Method for calculating power capability of battery packs using advanced cell model predictive techniques
View Patent ↗The present invention relates to a method and an apparatus for estimating discharge and charge power of battery applications, including battery packs used in Hybrid Electric Vehicles (HEV) and Electric Vehicles (EV). One charge/discharge power estimating method incorporates voltage, state-of-charge (SOC), power, and current design constraints and works for a user-specified prediction time horizon Δt. At least two cell models are used in calculating maximum charge/discharge power based on voltage limits. The first is a simple cell model that uses a Taylor-series expansion to linearize the equation involved. The second is a more complex and accurate model that models cell dynamics in discrete-time state-space form. The cell model can incorporate a inputs such as temperature, resistance, capacity, etc. One advantage of using model-based approach is that the same model may be used in both Kalman-filtering to produce the SOC and the estimation of maximum charge/discharge current based on voltage limits.
1. A method for estimating a maximum discharge power of a battery, comprising:
generating a signal indicative of a present state-of-charge of said battery, utilizing a sensor;
calculating said present state-of-charge of said battery based on said signal, utilizing an arithmetic circuit operably coupled to said sensor;
calculating a maximum discharge current of said battery utilizing said arithmetic circuit based on at least a minimum state of charge limit associated with said battery, said present state of charge of said battery a minimum voltage limit associated with said battery, and of a cell model that is solved by a Taylor series expansion of an open cell voltage of the battery to determine a future output voltage at the battery, such that the future output voltage of said battery does not fall below said minimum voltage limit and a future state-of-charge of said battery does not fall below said minimum state-of-charge limit associated with said battery; and,
calculating said maximum discharge power based on said maximum discharge current value, utilizing said arithmetic circuit.
2. The method of claim 1 wherein said cell model is solved by using a discrete time-state space model.
3. The method of claim 1 wherein said battery is a battery pack comprising at least one cell.
4. The method of claim 3 wherein said cell model is
v k ( t+Δt )= OCV ( z k ( t+Δt ))− R×i k ( t )
wherein v k (t+Δt) denotes the cell voltage for cell k for the time period Δt units into the future, OCV(z k (t+Δt)) denotes the open cell voltage as a function of the state-of-charge z k for cell k for the time period Δt units into the future, R is a constant that denotes an internal resistance of said cell k, and i k (t) denotes a cell current for cell k.
5. The method of claim 4 wherein said cell model is solved by using a discrete time-state space model.
6. The method of claim 5 wherein said discrete time-state space model is
x k [m +1]=f( x k [m],u k [m ])
v k [m]=g ( x k [m],u k [m ])
wherein m denotes the discrete time sample index, x k [m] denotes the vector function of time and the state of the battery, u k [m] denotes the input to the battery and includes cell current i k [m] as a component, and f(•) and g(•) are functions chosen to model the cell dynamics.
7. The method of claim 6 wherein
i
max
,
k
dis
,
volt
is found by looking for i k that causes equality in
v mm =g ( x k [m+T],u k [m+T ])
wherein g(x k [m+T], u k [m+T]) is utilized to determine the cell voltage for the cell k at a predetermined time in the future.