BQ76930EVM: Basic Use with BQ76200EVM

Hi Shelby,

1. The bq76930EVM and bq76200EVM are not designed to connect together directly, but there are features which may help if you choose to do this.  Check the schematics and modify appropriately.  Realize that the modified board will not have the original features as delivered, and the assembly can be fragile.  Be sure to take appropriate safety precautions
   1. Populate the components on the '930 board to bring signals to J13.
   2. Bypass the FETs on the '930 board to use the PACK- terminal for the connection after the sense resistor
   3. Connect the '930 and '200 boards as shown in the bq76200EVM user guide figure 3.  J13 of the '930 board will provide the signals in sequence for J7 or J8
   4. Remove the ground shunt on the '200 board as described in the user guide.
2. It sounds like you want a split path FET configuration similar to the ap note FET Configurations for the bq76200 High-Side N-Channel FET Driver figure 18. The bq76200EVM does not support this split path configuration, however if you remove the surface mount FETs you can connect your FET array at the through holes for Q7 and Q8.  This lets you make the appropriate FET connections but the ESD paths will need to be made off the board to PACK- if desired.  Again the assembly may be fragile.  Notice that the pre-charge feature of the bq76200 is not useful with split paths.
3. If you connect as described, the discharge positive terminal would be the J5 PACK+ terminal of the '200EVM. The charging positive terminal would be at the drain of the charge FETs on your FET board. Both paths share the PACK- terminal of the as the negative reference.
4. A BMS will typically allow either charge or discharge enabling both FETs unless there is a fault.  The 2 EVMs discussed here are set up for series FETs.  The bq78350 gauge understands only series FETs and includes a body diode protection feature where a protected FET will turn on again if the current direction would heat the body diode of that FET, this is to avoid damage to the FET. The series path design will allow control of charge or discharge since the path is common.  A split path design will need a MCU for appropriate FET control instead of the gauge.  Both FETs might still be kept on unless there was a fault, or the FETs could be enabled only when needed for the system mode.  Remember the body diode of the FETs can not be turned off, so the BMS will not be able to protect from a discharge on the charge path or charge on the discharge path. Your system design must prevent those conditions.

Hi thank you for the help! I had a couple more questions regarding a few of your instructions.

1) As you described in your first point, I need to populate the components on the '930 board to bring signals to J13. How do you do that?

2) After reading over the instruction manual for the '200 board regarding connecting it to a BMS system (Section 3.2), it says that the settings for the BMS board ('930 board) need to match the settings to the driver board ('200 board), how exactly do you do that?

Thank you again for the support.

I meant to reply to your answers, so I apologize for the extra posting below.

Hi thank you for the help! I had a couple more questions regarding a few of your instructions.

1) As you described in your first point, I need to populate the components on the '930 board to bring signals to J13. How do you do that?

2) After reading over the instruction manual for the '200 board regarding connecting it to a BMS system (Section 3.2), it says that the settings for the BMS board ('930 board) need to match the settings to the driver board ('200 board), how exactly do you do that?

Thank you again for the support.

Hi Shelby,

1. Look at figure 29 in the bq76930EVM user guide.  

• Install a connector at J13 if desired, or solder wires to appropriate  pins.  
• CHG and DSG will normally be used, so you would want to populate R80 and R81.  
• If you want to use PCHG from bq78350-R1, connect R113.  The net needs a pull up: Q18 and R112 will function as a pull up resistor for the /PRECHG net but will draw more current than a normal design, so you may wish to remove one or the other and add a 100k to 510k pull up to GG_PWR.  If you are not using the bq78350 you probably want to connect directly to your host board rather than J13.
• Enabling the charge pump is good with the bq78350-R1, install R98 and R100. 
• If using the pack voltage measurement for the bq78350-R1, install R99 to enable the signal.  You may need to change R104 to a lower value such as 100k to be sure the PMON_EN voltage reaches high in all conditions.
• If using the pack voltage measurement for the bq78350-R1, install appropriate resistors at R84, R88 and R93 patterns: Since there is R36 on the '200 board it will parallel R96 on the '930 board and give an in-range VAUX voltage if you short the R84, R88 and R93 patterns. Or you could optimize the installed resistors between the 2 boards.  Note that D10 on the '200 board prevents a high voltage on the cable. Installing C38 will stabilize and filter the voltage. (For future readers who may be using a bq76920 or bq76940, it is more likely to need to use values at these resistor positions or change other values.)

