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+# Upgrading to VictronBLE v0.4
+
+v0.4 is a breaking API change that simplifies the library significantly.
+
+## Summary of Changes
+
+- **Callback**: Virtual class → function pointer
+- **Data access**: Inheritance → tagged union (`VictronDevice` with `solar`, `battery`, `inverter`, `dcdc` members)
+- **Strings**: Arduino `String` → fixed `char[]` arrays
+- **Memory**: `std::map` + heap allocation → fixed array, zero dynamic allocation
+- **Removed**: `getLastError()`, `removeDevice()`, `getDevicesByType()`, per-type getter methods, `VictronDeviceConfig` struct, `VictronDeviceCallback` class
+- **Removed field**: `panelVoltage` (was unreliably derived from `panelPower / batteryCurrent`)
+
+## Migration Guide
+
+### 1. Callback: class → function pointer
+
+**Before (v0.3):**
+```cpp
+class MyCallback : public VictronDeviceCallback {
+    void onSolarChargerData(const SolarChargerData& data) override {
+        Serial.println(data.deviceName + ": " + String(data.panelPower) + "W");
+    }
+    void onBatteryMonitorData(const BatteryMonitorData& data) override {
+        Serial.println("SOC: " + String(data.soc) + "%");
+    }
+};
+MyCallback callback;
+victron.setCallback(&callback);
+```
+
+**After (v0.4):**
+```cpp
+void onVictronData(const VictronDevice* dev) {
+    switch (dev->deviceType) {
+        case DEVICE_TYPE_SOLAR_CHARGER:
+            Serial.printf("%s: %.0fW\n", dev->name, dev->solar.panelPower);
+            break;
+        case DEVICE_TYPE_BATTERY_MONITOR:
+            Serial.printf("SOC: %.1f%%\n", dev->battery.soc);
+            break;
+    }
+}
+victron.setCallback(onVictronData);
+```
+
+### 2. Data field access
+
+Fields moved from flat `SolarChargerData` etc. into the `VictronDevice` tagged union:
+
+| Old (v0.3) | New (v0.4) |
+|---|---|
+| `data.deviceName` | `dev->name` (char[32]) |
+| `data.macAddress` | `dev->mac` (char[13]) |
+| `data.rssi` | `dev->rssi` |
+| `data.lastUpdate` | `dev->lastUpdate` |
+| `data.batteryVoltage` | `dev->solar.batteryVoltage` |
+| `data.batteryCurrent` | `dev->solar.batteryCurrent` |
+| `data.panelPower` | `dev->solar.panelPower` |
+| `data.yieldToday` | `dev->solar.yieldToday` |
+| `data.loadCurrent` | `dev->solar.loadCurrent` |
+| `data.chargeState` | `dev->solar.chargeState` (uint8_t, was enum) |
+| `data.panelVoltage` | **Removed** - see below |
+
+### 3. panelVoltage removed
+
+`panelVoltage` was a derived value (`panelPower / batteryCurrent`) that was unreliable (division by zero when no current, inaccurate due to MPPT conversion). It has been removed.
+
+If you need an estimate:
+```cpp
+float panelVoltage = (dev->solar.batteryCurrent > 0.1f)
+    ? dev->solar.panelPower / dev->solar.batteryCurrent
+    : 0.0f;
+```
+
+### 4. getLastError() removed
+
+Debug output now goes directly to Serial when `setDebug(true)` is enabled. Remove any `getLastError()` calls.
+
+**Before:**
+```cpp
+if (!victron.begin(2)) {
+    Serial.println(victron.getLastError());
+}
+```
+
+**After:**
+```cpp
+if (!victron.begin(2)) {
+    Serial.println("Failed to initialize VictronBLE!");
+}
+```
+
+### 5. String types
+
+Device name and MAC are now `char[]` instead of Arduino `String`. Use `Serial.printf()` or `String(dev->name)` if you need a String object.
+
+### 6. addDevice() parameters
+
+Parameters changed from `String` to `const char*`. Existing string literals work unchanged. `VictronDeviceConfig` struct is no longer needed.
+
+```cpp
+// Both v0.3 and v0.4 - string literals work the same
+victron.addDevice("MySolar", "f69dfcce55eb",
+    "bf25c098c156afd6a180157b8a3ab1fb", DEVICE_TYPE_SOLAR_CHARGER);
+```