Hello folks,
Can't say I am an expert on the BC-900, but judging from the photos posted I believe the problem was not due to a "missed termination."
In the BC-900 design, it uses a very small package MOSFET (SOT23) as a linear step-down regulator which has to disspate power due to the difference in voltage between the power supply (4V) and the battery (1.5V). The power disspated by the MOSFET is the current times the voltage difference. At peak charging current, the wattage disspated is considerable.
This is not a typical design as most chargers today use a "switching buck design" which uses a inductor, transistor, diode and capacitor to convert the voltage. This design usually gives 70-80% conversion efficiency even at a large voltage difference, but more expensive to make.
Back to the point, I believe it is possible that the MOSFET failed due to either a surge in the Gate-to-Source voltage causing the gate oxide to break. Even more likely it is due to the aging of the MOSFET since it is taking a huge toll close or beyond its power disspation limit. As a result, the probability of MOSFET failure increases.
When a MOSFET fails, it can either become always OPEN or always SHORTED. In the latter case, there would no longer be a current limiting component between the charger and the power supply causing the all the current to rush to the batteries. Furthermore, since the MOSFET is always SHORTED there's no way to stop the charging even if the microcontrolluer unit sends the signal to stop the current. Therefore, it doesn't matter if there is a thermocouple or voltage-based termination, since the MCU is no longer able to control the charging current.
Just my two cents.
William