Ok now I see that payload is a JSON (string), you can use a json node to convert to JS object, then there would be no need for the string node and you would have all the properties at hand.
Saying that, if you are using a mqtt in node it should recognise a JSON and parse it. What version of node-red are you using. Maybe post a export of your flow for clarification.
example
[{"id":"c0a4103fc6bdf1d0","type":"inject","z":"d1395164b4eec73e","name":"json (string)","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{ \"Device\": { \"Name\": \"Daly2MQTT\", \"IP\": \"192.168.1.48\", \"ESP_VCC\": 3.375, \"Wifi_RSSI\": -69, \"Relais_Active\": false, \"Relais_Manual\": false, \"sw_version\": \"2.13.0\", \"Flash_Size\": 4194304, \"Sketch_Size\": 448544, \"Free_Sketch_Space\": 3723264 }, \"Pack\": { \"Voltage\": 26.8, \"Current\": 0, \"Power\": 0, \"SOC\": 96.7, \"Remaining_Ah\": 58, \"Cycles\": 114, \"BMS_Temp\": 13, \"Cell_Temp\": 13, \"cell_hVt\": 2.75, \"cell_lVt\": 1.8, \"cell_hVt2\": 2.66, \"cell_lVt2\": 1.9, \"pack_hVt\": 30.3, \"pack_lVt\": 19.8, \"pack_hVt2\": 28.8, \"pack_lVt2\": 19, \"High_CellNr\": 8, \"High_CellV\": 2.445, \"Low_CellNr\": 11, \"Low_CellV\": 2.442, \"Cell_Diff\": 3, \"DischargeFET\": true, \"ChargeFET\": true, \"Status\": \"Stationary\", \"Cells\": 11, \"Heartbeat\": 191, \"Balance_Active\": false, \"Fail_Codes\": \"\" }, \"CellV\": { \"CellV_1\": 2.443, \"CellV_2\": 2.444, \"CellV_3\": 2.444, \"CellV_4\": 2.444, \"CellV_5\": 2.444, \"CellV_6\": 2.444, \"CellV_7\": 2.444, \"CellV_8\": 2.445, \"CellV_9\": 2.445, \"CellV_10\": 2.445, \"CellV_11\": 2.442 }, \"CellTemp\": { \"Cell_Temp_1\": 13 } }","payloadType":"str","x":210,"y":5540,"wires":[["66fe1da7dd414ea1"]]},{"id":"66fe1da7dd414ea1","type":"json","z":"d1395164b4eec73e","name":"","property":"payload","action":"","pretty":false,"x":450,"y":5540,"wires":[["90d82b0a110b1408"]]},{"id":"90d82b0a110b1408","type":"function","z":"d1395164b4eec73e","name":"function 156","func":"msg.payload = {idx: 481, nvalue: 0, svalue: msg.payload.cell_hVt};\nreturn msg;","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":510,"y":5620,"wires":[["67488a1ff093273b"]]},{"id":"1e8f7c71650943cc","type":"inject","z":"d1395164b4eec73e","name":"javascript object","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{ \"Device\": { \"Name\": \"Daly2MQTT\", \"IP\": \"192.168.1.48\", \"ESP_VCC\": 3.375, \"Wifi_RSSI\": -69, \"Relais_Active\": false, \"Relais_Manual\": false, \"sw_version\": \"2.13.0\", \"Flash_Size\": 4194304, \"Sketch_Size\": 448544, \"Free_Sketch_Space\": 3723264 }, \"Pack\": { \"Voltage\": 26.8, \"Current\": 0, \"Power\": 0, \"SOC\": 96.7, \"Remaining_Ah\": 58, \"Cycles\": 114, \"BMS_Temp\": 13, \"Cell_Temp\": 13, \"cell_hVt\": 2.75, \"cell_lVt\": 1.8, \"cell_hVt2\": 2.66, \"cell_lVt2\": 1.9, \"pack_hVt\": 30.3, \"pack_lVt\": 19.8, \"pack_hVt2\": 28.8, \"pack_lVt2\": 19, \"High_CellNr\": 8, \"High_CellV\": 2.445, \"Low_CellNr\": 11, \"Low_CellV\": 2.442, \"Cell_Diff\": 3, \"DischargeFET\": true, \"ChargeFET\": true, \"Status\": \"Stationary\", \"Cells\": 11, \"Heartbeat\": 191, \"Balance_Active\": false, \"Fail_Codes\": \"\" }, \"CellV\": { \"CellV_1\": 2.443, \"CellV_2\": 2.444, \"CellV_3\": 2.444, \"CellV_4\": 2.444, \"CellV_5\": 2.444, \"CellV_6\": 2.444, \"CellV_7\": 2.444, \"CellV_8\": 2.445, \"CellV_9\": 2.445, \"CellV_10\": 2.445, \"CellV_11\": 2.442 }, \"CellTemp\": { \"Cell_Temp_1\": 13 } }","payloadType":"json","x":220,"y":5600,"wires":[["90d82b0a110b1408"]]},{"id":"67488a1ff093273b","type":"debug","z":"d1395164b4eec73e","name":"debug 2488","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":590,"y":5660,"wires":[]}]
How to import/export a flow