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BQ77PL900 Circuit Suddenly Not Working

Travis Pollok
Prodigy 110 points
Other Parts Discussed in Thread: BQ77PL900

So we're using the BQ77PL900 circuit for 9 lithium ion cells.  We connected it up to the resistor "batteries" system to turn it on and program the EEPROM to configure the following:

4 to register 6 (Sets max voltage to 4.35V in OV CFG Register)

b to register 7 (Sets lowest voltage to 2.5V in UV CFG Register)

0 to register 8 (Default OC and UV Delay settings)

0 to register 9 (Default OCD CFG settings)

0 to register a (Default SCD CFG settings)

For the first board we were using, the programming went successfully.  We then put some touch-up conformal coating and vacuumed it in a bell jar.  We let it cure a day and then tried to integrate it with the cells.  With it hooked up we tried to turn it on to make sure it was properly connected by applying a 20 V charge across the PACK.  The light turns on and turns off after the voltage is removed.  This indicates to us an undervoltage reading for some reason.  The battery voltages, the series connections, the monitor wires on the positive tabs, and the wires on the ends seem to all be correct. 

We thought it may be that since the lowest voltage it needs is to be 22.5 V altogether that it needs to be at least that in order for the charging voltage to turn it on.  We tried it at 25 V instead of 20 V but the same thing happened. 

Another thing is that the overall battery voltage dropped 0.1 V overnight and drops about 0.01V every hour or so. 

One of the incidents with this board was that while soldering the batteries, the PACK- touched a common ground to the soldering iron and shorted some batteries.  The metal of the PACK- was fused to the common ground connection.  But when we turned it on and programmed it we thought it was fine.

The only thing I can think of is that there may have been some preexisting damage from the short and that was exacerbated during conformal coating and vacuuming.  It's just weird because it's the third circuit to do this where everything is connected but it's still going into shutdown due to undervoltage. 

Help?

  • 0
    TI__Guru 54715 points

    I would not expect the conformal coat to cause a problem.

    If multiple (all?) boards are having the same apparent undervoltage, I'm sure you have checked the obvious cell count configuraiton.

    The IC is sensitive to the absolute maximum differential voltage.  A common damage is a resistive short between 2 inputs.  This can reduce the voltage measrued on the cell and raise that of the adjacent cells. The leakage may be measurable with an ohm meter (use one which won't force excessive voltage). On the board which shorted the cell- to soldering iron ground, if the iron tip touched the pad before the cell terminal, it would most likely have damaged one of the differential inputs. A resistive short between the inputs would discharge one cell with time and show as a battery voltage drop.  The IC would seem to be in shutdown mode from your description and should be in a low current power state, so  some resistive leakage would seem the likely cause..

    Programming is typically not affected by an input short, so success of programming does not indicate the part is damage free.

    If your conformal coat leaves the programming interface and VOUT available, you might put the part in host mode and look at the voltage of the cells.  If one cell shows a higher VOUT (lower cell votlage), it is the likely damaged input pair.  If a pattern shows up on the damaged boards, it may indicate where to look in the board assembly or test process for unexpected voltage levels. 

    Zener diodes at the board input either in the assembly fixture or as part of the product could help prevent overvoltage and damage.