Monday, July 25, 2011

Improvements - Part 2

Adding cells to my pack forced me to purchase a power supply to individually charge each new cell to balance with the rest of the pack. I opted for the Mastech HY1520EX DC Power Supply. At first, I had trouble understanding how to correctly set the unit to safely charge my cells. After reading and deciphering the brief instructions and getting pointers from contacts, I managed to figure out how this unit works. I also had to be very patient because the unit was self adjusting the current based on the set voltage and it took many hours before the cell reached a full charge.

Now for the BMS. As many will admit, shopping around for the right BMS is not an easy task. There are many different types and models on the market many of which are relatively new.

Before making a BMS choice, I had prepared a short list of desired features or functionality.
  1. Programmable and flexible for resizing battery pack and customizing charge voltages (CAN Bus). 
  2. System that is not susceptible to interference (false alarms).
  3. Good Graphics to display cell monitoring and charging data.
  4. Logging capability for tracing overall system or cell behavior under charge/discharge.
Based on the above, I found myself looking for a digital BMS with CAN bus control for charging. The system that I chose and installed is the EMUS BMS made by a startup company in Lithuania named Elektromotus. This product is still in the development phase but has some very attractive features with additional features planned in future releases.

The Control Unit is very compact and uses small gauge (24 awg) wiring that requires carefull handling. If you have to redo any connections, you will need a special extraction tool and mini style (3mm) crimper. Proper fastening is also very important to prevent movement and connections from breaking loose.

The cell modules are also compact and install with a particular order (one Bottom module, one Top module and alternating A and B modules). Note: The boards will come with longer positive terminal wires in the future to allow for more slack.

Early on, there were some delays integrating the BMS into the car due to incorrect buggy software initially shipped with the cell modules. Also being new to the system and referencing a draft version of the documentation, I received good online support to gain an introduction to the system and get started. At first I chose the default configuration parameters which eventually needed to be modified to better suit my specific setup and to also minimize some communication errors reported by the system software between the BMS control unit and the cell modules. The system was also intermittently reporting unexpected and frequent charging state changes. These issues were alleviated by making some timing configuration changes but not completely resolved and still under investigation. Nevertheless, I am still able to use the system and operate the vehicle on a daily basis and have learned a few new things from the experience with this BMS.

Configuring the BMS is done via a the EMUS "Control Panel" program on a windows operating system (running on a laptop is ideal for mobility).

The "Update" screen (above) is used to upload the Control Unit image file.

The configuration screen of the "Cells" (above) allows you to enter information about your cells and desired parameters for "Low" and "High" voltage, temperature, etc. If you ever resize your pack, all that is needed is reconfiguration.
The configuration screen of the "Charger" (above) allows you to enter information about your charger and desired parameters for Slow/Fast charging current, etc. Updates to the documentation should help determine how to calculate and set these values correctly for your specific needs. In my case, slow charging at 9 amps draws 12.8-12.9 amps at the AC socket which I found to be ideal for opportunity charging since standard north american AC circuits are 15 amps. I have fast charging set to 20 amps (or charger max.) since I have a dedicated 20 amp AC circuit for fast charging at home.

The "Timing" configuration screen (above) allows for setting the duration of charging phases as well as cell polling and display period. The polling and display timer defaults were set at 2.0 sec but this eventually was lowered down to 0.1 sec to minimize some errors I was getting. I am not certain but I believe that having the timers this low or frequent could result in larger log files.
The "General" tab within the "Status" window (above) shows the overall state of the system similar to the EVGUI main screen.
The "Cells" tab within the "Status" window (above) shows bars for the cells, temperature and balancing current (shunt current) of the system similar to the EVGUI cells screen.
The "Elcon Charger" tab within the "Status" window (above) shows the charger state, set and actual voltage/current similar to the EVGUI Charger screen.

Monitoring the BMS inside the car is done via the EMUS EVGUI.
The EVGUI runs on an HP iPaq 5700 PDA (above) which I have mounted to the windshield using a universal car mount.
Here is a shot of the EVGUI touch screen menu.
The "Main" screen above is showing that the BMS is in "Charging" state.
The "Cells" screen above shows bars for the voltage, temperature and balancing current of each cell.
The "Charger" screen above shows the charge state, pre-set and actual voltage, current and power of the charger as well as the minimum, maximum and average cell voltage.
The "Consumption" screen (above) is empty since this is for future use in upcoming releases (TBD) as there are provisions for connecting a current sensor and speed sensor to the control unit for data to be calculated.

All data gathered by the evgui is logged and saved on the iPaq SD card. Similarly, the control panel logs are saved on your windows PC/Laptop. Currently, the log files need to be converted to csv format by a proprietary log file converter program provided by elektromotus and then opened in excel. If the csv file exceeds 65,536 rows, excel will report that it cannot load the entire file and will only display up to 65,536 rows. In my case, I had to split my logs into multiple files using Linux "split" (command line utility). The drag about this is that the data is spread across files and somewhat inconvenient. There might be a future release that includes a feature to easily identify low cells avoiding sifting through logs to find low, defective or damaged cells.

For reference, below are some wiring diagrams showing how I integrating things in the e-Protege:

At this point I am anxious to see improvements in the BMS and looking forward to any future add-ons. I shall post more news as things develop.

1 comment:

D. Morche said...

Very ambitious project and an excellent and well documentated commissioning report of the EMUS BMS