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Discharging Issue after parallelling battery

Recently I found an issue with BR (4S6P) batteries, one cell is not discharging. I charged the battery to 16.8V using BR battery charger. After charging I checked the battery voltage across the XT90 terminals, the voltage is 16.8V and individual cell voltages are cell no.(1) 4.2V cell no .(2) 4.2V no.(3) 4.2V cell no .(4) 4.2V. After that I discharged the battery with 2 methods.

  1. discharged the battery using BR battery charger itself(using discharge option).
  2. discharged using operating ROV.
    While using ROV battery suddenly discharges to 12.9V in QGC. Then I checked the battery voltage across XT90 terminal and checked individual cell voltage. Total voltage is 12.9V and cell voltages are cell no.(1) 3.2V cell no (.2) 3.2V cell no.(3) 2.5V cell no .(4) 4V. On of the cells (cell 4) are not discharging during operation. Before that we used to connect the batteries parallel. Is there any issue for paralleling batteries or why the cell no 4 is not discharging?? @jwalser @patrickelectric

It seems your battery went through something bad. This usually happens when the battery is put under too much stress, such as completely or almost completely discharging it, or putting way too much current through it. What happens in these cases is that the internal resistance of the cells rises, some more than others.
This causes the battery to charge and discharge unevenly. Up to a point balancing chargers can deal with it, but in my experience these batteries gradually get worse until the charger is not able to balance them anymore.

You should take care when connecting batteries in parallel. They must be both healthy and at the same voltage levels. If they are not both at the same charge, as soon as you plug them together one will try to charge the other with a huge current (due to their low internal resistance) until their voltages match.
If they are not both healthy, one will discharge more than the other under load, and when you take the load off, they bad one will slowly charge the good one again, to get back to balance. Neither of these things are good for the batteries.

About your battery, I would check the internal resistance of the cells and discard this battery if they are not close to each other.

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