BQ76952: BQ76952

Hello John,

The EVM actually has the ability to do both, series and parallel connections. It has unplaced components for parallel paths, however, as the default configuration is series FETs.

Yes, you can use pre-charge/discharge FETs as well, you just have to ensure that you disable body-diode protection in this mode (by making Settings:FET Control[SFET] bit to 0). 

We do have an application note that goes over different considerations for this type of configuration:

• Parallel Paths with the BQ769x2 Battery Monitor

The FETs are controlled the same as if the FETs were in series, however body-diode protection has no effect. This is the biggest difference from the perspective of the FET control.

Hope this helps .

Best Regards,

Luis Hernandez Salomon

Hi Luis,

So the parallel configuration MUST have separate terminals for the charger? Is there a way to have parallel yet with only the two battery terminals? I was kind of hoping that I could size the charge FETs smaller than the discharge FETs for cost/power but it I need to have a separate charge terminal, that may be a show stopper.

Thanks again!! Most appreciated.

John

Hello John,

I am not quite too sure what you mean, mind drawing out what you had in mind for your system? Then maybe I can understand if it is possible to implement with our device.

Of course! No problem.

Best Regards,

Luis Hernandez Salomon

Hi Luis,

Sorry for the delay. I was on travel.  In the  Parallel Paths document, in Section 2, it states :

With a parallel FET configuration, there are different connections for the charge and discharge current paths. The protection FETs can be sized appropriately for the current of the protected direction. The BQ76952 family of devices can support parallel FET operation. An example of a parallel FET schematic with the BQ76942 is shown in Figure 2. When using a parallel FET circuit, the body diode protection feature must be turned off in the BQ76952 configuration by setting the Settings:FET Control SFET bit to 0.

My question is do you NEED "different connections for the charge and discharge current paths"?? I'd prefer to have just two terminals to the battery, not a terminal for charge and terminal for discharge.  Is that possible?

Thanks again!!

John

Hello John,

Well, if I am understanding correctly, you essentially want to have a separate charging and discharge path, while keeping the same load/charger terminal, correct?

I've personally never seen it this way, but it may be possible to do.

You will likely need to have additional circuitry to block unintended leakages (Like discharge current flowing through the CHG FETs or charge currents flowing through the DSG FETs), that'd be my biggest concern.

For example, if you just have them in parallel connected to the same terminal, then you can have a charging current flowing through the discharge FETs, even if both the CHG and DSG FETs were to be off (The DSG FET through the body-diode). Like-wise, you could have discharge current at any time, even if both FETs are OFF (Due to discharge current flowing through the body-diode of the CHG FETs).

That may be somewhat challenging to solve, specially at the higher currents that you want to use these at.

Best Regards,

Luis Hernandez Salomon

Hi Luis,

Thanks for the insight. Seems like I need the serial topology which is fine.  

Another question, my end customer is looking to monitor all cells in the battery individually.  There are a few battery configurations including 4S1P, 4S4P, 10S4P and 14S4P. We are thinking about using one BQ76952 for each stack.  So there would be one in the 4S1P and four in the other batteries. Kind of a modular system with a single controller monitoring all the BQ76952 via I2C.

With that said, we were thinking that each "module" would have it's own set of FET's that would support the maximum discharge/charge current of each battery stack. The thought would be any fault (short circuit, OCD,OCC, temperature, COV, CUV) would interrupt the processor which in turn would turn off each cell stack. I was thinking of simply asserting the reset but not sure if this would cause problems. Is there a simpler/more controlled way of doing that?  To restart, we would have a button on the battery as I don't think the LD pin will work. Thoughts/comments?

Or the other idea, would be to have only 1 module control all the FETS required for the total current in the system (250A).  Can the other BQ76952 devices NOT control the FETs and basically exist solely as a cell balancer along with a fuel gauge?

Thanks!!

John

Hello John,

Typically you do not need to have more than one BQ76952 if you put the cells in parallel. With all of the cells in parallel they would share a voltage, so they would balance each other out between each other and measure the same voltage. 

Now, if you are just putting in parallel at the top-of-stack level, that may be different. You'd likely only do that in modular systems where you can swap out modules.

If you are using a single set of FETs, then I'd imagine all of the cells are directly in parallel with each other, so you would not need to have balancers for all of them and just have a single BQ76952 do the balancing for all of them.

If you want to connect them on the PACK+/PACK- sides, then yes, you could have them connect after the series FETs. You would just have to be careful in the case where the pack voltages are widely different, since the batteries could discharge/charge between each other due to their differences.

John DeCesare said:

The thought would be any fault (short circuit, OCD,OCC, temperature, COV, CUV) would interrupt the processor which in turn would turn off each cell stack. I was thinking of simply asserting the reset but not sure if this would cause problems. Is there a simpler/more controlled way of doing that?  To restart, we would have a button on the battery as I don't think the LD pin will work. Thoughts/comments?

For this question specifically, there is an ALERT pin, which can be asserted when a fault occurs to provide an interrupt for the microcontroller. For RESET, if you'd like to perform a full reset, you can send a RESET() command, if you want a partial reset, you can assert RST_SHUT for less than one second. There is the following FAQ that may be helpful to understand RESET: 

• How do I RESET the BQ769x2 device?

Best Regards,

Luis Hernandez Salomon