Electric vehicle battery charge and discharge management

1. An electric vehicle comprising:

a motor coupled to one or more wheels of the electric vehicle;

an inverter coupled to the motor;

at least a first power bus coupled to the inverter; and

a plurality of battery strings including a first battery string having an output that is coupled to the first power bus through a first switch, a second battery string different from the first battery string having an output that is coupled to the first power bus through a second switch different from the first switch, and a third battery string different from the first and second battery strings having an output that is coupled to the first power bus through a third switch different from the first and second switches;

control circuitry coupled to at least the first switch, the second switch, and the third switch, the control circuitry configured to selectively and independently control the open or closed state of the first, second, and third switches, thereby selectively and independently connecting the output of the first, second, and third battery strings, including:

opening the first switch and, with the first switch open, measuring a first voltage across the first battery string,

opening the second switch and, with the second switch open, measuring a second voltage different from the first voltage across the second battery string,

opening the third switch and, with the third switch open, measuring a third voltage different from the first and second voltage across the third battery string,

closing the first switch, thereby connecting the first battery string to the first power bus, based at least in part on a determination that the first voltage is higher than both the second and third voltage,

closing the second switch, thereby connecting the second battery string to the first power bus, based on a determination that the second voltage is within a first threshold voltage difference below the first voltage,

not closing the third switch, and thereby not connecting the third battery string to the first power bus, based on a determination that the third output voltage is not within the first threshold voltage difference below the first voltage,

upon determining that power is being delivered to the electric motor from at least the first and second battery strings and with the first and second switches closed, measuring a power bus voltage on the first power bus,

determining a derated power bus voltage based on the power bus voltage on the first power bus;

with power being delivered to the electric motor and with at least the first and second switches closed, closing the third switch, thereby connecting the third battery string to the first power bus, based on a determination that the third output voltage is within a second threshold voltage difference below the determined derated power bus voltage.

2. The electric vehicle of claim 1 , comprising a first current sensor in series with the first switch, and a second current sensor in series with the second switch.

3. The electric vehicle of claim 1 , wherein the first battery string and the second battery string have outputs coupled to a second power bus through respective fourth and fifth switches.

4. The electric vehicle of claim 3 , wherein the control circuitry is configured to control the open or closed state of the fourth and fifth switches.

5. The electric vehicle of claim 1 , wherein the first battery string, the second battery string, and the third battery string each comprise a plurality of modules of cells connected in parallel and wherein each of the plurality of modules of cells comprise a plurality of cells connected in series.

6. The electric vehicle of claim 1 , wherein the control circuitry is configured to measure a first output current of the first battery string when the first switch is closed and an output current of the second battery string when the second switch is closed.

7. A method of powering an electric vehicle, the electric vehicle comprising an electric motor driven with a power bus, the method comprising:

determining a plurality of separate output voltages for a corresponding plurality of separate battery strings of the electric vehicle including a first battery string having a first output voltage, a second battery string having a second output voltage, and a third battery string having a third output voltage wherein the first, second, and third output voltages are different from one another;

connecting the first battery string to the power bus based on a determination that the first output voltage is the highest of the plurality of separate output voltages;

connecting the second battery string to the power bus based on a determination that the second output voltage is within a first threshold voltage difference below the first output voltage;

not connecting a third battery string to the power bus based on a determination that the third output voltage is not within the first threshold voltage difference below the first output voltage;

delivering power to the electric motor from at least the first and second battery string;

while delivering power to the electric motor from the first and second battery string, determining a power bus voltage of the power bus;

also while delivering power to the electric motor from the first and second battery string, connecting the third battery string to the power bus based on a determination that the third output voltage is within a second threshold voltage difference below the determined power bus voltage; and

disconnecting different ones of the plurality of battery strings to the power bus based at least in part on the determined plurality of separate output voltages.

8. The method of claim 7 , comprising, prior to delivering power to the electric motor from the first and second battery string, determining that a first set of the plurality of battery strings have output voltages that are within the first threshold voltage difference below the first output voltage, with the remaining plurality of battery strings making up a second set of the plurality of battery strings, and connecting all of the first subset of battery strings to the power bus based on that determination.

9. The method of claim 8 , comprising, prior to delivering power to the electric motor from the first and second battery string, not connecting any of the second set of battery strings.

10. A method of powering an electric vehicle comprising:

initially connecting some but not all of a plurality of battery strings of the electric vehicle to a power bus;

loading the power bus with an electric motor of the electric vehicle;

while the power bus is loaded with the electric motor, measuring the voltage of the power bus;

measuring a voltage of an unconnected one of the plurality of battery strings; and

while still loading the power bus with the electric motor, connecting the unconnected one of the plurality of battery strings to the power bus if based on a determination that the voltage of the unconnected one of the plurality of battery strings is within a threshold voltage below the measured voltage of the power bus.

11. The method of claim 10 , further comprising measuring at least one discharge current for at least one connected one of the plurality of battery strings.

12. The method of claim 11 , further comprising: disconnecting a connected one of the plurality of battery strings if the discharge current of the connected one of the plurality of battery strings is below a threshold.

13. The method of claim 11 , further comprising:

storing a measured initial voltage of at least one of the plurality of battery strings; estimating a state of charge of at least one of the plurality of battery strings based in at least part on the stored initial voltage and the measured discharge current of the at least one of the plurality of battery strings;

and disconnecting the at least one of the plurality of battery strings if the estimated state of charge of the at least one of the plurality of battery strings is below a threshold.