BQ76972: IC blowdown when connecting BAT power wires AFTER Cells wires

Hi again,

I urgently need some clarification on this topic.

Thanks.

Hello Roberto,

How are you connecting the battery cells to the actual board?

Roberto Carrasco said:

The IC doesn't suffer any damage when I just connect battery power wires. I'm not sure if I have tried to connect power wires before cells, so I can't tell you what happen in this case.

Also, can you clarify this statement a little further. Does the IC continue to work when conducting the cell simulator? Or is it completely damaged?

Regards,

Rohin Nair

Hi Rohin,

The cells are connected in the actual board as shown in the schematics through the red labels. Cells' wires are arranged in a Molex Microclasp 15 pins, 1 row connector.

What I meant with the statement you mentioned before is that, in other boards, when I just connect battery's power wires, the BMS survived and starts working then when connecting the cells connector. It only blowdown when the cells connector is plugged in BEFORE the battery's power wires.

Regards

Hi again Rohin,

I know that you are probably overloaded, but please, I need assistance on this topic.

It has been 3 weeks since I opened this thread and I haven't gotten any indication or possible reason for this problem.

Now, I can confirm you that the IC survives and works fine when battery power wires are connected BEFORE cells wires. 

I need to get a solution for forgetting about the connection sequence, it needs to work however you connect these two harnesses.

Thanks in advance.

Hello,

I apologize for the delayed response.

I am trying to better understand your situation here. When connecting to BAT+/- before the cell connectors it works properly correct?

Are you using the charge pump? I noticed there was no capacitor located there to ground. Also, what is the diode you are using on your BAT pin rated for? And what is your expected voltage per cell?

Regards,

Rohin Nair

Hi,

Exactly, when connecting BAT +/- before cells connectors it works properly. The issue comes when the connecting is done inversely: Cells connector before BAT +/-.

No, I'm not using the charge pump. As you can see in the schematics, I'm not using external MOSFETs. This IC just needs to perform cell balancing in my PCB.

The diode is 100 Vr rated, so it should fit. I'm working with a LiPo battery, so max. expected voltage per cell is 4.2 V.

Best regards.

Hello Roberto,

Just to clarify, when you are connecting the connectors together R110 is always connected to ground from VC0, correct? Our VC0 pin has a lower voltage tolerance. If the VC0 does not have the resistor to VSS on the PCB, then the cells cannot be connected in a random sequence as the pin voltage is expected to be close to VSS.

Also, can you measure the voltage between the VC1-VC0 pins? I am curious to see if there may be a negative voltage transient occurring causing the device to become damaged.

Regards,

Rohin Nair

Hi Rohin,

R110 is always placed on the PCB, it is soldered and fixed, not removable. 

You mean, to measure across VC1-VC0 when attaching the BAT+/- harness, once cells are already connected?

Regards,

Hello Roberto,

Correct. I am wondering if there is a negative voltage transient occurring across the pins. If you can record a waveform across VC1-VC0 without BAT +/- attached, then connect the BAT pins and stop the recording. This will help me better understand what may be occuring here.

Regards,

Rohin Nair

Hi Rohin,

I have just taken the scope you asked, but in a new prototype with some more parts intended to avoid this phenomenon. I don't have any old board. Here you have the updated schematics:

As you said, VC1 drops when I connect BAT power wires AFTER cells one, but it doesn't look to go negative.



In this case, the BMS seems to have survived to this deadly connections.

Hello,

This is interesting. I just noticed this on your schematic, but what is the relation between PGND and SGND? Are they the same?

In this test, did you connect BAT- first?

Regards,

Rohin Nair

PGND and SGND are connected through a net tie in a single point in the board.

BAT+ and BAT- are connected almost at the same time. They are arranged in the same connector.

Hi again Rohin,


I have been able to replicate the issue on a new prototype: IC blows down when connecting BAT power wires AFTER cells wires AND if the IC is already balancing the cells.

And spark can be seen close to R110 (I think, not fully sure of the exact spark place). I haven't been able of scoping VC1 when so.

Is this possible to avoid?

Regards,

Hello Roberto,

Can you clarify what the Bat+/- are? Is it the same as Cbat+ and Cbat- on your schematic? If so, are these not the cells being connected? Or is VC0 and VC16 lines not being connected?

Can you measure the voltage of Sground, Cell0, and the BAT pin with a DMM prior to connecting the BAT+/-? Likely what we will see here is that the ground starts floating up.

Regards,

Rohin Nair