Mod #6: Battery Cutoff Switch

Mod #6: Battery Cutoff Switch

Submitted on: 06/10/08

     Category: electrical
Mod Rating: 12345

(64 ratings)

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Mod Description:

Ever wonder why you take your RV out of storage after a few weeks and find that the batteries are deader than a door nail? You went through your pre-storage checklist and made sure all breakers and lights were off but your batteries are still DOA. You even plugged in the RV and charged the battery for a few days to make sure it was topped up before storage. What’s going on?

Mod Difficulty:

Almost every RV has some sort of parasitic current draw on the 12-volt electrical system. The most likely offenders are the LP gas detector, the refrigerator, and the radio. Even an inverter, although switched off, can have a small current draw. Over time, these seemingly harmless draws can eventually bring even the largest of battery banks to their capacity death. A simple device called a battery cut off switch will prevent this from happening.

Battery switches come in a variety of sizes and functions. There is the simple cut off switch that goes in between the positive connection to the battery and the battery itself. One type has a removable key that is turned to break the connection and you can take the key with you. This prevents someone from maliciously switching back on your power when say your RV is in storage.

Other types of switches feature not only a power disconnection from the battery, but also switching between battery banks. These types will make the connection to the battery you are switching to first before breaking the battery connection you are switching from. This ensures that any 12-volt devices that are being powered do not suffer from a momentary power loss. This type of switch, while still straightforward to install, requires a bit more planning to ensure you get the wiring correct. See the example image for suggested wiring configuration using multiple switches.

A battery switch can also be used to disconnect power from an inverter DC connection. Larger inverters are often wired with separate cabling from the battery to the inverter. Even when the inverter is switched off, there can be a small current drain. Placing a switch between the inverter and the battery will prevent this from happening.

When installing a cut off switch, it is generally recommended to place it in the positive side of the DC circuit. There is some debate however on whether to install it on the negative side. There are valid arguments for each way. The bottom line is that you can do it in either the positive or the negative side. It’s your decision.

ModMyRV recommends these parts for this mod:
Battery Disconnect Switch with Key
Multiple Battery Disconnect Switch

Battery Cables
Battery/Inverter Cables
Battery Cables

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16 Comments For This Mod

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  1. jmt

    I did this one and so far love it. I was able to hit the cutoff switch under the over so kids can get at it, just before the inverter. I just need to remember to use it since it is in a hidden spot. Totally cuts off power and now I have no battery drain.

  2. Bob Vaughn

    I have this mod on my TT and thought it was such a good idea that I put one on my tow vehicle….I don’t drive my truck that often since I retired and something kept running the battery down….I went to the auto parts store and bought a knife switch to isolate the battery……It also keeps anybody from stealing my truck…..

  3. jmt

    Anyone else with experience with this mod?

  4. Bob Vaughn

    My switch is on the inside of the A frame….I have to remove the cover that covers my propane tanks and batteries…..It is protected from the elements but would be more convenient on the outside….
    http://community.webshots.com/album/570450720QTvKTP

  5. Robert

    Risks involved with battery disconnect devices:
    1. When power is interupted to the electronic engine control unit (computer), all of the information it has programmed is lost and the EEC must reprogram itself. This does not always happen and the result is a very sick vehicle and EEC replacement is expensive. This can occur in Fords during a simple battery replacement.
    2. Information and message displays are lost. Rapid presets, seat and steering wheel settings, clocks, trip odometers, etc. are wiped out. Batteries should stay charged for months. If not, a problem exists that needs attention. A much better idea is a simple 1 or 2 amp battery maintainer for $25.00 at most auto parts stores.

  6. ModMyRV

    @Robert: Yes, if the battery cutoff switch was used to disconnect the vehicle/starting battery, then what you say is true. The main intent of installing a cutoff switch is so the “house” batteries don’t run dead from parasitic power draw, such as the LP detector.

    A battery maintainer is a good idea if you store your RV at your house. But some store theirs at a storage facility or lot and there is no access to AC power for the charger to run. You probably don’t want to leave something like that running unattended anyway. If there was a problem, like the batteries being boiled dry because of a charger malfunction, you wouldn’t know about it until the next time you dropped by to check on your RV. A simple knife or rotary switch is a low-cost way to ensure that no matter where you store your RV, the house batteries won’t run dead.

    If you have a class A, B, or C, you can keep the starting battery charged up by periodically starting the RV and letting it warm up to full operating temperature. The parasitic power losses on the vehicle battery are extremely small to begin with so a dead starting battery is much less likely than dead house batteries!

