Design Ideas: June 23, 1994
To assemble superior NiCd batteries, you need to form and match the individual cells making up a battery. First, formation takes place in a NiCd cell only after numerous charge-discharge cycles. Second, you must collect voltage data during these cycles to match up cells having uniform characteristics.
To keep costs under control and to minimize human error, the circuit in Fig 1 processes cells in batches. Fig 1 shows a single cell channel. You can place as many channels on a pc board as suits your purposes. The ZIPfile attached to EDN BBS /DI_SIG #1446 contains a write-up of this Design Idea and the circuit diagram in OrCAD format.
In addition to the circuit, you will need a computer-controlled, data-acquisition system to cycle the cells and record data. If you were to build a pc board bearing 16 channels to handle 16 NiCd cells simultaneously, for example, your data-acquisition system would need one digital output per channel (D1), one digital output per pc board (D2), one analog output per board to set the charging rate, and one analog input per cell.
As the system performs timed cycling, it records individual cell voltages—the only data it needs. The system can then calculate each cell's internal impedance using dV/dI and calculate each cell's capacity in ampere-hours (Ahr). Further, the computer can determine each cell's voltage-cutoff point during discharging.
The circuit provides closed-loop current regulation. The master analog-input voltage, AIN, dictates the magnitude of charge or discharge currents—1V=1A. IC2B buffers the analog-input voltage. Because all channels on the pc board use the same analog-input voltage, the LC filter, L1 and C1, minimizes reflected noise.
A 5V power supply provides up to 2.5A charging current. Digital input D2 configures relay K2 for charge or discharge. All channels share D2. Digital input D1 controls each channel's current flow.
Applying a high input to both D1 and D2 (the controlled-discharge state) causes the relays to place the 5V supply in series with the cell to bring the effective voltage to 6.2V nominal. The relays also create a path that allows the cell to discharge through the same circuit that charges it. The 6.2V ensures that the cell maintains a constant discharge current. (DI #1446)