3.0
Installation and Commissioning
16
GenStar MPPT
17
Battery Voltage Sense
is a high impedance connection that enables the GenStar MPPT to measure
the battery terminal voltage precisely with small gauge wires that have no voltage drop. When
connected directly to the battery, the sense wires will improve battery charging accuracy. Battery
voltage sense wires are recommended, but may not be necessary when using larger battery cables
and/or shorter battery wire lengths.
Generally accepted wiring practice is to limit voltage drops between the charger and the battery to
2%. Even properly sized wiring with 2% drop can result in a 0.3 volt drop for 14.4V charging. Voltage
drops will cause some undercharging of the battery.
The controller will begin Absorption or limit equalization at a lower battery voltage because the
controller measures a higher voltage at the controller’s terminals than is the actual battery voltage.
For example, if the controller is programmed to start Absorption at 14.4V, when the controller, "sees",
14.4 volts at its battery terminals, if there is a 0.3 volt drop between the controller and battery, the
true battery voltage would only be 14.1 volts.
Note that the battery sense wires will not power the controller, and the sense wires will not
compensate for losses in the power wires between the controller and the battery. The battery sense
wires are used to improve the accuracy of the battery charging.
The two sense wires, can range in size from 1.0 to 0.25 mm
2
(16 to 24 AWG), and should be cut to
length as required to connect the battery to the voltage sense terminals. The 2-position terminal - to
the left of (beyond the RTS terminals) the Load (+) terminal (see Figures 3-3 and 3-4) is used for the
battery sense connections. A twisted pair cable is recommended but not required. Use UL rated 300
Volt conductors. The voltage sense wires may be pulled through conduit with the power conductors.
Observing correct polarity, connect both battery voltage sense wires to the GenStar MPPT at the
2-position
Battery Sense
terminal, and to battery (+) and (-) terminals. No damage will occur if the
polarity is reversed, but the controller cannot read a reversed sense voltage.
Tighten the connector screws to 5 in-lb (0.56 Nm) of torque.
The maximum length allowed for each battery voltage sense wire is 98 ft (30 m).
Connecting the voltage sense wires to the RTS terminal will cause an alarm.
NOTE:
If the battery input voltage is greater than 5 Volts different from the Battery Sense,
due to voltage drops or faulty connections, the Battery Sense input will not be recognized by
the GenStar MPPT. An alarm will be set until the condition has been corrected.
Although not required to operate the GenStar MPPT controller, a battery voltage sense connection is
recommended for best performance.
Accepted practice for system design requires that DC conductors are sufficiently sized to limit
voltage drop losses to 2% or less. The table in Appendix C provides wire sizing information and
distances for required maximum 2% voltage drop in GenStar MPPT wiring applications.
Also see Appendix C, “Wire Sizing”, for copper wire sizing instructions, including minimum wire
sizing requirements.
Accessory Wiring -
refer to schematic in Figure 3.3 below when following 3.4.1 and 3.4.2 on
next pages.
Figure 3-3. Accessory installation detail
3.4.1 Battery Voltage Sense Wires -
refer to Figure 3-3 above
WARNING: Fire Hazard
When connecting Battery Sense Wires, install a 5 Amp fuse in the (+) sense wire, within six
inches (150 mm) of the (+) battery terminal.
Due to connection and cable resistance, voltage drops are unavoidable in all power cables that
carry current. If
Battery Sense
wires are not used, the controller must use the voltage reading at the
battery power terminals for regulation. This voltage may differ from the actual battery bank voltage
due to voltage drop.
BATTERY
SENSE (+) (-)
5 Amp fuse
RTS
LUGS
Install ferrite core
with at least one
RTS wire loop,
then close and
lock core.
