How do 12V marine fridges work and why do they drain batteries?
My older sailing yacht has a top-loading fridge built into the galley worktop, but I am not entirely sure how it operates compared to a domestic one. It seems to drain my batteries quite fast. How do these marine fridges actually work, and what should I check to make it more efficient?
Answer
Marine fridges use a 12V or 24V direct current compressor to pump refrigerant through a closed loop, absorbing heat from the insulated box and expelling it into the cabin or water. Most units rely on Danfoss (now Secop) BD35F or BD50F compressors, which handle constant motion and run directly off the domestic battery bank.
The Refrigeration Cycle
A boat fridge does not actually make things cold. It removes heat from the insulated box using three main components and a refrigerant gas, almost always R134a.
- The Evaporator: This aluminium plate sits inside the fridge box. Liquid refrigerant expands into a gas here, dropping rapidly in temperature and absorbing heat from food and drinks.
- The Compressor: Usually tucked away in a locker, this pump sucks warmed gas from the evaporator and pressurises it. This action significantly raises the gas temperature.
- The Condenser: The hot, pressurised gas passes through a radiator coil. As it cools, it condenses back into a liquid, shedding the absorbed heat.
Air and Water Cooling
The condenser needs a cooling medium to shed heat. This choice dictates fridge efficiency in warmer climates.
Air cooling is common and easy to install. Standard Dometic, Isotherm and Vitrifrigo units use a small fan to blow ambient cabin air over the condenser coil. These work perfectly well in the UK and Northern Europe, provided the compressor locker has adequate ventilation. Trap a compressor in a sealed cockpit locker and it will bake in its own heat, running continuously.
Water cooling suits the Mediterranean or Tropics. Water transfers heat far better than air. Pumped systems use a small 12V pump to circulate seawater through the condenser. This adds moving parts and a strainer to clean. Keel cooling uses a sintered bronze plate mounted through the hull below the waterline. Refrigerant runs directly through this plate, transferring heat to the ocean silently with no pumps required.
Power Consumption and Duty Cycle
A Secop BD35F compressor draws 3.5 to 4.5 amps when running at 12V. It does not run constantly. The thermostat switches it on and off, creating a duty cycle.