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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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 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 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.
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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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
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.