Stock Photo Victron Battery Monitor

The RV battery monitor is the most important part

The battery monitor is the missing critical part. If your RV has a battery it needs a battery monitor. If you don’t have a battery monitor then you are guessing. The most important part of an RV solar install isn’t the panels, controller, or battery, it is the battery monitor. If you install a solar charger without a battery monitor you are guessing at the state of charge.

Every RV needs a battery monitor

This should be on every RV from the factory and I don’t know of any RVs that have this critical component. Without a monitor, you are guessing. Two weeks ago I met a guy that killed eight batteries (two sets) all in one year — all because he didn’t have a battery monitor and never recharged them fully.

What is a battery monitor?

If you are lucky — you will have a voltmeter. Even then not all RVs have voltmeters. Voltmeters don’t tell even half the story. Some RVs don’t even have a manual shutoff switch. So perhaps I am just dreaming that they would have a monitor.

A good monitor is more important than another battery and more important than any other item in your electrical system, with the one exception of fuses. I even saw pictures of a “professional” solar install last week without fuses. That is not acceptable.

Workarounds don’t work

I have tried to live without a battery monitor and tried to use a voltmeter as a substitute; this doesn’t work.

stock photo voltmeter
The RV battery monitor is the most important part 18

The only method that can work, only on flooded lead-acid batteries, is to measure the specific gravity of the electrolyte. This is accurate but is only done as a maintenance measurement. It is nothing you would do frequently.

How Electricity “Works”

Volts = Pressure = Potential to create work

Volts are like measuring the depth of the water behind a dam. The deeper the water, the higher the pressure. It is the same with voltage. Higher voltage means more pressure and more potential to create work. Pressure alone does not do anything, except sit there, until you have current.

Voltage (pressure) is only one measurement that affects performance.

Capacity = Volume

The measurement of volume, like water backed up behind a dam, is the starting point. The more volume the longer you can sustain work. Bigger battery banks have more volume and thus can sustain current for a longer period. Current, measured over time, is how to measure how much of the capacity has been used. This is the critical measurement. Subtract the energy consumed to determine the energy remaining.

The RV battery monitor is the most important part 19

Amperage = Current = Work in process

Amperage is the measurement of current flow, like water in a river — how much current is flowing. This is a very important measurement but still is not the whole story. You need to know the voltage, current, and capacity.

Volume, not pressure measures the ability to sustain work

Grand Lake, on the Colorado River, (in Colorado) is 389 feet deep and holds sixty-eight thousand acre-feet. = One hundred sixty-nine pounds-per-square-inch pressure at the bottom.

Lake Havasu, on the Colorado River, (in Arizona) is 90 feet deep and holds more than six hundred thousand acre-feet. = Thirty-nine pounds-per-square-inch pressure at the bottom.

To create sustained work you need both pressure and volume. In terms of the ability to create work, Lake Havasu has ten times the ability to create work than Grand Lake. Even though the pressure at the bottom of Grand Lake is more than four times greater. Lots of pressure does not equal the ability to create more work. Enough pressure, coupled with ample volume, creates the formula for sustained work. You would have to empty Grand lake down to 25% of its normal capacity to have lower pressure, (equal to Lake Havasu), and then it would soon run dry and have very little potential for sustained work.

True, as you remove water from Grand Lake, the pressure at the bottom will gradually change relating to the loss of water. The problem is that with electricity, especially with lithium batteries, the voltage doesn’t change until most of the discharge has already occurred.

Lake Mead has more depth than Grand Lake and forty-six times the volume of Lake Havasu and dwarfs these lakes in terms of ability to create work. This would be the best of all worlds, with lots of pressure and large volume to create lots of work.

How a battery monitor works

State of Charge or State of Discharge

A battery monitor measures the battery state of charge and measures how much volume remains and the battery state of discharge measures how much is already used.

12-volt Battery graphic
If it were only this simple.

How a battery monitor works

All RV solar installs include solar panels, wires, a charge controller, and a battery. Most solar installs lack an accurate method to determine how much volume has already been used. To measure this you need to record current and elapsed time. If you could see the energy, like sand in an hourglass, then you would know when you are about to run out.

