Passive hybrid energy storage system for electric vehicles at very low temperatures

In modern electric vehicles (EVs), the storage system is usually composed only of lithium ion batteries (LiBs), which are characterized by a high energy density but medium power density. In order to increase also the power density, a hybrid energy storage system (HESS) that combines LiBs with supercapacitors (SCs) could be produced. In particular, at very low temperatures SCs can be very helpful because, unlike LiBs, they maintain good performances. In the literature, it is possible to find three different ways to combine LiBs and SCs: passive, semi-active, and fully active HESSs. Despite the passive solution seems to have been abandoned, this paper analyses the direct parallelizing of LiBs and SCs focusing on the advantages that this simple solution can involve at very low temperatures. Indeed, LiBs fail at those temperatures, which could make it impossible to start the EV unless to use a pre-heating system for the batteries that can take up to several minutes. Based on the results of experimental tests, this paper shows that, at low temperatures, adding a relatively small SC unit to the battery pack makes it possible to start immediately the EV without waiting for the pre-heating of batteries, greatly increase the EV range, and thus use the vehicle under very harsh conditions.