D250SE (1044 / 40-315)
CTEK D250SE 20A wiring diagram
20A house-battery charger accepting alternator and solar input; 25A maximum input and a 23V solar open-circuit ceiling. Supports 12V lead-acid or LFP house banks and a lead-acid starter battery. A common earth connection and separate smart-alternator/profile wires complete the installation.
- Nominal voltage
- 12 V
- Compatible systems
- 12 V
- Continuous draw
- 25 A
Wiring diagram
How the D250SE 20A is wired.
Open this example in Crimp to use your own parts and cable lengths. Review the assumptions and installation notes as you adapt the circuit.
The example connects starter and house batteries through the D250SE, with solar disconnected. Both battery-positive paths use the manual's 30 A protection. The 2 m output uses 10 mm²; the 3.048 m input also uses 10 mm², above the table's 6 mm² minimum for runs up to 5 m. Verify battery chemistry, control wiring and protection location before installation. Manufacturer installation manual
Edit this wiring exampleCable sizing for this example
Use the diagram as a starting point for your installation. Replace the example equipment and cable lengths with your own, and check the connections against your exact model's instructions.
2 m one way
12 V system
- Wire
- 10 mm² · 8 AWG
- Example fuse
- 30 A
- Design current
- 20 A
- Voltage drop
- 1.2%
Sized against ABYC E-11. The design current is the unit's rated output. The length is one way; voltage drop includes an equal-length return.
Run-length and manufacturer assumptions
The editable example uses 3.048 m (10 ft) runs unless a shorter run is stated. Metric overviews round these to 3 m. Adapt the parts and lengths to your installation before choosing cable and protection.
The example connects starter and house batteries through the D250SE, with solar disconnected. Both battery-positive paths use the manual's 30 A protection. The 2 m output uses 10 mm²; the 3.048 m input also uses 10 mm², above the table's 6 mm² minimum for runs up to 5 m. Verify battery chemistry, control wiring and protection location before installation. Manufacturer installation manual
Supporting parts
- 4 m total of 10 mm² (8 AWG) marine cable
- Terminations for 10 mm² cable, matched to the equipment's screw terminals, leads or studs
- 30 A fuse and holder on the positive run
- Heat-shrink, strain relief and terminal boots sized to the cable
Check stud sizes, fuse type, run length and how warm the space gets against the gear you actually have before you order.
Installation notes
Before you install.
Open a question for model-specific guidance and sources.
Verified against the manufacturer's documentation (rev. D250SE / SMARTPASS 120S UK English manual; models 1044 / 1058), checked 2026-10-01. Manufacturer documentation →
What does the D250SE do?
The CTEK D250SE charges a 12 V house battery from alternator power, solar, or both, with 20 A maximum output. Its 25 A input limit differs from that output rating. The starter battery must be lead-acid; the service bank can use a supported lead-acid or LFP profile.
Which terminals and fuses are required?
Connect alternator/start-battery positive to ALT +, panel positive to SOLAR +, service positive to SERVICE +, and returns to the common earth connection. The manual’s cable table specifies 30 A battery-side fuses and length-dependent conductors. The output table reaches 2 m using 10 mm²; route longer installations from a fresh conductor/drop assessment rather than extrapolating silently. Main M8 connections tighten to 8 Nm.
How are the control wires set?
The red smart-alternator lead needs the manual’s ignition connection when that mode is required. The black battery-select lead selects Normal, AGM or lithium behavior; follow the wiring combinations for the actual battery. Attach the supplied temperature sensor near the service battery’s positive post. These control/sensor wires are not represented as power terminals on the symbol.
What limits matter in use?
The solar array’s cold open-circuit voltage must stay below 23 V; panel watts alone cannot establish compatibility. Charging derates with temperature. Mount in ventilated surroundings and check source LEDs when output is absent. The unit lacks reverse-polarity protection and is not spark-free. A SMARTPASS adds a separate power path; it does not increase this converter’s own 20 A rating.
Technical reference
Specifications and terminal map.
Check the model, ratings and connection points against your own unit before choosing installation hardware.
Catalog specification
20A house-battery charger accepting alternator and solar input; 25A maximum input and a 23V solar open-circuit ceiling. Supports 12V lead-acid or LFP house banks and a lead-acid starter battery. A common earth connection and separate smart-alternator/profile wires complete the installation.
- Nominal voltage
- 12 V
- Compatible systems
- 12 V
- Continuous draw
- 25 A
- Rated output
- 20 A
- PV open-circuit limit
- 23 V
Terminal map
Catalog connection points, current direction and planning limits. Check the manufacturer's pinout and connector ratings before installation.
ALT +
positive · input
SOLAR +
positive · input
SERVICE +
positive · output
EARTH −
negative · bidi
Sources and sizing method
Equipment specifications. Model details, ratings and terminal roles come from the catalog. Check your unit’s label and current manufacturer documentation for its exact revision.
Example sizing. Sized against ABYC E-11. Boats built to UK or EU rules answer to ISO 13297 — check these figures against it before you build. Results use the example current and run length shown above. Set your actual equipment, lengths and installation conditions in Crimp before choosing hardware.
Read how the sizing worksNext step
Plan your installation with D250SE 20A.
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