Phoenix Turbine Builders Club

Fiero HEV Conversion Project

Electrical system, batteries, voltages, power controllers

November 2007

Let's take a look at the electrical system requirements and how to achieve the performance we are looking for at the least possible cost.

First of all, we need to acquire a motor/generator set at a reasonable cost. Most EV hobbyists use electric motors from Advanced DC. These are good motors, but a bit pricey. That also does not get you the generator part.

In our build, we located surplus motor-generators from Surplus Center. These are Jack and Heintz G29 aircraft surplus. A pair of the ran about $500 with shipping. These units are rated for 30 volts and 400 amps, or 12 kW or around 17-18 hp. Although they are rated at 30 volts, we have seen some from the EV crowd running them at 72 volts without any reported ill effects.

We've also decided to run our system at 72 volts. For this voltage we'll need (6) 12 volt batteries (12.8 volt) which will place the full charge of the battery pack around 78 volts. Speaking of batteries, there are a number of choices including:

To keep our costs low, we chose lead-acid. There are also many lead-acid battery choices, but only one practical path -- AGM deep-cycle. These are extremely low venting batteries typically used in wheel chairs and UPS power supplies. They have superior charge characteristics compared to auto batteries, and are much smaller, better fitting our compartment constraints.

Next on our list is a power controller. EV builds typically call for a sledgehammer approach, since all of the vehicle power flows from the batteries to the traction motor. Curtis, Alltrax, and Zila controllers are the favorites of the EV crowd. 

The problem with these controllers is that they are pro-rated. They can deliver their maximum rated current for only a couple of minutes before they electronically "shift down" to a lower rated flow. They can only deliver half their rated power on a continuous basis. So if you want to maintain highway speeds for any length of time, you will need a controller with twice the expected power rating.

Since HEV's use a combined power approach, they do not use battery power for extended periods. Instead, they use battery power for stop-and-go situations, ICE power for extended highway use.

In our system, we use battery power only at startup and for high load situations, such as hill climbing. The rest of the time, the main generator supplies the majority of power through a direct generator-to-motor heavy cable link. Vehicle speed is controlled by transmission gearing, motor-generator field control and, to a lesser extent, the battery power controller. Since our battery power controller delivers a maximum of around 100 Amps intermittently, our power controller is much smaller and only a fraction of the EV controller cost.

Of course, to make sure the entire system functions properly, we are designing a complete embedded processor network to control all of the individual components -- to ensure we reach the best performance at the lowest fuel costs.

That completes our introduction to HEV systems and how to initiate a build. 

Look for updates to the project in future articles -- also watch for our complete how-to book coming in 2008.