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Cell PTC vs BMS voltage violation

Fred P
Expert 2070 points

Hi folks,

Consider a 10S pack with 3V per cells.
Each cell has a builtin PTC with a lower trip current than what the BMS overcurrent protection is set to.
Apply a short between pack+ and pack-.
One PTC will kick in for sure.
The pack voltage will collapsed and split it in an uncontroller way depending on which PTC kicked in.
For example, if the 4th cell PTC kicks in, you end up with the following voltages at the top of each cell (1st cell neg terminal is ground):

c1: 3V
c2: 6V
c3: 9V
c4: -18V
c5: -15V
c6: -12V
c7: -9V
c8: -6V
c9: -3V
c10: ~0V (almost no current through a short)

This situation will for sure violate BMS IC specifications on most cell voltage sensing terminals.  Not to mention FETS and capacitors of any external balancing circuitry present.

I feel like a PTC kicking in would be a disaster.  Kind of paradoxical when you think that a PTC is meant to avoid a disaster..

Does anybody have an opinion on that?

Regards,

Frédéric

  • 0
    TI__Guru 54715 points

    Yes, an open PTC or a broken strap weld or a fuse between cells opening will cause a cell reversal under load and likely damage the BMS IC if the voltage exceeds the abs max.  It may be possible to design so that external clamp didoes would carry the load current and keep the IC voltages within safe operating limits, but those components may need to be very sturdy.  It may be better to consider system design where the PTC opening is one of the last mechanisms to actuate rather than one of the first.