Boat ElectricsDC Converter and Equaliser 

by Kurt Küpper*



DC Converters and Equalisers

DC to DC converters are used in power systems for a range of purposes. These include lowering or raising the voltage, filtering and stabilising the voltage and charging auxiliary batteries.
Probably the most common application is on boats with 24V systems, where there is a requirement to power some 12V equipment. Nowadays both converters and equalisers utilise high frequency switching technology as is also used in battery chargers, DC to AC inverters etc.
Converters draw power from a 24V battery bank and typically have a fixed voltage output of 12.5V or 13.6V. The output voltage is regulated, so it will remain constant regardless of the input voltage. The converter is wired in series with the load, i.e. all the power passes through the converter and then flows to the load. Good quality converters will also remove any spikes and electrical ‘noise’ from the power supplied, so they are very good for supplying sensitive electronic equipment and halogen lamps, etc.
Because the full power passes through the converter, care must be taken to ensure that its current carrying capacity is high enough for the requirement. Typically they are rated between about 5A and 40A. Some products can be parallelled with a second unit to allow higher loads to be drawn. Some units that do not have a common negative between the 12V and 24V circuits (isolated converters) can be used for charging 12V batteries, although this is not ideal, as the charging voltage is fixed to under 14V, and the 12V battery will therefore never be fully charged, leading to shorter battery life.
An equaliser is used where a 24V battery has been centre-tapped to draw 12V off the lower half of the battery cells. It is wired across both the full 24V bank as well as the 12V half. Note that the nominal 12V load is drawn directly off the battery, not through the equaliser.
In contrast to a converter, an equaliser is designed to put out half its input voltage. So if the overall input drops to 23.8V, the output voltage will be 11.9V. The equaliser senses the imbalance in the two halves of the 24V battery caused by the 12V loads drawing power off the one half. It draws power from the more charged half and feeds it to the less charged half until the voltages in both halves is equalised.
The major advantage of using an equaliser is that the loads are not drawn through the unit, but directly off the battery. If there are significant peak loads on the 12V circuit, a converter would have had to be rated high enough for the high current to pass through it. But the battery can easily supply the peak loads, and a relatively small (i.e. cheaper) equaliser can take its time to afterwards restore charge equilibrium. It is even possible to start an engine with a 12V starter (e.g. for a genset).
The major disadvantage of using an equaliser is that all the 12V loads have to be run back to the battery bank. This partly negates the purpose of having a 24V system, i.e. to reduce the cost and weight of cables. If a converter is used, this can be mounted remotely from the battery, close to the load(s), keeping the 12V wiring to a minimum. If the loads are located in different areas, consideration may be given to fitting several smaller units in different locations close to those loads rather than one larger unit.
If used extensively over time, using an equaliser will mean that one half of the battery will be cycled more than the other, which could shorten the lifetime of both halves of the battery.
If a converter fails, the 12V circuits will immediately be inoperative. This is an important consideration, as safety critical equipment such as navigation instruments and radios are often powered by 12V. If an equaliser fails, the systems will continue to work, even if the charge imbalance between the two halves of the battery will progressively increase. As long as the problem is rectified fairly soon, this should not be a problem. Beware, though, that while converter failure will immediately become apparent, equaliser failure can remain undetected until a serious charge imbalance between the two halves of the 24V battery has resulted.


*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455