Economy drive

Retired engineer customises his Toyota Prius drive system, improving fuel economy and making move towards full electric power. Siobhan Wagner reports


A retired electrical engineer has revamped his Toyota Prius drive system to improve its fuel economy from the normal 60mpg to 100mpg — with the addition of a Li-ion battery pack and off-the-shelf control technology.

Jim Fell, from Peterborough, said he first had the idea of converting a Prius into a full electrical car in 2005. ‘The interesting thing about the Prius was that it could run for a limited period as an electric vehicle,’ he said.

The Prius, he explained, works like a traditional car with the addition of an electric motor/generator in the drivetrain. When the driver slows down, the brake pedal puts the ‘motor’ into generator mode, which charges the battery. At low speeds the motor assists the conventional petrol engine, which decreases fuel consumption.

Fell said the vehicle, along with most modern cars, has a complex electronic control system with CANbus to manage the drive and battery. The operation of the drive is based on the state of charge (SOC) of the battery pack.

If the SOC is low, Fell explained, the management system will recharge. If it is high, the pack will drive the car at low speed or supplement the engine when driving, climbing hills or overtaking.

Before he considered adding a large battery pack to his car, Fell first questioned how the Toyota management system would handle charge from a second battery. He then wondered how to control the second charging source.

The control system needed to be manipulated so that the management system saw a high SOC and used the battery instead of the engine whenever possible.

The first problem was easily solved. Fell charged up the battery pack with an EV charger, and the battery manager took into account the pack temperature and voltage and computed the SOC.

The main area of work left was transferring energy to the Toyota battery. For his second battery, Fell used a set of 56 Thunder Sky Li-ion cells, which he connected in series to give a DC voltage of around 210V. ‘The Toyota’s NiMh battery produces around 240V DC so I knew that I would need an inverter to allow the additional battery pack to charge its own battery,’ he said.

In addition, he needed a unit to recharge the Li-ion batteries overnight and a circuit to control the flow of charge into the Prius’ own battery.

Fell connected the extra battery pack to the existing one using four single pole, high-voltage power contactors and a high-power DC/DC converter.

To control the system he had to hack into the CANbus and create a customised device that could read parameters on the system and move charge into the existing battery pack at the right time. He did this with a CAN system called Flowcode from MatrixMultimedia, which allowed him to set up parameters for the bus and read specific messages with pre-written macro commands.

Fell said he set up a system that mimicked the function of the CANbus in the Prius. One part continuously transmitted a SOC message in the same format as the Toyota’s, while the other was an LCD display unit showing the system parameters. the display was fitted into the car’s radio compartment.

The second stage of the programme relied on information about the per cent of SOC and gave an output of either high or low, depending on the value.

To maintain the existing battery SOC at around 70 per cent, Fell used a pair of decision instructions in Flowcode to disconnect Li-ion cells from the charge circuit if SOC was more than 70 per cent. If it dropped below 65 per cent, the cells were instructed to be switched into the circuit while the the NiMh Prius battery charged. In this ‘assist mode,’ the cells are able to run for about two to three hours before the car runs in normal hybrid mode.

Road tests showed that the car will return about 60mpg in normal hybrid mode and about 100mpg in battery boost mode. ‘Unfortunately the Prius’ read-out only goes to 99.9mpg so you are a bit blind as to how well it’s really doing,’ said Fell.

One of the drawbacks of the system, said Fell, are the batteries, which can cost several thousand pounds and tend to take up a great deal of boot space.