Battery Switches
Shop Battery Switches
Battery switches are used to disconnect and connect a boat’s batteries to the rest of the electrical system. Battery switches are connected to large cables which carry the large amounts of electrical current that boat batteries produce.
Why Battery Switches Are Important
There are several reasons why battery switches are important.
- Most boats have more than one battery, so a battery switch allows you to select which battery (or both) receive charge current, and which battery runs the loads onboard.
- Batteries contain lots of energy, which can cause and sustain a fire onboard if a problem develops. A battery switch allows you to disconnect the batteries from all other circuits.
- Loads like windlasses, engine starters, and inverters use lots of current, and standard marine switches would melt if subjected that much current flow.
The Two Main Varieties of Battery Switches
Battery switches come in two main varieties: single and dual battery. Single battery switches have two positions: OFF and ON. Dual battery switches have four positions: OFF, 1, BOTH, and 2. This type of switch is popular, since most boats have two identical general-purpose batteries, and the switch allows you to select any combination including having both batteries connected at the same time. For somewhat complicated reasons, we think the ideal setup is to have three ON-OFF switches in most instances, because it allows an engine battery to be used specifically with the engine, and a house battery (or bank) to be used for all the house loads, without interconnecting them. However, in practice, the majority of boats will have a dual battery selector switch.
Battery Switch Ratings
Amperage ratings are generally given for continuous and intermittent use. Continuous ratings mean that a load like an inverter, which may run for an hour at a time, can be powered through the switch at the stated amperage. Most battery switch ratings range from 100A to 450A continuous. The intermittent rating is based on loads that operate for five minutes.
The Importance of an Alternator Field Disconnect
It is much easier to make a switch which is intended to be switched with no current flowing than one which is designed to be switched under load. It is the arcing which occurs when a switch is changed under large loads that tears up the terminals inside the switch. If you are going to switch a battery switch from position 1 to position 2 to start your engine, do it with the ignition key in the off position.
To protect your boats alternator, a feature to look for in battery switches is an alternator field disconnect. This is a second set of terminals that disconnects your alternator’s field when the battery switch is turned off. Without this feature (and assuming no Combiners or Isolators are being used) it is likely that turning OFF your battery switch with the engine running will damage your alternator’s diodes.