Vectronics VEC-412K - Manual (Page 31)

VEC-412K Owner's Manual Rapid Battery Charger/Conditioner Kit
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THEORY OF OPERATION AND SPECIFICATIONS
Theory:
The VEC-412K is a rapid battery charger and battery conditioner kit for NiCd
(nickel cadmium) or NiMH (nickel-metal hydride) rechargeable batteries. The
design incorporates a BQ2003 integrated circuit. This device is configured as a
Switch-Mode-Current-Regulator—providing efficient energy transfer, reducing
power dissipation and associated heating. It operates as a frequency modulated
controller for switched regulation of the charging current. The unit uses three
different fast-charging determination methods. For NiCd batteries a maximum
voltage, maximum time or a negative increment of voltage detection are
available. For Ni-MH batteries the drop in voltage as full charge is reached is
not very big. In order to avoid overcharging NiMH batteries this method should
be disabled via the HD6 jumper setting.
The charge controller uses the single cell voltage provided by this adjustable
resistor divider network connected between the positive and the negative
terminals on the battery. The divide count is controlled by range switch SW3
and rotary switch SW1. The BQ2003 IC views a battery pack as being a single
“cell”. The range switches divide the pack voltage to that of a single cell for
measurements. For example, to detect the 1 volt-per-cell full discharge point of
a battery pack with ten cells, the battery pack voltage is divided by ten if the
range switches are set properly. When the pack reaches 10 volts, or one volt per
cell, the IC “sees” a 1 volt-per-cell reading for the pack.
The fast charge is only initiated if each cell in the pack is greater than 1 Vdc. If
the cell voltage is less, the controller applies a trickle current until it senses the
cells have reached 1 volt. The trickle charge is supplied to the battery through
R20 from the DC supply to the positive battery terminal.
During fast-charge the red status LED CR1 will be continuously ON. For the
initial period of fast charge or hold-off, the voltage charge determination
methods are disabled. The hold-off is a function of the charge rate selected by
the Time-Out setting of the jumpers on HD2 and HD1.
The maximum cell voltage (MCV) is set to 1.82V by the R4-R5 divider
network. Charging activity is halted if the cell voltage (BAT) is higher than the
MCV. The BAT voltage is used to determine fast charge initiation and
termination.
The discharge-before-charge function is used to condition NiCad batteries
exhibiting a memory condition. The battery is discharged through a constant-
current discharging circuit. The HD3 jumper sets the DISCHARGE rate to
120mA or 240mA.