Electric Car Battery Testing

An electric utility company was interested in using electric vehicles for its meter readers and other customer service personnel. In order to determine the feasibility of this idea, engineers needed to evaluate the vehicle’s operating parameters during operation and measure what effect they would have on its electrical requirements. In other words, they needed to determine how long the batteries would be able to power the vehicle under normal operating conditions.

Application Summary
After an electrical propulsion system was installed in a small pickup truck, the engineers began to research methods for increasing the truck’s operating efficiency and its operating range between battery chargings. Some initial possibilities they considered were: solar panels to extend the battery life, regenerative braking to recover kinetic energy, lighter plastic components replacing the heavier stock vehicle components, and low-resistance tires. All of these methods would extend the range of the vehicle; however, the relative merits of each needed to be quantified so that the value of each modification in terms of performance could be compared to the cost of its implementation. To obtain this data, the vehicle’s operation needed to be measured both before and after each modification.

To make the measurements, the researchers needed a data acquisition system able to meet a demanding set of performance criteria. These criteria included portability, operability from a battery, ability to accommodate mixed analog signals, a high channel count, and expandability. The system needed to run unattended after a relatively simple setup procedure. Furthermore, the system’s sampling rate, resolution, accuracy, and data transfer speed needed to be sufficient.

IOtech’s Solution
The data acquisition system that best met these criteria was IOtech’s PC-based DaqBook data acquisition system. The DaqBook system was equipped with IOtech’s thermocouple card and universal voltage and current card. The thermocouple card provided auto zero, cold-junction compensation, and programmable gain for a variety of temperature measurements. The signal conditioning card accommodated analog signals from transducers placed on the vehicle to measure voltage, current, temperature, and other variables. The data acquisition was controlled by a notebook PC using a custom software program; the PC’s hard drive provided data storage.

The DaqBook data acquisition system provided the accuracy needed to capture even the most rapidly changing variables. It also provided sufficient channels in the form of 8 differential or 16 single-ended analog inputs, plus many additional output and digital input channels. Expandable to 256 analog-input channels, the DaqBook system was capable of multiplexing all channels with individual gains for each channel.

The DaqBook’s rugged metal enclosure is roughly the same form factor (8 1/2” x 11” x 1 3/8”) as the notebook PC, as is the optional expansion card enclosure and battery pack. The system was installed behind the driver’s seat, and combined with the PC, weighed no more than 35 pounds, of which 15 pounds was mounting hardware.

Data Acquisition System Measurements
Battery charge and discharge characteristics were perhaps the most important variables measured on the electric vehicle, which used 20 six-volt lead/acid batteries connected in series to drive a 120 VDC motor. A voltage divider was used to scale down the aggregate voltage to 5 VDC for input into the data acquisition system. A clamp-on current sensor measured battery charge and discharge current, up to a maximum of 400A. The sensor output was set up for ±5 VDC for 500A full-scale current flow in either direction. Voltage and current were also measured on a 12 VDC accessory battery used to operate windshield wipers and lights.

To help determine the effects of charge and discharge characteristics on the batteries, temperatures were measured in various locations. Thermocouple probes were installed in selected cells to measure electrolyte temperatures inside the front and rear battery assemblies. Flush mount thermocouples measured the batteries’ casing temperatures. Other temperature measurements included a flush mount thermocouple on the drive motor, and a thermocouple probe in the bed of the truck for ambient air temperature.

Charge/discharge data was correlated with truck loading. Although the truck load itself was virtually constant, terrain affected both motor load and speed. Terrain was measured with an inclinometer installed behind the truck’s seat in the center of the cab. The output was scaled 0 to 3.6 VDC for 0 to 360° of rotation (incline). Vehicle speed was measured by attaching a pulse generator to the speedometer cable. The DaqBook data acquisition system’s counter timer created the appropriate speed scale. The output from an electronic tachometer connected to the motor was used in a similar fashion to obtain motor speed.

Conclusion
Using a single portable PC-based data acquisition system, the researchers quickly and easily acquired many channels of data of mixed signal types. IOtech’s DaqBook data acquisition system worked so well that the utility is considering alternative uses for it. One use would be to add a GPS (Global Positioning Satellite) system that would determine the vehicle’s coordinates and those of its proposed destination. This information, combined with terrain data and battery-charge data, would allow the driver to decide whether the truck could make a trip of a certain distance on its current charge.