Seems I've fried a battery :(
I have the charger and parallel board shown below. Was planning on charging 3 batteries. Plugged in the first 2 just fine, but plugging in the 3rd got a 'poof!' One of the connectors came off the board, and the battery connectors are black'ish now. One of the connectors from the board broke/melted off into the battery balance connection, so I can't even test the charge now.
What did I do wrong? I used the charger to charge 1 battery successfully.