CHARGING AHEAD!

How to ensure your caravan battery gets fully charged while travelling

WORDS BY JOSEPH WOERKOM, PHOTOGRAPHY BY CARAVAN AND MOTORHOME ON TOUR

A couple of issues back I talked about the heavy cable needed to get the caravan battery charged from the tow vehicle’s alternator. The reason for the heavy cable is mostly to account for the extent of voltage drop along the long cable run between the alternator and the battery.

The problem is, even when a suitably heavy gauge cable is used, it is impossible to get the caravan battery fully charged. In fact, it’s impossible to get the starting battery in the tow vehicle fully charged with a standard alternator setup. The best you can hope for is around 80%. Starter batteries can cope with this charging regime as they are only used for a brief time during starting and then the alternator takes over while the engine is running. However, as most people know, starter batteries only last for about three years. One of the reasons for their relatively short life is the fact that they never get fully charged. If they were fully charged on a regular basis they should last a lot longer.

Anyway, to get the best performance and longest life from your caravan batteries they need to be regularly fully charged and for that you need a multi-stage battery charger. These devices vary the voltage at the battery according to how much current the battery is accepting. Sometimes this voltage is well above the voltage produced by the alternator (around 14.8V to 15.2V). Since the battery would be damaged if it was continuously subjected to this voltage, the charger reduces the voltage when it detects the battery current has fallen to a low enough level.

Given the above it’s easy to see that the caravan battery has no hope of being fully charged by the tow vehicle’s alternator alone as the voltage ‘seen’ by the battery always has to be less than the alternator voltage. Since mains power isn’t available when on the move, what’s needed is a multi-stage battery charger that runs from the nominal 12V DC power from the tow vehicle’s alternator. Such a device is known as a DC/DC charger.

DC/DC chargers for automotive or RV use are designed to accept a wide input voltage range to cater for the voltage drop on long cable runs. Many provide a control input that is linked to the tow vehicle’s ignition circuit and will only charge the battery when the ignition is ‘on’. Others protect the starter battery by constantly monitoring the input voltage and will only turn on after the input voltage rises above a specified threshold e.g. 13.5V, and will turn off again if the voltage falls below another threshold e.g. 12.6V under no load.

Some DC/DC chargers also accept DC input from solar panels and will use alternator power and/or solar power to charge the batteries, according to what’s currently available. This can be quite handy for solar equipped caravans.