Dear sir.
I can propose to use ESP8266 WiFi module with inexpensive step-down buck power DC-DC module. You will have a very low power consumption and wide input voltage range. That's extremely low-cost module, it has a single ADC input, so it's good for measurement of one battery on the moving device (cart or something). Also, it have very good low-power sleep mode, so if you will measure the battery, for example, one time per several minutes, the power consumption will be extremely low.
In that case, you can use public MQTT service (there is a lot of them) or Thingiverse to collect and display a voltage for all your devices.
I will provide a link to the components (ebay/aliexpress) and make a working prototype at my side as the proof of work.
I will provide a connection diagram.
Also, I will make an initial setup of the web part and provide an instruction how to add new sensors.
I also can make some batch quantity of the working modules, in that case, the price will be higher (the cost of the components, assembling and delivery will be included).
If you need to have a design of the single PCB to install all components - that's also discussable.
The range of WiFi connection can be 200 or more meters, but that also depends on the antenna of the WiFi access point. (the longest distance for ESP8266 modules was 1.5 Km with powerful access point).