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The Solar Power Expert Blog

Choosing a Battery Disconnect Switch

by Chris - July 8th, 2011.
Filed under: Equipment. Tagged as: , , , , , , .


A common component that DIY solar builders often forget to include is a battery disconnect switch. These electrical switches are specially designed for switching large currents and are frequently used in 12 volt power systems such as boats, heavy duty trucks, and solar systems.

It’s a good idea to install these switches in even low-current 12 volt systems. They aren’t too expensive and they allow you to safely disconnect the batteries from the rest of the system. You will need to do this far more often then you think you will… trust me. Plus, even an inexpensive battery disconnect switch will probably handle as much current as you would ever want to generate – so you won’t have to buy another one if you scale your system up later.

Electrical Switches

A Hella Battery Disconnect Switch


Boat switches as well as automotive battery disconnect switches will often have additional features, such as a double pole double throw switch (DPDT) which gives it the ability to switch between multiple battery banks. Another common feature is the alternator field disconnect. Electrical generators and alternators can create large, damaging voltage spikes when rapidly disconnected, and the alternator field disconnect feature prevents this from happening.


12 Volt Switches

If you are connecting a single battery pack to your solar system, then you won’t need these features. However, it’s a good idea to know what they are and how they’re used. The main purpose of installing a battery disconnect switch is to isolate your battery bank from the rest of the circuitry, and for that purpose you just need a simple single pole single throw (SPST) switch like this Perko battery switch:



If you’re looking for the least expensive option possible, you might also consider this battery disconnect switch:



However, be sure it is mounted away from weather and any possible sources of corrosion such as salt water or coastal air. If the switch is going outside, will be serviced very infrequently, or you just want the highest quality disconnect switch possible, then you should seriously consider going with marine switches. Boat switches are designed for use in much harsher environments then a typical solar setup, and will be much more reliable as a consequence.




Further Considerations

When mounting the battery disconnect switch, you want to ensure that it is located as close as possible to the battery bank. This is good safety practice as it minimizes the length of cable that is ‘hot’ after the switch is opened.


Be sure to verify both the voltage rating and the continuous current rating before you buy a switch. For example, if you use a switch rated for 12 volts for a 120 volt system, then you risk arcing. Exceeding either the voltage rating or the continuous current rating of the battery disconnect switch invites a fire risk.

If you can’t immediately find these ratings, then don’t buy the switch! Not stating these two critical specifications is a dead give away that the manufacture is trying to scam people.






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