DC Metresby Kurt Küpper*




12 Volts DC or 24 Volts DC?

Remember when it was quite normal for a house to have three bedrooms, one bathroom, one lounge room and a kitchen? You ate either in the lounge room or in the kitchen. Take a look around you today or read the property classifieds. You would struggle to sell a house like that, because nowadays one needs four bedrooms, a second en-suite bathroom, separate dining room, second living area in the kitchen, rumpus room, media room and a study, apart from at least a double garage!
Just like our houses, boats seem to be getting ever larger. A boat that was large twenty years ago is nowadays of average size. And with the increase in size come the power hungry trappings, such as large anchor winches and thrusters and refrigeration.
Given that we have always powered such loads off our 12Vdc systems, we get to task and size the cable using our voltage drop tables. Apart from the current strength (Amps), the voltage drop is proportional to the distance of the cable run. Remember that the current does not just have to travel from the batteries near the stern all the way forward to the anchor winch, but also back again in order to complete the electrical circuit.
So we discover to our horror that the larger loads and longer distances mean that in order to limit the voltage drop to an acceptable level, we have to run cables of 70mm² up to above 100mm². Apart from costing an absolute fortune, that adds up to a huge weight at a time when we are trying to keep our boats as light as possible.
Fortunately there is a ready solution at hand, namely to change to a 24Vdc system. Since electrical power is the product of the voltage and current, by doubling the voltage, we reduce the current that we need to carry by half. This in turn means that we can reduce the cable size back down to a size that won’t cause the boat to list to one side.
So all is well, or so we thought, until we set out to buy all the 24Vdc components required for the boat. As many components used on boats are primarily used in the automotive industry, economies of scale have driven down prices of 12V components such as switches or relays to lower levels than the equivalent 24V items can be purchased for. But in the overall scheme of things, this on-cost is more than offset by the lower cabling and running costs.
However, 24V versions of most electronic equipment such as radio transceivers, chart plotters, navigation instruments and even some engine monitoring instruments are well nigh impossible to find. So the option is to stick with 12V and live with the cost and weight penalties this brings with it, or run dual 12V and 24V systems on the boat.
Dual systems can be achieved by running completely separate systems, i.e. loads, batteries and charging systems (alternators, battery chargers etc.). But duplicating everything is expensive and not really necessary.
Years ago in the USA, when many fishing boats had 32V systems, they used to count off six of the 2V cells and drive a nail into the sixth battery terminal to give a 12V supply, which may be where the term tapping off 12V came from. This is a strict no-no!
Centre tapping a 24V battery will result in the two halves of the 24V battery being charged and discharged unequally, and very soon premature battery failure or even catastrophic thermal runaway.
The better way of using a 24V battery to feed 12V loads is to use DC-DC converters such as voltage converters (reducers) or battery equalisers.
These are electronic devices that enable us to use 24V batteries to power 12V equipment. Voltage converters draw 24Vdc off a 24V battery and supply 12Vdc. Equalisers allow the otherwise unthinkable, i.e. centre tapping a 24V battery to draw 12V directly off one of the two 12V batteries that have been wired in parallel.
Next month we will discuss the differences between these devices as well as their relative advantages.


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