_ts());
     } else {
+      serial_debug_printf("ack: failed batch_id=%u policy=%s", g_inflight_batch_id,
+                          BATCH_RETRY_POLICY == BatchRetryPolicy::Drop ? "drop" : "keep");
       if (BATCH_RETRY_POLICY == BatchRetryPolicy::Drop) {
         finish_inflight_batch();
       } else {