State of charge tracking system for battery systems based on relaxation voltage

1. A battery control module for a battery system, comprising:

a voltage measuring module that measures battery voltage;

a current measuring module that measures battery current;

a state of charge (SOC) module that communicates with said current and voltage measuring modules, that calculates a charge power limit during a charging event based on a first voltage measurement and a first current measurement measured before the charging event ends, that calculates a discharge power limit during a discharging event based on a second voltage measurement and a second current measurement measured before the discharging event ends, that calculates a power limit ratio, and that estimates SOC based on the power limit ratio, wherein:

said power limit ratio corresponds to a ratio of said charge power limit to said discharge power limit and wherein said charging and said discharging corresponding to said charge power limit and said discharge power limit occur in adjacent events;

said SOC module accumulates charge swing during charging, identifies a maximum current divided by K during said charge swing, where K does not equal zero, and stores said charge current, said charge swing and said charge power limit;

said SOC module determines whether said charge swing is greater than a negative of a retained discharge swing;

said SOC module determined whether said charge current is approximately equal to a negative of a retained discharge current;

said SOC module includes a lookup table; and

said SOC module looks up said SOC as a function of said power limit ratio when said charge swing is greater than a negative of said retained discharge swing and said charge current is approximately equal to a negative of said retained discharge current.

2. A battery control module for a battery system, comprising:

a voltage measuring module that measures battery voltage;

a current measuring module that measures battery current;

a state of charge (SOC) module that communicates with said current and voltage measuring modules, that calculates a charge power limit during a charging event based on a first voltage measurement and a first current measurement measured before the charging event ends, that calculates a discharge power limit during a discharging event based on a second voltage measurement and a second current measurement measured before the discharging event ends, that calculates a power limit ratio, and that estimates SOC based on the power limit ratio, wherein:

said power limit ratio corresponds to a ratio of said charge power limit to said discharge power limit and wherein said charging and said discharging corresponding to said charge power limit and said discharge power limit occur in adjacent events;

said SOC module accumulates discharge swing during discharging, identifies a minimum current divided by L during said discharge swing, where L does not equal zero, and stores said discharge current, said discharge swing and said discharge power limit;

said SOC module determines whether said discharge swing is greater than a negative of a retained charge swing;

said SOC module determined whether said discharge current is approximately equal to a negative of a retained charge current;

said SOC module includes a lookup table; and

said SOC module looks up said SOC as a function of said power limit ratio when said discharge swing is greater than a negative of said retained charge swing and said discharge current is approximately equal to a negative of said retained charge current.

3. A method for operating a battery control module for a battery system, comprising:

measuring a battery voltage;

measuring a battery current;

accumulating a charge swing during a charging event;

identifying a charge current equal to a maximum current divided by K, where K does not equal zero, during said charge swing;

storing said charge current, said charge swing and said charge power limit;

calculating a charge power limit during the charging event based on a first voltage measurement and a first current measurement measured before the charging event ends;

calculating a discharge power limit during a discharging event based on a second voltage measurement and a second current measurement measured before the discharging event ends;

calculating a power limit ratio;

determining whether said charge swing is greater than a negative of a retained discharge swing;

determining whether said charge current is approximately equal to a negative of a retained discharge current;

looking up said SOC as a function of said power limit ratio when said charge swing is greater than a negative of said retained discharge swing and said charge current is approximately equal to a negative of said retained discharge current; and

estimating SOC based on said power limit ratio, said battery voltage and said battery current, wherein said power limit ratio corresponds to a ratio of said charge power limit to said discharge power limit and wherein said charging and said discharging corresponding to said charge power limit and said discharge power limit occur in adjacent events.

4. A method for operating a battery control module for a battery system, comprising:

measuring a battery voltage;

measuring a battery current;

accumulating a discharge swing during discharging;

identifying a discharge current equal to a minimum current divided by L, where L does not equal zero, during said discharge swing;

storing said discharge current, said discharge swing and said discharge power limit;

calculating a charge power limit during a charging event based on a first voltage measurement and a first current measurement measured before the charging event ends;

calculating a discharge power limit during a discharging event based on a second voltage measurement and a second current measurement measured before the discharging event ends;

calculating a power limit ratio;

determining whether said discharge swing is greater than a negative of a retained charge swing;

determining whether said discharge current is approximately equal to a negative of a retained charge current;

looking up said SOC as a function of said power limit ratio when said discharge swing is greater than a negative of said retained charge swing and said discharge current is approximately equal to a negative of said retained charge current; and

estimating SOC based on said power limit ratio, said battery voltage and said battery current, wherein said power limit ratio corresponds to a ratio of said charge power limit to said discharge power limit and wherein said charging and said discharging corresponding to said charge power limit and said discharge power limit occur in adjacent events.