How do battery switches work on a sailing or motor boat?
Master switches enable power to be systematically managed on board. Their primary purpose is to connect or disconnect batteries to the on-board power supply. Depending on the type of switch and their installation, they allow two or more batteries to be used for a particular purpose. During periods when the boat or vehicle is not in use, they prevent batteries from discharging. When the alternator is in operation or in combination with other charging sources, these switches are used to distribute charging current selectively. In an emergency, they make it easier to connect batteries in parallel to provide assistance for starting or they can be used to quickly and easily disconnect the entire on-board network.
Battery switches with a removable key prevent the accidental use of the electrical circuit.
What installation regulations are there for battery switches on sports boats?
Boats and vehicles may be equipped with one or more batteries whose primary function is for starting the engine and supplying power to electrical consumers such as lights, pumps, electronics, etc. However, the electrical energy stored in these batteries poses a certain operating risk. For this reason, we recommend using switches with sufficient strength to disconnect batteries from the on-board power supply.
It is also permitted to use remotely controlled disconnecting relays, but these must also be manually operatable on the relay itself.
According to DIN EN ISO 10133, a master switch must be installed in the positive wire of a vehicle electrical system with negative grounding (engine block/alternator housing = battery negative = ground).
Boats with metal hulls and special vehicles are often fitted with fully insulated two-wire DC on-board power systems. For these, a two-pole dual-circuit switch must be installed in the positive AND negative wires.
Fig. 1 Battery master switch with ON / OFF switching function
Source: BLUE SEA SYSTEMS
Are there exceptions where no switch is required?
Fig. 2 Two-pole battery master switch with switching function 2x ON / OFF
Source: BLUE SEA SYSTEMS
Yes, there are:
- Vessels powered by outboard motors and fitted only with circuits for starting the engine and for navigation lights do not require a circuit breaker.
- Similarly, unmonitored electronic equipment and electrical devices, whose connecting lines are individually protected by a fuse or circuit breaker, if possible directly at the battery terminal.
- This includes charging equipment such as land power chargers, solar or wind generators,
- electronic devices with voltage-supported memory function or alarm functions
- electrical equipment, such as bilge pumps or engine room ventilators.
What is the difference between a battery selector switch and a battery master switch?
Some boats and vehicles have two or more batteries that can be used alternately or simultaneously on board.
A battery selector switch is used to direct specific drain and charge currents to and from different batteries, to connect batteries to each other or to disconnect both batteries from the on-board power supply at the same time. You could also achieve this by using two or three individual switches, but this would require much more in terms of materials and installation effort. Battery selector switches have three positions, one for battery 1, one for battery 2 and a common position from which it is possible to switch off from battery 1 or battery 2 or on to both batteries (1+2).
Fig. 3 Battery selector switch with switching function 1/2/1 + 2 / OFF
Source: BLUE SEA SYSTEMS
What is a 2-pole dual circuit battery switch?
A dual circuit switch consists of two individual, galvanically separated ON/OFF switch contacts, arranged in a single switch housing. When the switch is operated, the two switching contacts are closed or opened simultaneously. These switches are used ...
- ... in two-wire vehicle electrical systems where simultaneous disconnection of the positive (PLUS) and negative (MINUS) battery/vehicle electrical connections is required.
- ... on small boats with little space to switch starter and consumer batteries on and off at the same time.
The dual circuit switches are also available as a special version with an additional switch position "Connect/Combine". In this position, the two galvanically isolated circuits within the switch are purposely connected to each other, for example for starting assistance.
If an additional battery is intended to be installed on small boats fitted with only one battery, this special type of dual-circuit switch is often used together with a charging relay. Both components (switch and charging relay) are available as "Add A Battery"
CAUTION, this special type of switch with the "Connect/Combine" position must NOT be used in two-wire vehicle electrical systems!
Why not? Remember, installation regulations require a two-pole, dual-circuit switch that disconnects the battery positive and negative simultaneously from the vehicle electrical system. In the switch position "Connect/Combine" the battery will be short-circuited.
What do the manufacturer's specifications mean?
The information is virtually self-explanatory.
- Continuous load capaciity with continuously flowing power.
- Short-term load capacity for a few minutes.
- Short-term load capacity for a few seconds, for example when starting the engine.
- Maximum permitable voltage
Attention!!! This is not the nominal voltage of the vehicle electrical system, but the maximum permissible voltage! According to DIN EN ISO 10133 the voltage measured at the battery poles may be 75% to 133% of the nominal voltage.
- In 12 V electrical system max. 16 V
- In 24 V electrical system max. 32 V
- In 36 V electrical system max. 48 V
- In 48 V electrical system max. 64 V
What load capacity must a switch have?
The continuous load capacity of a battery switch must at least correspond to the nominal current of the battery main fuse.
A switch for the motor starter circuit should be chosen such that its capacity is approximately equal to the nominal current of the starter.
The starter is part of the motor. Normally, there is no information on the current or power consumption of the starter in the motor documents. This data must therefore be obtained from the motor manufacturer or, if necessary, the motor type can be obtained from spare parts suppliers.
The power specifications of the starter are often given in kilowatts (kW). Dividing this value by the nominal voltage gives the nominal current. Example: a 12 volt 1.5 kW starter
=> 1.5 kW = 1500 W (Watt is Volt x Ampere) => 1500 V x A / 12 V = 125 A
Where must the switch be installed?
The switch should be installed as close to the battery as possible in an easily accessible location where it can be operated quickly and safely without the use of tools.
What should be considered when installing?
Depending on the housing design, it can be mounted behind, in or on top of the surface.
Fig. 4 Installation options, left = surface mounted, middle = recessed , right = behind
Source: BLUE SEA SYSTEMS
The switch is located in the main feed line between battery and on-board network. High currents flow here. As a result, the connecting cables will have large cable cross-sections and be fitted with robust ring cable lugs. In our experience, connecting cables are somewhat "bulky". They should therefore be arranged and fixed so that no strong pulling or pushing can occur at the terminals of the switch.
- All connection cables at the main switch must be fitted with suitable cable lugs. No bare wires may be attached to screw connections.
- Exposed connection terminals must be protected against accidental short circuits by suitable measures.
Avoid turning off a battery switch while the engine is running.
Switching off breaks the connection between the running alternator and the battery. Without a battery, the alternator produces a heavily pulsating, high DC voltage or voltage spikes which can cause the diodes on the rectifier plate of the alternator to blow. Using an alternator protection switch
can prevent the risk of alternator failure. Installation is easy as only 2 "thin" wires and a 5 A fuse need to be connected to the alternator terminals B+ and B-.
For alternators with a separate regulator, circuit breakers with integrated breaker contact can be used for alternator field disconnect. Installation requires specialist expertise! BLUE SEA SYSTEM can provide appropriate switches with "Alternator Field Disconnect" on request.