Before a car battery fails, its fluid levels and condition can only be checked by visual inspection, says the German engineering company Siemens. So it has developed a way of monitoring fluid levels remotely.
They say that the acoustic properties of the battery change as the level of liquid electrolyte fall. So the trick to spotting a failing battery is to send acoustic pulses regularly through the battery and listen for the response. In that way it should be straight forward to spot low levels of electrolyte and warn the driver that a top up is needed.
Read the full acoustic batter checker patent application