Mod #5: Battery Monitor

Mod #5: Battery Monitor

Submitted on: 06/09/08

     Category: electrical
Mod Rating: 12345

(34 ratings)

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Mod Description:

If you have invested the mod-time and mod-money to upgrade your battery bank and charging system, then you should also invest in a battery monitor. There are a variety of monitors available covering a broad range of features. A simple volt meter (see image) can quickly tell you at a glance what your battery voltage is and therefore the assumed percent of charge remaining. This can be a bit misleading though since voltage alone doesn’t tell the whole story. A simple voltage reading would need to be performed when there is no load on the battery for at least 24 hours to get an accurate reading. This is not very practical and probably not something you would want to do frequently.

Mod Difficulty:

What is needed is a way to tell how much capacity you have left in your battery bank, much like a car’s gas gauge. At a quick glance, you can see how much gas you have left so you know when to fill up. Battery monitors work much the same way. Not only can they tell how much battery capacity you have left, but they also can monitor real-time current draw, how much power has been consumed, and how much current going in to the battery during charging.

Performing this mod requires both patience and high quality electrical connections. These type of battery monitors use what’s called a shunt, which is a high-precision resistor that goes in between the negative battery cable from the battery bank and the battery bank terminal, such that all the current going into the battery bank (charging) and out (discharging) must pass through the shunt (see image). The current voltage difference between the shunt terminals is measured and sent to the monitor for current calculation and display. It is essential that all connections are properly made to ensure accurate readings. It should be noted that the shunt, wiring, connectors, and mounting boxes are all sold separately so plan on purchasing these items in addition to the monitor itself.

Mounting options for most monitors include box mount and surface mount. Surface mounting provides a clean and finished look. Just be sure you have enough space behind the mounting surface to accommodate the monitor circuitry and connections. Follow the instructions provided with the monitor as they usually come with many diagrams and examples (see image), especially the TriMetric brand.

After you have installed the monitor, the fun begins. There is a bit of tweaking to be done, like programming in your battery bank capacity in amp-hours, charging parameters, time-to-discharge sample intervals, and alarms. After a few discharge-charge cycles, you may find that you need to tweak the parameters again to get them just right. But modding your RV is supposed to fun, right?

ModMyRV recommends these parts for this mod:

Battery Monitors
Equus 3721 Battery and Charging System Monitor
Trimetric Battery Monitor
Xantrex Battery Monitor

Monitor Accessories
Shunts and Wiring
More Shunts and Wiring

General Reference
The 12-volt Side of Life

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13 Comments For This Mod

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  1. cccale

    I’d love to do this mod but i have one problem. I’d like to put the display above the fridge where the other system monitors are. I just don’t know how or where to run the wires from the batteries (under the stairs) to over the fridge (near the ceiling). I just don’t know how to open up the walls. Are there any good resources on RV interiors?

  2. ModMyRV

    In an RV refrigerator installation, there is always a space above and behind the refrigerator where you could run your wiring. If you take a look at Mod #41: Refrigerator Cooling Fan:

    In one of the pictures, there is a drawing that shows these spaces. To run your wiring, you can use flex tubing to run the wiring within underneath your coach. Usually the power center is near the fridge so you can run the wiring up through the floor using the opening that’s already there. Take a look under your coach to see what I mean.

  3. cccale

    Would i install the flex tube from the fridge access panel outside? I’m still not sure how i can get from behind the fridge to the top. Can i upload some pictures so you can see what i’m dealing with?

  4. ModMyRV

    Sure cccale. Go to the Mods Wanted forum and post your situation there. You have the ability to upload images in to the post. If that doesn’t work for you, you could email me at and I’ll take a look.

  5. jmt

    This mod is so nice, since you always have a good idea of where you batteries are at. Highly recommend it. Pretty easy to complete also.

  6. Andy Baird

    The author of this aricle is too tactful in saying that a voltage reading “can be a bit misleading” as a guide to battery status. Voltmeters are all but worthless for this job. A battery monitor of the type described here, which keeps a running total of every milliamp that goes into or out of your battery bank, is the *only* practical way to really know where you stand. If you do much boondocking, you should consider a battery monitor to be a mandatory addition to your electrical system. I can’t recommend them too highly!

  7. Bill

    The author made an error in the third sentence of the third paragraph. The voltage difference is what’s measured, not the current difference. There will be no current difference between the shunt terminals, but there will be a voltage difference; or voltage drop across the shunt. Since E = IR (voltage = current x resistance) the current can be determined from the shunts resistance and the voltage drop across the shunt.

  8. ModMyRV

    Good catch Bill! I meant voltage and am surprised no one caught this earlier. I’ll make the correction. Thanks.

  9. madsci

    I built my own with 3 shunt resistors and a 4-channel current sense amplifier. A microcontroller ties it in to the RS-485 data network used by my monitor panel.

    Two channels are used for bidirectional measurement of the battery current (since it can be charging or discharging), one is connected to the solar panel, and the other is reserved for the wind turbine which I haven’t gotten around to setting up on this rig.

    State-of-charge estimation gets tricky. I haven’t gotten around to implementing coulometric metering yet, but maybe I’ll try that this year.

  10. tomharg

    I just did this on my 5th wheel and I love it. One of the best things about it is that I can turn something on and tell how many amps it is taking then either minimize its on time or replace it with something that draws less current. We boondock a lot. Presently I am replacing the lights that we like to keep on with LED’s and drastically reduce the power draw from about 2 amps per bulb to around .2 amps per bulb! That should let us only run the generator every other day instead of every day.

  11. Warren

    I have a utility trailer that I am putting power in and a battery monitor would be nice. I like the battery monitor that is shown on your site. the one with the little meter, momentary switch and the chart under it.. It would be nice to know the schematic and the parts list with maybe the links for the parts. Any chance of that? Thank you

  12. mrgehring

    There is now a way cheaper option! $30 ( including shipping ) for the monitor and the shunt !
    Look for the AILI battery monitor on Amazon:

    I ordered one, will install in the next month.

  13. joseph milyko

    how do i figure what size shunt i need ,also if a bigger shunt is used {up grading batt bank in future }will I get an accurate reading ,,thanks

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