After @eldorado came up with this excellent idea I quickly set about looking for a suitable range strip that might be useable on each battery and can be permanently left attached. Found these but there maybe more. The thing that is important is a range that covers something like 20 degrees c down to zero and is small enough to stick onto each battery and be easily readable etc. Although we all have the app that tells us the current temp of the pack this method allows a quick visual indication of pack temp before we even load it into the aircraft. May order a few and see how they perform Reversible Liquid Crystal 7 level Horizontal Thermometer, self-adhesive (top one on the page 0-30 degrees) Or here (cheaper) Colour Changing Products | Color | Eco | Room Thermometers | Thermometer strips | Liquid Crystal | Temperature | Gas level indicator - 0-30°C Liquid Crystal thermometer Edit: OK - have ordered 4 to be getting on with (from the cheaper website ). Will see how that goes. If they are great will order the rest if they are poo I won't!