JK BMS: only publish changed values to MQTT broker

all values are still published once every minute if the MQTT retain flag
is NOT set. otherwise, the constant values are only published once on
startup.

include/BatteryStats.h

@@ -95,4 +95,6 @@ class JkBmsBatteryStats : public BatteryStats {
 
     private:
         JkBms::DataPointContainer _dataPoints;
+        mutable uint32_t _lastMqttPublish = 0;
+        mutable uint32_t _lastFullMqttPublish = 0;
 };

src/BatteryStats.cpp

@@ -2,6 +2,7 @@
 #include <vector>
 #include <algorithm>
 #include "BatteryStats.h"
+#include "Configuration.h"
 #include "MqttSettings.h"
 #include "JkBmsDataPoints.h"
 
@@ -156,13 +157,25 @@ void JkBmsBatteryStats::mqttPublish() const
         Label::BatterySoCPercent // already published by base class
     };
 
+    CONFIG_T& config = Configuration.get();
+
+    // publish all topics every minute, unless the retain flag is enabled
+    bool fullPublish = _lastFullMqttPublish + 60 * 1000 < millis();
+    fullPublish &= !config.Mqtt_Retain;
+
     for (auto iter = _dataPoints.cbegin(); iter != _dataPoints.cend(); ++iter) {
+        // skip data points that did not change since last published
+        if (!fullPublish && iter->second.getTimestamp() < _lastMqttPublish) { continue; }
+
         auto skipMatch = std::find(mqttSkip.begin(), mqttSkip.end(), iter->first);
         if (skipMatch != mqttSkip.end()) { continue; }
 
         String topic((std::string("battery/") + iter->second.getLabelText()).c_str());
         MqttSettings.publish(topic, iter->second.getValueText().c_str());
     }
+
+    _lastMqttPublish = millis();
+    if (fullPublish) { _lastFullMqttPublish = _lastMqttPublish; }
 }
 
 void JkBmsBatteryStats::updateFrom(JkBms::DataPointContainer const& dp)