At the present time most hybrid buses have an alternator which provides energy for 24VDC
system of the bus. The requirement in this case is a minimum of 250A current, but the
requirement tends to grow up to 500A. Alternators for this application have an efficiency of 65%
and cost around $2000 (250-300A alternator); a more powerful alternator has a higher cost. We
must also add accessories around, $700, furthermore they have a one to two year life time and
the cost of other parts and labor to replace it is around $2000. Also an alternator produces
stress on the 24V battery that significantly reduces the life time of the battery. The typical life
time of the 24V battery is 6-12 months. With an alternator system, bus manufacturers use cable
from alternator to battery with current density maximum of 1.5A/mm² because there has to be
minimum voltage drop in the cable. In this case, an alternative would be an isolated DC-DC
converter which provides energy for the 24V system of the bus directly from the main Hi-Voltage
system of the bus.