Ohm’s Law, Cable Connectors and Cable Sets
According to Ohm's Law, the power dissipation of a cable is proportional to its electrical resistance and is related to the square of the electricity flowing through it. Expressed more simply, double the current results in four times the loss, and ten times the current in 100 times the loss.
Boats often use low voltage (< 60 volts) for safety reasons. This is why, depending on the battery configuration, peak currents of 80 to 100 amperes flow throw electrical drives with an input power of 2,000 to 4,000 watts.
In comparison – a 1,100 watt power drill draws 5 A from a 220 V power socket. If an electric drive were run using common household cabling, there would be approximately 15% power dissipation from the resistance in the cables alone. In order to keep losses low and avoid the risk of local overheating, manufacturers must cut cable resistance to a minimum. There are two levers that can be used: minimise the length of cable between batteries and motor and select a cable with a suitable cross-sectional area.
Due to the squared dependence of cable power dissipation, it is generally advisable to use higher voltages, i.e. more batteries connected in series, with higher input power. The power drawn from the battery is calculated from battery voltage x battery current. A motor with 2,000 W input power operated with 2 serially connected 12 V batteries would therefore draw 2,000 W / 24 V = 83 A of peak power.
With a 4,000 W motor this would be 166 A, meaning four times the amount of cable dissipation. You would have to invest in significantly thicker cables in order to keep cable dissipation as low as possible. Operating a 4,000 W motor with 4 serially connected lead batteries of 12 V each, i.e. 48 V, would lead to peak current of 4,000 W / 48 V = 83 A. With this layout of the battery bank it is possible to use the same set of cables as for the 2,000 W motor.
This is the reason why our motors are optimised for higher performance and also for higher voltages.
The cable connections of a Cruise 2.0 outboard must cope with peak currents of over 80 A (2,000 W at 24 V). For this reason, Torqeedo designed the cable connections and sets with a cross-sectional area of 25-35 mm². With a cable length of five metres between motor and battery, there will be power dissipation of approximately 17 W. In the case of the Cruise 2.0 this corresponds to a loss of 0.8 percent of total power and 3.4 W per metre of cable. Minimising power dissipation in this way ensures higher efficiency of the overall system and provides added safety since the risk of local overheating rises with higher losses.