2. For the bq76930 EVM it will depend on whether you are using the bq78350-R1 or your own host.

• One part of the settings would be the hardware configuration described above in #1 and the prior post.
• /PRECHG polarity is configurable in the bq78350-R1, default is active low, it must be set active high for the bq76200. See the bq78350-R1 technical reference manual (TRM) for descriptions and configuration settings
• VAUX is the pack voltage input for bq78350-R1.  If planning to use this voltage for the supported purposes, be sure to select them, see the TRM for details.

If using your own host the precharge and pack monitor signals would route to that board rather than the bq76930 board, make any relevant configuraiton on that board.

That was extremely helpful, thank you.

I had one more question regarding the system as a whole. How does the system decide when to charge or discharge? Is there someplace to set a condition of some sort or does it do it automatically? 

Hi Shelby,
For a typical battery in normal conditions, the battery has only 1 set of power terminals (PACK+ and PACK-) and will charge or discharge depending on the external conditions. The system should be designed to operate within the capability of the battery.
If the battery is pushed up to a too high voltage it will disable charging but allow discharging. If the battery is discharged to too low of a voltage it will disable discharging but leave charging enabled. There are other protections such as current and temperature which may affect one or the other current directions and have specific recovery conditions. With the bq76930EVM, the bq78350-R1 on the board will provide the various protection thresholds, see its technical reference manual. If using a MCU and the bq76930, some levels can be set in the bq76930 registers, other thresholds or protections might be implemented in firmware using measurements from the bq76930. The system where the battery is installed may determine whether the battery is allowed to charge or discharge depending on the system mode, or the battery may be physically removable to be attached to either a charger or load.
Battery designs may separate the charge and discharge paths as described in the FET Configurations for the bq76200 High-Side N-Channel FET Driver application note mentioned before. These designs will frequently require a MCU to control charge and discharge during the desired system modes. For example such a system might not allow discharge if the charger is attached. These systems may require additional circuitry from the battery, for example since the battery can't prevent discharge through the charge port the system or charger must prevent this.

Hi, 

So I followed all of your instructions on connecting the 930' and 200' board and it won't power up the boards. I populated J13 on the 930' board, I configured the settings so that the boards match, and I followed the figure 3 on the 200' user manual. Is there something else I am missing?

When I connect the battery to the BATT+/- connections on the 930' board alone, the power light does turn on so there is something in the connections between the two boards that is not working. Any help would be greatly appreciated, thank you.

So I was checking the voltages on each board and I noticed that I was not getting a voltage drop from BATSR- to PACK-, so I used an external wire to connect BATT+ and BATSR- of the 200' board and it turned on the light of the 930' board. Is there something wrong with the 200' board that I can fix so there would not be any need to connect the external wire? Thank you.

Hi Shelby,

The boards are probably OK if you have modified correctly. The "battery" must connect to the bq76930 board to power the components on the board.  If the cell simulator switches are closed BATT+ will power the resistor divider and the components.  If the cell simulator is not used power comes from the cell monitor terminal block, but BATT+ still has transient suppression diodes which may be beneficial in the circuit and the route across the board to "PACK+" may be a convenient connection to the bq76200 board.  BATT+ of the bq76200 board could connect to the '930 board BATT+ or PACK+ or directly to the battery supply, but both boards will need power.

Hi, so I believe I got the boards to power up correctly while hooked to one another, but I am having a problem getting the battery to discharge/charge. Based off of an earlier comment you have made, the charge/discharge functionality is based off of the external environment, so I hooked up a series of resistors to the PACK+/- terminals of the 200' board and the system is not registering the external load being attached and it is not discharging the battery. I am using a 16000 mAh battery that has 6 cells, 22.2 volts and discharges at 20C. Are there settings or configurations in the bqstudio that I need to adjust to get this system to perform correctly? Thank you for the help.