Traveling Light Reflections
Why You Need a Battery Monitor in Your RV
by J. Tracey
What is a Battery Monitor?
A battery monitor is like the fuel gauge in your car or truck. It allows you to precisely see how much energy is in your battery at a given moment. Additionally, a good battery monitor will tell you the amount of electrical current leaving your battery while it is being discharged. It will also tell you the amount of current going into your battery while it is charging. It may even tell you the amount of time left on your battery. Some battery monitors will store data about the battery usage and allow you to refer back to the battery usage by looking at tables or graphs.
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Why Not Just Use the 4-Lights on the Stock Battery Gauge or a Voltmeter?
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The stock battery gauge with the 4-lights is NOT accurate! It may tell you when the battery is fully charged and it may tell you when the battery is dead BUT, everything in between is a rough guess. Using a voltmeter is a better but still highly inferior to a good shunt style battery monitor. Both of these gauges use voltage, however a voltmeter gives you a numerical value as opposed to a series of 4 lights. There are a few reasons why these 2 types of gauges or meters are not accurate. The first occurs when a battery is being charged. The charger has a higher voltage than the battery’s voltage. The charge voltage needs to be higher because the difference in these voltages is what causes current to flow into the battery. The problem is that this causes a false voltage reading when that battery is being charged. This causes the voltmeter or the 4-light gauge to show the battery voltage to be higher than it really is. We falsely believe the battery to have more charge than it really does. The second problem occurs when the battery is being discharged. As soon as any load is put on the battery, there will be a drop in the battery's voltage. The more current drawn from the battery, the greater that voltage drop is. Again, the voltage reading is false and causes an inaccurate reading of the amount of charge left in the battery. In order to accurately determine the true charge of a battery, using voltage, you must read the “Resting Voltage” of the battery. This means the battery has been resting for a while with no charge going in or out of the battery. As you can imagine, in an RV, the battery is rarely in this state.
Why is a Shunt Battery Monitor More Accurate?
What is a shunt? The shunt in a battery monitor is a large resistor with a very low impedance. What does this mean? A resistor impedes electrical current from flowing through it. A resistor with a high impedance lets very little current flow through it. A resistor with a low impedance lets a lot of current flow through it. The shunt in a battery monitor has an extremely low impedance. This is important. It means that it will allow a high current to pass through it. What is also important is that the impedance, like all resistors, is constant. This means it does not change. Ohm’s Law, V=IR can also be stated as, “at a constant resistance, any change in voltage is directly proportional to the change in current”. Simply put, even the slightest resistance in a shunt will cause a slight voltage drop. This voltage drop is proportional to the current passing through the shunt. So, by knowing the impedance of the shunt, the manufacturer is able to use even the slightest voltage drop to calculate, very precisely, the current through the shunt.
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How is the Shunt Type of Battery Monitor Different from a Voltage Meter?
First, a voltage meter displays voltage while a shunt primarily displays current. As discussed previously, using voltage to determine the State of Charge (SOC) of a battery can be very inaccurate. On the other hand, using a shunt to determine the current flow can be very accurate. When you first set-up a shunt you simply need to program the battery monitor to know a few things. It needs to know the capacity of the battery. It also needs to know the resting voltage both when it is full and when it is empty. These are usually given in the specifications of the battery from the battery manufacturer.
In summary, the shunt type battery monitor is more accurate than a voltage meter type. Using the analogy of a fuel gauge in a car, using a shunt battery monitor is like reading the fuel gauge in a modern car. You can see how full your tank is, how many miles per gallon you are getting, how many miles to empty and how many miles per gallon you are getting at a given moment. The voltage type meters are analogous to a gas gauge in a 1976 Dodge van. Your gauge says you have “half of a tank”. You know this is a rough estimate and you can only guess how many miles you have left before empty.
How do Shunt Type Battery Monitors Differ?​
Today there are a number of different shunt type battery monitors available. Some are better than others. More and more keep appearing on Amazon. The quality and features of each differ in various ways. For example, the Victron Smart Shunt is available in current ratings from 500 amps up to 2000 amps. This monitor does not have a display but instead uses Bluetooth to connect to the Victron Connect app. You can see lots of valuable data with this free app. Victron also has their BMV 7xx series that has a small panel to observe data. Some of these within this series also have Bluetooth to connect to the app. All of these will also communicate with other Victron devices. There is a clone of the Victron Smart Shunt available on Amazon. You can also find many clones of the Victron BMV 7xx series monitor available from many other manufacturers. For example Furrion has a similar model, as well as Camway. A search on Amazon will produce a multitude of different shunt style battery monitors.
Choosing a Battery Monitor
A few things to think about when buying a battery monitor are: ease of use, set-up process, cost, functionality and ability to interface with other equipment. In my opinion the Victron brand meters have all other brands beat in every category except for price. They are expensive but the quality is worth it. Personally, our entire power system in our RV is built with Victron components. The main reason is the ability of all the components to communicate with each other. The system is totally integrated. The second reason is the quality. Renogy is another popular company for RV power components. They have also begun to integrate some of their components with their newly developed Renogy One. This is a work in progress but they appear to be on track toward developing a more affordable, integrated, system than Victron.
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Back to the things to think about when looking for a battery monitor. The more data that your monitor is able to tell you, the better it may be. However, you may want one that simply displays your State of Charge and does not show stuff that you may not care about. Set-up is a serious consideration too. Monitors that offer Bluetooth communication are often the easiest to set-up. This is because running a wire for a display screen may be quite difficult depending on where the battery is, relative to where the display will be mounted. If your monitor uses Bluetooth you do not need to install a display and you will use your phone as the display.
Regardless of which battery monitor you choose, you will be glad you decided to upgrade the battery gauge your RV came with. Knowing how much energy you have, and how much energy you are using, will go a great distance toward helping you get the most out of your RV.
