Wiring: INPUT+ to BAT+, INPUT- to BAT-, OUTPUT+ to LOAD+, OUTPUT- to LOAD-
Reminder: Never reverse the polarity. Use the enough big cable to avoid the voltage drop. Never connect to solar panel or wind turbine. Never charge the battery.

Daygreen P/N: B40-24-12A
Efficiency: 95%
Input Voltage: 21-35VDC
Output Voltage: 12VDC
Output Current: 40A
Ripple: 200mVp
Line Regulation: ±0.2%
Load Regulation: ±0.2%
Voltage Accuracy: ±1.5%
Enclosures: IP68
Dimension: 100*80*40mm
Net Weight: 0.5kg
Case Operating Temperature: -40~85°C
Certificates: CE Rohs
Warranty: 1 year1
MTBF: 100,000 hours

Application:
24V DC battery of vehicles as input power to 12VDC appliances as output load.

Questions
How to choose the correct current for loads?
For example, if the max current of your load device is 10A, then choose 10A/0.8=12.5A at least for resistive loads according to international standard.
If the load device is motor or compressor, then you may choose 10A/0.5=20A according to real testing result since the motor and compressor starting current may be 2-3 times than nominal current.