In high temperature electrical generators such as thermal batteries, the molten salt electrolyte needs to be retained by a binder, e.g. MgO. We have pointed out that the magnesia volume fraction was a more accurate parameter than the usually used weight fraction. Moreover, based on our technique measurements, we defined a magnesia volume fraction range (27–30vol.%) where the electrolyte retention could be considered as efficient whatever its nature. And finally, we proposed a microstructure description of the retained electrolyte.