Controlled power fade for battery power devices

1. A method for operating a power tool having a motor powered by a battery pack, the method being executed via a controller residing in the power tool, the method comprising:

setting the duty cycle of a pulse width modulated (PWM) control signal for driving the motor to a first PWM value in accordance with a desired rotational speed of the motor;

delivering power from the battery pack to the motor in accordance with the control signal;

monitoring a parameter associated with at least one of battery operation or tool operation while power is delivered from the battery pack to the motor;

detecting a condition of the parameter indicative of a shutdown condition; and

fading the power delivered from the battery to the motor, in response to the detected condition, wherein the fading step comprises:

computing a second PWM value based on a value of the parameter, and

setting the duty cycle of the control signal to the second PWM value when the second PWM value is less than the first PWM value, and retaining the duty cycle at the first PWM value set in accordance with the desired rotational speed of the motor when the second PWM value is more than the first PWM value.

2. The method of claim 1 , wherein fading of the power delivered is by an amount that is computed as a function of a value of the parameter.

3. The method of claim 1 , wherein fading of the power delivered further comprises decreasing the power delivered as a function of the value of the parameter, where the function is at least one of linear, step, exponential or polynomial.

4. The method of claim 1 , wherein the parameter is voltage of the battery, comprising: initiating fading of the power delivered when the battery voltage falls below a voltage fade threshold and decreasing the power delivered linearly as a function of the battery voltage until the battery voltage falls below a voltage cutoff threshold.

5. The method of claim 1 , wherein the parameter is temperature in the power tool, the method comprising: initiating fading of the power delivered when the temperature exceeds a temperature fade threshold and decreasing the power delivered linearly as a function of the temperature until the temperature exceeds a temperature cutoff threshold.

6. The method of claim 1 , wherein the parameter is temperature in the battery pack, the method comprising: fading of the power delivered when the temperature exceeds a temperature fade threshold and decreasing the power delivered linearly as a function of the temperature until the temperature exceeds a temperature cutoff threshold.

7. The method of claim 6 , wherein the battery pack includes a positive terminal, a negative terminal, a temperature sense terminal, battery cells coupled to the positive and negative terminals, and a temperature sensor electrically coupled to the positive terminal.

8. A power tool system comprising:

a tool assembly having a motor;

a battery pack that selectively couples to the tool assembly and operates to provide power to the motor;

a discharge control module that regulates power provided to the motor by setting a duty cycle of pulse width modulated (PWM) control signal of a switch disposed in series with the motor to a first PWM value in accordance with a desired rotational speed of the motor, wherein the discharge control module is configured to monitor a parameter associated with at least one of battery operation or tool operation while power is delivered from the battery pack to the motor, detect a condition of the parameter indicative of a shutdown condition, and fade the power delivered from the battery to the motor by adjusting the duty cycle of the control signal by an amount based on a value of the parameter,

wherein the discharge control module computes a second PWM value based on the value of the parameter, sets the duty cycle of the control signal to the second PWM value when the second PWM value is less than the first PWM value, and retains the duty cycle at the first PWM value set in accordance with the desired rotational speed of the motor when the second PWM value is more than the first PWM value.

9. The power tool system of claim 8 , wherein the discharge control module fades the power delivered by decreasing the power delivered as a function of the value of the parameter, where the function is at least one of linear, step, exponential or polynomial.

10. The power tool system of claim 8 , wherein the discharge control module monitors voltage of the battery, initiates fading of the power delivered when the battery voltage falls below a voltage fade threshold and decreases the power delivered linearly as a function of the battery voltage until the battery voltage falls below a voltage cutoff threshold.

11. The power tool system of claim 8 , further comprising a temperature sensor disposed in the tool assembly, wherein the discharge control module is configured to receive a temperature measure from the temperature sensor, initiate fading of the power delivered when the temperature exceeds a temperature fade threshold and decrease the power delivered linearly as a function of the temperature until the temperature exceeds a temperature cutoff threshold.

12. The power tool system of claim 8 , further comprising a temperature sensor disposed in the battery pack and in data communication with the discharge control module, wherein the discharge control module is configured to monitor temperature in the battery pack, initiate fading of the power delivered when the temperature exceeds a temperature fade threshold and decrease the power delivered linearly as a function of the temperature until the temperature exceeds a temperature cutoff threshold.

13. The power tool of claim 12 , wherein the battery pack includes a positive terminal, a negative terminal, a temperature sense terminal, battery cells coupled to the positive and negative terminals, and a temperature sensor electrically coupled to the positive terminal.

14. The power tool of claim 8 , wherein the switch is disposed in series with battery cells of the battery pack.

15. The power tool system of claim 8 , wherein the discharge control module resides in the tool assembly.

16. The power tool system of claim 8 , wherein the discharge control module resides in the battery pack.