Applications
900-0209-01-00 Rev A
67
Three-Stage Battery Charging
The FLEXmax 100 charge controller is a sophisticated, multi-stage battery charger that uses several
regulation stages to allow fast recharging of the battery system while ensuring a long battery life. This
process can be used with both sealed and non-sealed batteries. The FLEXmax 100 is a “buck” converter
which turns higher PV voltages into the lower charging voltages used by batteries (with correspondingly
higher currents). The chart in Figure 54 shows the voltage levels achieved by the PV array throughout a
typical day, and the battery voltages (by stage) during the same times.
The FLEXmax 100 has preset recharging voltage set points (Absorbing and Float voltages); however,
OutBack always recommends using the battery manufacturer’s recommended charging voltages.
Figure 54 FLEXmax 100 Three-stage Battery Charging
A new charge cycle is started any time the battery voltage decreases below the
Rebulk
set point for 90
seconds or more. (See page 42.) This usually occurs each night unless the batteries were maintained by
another means. (If so, they may not need to be recharged).
Bulk
This is the first stage in the three-stage charge cycle. It is a constant-current stage which drives the
battery voltage up. The DC current is the maximum current the charger can deliver. This stage typically
leaves the batteries at 75% to 90% of their capacity, depending on conditions.
In Bulk, the controller will charge the batteries to the Absorbing voltage setting (see page 42). This
stage is not timed. It will charge as long as necessary, regardless of any timer set points. If little PV
energy is available, it may take a long time for Bulk to complete. If the FLEXmax 100 is in a different
charging stage and little PV energy is available, it may return to Bulk. (See page 31.) This stage is
temperature-compensated. (See page 68.)
Absorption
This is the second stage of charging. It is a constant-voltage stage. Current varies as needed to
maintain the Absorbing voltage setting, but will typically decrease to a very low number over time.
This “tops off the tank”, leaving the batteries at essentially 100% of capacity.
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