By Mike Sokol
12-volt DC electrical systems
How to install a disconnect switch for 1-, 2- and 4-battery systems
I’m a paying member of RVtravel.com because I appreciate the hard and honest work you and Chuck do. Thank you. In your newsletter #20 you talked about disconnect switches and said you would do an extended article on the subject in a few weeks. Not sure if you have or I missed a newsletter, but I’m interested in a wiring diagram of two 6-volt batteries in series and connecting a disconnect switch. I have a 2-post on-off switch. Do I disconnect the negative cable that goes from my trailer frame to my battery and connect it to one post on the switch, then take another cable, connect it to the 2nd post and run this cable to the negative post on the battery? —Mark Eddy
Ah, yes. The series/parallel battery question about where to place the disconnect switch. But before we get into the details, let me first give you all a few precautions when working around 12-volt DC power, especially when you’re moving battery terminals around.
While 120-volts AC is primarily thought of as a shock hazard due to its ability to punch through your epidermis (top skin) layer and interfere with your heart rhythm, 12 volts generally doesn’t pose such a threat. Its danger lies in the amount of current available under a direct short condition. Dropping a wrench across the battery terminals or getting your wedding ring trapped between anything contacting the positive battery terminal and the chassis will result in many hundreds of amps flowing through that connection.
Think about it a bit. In a 120-volt AC system you have a 20-, 30- or 50-ampere circuit breaker (and a long run of wire back to the service panel) that limits the amount of short-circuit fault current. So get a screwdriver across the hot and neutral of a 120-volt AC wire and while you’ll probably blow a chunk of metal of your screwdriver, the circuit breaker will trip within a few line-cycles (a fraction of a second) and the wiring won’t vaporize in front of you.
When you’re working around your house batteries, there’s no such current limiting device. So the battery (or batteries) will deliver all of those CCA (Cold Cranking Amps) you paid so much for, and it can reach thousands of amperes in a dead short. That’s enough current to weld a wrench to the chassis, turn your wedding ring cherry red while it’s still on your finger, and vaporize any small wires that get in the way of the current.
That creates something called an arc-flash, basically a ball of super-heated copper plasma that’s hotter than the surface of the sun coming at you faster than the speed of sound. I won’t post pictures of any arc-flash injuries and autopsies here because they’re really gruesome, but just know that you don’t want to be anywhere near one. And certainly when working around any kind of electrical system, either 12 or 120 volts, you want to be wearing safety glasses and remove any jewelry. So, safety first, all right?
And whenever you’re disconnecting batteries with a wrench, always remove the negative terminal first and put it back on last. That way you isolate your batteries from the chassis ground of the RV, and if you do accidentally get your wrench between the battery terminal and the RV chassis or frame, you won’t have a spectacular (and dangerous) meltdown/arc-flash.
Now to your question about where to place a disconnect switch for your batteries. Let’s review the four types of battery banks typical in RV house systems: single 12 volt, double 12-volt, double 6-volt, and four series-parallel 6-volt. Yes, they all add up to 12 volts, but in different ways.
Single 12-volt battery
For a single 12-volt battery placing a disconnect switch is usually pretty simple. Typically the safest position is on the negative terminal wire between the battery and where the negative black wire connects to the frame/chassis of the RV. Yes, it can be on the positive terminal as well, but usually there’s a fuse block of some sort there which can make placement challenging.
Two 6-volt batteries in series
Next let’s examine a pair of 6-volt batteries in series. As you can see, there’s a jumper wire between the positive and negative terminals on the battery pair. This essentially stacks up the batteries just like in your flashlight. In this case, 6 volts plus 6 volts will add up to a nominal 12 volts. And while you could place your disconnect switch in any of these wires and it would work, best practice is to place it on the negative terminal of the battery that connected to your RV chassis. In the graphic that’s the lower right wire as it leaves the battery.
Two 12-volt batteries in parallel
A pair of 12-volt batteries in parallel hooks up a little differently since there are two sets of jumper wires between the batteries. In this case the voltage doesn’t increase, but the available current does add up. You treat this electrically like one big battery, and you can place your disconnect switch on either the outgoing negative (black) or positive (red) battery terminals. But once again, best practice is to place your disconnect on the negative battery lead that’s connecting to your RV’s chassis. In the graphic that would be the lower left wire as it leaves the battery.
Four 6-volt batteries in series/parallel
The four-battery series/parallel setup is a little more complicated, so be sure to take a picture of how everything is hooked up BEFORE you disconnect anything. The trick to understanding this is to treat it like a pair of 12-volt batteries in parallel that just happen to be made up from 6-volt batteries in series. In the graphic that would be the lower left terminal of the battery.
To wrap it up, usually the best place to install a battery disconnect switch is at the negative battery terminal where it connects to the chassis. If the geometry of the situation forces you to put it on the positive terminal, that works just as well. Just be careful when making ANY connections to house batteries.
OK everyone. I hope this helps you understand how house batteries are hooked up. See you next week. In the meantime, let’s play safe out there.
Email me at mike (at) noshockzone.org with your questions.
Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.