The nickel-metal-hydride (NiMH) battery model in Listing 1 accurately predicts the discharge characteristics of an NiMH cell (or groups of cells). The model accounts for
The nickel-metal-hydride (NiMH) battery model in Listing 1 accurately predicts the discharge characteristics of an NiMH cell (or groups of cells). The model accounts for
The basic cell model is accurate for 0.001 to 5C discharge rates, where the 1C rate is a battery-industry term for the current that will discharge a cell to its cutoff voltage (usually 1.1V) in exactly one hour.
Fig 1's subcircuit is the basis for the cell model. You can probe four nodes of this subcircuit. +OUTPUT and -OUTPUT are the usual positive and negative terminals of the cell. You may connect these terminals to your circuit or connect the terminals in a series with other cells to model batteries.
SOC is the state of charge of the battery at its present discharge rate. 1V at SOC indicates 100% state of charge; 0V represents no capacity remaining. Depending on the discharge rate, E_LOST_RATE lowers cell capacity at high rates of discharge. RATE is the instantaneous discharge rate of the cell in C units. So, 1V indicates that the battery is discharging at the 1C rate.
The parameters CAPACITY and RESISTANCE, passed to the NiMH subcircuit, represent the cell's nominal capacity and 100% charge. R2 and C1, connected to node 60, model a delaying action when E_LOST_RATE corrects the cell's capacity. You can see this effect in a real cell. A real cell initially exhibits a high terminal voltage that lasts for several seconds when first connected to a load. Then the cell's voltage quickly decays to a lower steady-state-discharge value.
To use the model, connect the cell's output terminals to your load, initialize node 50 to the cell's desired state of charge (1V=100% charge), and set node 60 to zero, initially.
The two most common NiMH cell types are the (4/5)A and AA types. These cells have capacities of 1.5 and 1.1 Ahr (5-hour rate) and resistances of 0.02 and 0.03(ohm), respectively. Consult manufacturers's data to model other cell sizes. Use the manufacturers's listed 5-hour discharge rate in Ahrs for the parameter CAPACITY.
Note: When simulating series-connected cells, connect the SOC and RATE of each subcircuit cell to different nodes in your circuit. Do not connect these nodes in common between cells. The ZIPfile attached to EDN BBS /DI_SIG #1646 contains documentation, schematics, a test circuit and test results, and the listing.(DI#1646)