If you want just the one value you can use a change node to set payload to that value.
Or
If you want several different ones you can use a split and switch node to filter them out.
e.g
[{"id":"172551ac1ab40d8a","type":"inject","z":"d1395164b4eec73e","name":"","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":490,"y":6480,"wires":[["7b405ffd604b6d5d","4fb763d1a8be3fc4"]]},{"id":"7b405ffd604b6d5d","type":"change","z":"d1395164b4eec73e","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload.Pack","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":640,"y":6480,"wires":[["c0987e33ce90b79f"]]},{"id":"4fb763d1a8be3fc4","type":"change","z":"d1395164b4eec73e","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload.Pack.cell_hVt","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":640,"y":6560,"wires":[["faedbd4890a278aa"]]},{"id":"c0987e33ce90b79f","type":"split","z":"d1395164b4eec73e","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":790,"y":6480,"wires":[["07eb262f81bec0cc"]]},{"id":"faedbd4890a278aa","type":"debug","z":"d1395164b4eec73e","name":"debug 2489","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":810,"y":6560,"wires":[]},{"id":"07eb262f81bec0cc","type":"switch","z":"d1395164b4eec73e","name":"","property":"parts.key","propertyType":"msg","rules":[{"t":"eq","v":"cell_hVt","vt":"str"},{"t":"eq","v":"ESP_VCC","vt":"str"}],"checkall":"true","repair":false,"outputs":2,"x":910,"y":6480,"wires":[["e6abb1e4cfa61499"],[]]},{"id":"e6abb1e4cfa61499","type":"debug","z":"d1395164b4eec73e","name":"debug 2488","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":1070,"y":6480,"wires":[]}]
If this is not what you mean, you may want to explain with more detail.
p.s. When posting data/code etc please use the correct format, as it makes copying easier. more info about this below.
In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```)
```
code goes here
```
You can edit and correct your post by clicking the pencil 
icon.
See this post for more details - How to share code or flow json
