ebbfolls,

1. The no-load voltage of the usual "unregulated" 12V dc adapter is near 16 volts and would, in theory, be capable of charging the battery to 16 volts. But the internal resistance of the adapter coupled with the voltage drop on the external diode limits the voltage down to 14 volts, which turns out to be a safe level.

2. The diode does two things. 1) It gives a voltage drop of 0.6 volts, hence the battery will not be abused with overvoltage, and 2) It prevents the battery from discharging onto the adapter when AC power is not present.

Although the internal circuitry of the adapter is already sufficient for the task, I decided to add the resistor for testing purposes. It serves as convenient test point for measuring charging current.