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PORTAFLOW 216 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-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.