A battery monitor is the missing critical part

A battery monitor isn’t on this list of standard components and it is the number one missed item. Any solar installation that does not include a battery monitor is incomplete. Any battery without a monitor may be full or nearly drained. How could you know?

A voltmeter is not a good substitute — not even a close substitute. A battery monitor will give you a precise measurement of the battery’s state of charge by measuring the “volume” consumed. Without a battery monitor, you are merely guessing. As in the above water example, the larger the capacity of your battery, the more you will be guessing if you only measure the voltage.

Victron 712 Battery Monitor Shunt
Victron 712 Battery Monitor Shunt

A battery monitor is critical regardless of which battery type you choose. In fact, the more you spend on your battery, the more you need a battery monitor. The bigger the battery the more you need a monitor.

A battery monitor is the only tool that can measure the state of charge. Nearly all RV solar installations are missing this critical component.

Voltmeters only make people feel good

Sometimes, you can tell something about the state of charge of a lead-acid battery by using a voltmeter. The only way to do this is to disconnect the battery from all charging and discharging for several hours and then measure the voltage. This will remove the surface charge. Who does this? It just isn’t practical. Voltmeters on systems while being used don’t give you even part of the story. Voltmeters read high during charging and read low during a discharge. Charge rapidly and voltmeters read very high. Discharge rapidly and voltmeters read very low.

SeeLevel Tank Monitor AGM battery voltage
SeeLevel Tank Monitor on my AGM battery. The voltage here indicates a charging voltage, not the voltage currently in the battery.

Since lead-acid batteries suffer life-shortening damage if you ever remove more than fifty percent of their charge, a battery monitor is critical to measure how much of the capacity has been used. Voltmeters cannot accurately measure the capacity consumed.

Lithium makes this a critical component

You cannot (ever) determine the state of charge of a lithium battery using voltage. On a lithium battery, the voltage remains the same for the first 90% of discharge and then falls like a rock for the last ten percent of the charge.

Magnum Inverter Control Panel Lithium battery voltage
Magnum Inverter Control Panel – my lithium battery voltage. This battery was 100% full but given that it is a lithium battery the same 13.9 volts could indicate that it was about 20% full or may read higher while charging. Voltage measured on a battery being used or charged means almost nothing.

We had a monitor but were not watching it and this happened to us. The only way we knew that it was happening was when the inverter low voltage alarm went off.

Low Voltage Failure

  • August  7, 10 pm, 629 amp hours used, 12.93 volts
  • August  8,   6 am,  773 amp hours used, 12.69 volts
  • August  8,   9 am,  847 amp hours used, 10.12 volts

We document the entire story in this post. Boondocking without Solar

We have done the same thing with lead-acid batteries and never had the low voltage alarm because the voltage dropped off gradually and we put a stop to it before the voltage dropped to an unacceptable level.

Pretending isn’t the same as measuring

I have two ways to measure current (voltage isn’t one of them) I can measure current using my clamp meter which uses the principle of reading the magnetic field which is created when electricity flow through a wire on my real fancy voltmeter. This fancy voltmeter is called a “Clamp-meter”. My clamp-meter measures either alternating current or direct current. The clamp creates an induction coil that is around the wire. As electricity flows through the wire, it creates a magnetic field. The induction coil measures the change in the magnetic field and reports the magnetic field as amps. You could create a battery monitor with an induction coil but I am not aware of good monitors that use induction coils.

Klein Volt AC/DC Clamp Meter
Klein Volt AC/DC Clamp Meter. The clamp over the outside of the wire measures the current flowing through the wire. In this picture, the current flowing was 10.5 amps per hour. Since the battery voltage was 13.9 volts the current was 146 watts. Just a little more than two typical incandescent light bulbs.

My other tool is to measure the current flowing through a (small resistance) resistor called a shunt and this device is attached to a very small computer which adds up over time how much energy is consumed.

