Batteries are very important elements for the functioning of many devices, among which vehicles occupy a relevant spot. In this thesis work it is presented the development of a simple yet smart electronic device for 12V lead-acid batteries for automotive use. This after-market device, named BAT-up!, samples the voltage of the battery and is able to provide to the final user, thanks to a smartphone app, some useful informations such as the State Of Health and a new parameter named Time To Criticality. The first indicates the aging of the battery and is obtained by utilizing the open circuit voltage function, while the second is a time prediction of when the battery might be starting to work under a critical voltage threshold, and is obtained thanks to the reconstruction of the voltage profile with a RLS estimator.