  7. Harvey H.

    I have considered installing a cut-off switch but haven’t yet. But, if one is having trouble with their trailer batteries running down or needing to have water, the problem could very well be with the inverter such as in my case. Since I bought the trailer new, it seemed to need water often, sometimes taking 8 bottles of 1 pint size to fill up both size 27 batteries. I had my trailer in for warranty service(refrig recall) and told the tech about that problem. He said it sounds like a bad module in the inverter. He checked it out and it did have a faulty module. It cost as I recall about $45 for the module. I have spent more than that on one new battery. In five years, I went through 4 batteries that had cells burnt up. Since the fix a year ago, no water has been needed. period. A cut-off switch will not fix a bad inverter other than stop it from burning up batteries when not in use and AC to to the unit is on. For years when trailer is stored, I have plugged in the AC for a few days, then unplug it to prevent burnt up cells, but happened anyway until now.

  8. ModMyRV

    Harvey, what you suggest is a great idea to confirm. However, you refer to the misbehaving part as an inverter which is likely not correct. I think you mean converter.

    An inverter inverts DC voltage to AC voltage. A converter converts AC voltage to DC voltage. Most RV converters are not very good quality and contain only a single stage charger. It is this charger that overcharges the battery because of the constant voltage supplied is too high for long term charging of the batteries. Thus it boils the water right out of the batteries if left charging long term.

    A good 3-stage charger, like the Intellicharger PD9200 series will treat your batteries much better and recharge them much more quickly than a standard single stage converter charger.

    Here is a link to the Intellicharger:

    http://www.amazon.com/gp/product/B000ITUZGM?ie=UTF8&tag=moco00f-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=B000ITUZGM

  9. bighig

    This was an ez mod .I put it in the frame behind the battery box.I found A hole already in the frame I had to drill two small holes to mount. I used stainless bolts .This took about twenty minutes to complete. Its hidden an has A key to turn on an off. it cant be seen unless you know where its mounted.

  10. ksmith1123

    I did this mod and it works great but I have a question. I installed an 1800Watt power inverter about 3 feet from my batteries using VERY thick wires (0 or maybe 4 gauge? can’t recall exactly). Does putting the cutoff switch (with it’s very small contact surfaces) in between the batteries and the inverter effectively negate the intent of using the thick wires? Seems the cutoff switch and it’s small contact surfaces would be a “choke point” for the current making the inverter not able to draw current from the batteries as fast nor supply as much power as effectively as if the switch were not in there… ???

  11. ModMyRV

    @ksmith1123: Much would depend on the cutoff switch current rating and lug size. Most battery cutoff switches are rated quite high, usually in the range of 300 amps or more. Be sure you have high quality cables and terminal ring connectors too. Any battery store, such as Batteries Plus, will be able to install the appropriate lugs for your cable size.

    You can do a simple test to determine if the switch is causing a current drop. Just bypass the switch and make a direct connection to the inverter. Then see if the inverter performs better.

  12. Scott.S

    Is it best to disconnect the batteries or levee them connected if you have a solar Battery charger

  13. kc7cbf

    I love this mod- and I put my switch in a weatherproof “in use” cover so it is safe from the rain. No more parasitic draw!

  14. Charlie Hendricks

    This is exactly what I did on my RV. It worked out really well and saved my batteries for boondocking. It’s also great for when you put your camper in storage.

  15. SonoranChris

    Did this Mod to my Travel Trailer over the Labor Day weekend.

    I put the cutoff switch on the inside of the A-Frame just in front of the battery boxes. Zip Tied the longer connector cable to the frame to keep it nice and tidy. Was hard to get the screws into the trailer frame, but was able to pilot the holes with a short screw that had the same thread.

    Since having it on the trailer, I turned the switch on and after nearly 6 months, the batteries (twin 6v) still had sufficient charge to turn lights on and start the fridge.

  16. Cho

    However, even when totally disconnected, all Rv batteries will continue to have a small amount of “surface” discharge.
    That is a current flow from post to post across the battery material itself; or through debris on the battery.
    A $25 battery maintainer-charger will save a couple of $100+ batteries. Be sure to use an “intelligent” 3-step charger.
    If your existing Converter/charger is “intelligent” or has “wizard”, keep it plugged in.

2 Trackbacks For This Post

  1. 10 Low Cost Mods You Can Make to Your RV

    [...] 3. Battery Cutoff Switch - This simple mod can save you a lot of grief. Having a battery cutoff switch removes any DC electrical loads from your RV battery or batteries. It can be easily placed inline between the battery’s positive cable and the battery positive post. Isolating the battery like this can prevent damage to batteries when the RV is in storage or not being used. Letting a battery run down completely can shorten its life and may even damage it beyond use. Adding a $20 switch can save you from having to by a new battery. Here is an example: http://www.modmyrv.com/2008/06/10/rv-battery-cutoff-switch [...]

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    [...] there is no right or wrong way with regards to + vs. -. It’s pretty much personal preference. Here’s a short article on the subject. __________________ 2011 F250 6.2L XLT Crew Cab 4×4, 3.73. [...]

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