Shunt without built-in computer
Shunt without a computer attached

The shunt works both ways

The shunt not only adds up how much energy has been drawn from the battery but also measures how much energy is replaced into the battery. This feature allows me to see energy consumed and replaced and how far I have to go before I reach full.

Install one before you purchase solar or a new battery.

If you only get one thing, get a battery monitor. A battery monitor should be your number one change (unless you don’t have a master shutoff or a main battery fuse, then install these first).

On our first RV solar install, the battery monitor was used with the old battery. The battery was already a year old when I purchased the RV. With this RV I learned to treat the battery so that it would last as long as possible. Even after two years of living on the battery full time, I learned from the new owner that the batteries were still performing fine more than a year after I sold the RV.

A battery monitor is so critical that it should be purchased and installed before you ever install solar panels or a new battery. The reason for this is that a battery monitor alone can tell you how much energy you use before you spend lots of money on a solar and battery upgrade.

Real Data (delivered real-time) No Guessing

Phone screenshot Victron Bluetooth Battery Monitor (AGM battery 2018)
Phone screenshot Victron 712 Bluetooth Battery Monitor (AGM battery 2018). In this picture, I have consumed 48 amp hours from my nominal 300 amp hour battery. Thus the remaining state of charge is about 86%. However, since this is lead-acid battery damage starts at about 50% state of charge. You could say that I have consumed 1/3 of the allowable energy. In this picture, I am only consuming 1.9 amps per hour and currently using 25 watts. A voltmeter just doesn’t have the ability to give you this level of detail.

I have had my shunt installed on both of my RVs and I installed both of them before I made my solar upgrades. In my new RV, I installed the shunt as part of the battery upgrade nearly a year before I installed the solar upgrade. Here is a link to my article about my year with lithium batteries before I installed solar panels. These two articles show you how I use my shunt to monitor my energy consumption. Electricity the Hard Way & Boondocking Without Solar

If it doesn’t have a shunt it is not a real battery monitor.

Voltmeters are not battery monitors. You need to measure the current as it passes a specific location. Voltmeters don’t do this. Marketing departments often lie; is this a surprise? Don’t look for the name, look for the shunt. And while looking at the shunt, you should look for a high-ampere-rated shunt. I will never draw 500 amps from my battery all at once, but my shunt is rated for 500 amps. Small shunts can burn through like a fuse, obviously dying in the process. The same circuit needs a fuse, rated well below the shunt rating. A good shunt should also have a mounting point and not just hang from the wires.

Shunt with mounting holes

Super Easy Install

Installing a shunt only requires you to make one small change to the wiring of the battery. Almost all the installation will be on the negative post of your battery. You will need a new cable between the battery and the shunt and then re-attach the cable that was on the negative post of your battery to the shunt. After that, you will need a small wire, to power the shunt from the positive terminal of the battery (or any other location that is on the positive side of the battery) to power the shunt.

Battery Monitor graphic
The RV battery monitor is the most important part 20

The critical issue in the installation is that all power from the battery goes to the shunt before it goes to any other item in the RV. The only permissible items between a shunt and the battery would be a switch and fuse. However, even though the switch and fuse would function in this location, they should be located on the positive side of the battery, for other reasons.

Wiring the shunt
Battery Monitor / Shunt Wiring

On my installation, the small wire leading to the positive side of my battery is also a temperature thermistor. This is a fancy resistor that changes the resistance over calibrated temperature changes. All you need is a simple connection. This thermistor allows me to monitor the temperature inside my battery compartment.

I went fancy; you don’t have to

The other part of our battery monitor is the remote screen and Bluetooth function that allows the monitor to talk to my phone and also my charge controllers. If I had a Victron inverter the Bluetooth would also talk to the inverter. You could get a battery monitor without a remote display and without Bluetooth to save a little money. Even if you never install solar you should get a battery monitor.

Victron Smart Shunt
Victron Smart Shunt with built-in Bluetooth does not have a display

We also have a small remote display so we could monitor the state of charge without using Bluetooth. Honestly, we never do this, we only look at the Bluetooth. You can save a little money if you don’t want the remote display.

