This information pertains to the SCC3-e1 kit (current production version).
These modifications should be performed by someone with a fair level of electronics skills, they involve tracing signals on a circuit board, detailed soldering work and mechanical work. The high currents involved in this project can cause risk of burns and fire hazards. Protective over-current devices (switches and fuses or circuit breakers) should always be used on the PV and battery wiring.
This circuit has been field-tested for several years in an off-grid house, it will work reliably if it is built correctly.
Resistors R24 and R25 on the SCC3-e1 board should be changed to new values. R24 should be 6.8K and R25 should be 3.3K. These two resistors work in conjuction with the capacitance of the MOSFET gates to low-pass the gate drive signal just enough that radio frequency emissions are not created in the high current signal lines when they switch on and off.
If you build this modification for 40 amps, R24 should be 10K and R25 should be 4.7K. If you build this modification for 80 amps, R24 should be 4.7K and R25 should be 2.2K.
When running the modified SCC3 at higher currents, it is important to mount the SCC3 thermistor in thermal contact with the battery. This wiring is is visible in the photo as a red/black twisted pair. A 2 pin connector and matching header were used to allow easy removal of the thermistor wires.
Note that the transistor and diode mounting screws are electrically live, it is a good idea to cover them with an insulated block on their back side to prevent the possibility of a short-circuit should they come in contact with metal objects. Another approach is to mount the heat sink inside of a larger metal box, be sure to allow sufficient air flow through the box to dissipate the heat.
The three MOSFET/Schottky diode pairs should have equal wire lengths to insure equal current distribution. Note the arrangement in the photo with three diodes, three MOSFETs and equal length red wires. The wiring should be done with heavy wire (12 to 18 gauge) to handle the high currents. The three MOSFET gate terminals should be tied together and run back to the original Q2 gate pin on the SCC3 board. A heavy-duty terminal block should be used for the external PV and battery connections. These should be connected back to the original SCC3 four-pin connector, light gauge wire is acceptable here since the current to the SCC3 board is now low.
The 60 amp fuse in the schematic is not shown in the photo, it has been replaced by a 60 amp DC-rated circuit breaker. Another fuse/switch or circuit breaker should be installed on the PV + line. If you decide to use fuse and switch combinations instead of circuit breakers, the fuse should be a DC-rated class T type.
The wiring from the PV array to the charge controller and from the charge controller to the battery should be done with 6 gauge or heavier stranded wire, for handling the full 60 amps of current. If you will be wiring a system with 45 amps or less, 8 gauge wire may be used. It is recommended that all of the high current wiring be installed in metal conduit for protection from fire hazards.
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