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Hybrid Propulsion supports both AGM and Lithium Iron Phosphate (LiFePO4) battery technologies in the primary propulsion battery banks. In either case the banks are broken into 48 volt clusters to meetABYC safety requirements. For AGM banks, Hybrid Propulsion providesBattery Management Modules which individually monitor the voltage on each battery and redirect power during charging to bring all cells to a common voltage.
If an individual battery is marginal or bad, the BMM will communicate with the Central System Module via CANbus to report the state. The CSM then displays a message either by web page or an email notification of the defective cells.
LiFePO4 batteries and other Lithium technologies have several advantages. The power density is much greater and the discharge curve is much flatter than lead-acid meaning that you can have almost twice the number of hours of electric powering in a given space. Also, the total weight is significantly lower than lead batteries. The downside is that for a given power level, lithium batteriese cost over three times as much as lead-acid batteries. In 2010, a rule of thumb for the cost of batteries and battery management in a hybrid electric marine system is $300 per kilowatt-hour of top quality AGM battery storage versus about $1100 per kW-Hr for LiFePO4.
Most hybrid automobiles are more properly called a "parallel hybrid" system, in that the electric motor and the engine both can provide mechanical power to the drive train. The Chevy Volt and the Tesla sports car are "serial hybrid" systems in that the wheels are always propelled by an electric motor. A hybrid marine system may be of either type.
All modern motors for electric vehicles are brushless and powered by alternating current. They are either permanent magnet motors (often referred to as DC drive motors) or electromagnet-based induction motors. Both types of motors require an alternating current to operate. Since the 1980's, there has been a revolution in power electronics stemmed by the invention of the integrated gate bipolar transistor or IGBT. The IGBT has led to the development of low cost "drives" which can generate variable frequency, variable voltage AC at very high power levels to optimize the operation of both permanent magnet and induction motors. For a variety of reasons, permanent magnet motors are more desirable as propulsion motors at lower performance levels, typically less than 100 kW. Induction motors are universally preferred at higher power levels. Induction motors are almost always less expensive at a given power level since they do not use expensive rare earth magnets. However, the drive electronics are somewhat more complex. In marine applications, Hybrid Propulsion always recommends water cooled motors for both propulsion and and power generation. Besides reducing the build up of heat in tight engine rooms, the water cooled motor and drives are much less susceptible to the corrosive effects of salt air.
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