All RVs

All RVs should have a good battery monitor. The only RVs that don’t need a battery monitor — don’t have batteries. Some batteries have built-in state of charge indicators and some batteries (sometimes called solar generators) have built-in monitors.

Lion Energy Safari ME
This is a photo of an all-in-one Lion Energy Safari ME “solar generator. It is an all-in-one solution to the battery – solar controller- inverter and battery monitor for the built-in battery. If your RV only has a small battery but does not already have an inverter, this may be a good solution for your small power needs. A friend uses one of these to power his RV energy needs and then keeps it charged and ready to power the refrigerator in his house during power outages.

Lion Energy UT1300 and some other batteries have a push-button State of Charge LED display. When you push this button up to five LEDs will light up and tell you the state of charge for that battery. In my battery bank, I have to crawl into the storage bay to push this button and every time I have done this, the battery reads full.

Lion UT1300 State of Charge LEDs
Lion UT1300 State of Charge LEDs

As lithium batteries become more advanced I expect (and I have been promised) that some new batteries may have Bluetooth battery monitors built into the battery. When this happens, I will let you know and create an update to this article.

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Components List

This is what we used. We do not get commissions on the links and are provided only to help our friends.

BMV-712 Victron Battery Monitor w/shunt & temp sensor

Klein Volt AC/DC Clamp Meter

Smart Shunt Victron Energy (w/o remote display or optional temp sensor)

Lion Safari ME all-in-one solar generator (battery, inverter, solar controller) with a built-in battery monitor

Lion Energy UT1300 Lithium Iron Phosphate Batteries with push-button state of charge LED indicators.

42 thoughts on “The RV battery monitor is the most important part”

  1. Scott, your article literally blew me away. I took wires at the Naval Academy but never ever had this level of understanding. We have all sorts of battery-operated equipment at the helicopter museum: helos (of course), forklifts, carts and power tools. This definitely puts a whole new perspective on it for me. I’ll never look at the bank of batteries in our H-21 or the H-46 or forklift and cart batteries in the same way. In fact, I’m saving this post and will go back and read it again before I go back out to the museum or even back to North Island for the 60. Thanks so much. Keep truckin and stay safe. Chip
    P.S. Love your drawings 🙂
    P.S.S. I put our new museum website down for website

    1. Thanks, Chip, The other thing to remember is that lead-acid batteries want to be (float) charged 24 hours a day otherwise they leak ever so slightly every day.

  2. I am starting to think that I have been lied to about 48-volt systems having more power than 12-volt systems. Given your example of the lakes, and Lake Havasu being able to create more work than the higher pressure lake. Why was I told that a 48-volt system is better than a 12-volt system?

    1. George, if you have four one hundred amp-hour 12-volt batteries you can arrange them to create 12, 24, or 48 volts. You will have a 400 amp-hour 12-volt system, a 200 amp-hour 24-volt system, or a 100 amp-hour 48-volt system.

      All three arrangments will be able to create the same work. Given an equal load, they will all be empty at the same time.

      The higher voltage system has one benefit, in that it can deliver the same work at a lower current through smaller wires.

  3. I have killed dozens of batteries in my RVs over the last ten years. I installed a Trimetric Battery monitor about 2 years ago and now that I understand when they are empty (50%) and when they are full, my two-year-old lead batteries are still going strong.

    The biggest thing I learned was that I had never fully recharged the batteries that I had killed, instead, I turned off the charger when they hit about 90%. Some of those batteries lasted only one summer.

    1. Trimetric makes a good battery monitor. I chose Victron because the Bluetooth connects to my solar charge controllers.

      Magnum also has a good battery monitor, which could talk to my Magnum Inverter — but I value the connection between my monitor and my charge controllers more than I value the connection to my inverter.

  4. You are the only person I know that really understands how to treat batteries so that they will last for a long time. Now I understand how you did it.

    I have been limping along on my lead-acid batteries always afraid that they will fail at any moment and at the same time kinda wishing they would so I could get lithium batteries.

    1. Batteries die because people kill them… Noticed that I said kill them and not use them. Sometimes they kill them because they don’t use them but most of the time they kill them due to abuse.

