DC-DC battery chargers & smart alternators

12v charging systems: Introduction

My boat has an old diesel engine from the early 90’s. It is dead simple, reliable and above all easy to work on!  The simplicity in its design includes the electrical system and battery charger.  As with many older cars, the boat engine’s alternator incorporates a voltage regulator which ensures a constant steady output voltage regardless of the engine speed.

My boats leisure battery is connected via a voltage sensitive relay to the starter battery. Consequently, whenever the engine is running the voltage increases causing the relay to energise.  Both batteries are now electrically linked to the alternator and therefore charging. Simple!

Unfortunately things are slightly more complicated with my van.   In the ever important drive for vehicles to become more efficient, manufactures have had to devise new methods to extract more and more power from each litre of fuel.

One method of achieving this is to vary the load on the engine caused by the alternator.  The result is a varying output from the alternator meaning a voltage sensitive relay cannot be used.

Smart Alternator

If I were to switch on the headlights, heated rear window and heater blower at the same time and whilst the vehicle is stationary, I would notice a change in the engines note.  This change represents a greater load upon the engine, and consequently it will be burning more fuel.

Many modern vehicles are fitted with a smart alternator which constantly varies the load on the engine.  When driving down a hill for example, the alternator will operate under maximum load to bulk charge the battery. Conversely when driving up a hill or accelerating, the alternator’s load will reduce to allow maximum power to the wheels.

The constantly changing voltage means that a conventional voltage sensitive relay can switch on and off throughout a journey.  Consequently, whilst the leisure battery is likely to charge, it will not do so rapidly.  This is where a DC – DC battery charger comes in!

DC-DC battery charger

A DC – DC battery charger will take a wide input voltage, such as that given by a smart alternator.  It will monitor the target battery and ensure that it is charging at its optimum rate throughout the journey.  In addition, the charger will shut off when the input voltage drops below a set level to protect the starter battery from discharging when the engine is switched off.

A number of DC-DC battery chargers incorporate secondary inputs to enable a solar panel to be incorporated into the system.  A good example of this type of charger is the Ring RSCDC30 that I have installed in my motorhome.  Click on this link to read my review!


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