Method and system for charging an auxilary battery in a plug-in electric vehicle

1. A method of charging an auxiliary battery in a plug-in electric vehicle including an onboard charging system, the method comprising:

charging the auxiliary battery to an upper charge level using an external power source thereby reducing energy used from the onboard charging system to charge the auxiliary battery during a drive mode of the vehicle, the upper charge level being at least one of:

approximately an upper level of a predetermined operating range of charge levels; and

greater than the predetermined operating range of charge levels;

allowing the auxiliary battery to discharge to a lower charge level during the drive mode of the vehicle, the lower charge level being at least one of:

approximately a lower level of the predetermined operating range of charge levels; and

less than the predetermined operating range of charge levels;

wherein the lower charge level is based on a time that the vehicle is predicted to be in an off state.

2. The method of claim 1 further including determining parameters of the auxiliary battery in the vehicle to obtain the predetermined operating range of charge levels.

3. The method of claim 1 wherein the predetermined operating range for the auxiliary battery is a voltage range.

4. The method of claim 3 wherein the voltage range is between 11 volts and 13 volts and the upper charge level is a charge level between 12 volts and 15 volts.

5. The method of claim 1 wherein the upper charge level is a predetermined maximum charge of the auxiliary battery.

6. The method of claim 1 further including charging a high-voltage battery in the vehicle using the external power source while charging the auxiliary battery to the upper charge level.

7. The method of claim 1 further including charging a high-voltage battery in the vehicle to a predetermined charge level before using the external power source to charge the auxiliary battery to the upper charge level.

8. The method of claim 1 wherein allowing the auxiliary battery to discharge includes allowing the auxiliary battery to discharge from the upper charge level to the lower charge level while limiting energy usage from the onboard charging system to charge the auxiliary battery.

9. The method of claim 1 further including determining the lower charge level based on operating parameters of the auxiliary battery.

10. The method of claim 1 further including charging the auxiliary battery to an intermediate charge level within the predetermined operating range of charge levels with power generated from the onboard charging system.

11. A system for charging an auxiliary battery in a plug-in electric vehicle including an onboard charging system, the auxiliary battery having a predetermined operating range of charge levels, the system comprising:

at least one logic device configured to determine an upper charge level that is at least one of:

approximately an upper level of the predetermined operating range of charge levels; and

greater than the predetermined operating range of charge levels; and

to charge the auxiliary battery to the upper charge level using an external power source; and

to allow the auxiliary battery to discharge to a lower charge level during the drive mode of the vehicle, the lower charge level being at least one of:

approximately a lower level of the predetermined operating range of charge levels; and

less than the predetermined operating range of charge levels;

wherein the lower charge level is based on a predicted time until next charge of the auxiliary battery.

12. The system of claim 11 wherein the predicted time until next charge is based on when the onboard charging system in the vehicle is predicted to provide power for charging the auxiliary battery.

13. The system of claim 11 wherein the predicted time until next charge is based on when the auxiliary battery is predicted to be charged with power from the external power source.

14. The system of claim 11 wherein the predetermined operating range for the auxiliary battery is a voltage range, the upper and lower charge levels being voltage levels, and the method further including charging a high-voltage battery in the vehicle to a predetermined voltage level before using the external power source to charge the auxiliary battery to the upper charge level.

15. The system of claim 11 wherein the logic device is a controller including a processor operable to execute software instructions, a computer memory operable to store software instructions accessible by the processor, and a set of software instructions stored in the memory to determine the upper charge level and to charge the auxiliary battery to the upper charge level.

16. A method of charging an auxiliary battery in a plug-in electric vehicle including an onboard charging system, the method comprising:

charging the auxiliary battery to an upper charge level using an external power source; and

allowing the auxiliary battery to discharge to a lower charge level during the drive mode of the vehicle, the lower charge level being based on a predicted time until next charge of the auxiliary battery.

17. The method of claim 16 wherein the lower charge level being at least one of:

approximately a lower level of the predetermined operating range of charge levels; and

less than the predetermined operating range of charge levels.

18. A method of charging an auxiliary battery in a plug-in electric vehicle including an onboard charging system, comprising:

charging the auxiliary battery to an upper charge level using an external power source; and

allowing the auxiliary battery to discharge to a lower charge level during the drive mode of the vehicle, the lower charge level being based on a time that the vehicle is predicted to be in an off state.