      My number one recommendation right now is to fully recharge after any discharge. For a lead-acid battery, this means you need hours in float mode on a charger and then you need to keep them on a trickle charger 24/7 to keep them happy.

      Soon I will be working on an article soon on how to repair lead-acid batteries internally. Before I do this, I need to do it myself. I have friends that have done this but I am more driven by my own experience — before I tell others how to do things.

      Edit: here is the link to Zombie Batteries

  5. Great article, Scott! Thanks.

    I’m really looking forward to your article on repairing lead-acid batteries.

    Question: You mentioned that a trickle charger had to be going 24/7. Would that exclude a couple of small solar panels that would get no light at night? (Where we store our RV there is no electrical outlet for a shore=power trickle charger).

    1. Lead-acid batteries want to be charged 24/7 this is the ideal world. Real-world is you do what you can do. Sadly most batteries are ignored and then cussed at when they don’t work.

      If you fully charge your battery before you store it (including a few hours in float mode on the charger) then this is a great application for a small solar panel. Each day the battery will be returned to float mode and the charge will sustain the life of the battery.

      Since we haven’t tried it yet and will report on it as soon as I have data. I think the best thing for you to do would be to wait until my next report on pulse charging my starter battery comes out.

      Yes, this will be one more solar panel on the roof and I expect to install it soon. The same pulse charger will work on either the house or the starter battery. This will be a major improvement to a standard solar trickle charger.

      Edit: here is the link to Zombie Batteries

  6. Michael Rehmus

    A clamp-on ammeter, unless it has a Hall Effect transistor cannot measure DC current flow. It cannot work without the Hall Effect chip because the clamp-on loop is just one side of a transformer and requires an AC current to function.

    The magnetic field surrounding a wire does not vary with a DC current so it takes a Hall Effect transistor to measure the static magnetic field. The Hall Effect device is fragile and takes a bit of care to not damage it. Much of a current beyond the limits of the meter will destroy the transistor.

    1. Great comment. I love it when people smarter than I am, add to the discussion.

      Most clamp meters (and all of the less expensive ones) are unable to measure a DC amperage. A clamp meter only measures the current draw and has no ability to add up the draw over time. That is why a shunt with a microcomputer is critical to know the state of charge.

  7. We have the Victron monitor shown (and Victron charge controller) on our system.

    This is how I found out our charging parameters were not set right by the installer.

    Conferring with the battery manufacturer and using the Victron Bluetooth interface I was able to diagnose and correct that we were not getting a daily full charge from the solar panels.

    Now we are fully charged by noon on a sunny (or partly cloudy) summer day and can use the electric coffee pot and toaster off the inverter without worry.

  8. I have read a lot of articles on the subject of amps vs. volts and volts vs. current but these examples are by far the best I’ve seen.

    I’ve been trying to explain this concept to my wife for the past 18 months, without a lot of luck.

    I forwarded your article and suddenly she gets it and now when there is a problem — she suddenly is the expert. I guess thanks are in order — but did you have to do such a great job? Geoff

    1. I showed your comment to my wife… now she thinks that she might need to read the article. This could be trouble.

  9. This is exciting…sort of. The “battery monitors” are ammeters. That’s the proper name. You don’t have to go Deluxe and spend $130 for the Victron Smart Shunt with built-in Bluetooth. You can buy a great Bluetooth ammeter for less than $20 delivered, with a durable Android/Apple/PC app or program to back it up. I like the DT-24, Digital Display DC 0-380V Power Supply Voltmeter Ammeter Battery Capacity Tester Battery Fuel Gauge Power Meter with an available Relay Module. To do heavy AWG, consider the DT-3010, which runs the hefty 100A built-in shunt from 0-300V DC. I attach that to my new Interstate SRM27–EFB ON THE front of the trailer, and monitor strictly via Bluetooth. I use an auxiliary Optima Bluetop D34 inside the trailer with the DT-24. They are both available with all relays, software, and good manuals from ATORCH on AliExpress, based in China. I’ve never had a problem and it’s a cost-effective, elegant solution.

