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PORTAFLOW 204 Plus Battery Charge circuit Operation.
Charging Controller IC:
A Maxim IC MAX712 or MAX713 controls the Ni-Cd and Ni-Mh battery charger. It
has two modes, fast charge and trickle charge; an output indicates the fast-charge status. In
both modes it supplies, via a PNP power transistor, a constant current to the battery, by
keeping a constant voltage across a current sensing resistor. In fast charge mode it is
250mV, in trickle charge mode 31mV, so the trickle charge current is 1/8 of the fast charge
current.
By wiring up input pins on the IC, the number of cells is set to 5, the voltage sampling
interval to 168 sec, and the fast-charge time limit to 264 minutes (the maximum). The battery
temperature limits are not used.
The IC starts the fast-charge timer when a battery is connected or when power is
applied. It terminates the fast charge and returns to trickle charge, either after the 264 min
(~4.5 hrs) time limit, or when it senses that the battery voltage remains constant or begins to
decrease, meaning that the battery is fully charged.
Charging Voltage:
The voltage available to charge the 6V battery is restricted by the 9V charger input
and the two diodes in the input. The S2D silicon diodes had a fwd drop of 0.75V, limiting the
available charge voltage to 7.5V, which caused the MAX712 to sense that the battery voltage
had stopped rising, and therefore prematurely end the fast charge. With several days of trickle
charging the battery could however still reach its full capacity.
In Dec.2000 the S2D diodes were replaced by SS14 Schottky diodes with a fwd drop
of 0.35V, thus raising the available charge voltage to 8.3V. At the same time the current was
increased.
Instrument differences:
The current sensing resistor consists of either 2 or 4 parallel 1.2Ω resistors, giving
about 0.4A or 0.8A fast-charge current.
PF-300 and UFM610P:
Battery Capacity 3.5Ah, or 4.0Ah after Oct.2000
Current 0.4A before, 0.8A after Dec.2000
PF 204, PF-SE and 216:
Battery Capacity 1.2Ah
Current 0.4A
Software:
The fast-charge status output is not used by the present software (ver.3.06); in a
future software update a message will be added, indicating charging status.
Quicker full charge:
The fastest way to fully charge the battery is to charge for 4.5 hrs, then switch the
power supply off and on again, thus re-starting the fast charge for another 4.5 hr period,
followed by trickle charge.
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