• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 764 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ40Z80: Questions about VSS and PGND connection

Luc Th
Prodigy 10 points
Part Number: BQ40Z80

Dear experts,

We are developing a 6S BMS based on bq40z80, for high currents up to 100A.

We have a few questions regarding the connection of VSS and PGND:

1) On the EVM, why is PGND used as a ground on the SMBus connector, instead of VSS, while the bq has VSS as a ground?

2) On the EVM, VSS and PGND seem to be connected this way:

Is this diagram right?

This means VC1 reading relative to VSS will vary depending on battery current. Can the bq account for this voltage drop and compensate it?

3) Why not running an additional balancing wire directly from cell 1 (-) terminal, like this:

Here is a diagram of the complete BMS:

A few things to ask here:

4) Do we have to connect the PGND Net Tie near the shunt or near the BMS output connector (PACK-)?

5) Must other devices on the BMS, such as a µC, be connected to PGND or VSS, and why?

6) Must the host ground be connected to PGND or VSS through the BMS signal connector? Or no ground at all on this connector to avoid ground loops?

6) Do we need an isolated transceiver if using CAN bus to connect to our host through the µC?

7) If we run GPIOs between host and the BMS µC (such as here, "WAKE-UP_INPUT"), do we need to isolate these signals? As their reference ground may shift a bit (< 200 mV) because of high currents, and introduce noise (motor involved)

Last question:

8) On the battery pack (21700 cells), is it better to solder voltage sensing wires on C1+, C2+, C3°+... or on C2-, C3-, C4-...? Because voltage drop on the nickel strips, at high currents, can be as high as a few 10's mV

Thanks in advance!

  • 0
    TI__Mastermind 42060 points

    Hello Luc,

    I will look into these questions and then get back to you.

    Best Regards,

    Luis Hernandez Salomon

  • 0
    TI__Mastermind 34775 points

    Luc Th said:
    1) On the EVM, why is PGND used as a ground on the SMBus connector, instead of VSS, while the bq has VSS as a ground?

    It's an external connection and any current flow needs to be coulomb counted.

    Luc Th said:
    2) On the EVM, VSS and PGND seem to be connected this way: Is this diagram right?

    Yes based on the snipet provided

    Luc Th said:
    3) Why not running an additional balancing wire directly from cell 1 (-) terminal, like this:

    That's the VSS and should be connected to VSS (lowest terminal of the cell stack).

    Luc Th said:
    4) Do we have to connect the PGND Net Tie near the shunt or near the BMS output connector (PACK-)?

    No, the net tie allows you to connect two nets of different net names. You can choose to sue net name PACK- through out, instead of PGND.

    Luc Th said:
    5) Must other devices on the BMS, such as a µC, be connected to PGND or VSS, and why?

    It will depend on you and your application. If connected to VSS the current consumed by µC wouldn't be coulomb counted and can introduce some error in capacity accumulation, which the gauge can correct for. if connected the to PGND then the current consumed by µC would be coulomb counted, but can create noisy sleep or relax issue depending on the quit current and sleep current thresholds.

    Luc Th said:
    6) Must the host ground be connected to PGND or VSS through the BMS signal connector? Or no ground at all on this connector to avoid ground loops?

    Host ground must be connected to PGND/PACK-, since it's external to the BMS.

    Luc Th said:
    7) Do we need an isolated transceiver if using CAN bus to connect to our host through the µC?

    That's a discussion with µC and host IC vendor. You communicate to our gauge via SMBus.

    Luc Th said:
    8) If we run GPIOs between host and the BMS µC (such as here, "WAKE-UP_INPUT"), do we need to isolate these signals? As their reference ground may shift a bit (< 200 mV) because of high currents, and introduce noise (motor involved)

    It would be good practice to shield the signal in your layout.

    Luc Th said:
    8) On the battery pack (21700 cells), is it better to solder voltage sensing wires on C1+, C2+, C3°+... or on C2-, C3-, C4-...? Because voltage drop on the nickel strips, at high currents, can be as high as a few 10's mV

    That's your call, since the series resistance impact will be felt regardless. I'm assuming the connector for C1- to VSS will have the same effect as the nickel strips then I would recommend C1+, C2+, C3+,...