25
Jun

Gathering electrical data while driving

Written by admin. Posted in Other Details

Voltage and current waveforms of the Lithium Ion battery pack acquired while commuting to and from work:

These slow growing plots (200 seconds/division) represent my daily 41 mile commute

The 20.5 Mile trip to work above

The voltage waveforms (yellow traces) start out in their fully charged condition at left end. The yellow arrows indicate the 700V level (i.e. offset is set to 700V).  

The current waveforms (blue traces) show positive for acceleration (above the center horizontal line) and negative for regenerative braking (below center line).  

Return trip after work

The flat sections show where I was sitting at traffic lights; seen in top image at right end, and bottom image  at left end. Notice all the regen braking happening around town. The large humps on the right end of the bottom (blue) image are from the significant elevation changes over the last couple miles.  

The hard acceleration getting onto the freeway (and climbing in elevation) is the highest current peak on the drive in. Each feature in these curves correspond to a given road circumstance such as up or down grades, traffic, speed, stops etc.  

While on the freeway after work, there is usually slow traffic where I merge, as can be seen by the large impulses in both accel and regen polarity from speeding up and slowing down. Once on the open freeway, it settles into a relatively fixed level.   When turning off the freeway, more regen braking…

Power Calculations:

To calculate the average power at freeway speeds, I used the oscilloscope’s histogram measurement function.

When applied to the power curve (which was generated by multiplying the voltage and current waveforms), I can see a detail that is important for the range extender project. 

This Histogram (brown color) shows where the power level spends its time, so to speak.  The brown bar that extends furthest to the right illustrates the time sitting at traffic lights for example. 

Data for the Range Extender project

The Range Extender allows for driving continuously by generating the power equal to that needed to maintain highway speeds.  This necessary evil will only be attached when going out of town, i.e. rather infrequently.  It is in the form of a small trailer pod with a gas motor directly driving another AC motor but running continuously in Regen mode delivering power directly into the battery pack.  The intent is to add minimal aerodynamic load.

The question is: what power is needed to drive the BMW on level highway at 60MPH

The red horizontal line is positioned at the average value which is approx 16kW (from readout in upper right showing 16.8kW) which just so happens to be the specified value that the Honda V-twin gas motor is rated to deliver in continuous operation. 

Both plots agree in the value. In the upper power plot, I windowed in on the most level section of the freeway, which represents a 10 mile stretch. This excludes all other sections of the drive from the histogram.

A confident answer to the question!

Here is how I am capturing the voltage and current waveforms: 

The Tektronix P5205 HV differential probe (white box in upper right corner) is connected to the fuse and negative bus bar inside the contactor/fuse box (at right with black top).

The Tek current probe is a TCP404XL 500 Amp DC unit, the blue/white handle clamped over the fat negative pack conductor.

I added a 12VDC-120VAC power inverter to provide power to the measurement equipment.

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