    1. As pictured above my Klien voltmeter has a built-in clamp meter to measure amperage. Mine does both AC and DC. Less expensive voltmeters will only do AC. The difference between an ammeter and a battery monitor is that a monitor has a built-in coulomb counting.

      My Victron battery monitor also networks with my other Victron devices for more precise control.

  10. I found your article to be very informative.

    I’ve only just started boondocking, and am aware of the potential for battery damage if not properly maintained, and so have been shopping for a battery monitor.

    However, it seems to me there is still something missing – automatic low-voltage cut-off. It seems that no battery monitor includes this feature. I have a manual switch, and have played around with a cutoff module…but it sure would be handy to have a single gadget to handle everything. Is there one?


    1. Both my lithium batteries have low voltage cut-off and so does my inverter.  A battery monitor could have a low voltage cut-off with an additional relay for me I don’t have this need.  One of the cool things my Victron Monitor does is network with my Victron Controllers, which makes the controllers smarter. 

  11. I have 2 -6 v golf cart batteries in my RV. 115 Ah each.
    I have a shunt monitor.
    The battery measured 13.18 v
    The monitor reads 84.29 Ah and 73.3%.
    The monitor will not go above 73.3%.
    It did when installed 14 months ago.
    What’s wrong here?

  12. I found your article very informative but given my setup it raises a question.

    I have a 2013 Roadtrek RS E-Trek with 8 AGM 6 volt batteries. 4 are wired to supply 12v for the house lighting, etc. 4 are wired to supply 24 volts for the inverter. How can I wire a shunt to measure the state of charge with this setup?

    1. Elloit, quick answer, you need two shunts.

      We have somewhat of the same problem. This post has my battery setup.

      For me, my AGM batteries power my DC house, and my lithium batteries power my inverter.

      I don’t measure the draw on my AGM batteries because all the energy for my system comes from lithium via the shunt.

      For you, I don’t know why you have a split system but I would probably combine both banks into one 24 volt system and then use a buck converter to supply my DC house lighting.

  13. Good article! There is just one detail that I didn’t see included that I thought I would mention, which is charging efficiency. This mainly applies to lead-acid chemistries. Flooded lead-acid batteries have a charge “efficiency” in the neighborhood of 85%, or in other words, you have to put in about 15% more juice than you take out to get to 100% SOC. AGMs tend to be a bit better. The age of the battery and speed of charging affect the charge efficiency, too. The faster you charge a lead-acid battery bank (i.e. the higher the C rate), and the older it is, the more inefficient the charging will be.

    What this means is that you can’t just plug in your Victron monitor and assume it is going to be accurate. You need to do a little experimenting to see how close it is in real life to prevent the reported SOC from drifting up if charging is not adequate.

    To do this, disconnect your battery bank and charge with an appropriate charger (i.e. not the WFCO unit in your camper) overnight from shore power until it’s fully charged (with at least 3 or 4 hours in absorption mode). At this point, take specific gravity readings and record them as your baseline. All cells should measure within .05 of each other and be between 1.260 and 1.280 (temperature compensated). Then, reconnect the system and reset the Victron meter to ensure it’s reading 100%. Next, take the battery bank through a typical discharge cycle at a rate that is similar to how you normally use your system. If your typical loads are 5 amps, then discharge at that rate. After you take it down to 50% or so, reconnect your solar system, or however you normally charge and charge until the Victron meter says it’s at 100%. Then stop charging and measure specific gravity again to see if you’ve reached your baseline readings. If you have, then great, you’re done. If the specific gravity is below the baseline readings, you can adjust the Charge Efficiency Factor in the app and try again. The default value is 95% which will usually be too high for most flooded lead-acid batteries. You may need to repeat this exercise in a few years as your batteries age, or when you replace them. Happy camping!

    1. You are correct about charge efficiency being a huge factor for lead-acid batteries and not such a factor for lithium batteries. Lithium batteries are nearly 100% efficient at accepting a charge. Compared to lithium, lead-acid batteries need to be forced to take a charge and they take forever (really all day) to fully charge.

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