EV Consulting Inc.

HYBRIDIZING A DC SYSTEM USING STANDARD OFF-THE-SHELF DC HARDWARE
By Ken Koch, EV Consulting, Inc. Copyright 2008

At EV Consulting I get a lot of inquiries about turning battery-powered EVs into hybrid electrics. Many people think this is a straightforward, simple process: “You put a motor, a generator, and some batteries into a vehicle, and VOILA!…you have a hybrid.” Of course, this is a gross oversimplification. Just take a look at the hybrid system developed by Japanese engineers for the Toyota Prius hybrid. This Toyota “Synergy” system took 3-4 years and millions of engineering dollars (yen) to develop it into the superior system that it is today.

To modify a pure battery-powered EV that uses standard DC hardware (Advanced motors, Curtis controllers, etc.) into a hybrid is possible, but the results will not have the same efficiency and superiority that commercial hybrids enjoy. However, it can be done…with either of two methods shown below.

THE IDEAL DC HYBRID CONFIGURATION

It would seem that the ideal EV would be one which can satisfy most of a driver’s range requirements as a pure battery-powered electric. And for the time when the EV can’t meet all of the range requirements, use a generator-set installed in a small trailer, and towed behind the battery-powered vehicle. This way the user won’t need to haul around the extra weight of a gen-set in an EV when it isn’t needed. The hybrid trailer can be detached from the EV and stored in a side yard or garage, and reattached only for the trips which exceed the maximum range of the EV as a pure, battery-powered electric.

SELECTING AND SIZING THE GENERATOR SET

Most gen-sets technically are not generators, but rather alternators. A fuel-powered engine drives an alternator to produce alternating current. The AC power in turn can be used to drive either a high-powered battery charger, or, a high-current bridge rectifier, depending on which of the two methods is chosen. Both the battery charger and the bridge rectifier produce DC that can be driven directly into a battery pack. The battery pack, of course, further powers the electric motor that drives the vehicle. This configuration is referred to as a “series hybrid.” The average converted car or pickup truck with standard DC hardware requires approximately the following power at the constant speeds shown below, on level ground, without accelerating. (Acceleration and hill climbing both require power levels higher than that for level-ground driving.)

Speed	Power Required for Level Ground Driving	Gen-set Kilowatt Rating Required
30 mph	4.5 hp/3357 watts	3.5 kW
40 mph	6.5 hp/4849 watts	5.0 kW
50 mph	10 hp/7460 watts	7.5 kW
60 mph	15 hp/11,190 watts	12 kW

METHOD NO. 1 – GEN-SET DRIVING A HIGH-POWERED BATTERY CHARGER INTO THE BATTERIES

This is the simplest and most straightforward method. Select a gen-set that is rated for at least 5.5 kW. This can power most EVs up to about 45 mph without dipping into the battery pack’s energy. Make certain the gen-set puts out all of its power through a single 240 VAC plug. Purchase a Zivan #NG5 5 kW onboard battery charger (around $1500 from KTA Services) for whatever your battery pack voltage is. It will be compatible to drive straight into your battery pack, and no special considerations need to be taken for battery voltage limits so that the batteries are not overcharged. This 25 lb. charger can be mounted onboard in the vehicle, and it can draw its 240 VAC power from either the wall plug for standard charging, or, from the gen-set mounted in the trailer for hybrid charging/supplementing of battery power.

METHOD NO. 2 – GEN-SET DRIVING RECTIFIED VOLTAGE DIRECTLY INTO THE BATTERIES

This method is less expensive than Method No. 1, but is a bit more complicated. There are requirements that must be adhered to:

A battery pack must be employed which is no less than 96 volts, nor more than 108 volts. This requirement must be strictly adhered to for good compatibility and power balance. If a pack is less than 96 volts, the drive system will draw too much power from the generator, but not enough from the batteries. If the pack is more than 108 volts, the drive system will draw too much power from the batteries, but not enough from the gen-set. If no batteries are used, the voltage ripple of the power bridge rectifier output may be excessive and damage the motor controller.
The battery voltage must be monitored, and the gen-set power must be cut back if the battery voltage is more than 2.35 volts/cell. For a 96-volt pack, this voltage is 113 volts; for a 108-volt pack, this voltage is 127 volts. Exceeding this voltage while driving high current into the battery pack will cause excessive battery heat and excessive battery gassing.
The gen-set absolutely must put out all of its power at 120 VAC through one or more 120 VAC outlets. This gen-set configuration may take some extra research on the part of the user to find. If, for example, one chooses a widely available 5.5 kW gen-set, it might have two outlets: One for 120 VAC, and the other for 240 VAC. The 120 VAC outlet may be rated for only 15 amps, and the 240 outlet for 20 amps. But the 240 VAC output is incompatible and unusable. All power must be delivered through a single 120 VAC outlet…or more than one 120 VAC if each can deliver the amps and can be connected in parallel. A single 120 VAC outlet with a 5.5 kW gen-set could put out up to 45 amps. Three outlets with a 5.5 kW gen-set will put out the current at up to 15 amps each outlet.

Once the gen-set is identified and purchased, the 120 VAC outlet(s) may be driven into a high-powered bridge rectifier. This unit “converts” AC into pulsating DC, and consists of four power diodes that must be rated for at least 100 amps and 400 VDC. Batteries are necessary because they act not only as energy storage devices, but also as filter capacitors to smooth out the pulsating DC. Schematically the bridge rectifier assembly looks like the figure shown below. All diodes must be case-isolated from one another, and mounted to individual heat sinks.

POWER BRIDGE RECTIFIER PARTS LIST

Ref. Designation	Description/Rating	Potential Supplier
D1, D2, D3, D4	Rectifier Diode, #1N3292R, 100A, 500VDC NOTE: These diodes are high power-rated, and have 3/8” dia. studs at the base plus heavy-conductor leads at the top.	Digi-Key Electronics 1-800-344-4539 www.digikey.com
HS1, HS2, HS3, HS4	#345-1049 Heat Sink, Undrilled, 4.75” L x 3.00 W X 2.62”H. NOTE: Machining required to fit diodes to heat sinks.	Digi-Key Electronics 1-800-344-4539 www.digikey.com
B1, B2	AC Fan, 120 VAC, 60 Hz, 35 cfm NOTE: Each fan should be positioned so that it provides airflow for cooling of each pair of heat sink assemblies.	KTA Services 760-787-0896 www.kta-ev.com
P1, J1	Anderson #6326-G1 175A Double Pole Connector, Blue. NOTE: These two connectors make a mating pair.	KTA Services 760-787-0896 www.kta-ev.com
P2, J2	Anderson #6326-G1 175A Double Pole Connector, Red. NOTE: These two connectors make a mating pair.	KTA Services 760-787-0896 www.kta-ev.com
Miscellaneous	#4 GA Welding Cable, Insulated, Black	KTA Services 760-787-0896 www.kta-ev.com
Miscellaneous	#4 GA x 5/16” Hole Welding Cable Lugs	KTA Services 760-787-0896 www.kta-ev.com
Miscellaneous	Hardware, small gauge hookup wire, lugs, etc.	Radio Shack Stores (see Yellow Pages)

It is assumed that the reader of this publication has at least a basic understanding of electronics, DC and AC theory, and electronic symbology. If the reader’s understanding is below this level, it is suggested that he or she solicit help from an electronics technician or organization which services electric vehicles such as golf cars and lift trucks.