A kit for the standard 20 Amp SCC3 circuit board used in this project is available from CirKits.
(C) 2012, G. Forrest Cook
This article describes modifications that can be made to the 12V, 20 Amp SCC3 solar charge controller to allow operation at up to 60 Amps. Similar modifications can be performed to achieve a maximum operating current of 40 Amps (2 sets of IRF4905 MOSFETs and 20L15T diodes) or 80 Amps (4 sets of IRF4905 MOSFETs and 20L15T diodes). This article describes the modification for 60 Amps.
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.
A few minor changes need to be made to the SCC3-e1 circuit board. Note that the board in the photo is an earlier SCC3-c1 board that has been modified to the SCC3-e1 schematic. The charge current switch components Q2 and D1 need to be removed from the SCC3 board, if you are starting with an assembled SCC3, it is best to just unscrew both of these parts and clip the leads off where they meet the board.
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.
Three pairs of IRF4905 MOSFET transistors and 20L15T Schottky diodes need to be installed on a large aluminum heat sink. These parts should be insulated from the aluminum heat sink by using thermally conductive insulators (amber rectangles in the photo) and plastic shoulder washers. Heat sink grease should be used on both sides of the insulators to insure good heat transfer. Before wiring these parts, check the electrical continuity between the screw tabs and the heat sink and fix any shorts.
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 gauge) to handle the high currents. The three MOSFET gate terminals should be tied together and run back to the original Q2 gate pin (pin 1, blue wire) 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.
External connections should be made to the heavy-duty terminal block. If you cannot locate a sufficient terminal block, it is possible to build your own using a piece of insulating material such as phenolic block and 1/4-20 threaded bolts. Connections to the bolts should be made with heavy-duty circular crimp lugs, the bolts should be sufficiently tightened and inspected for heating during operation. Soldering the wires into the lugs after crimping is recommended. The insulating block should be mounted to the enclosure with spacers to ensure electrical isolation and physical strength.
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.
The IRF4905 MOSFET transistors and 20L15T diodes are available from Digi-Key and Mouser Electronics.
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