Alternators, Fuel and Partial State of Charge (PSOC) Batteries.

Every hour a 400-amp alternator is charging (not freewheeling) wastes almost 1 gallon of fuel. Alternator torque requires engines to produce one horsepower (HP) for every 25-amps of charge current produced. A 200-amp alternator requires about 8 HP, and a 400-amp alternator requires 16 HP from the engine. Diesel engines require, on average, .06 Gal (.21 L) or .40 lbs (.18 kg) of fuel per hour to generate 1HP.

Alternators in technology-rich vehicles will typically only charge the battery bank up to 90% due to the vehicle’s charge voltage regulation. The efficiency of standard alternators at medium speed is limited to 70-80% (at 77°F/25°C) by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. This efficiency reduces dramatically at higher temperatures and high speeds, mainly due to fan resistance. Combined, even by working as hard as it can, it is almost impossible for the alternator alone to fully charge batteries to eliminate Partial State of Charge (PSOC) conditions.

Avoid Prolonged Use of added Electrical loads during “Engine off” periods. Battery performance and service life are dramatically affected by the frequency and depth of battery discharge. Here are some typical electrical loads for popular devices used in commercial vehicle fleets to support driver health and comfort:

  • Refrigerator at 5 amps
  • Entertainment systems at 4 amps • Heating/AC at 19 amps
  • Sleep assist systems CPAP at 3 amps
  • Interior lighting at 4 amps

If these additional loads are used together for 6 hours while your vehicle is parked, they will collectively consume 210Ah (35-amps per hour x 6 hours = 210 Amp hours) from the battery pack.