I haven't seen a product that could check the voltage on the battery. Does anyone know if one is available?

I can understand why there would not be one due to the way the batteries are designed. You could not check the individual cell voltages only the total.

I think it would be more convenient than having to plug into the app to see the total and the lights on the battery only give you a rough idea of how much power is left.

I though of volt meter, but the ports for the pos and neg are very narrow. You would need something with the same blade shape as the charger.

Couldn´t you use the provided charging cable (to use when charging the RC from the battery) to build an adapter?

Well if you plug the battery in the inspire you can see all cell voltages in the app. And for charging maybe a computerized charger and a adaptor?