Higher voltage means higher efficiency (P for Power) = I (Intensity a.k.a. amperage) X E (Electromotive Force a.k.a. volts). So if you need more electrical power (wattage) to do an electrical function (heat the cabin) and that function requires an extra 1.2 KW (1,200 watts) of electrical power, a 12-volt system would require an additional 100 amps. (1,200 / 12 = 100) (Fig. 10) 100 amps is a lot SAE Nissan Leaf Owner's Forum For the Nissan LEAF, keeping the frost off your nose on a cold winter day can drop your range by as much as 30 miles. For that reason, Nissan Leaf 's PTC (Positive Temperature Co-Efficient) heater (Fig. 8) is connected to the Leaf 's full 392 volts. Ever Figure 8: The Leaf heats coolant for the heater core via a 392 V (DC) PTC heater assembly that can draw up to 5 KW off the HV (High Voltage) battery pack which means considerable range drop. Note the two large black cables. These are HV and are wrapped in an orange plastic conduit to indicate the HV status. HV system would need to be powered down and the cable connections double checked with a Cat III / 1,000 volt meter to verify HV power is off (while wearing Class 0 / 1,000 volt safety gloves) prior to accessing the PTC heater unit. Figure 10: Nissan Leaf's high voltage PTC uses the full voltage of the drive battery (around 392 Volts) to heat coolant. A 12-volt electric pump moves the heated coolant into the heater core to warm passengers. The BCM commands the action based on HVAC control head / amplifier request and ambient temperatures. Its efficiency is not the greatest but it works! wonder why such high voltages are used in newer vehicles such as hybrids and electric vehicles? The reason is POWER! In internal combustion engines power is measured in horse power. HP is torque X RPM X 5,252. Therefore HP is a 'rate of work'. Watts (electrical power) is similar. (Fig. 9) Watts Delphi of extra current to produce either from a battery or a generator so we raise the voltage. If the voltage is raised by a factor of 4 (for example) the extra current (amps) is only 25 amps instead of 100 amps. That means an easier electrical load and smaller gage wiring. Both contribute to fuel economy or range in the case of an EV or PHEV. The Leaf 's HV PTC heater is controlled via a PWM output of high current 4 IGBTs (Insulated Gate Bipolar Transistors) at a very conservative level. Nissan owners report ambient temps of 30 degrees F. with passenger compartment temps of slightly over 50 degrees F. That means you won't see your breath freeze on the windshield but you sure better wear a sweater on cool days! A HV / high current PTC is used in numerous EV / PHEV applications. In the case of the Nissan Leaf the PTC heater is located under the hood under the 12-volt accessory battery along with the electric pump which can move 12 liters of coolant per hour and a temperature sensor. Temps will Figure 9: Using the table above we can see why going to 48-volt systems for EVERY vehicle will help reduce weight (wiring size) of a vehicle's electrical high power accessories (such as PTC heaters) 4X as much voltage means ΒΌ the current draw for any given increases of electrical power (wattage). December 2017 3 MACS Service Reports