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RV Battery Monitors for 12v Electrical Systems

To avoid running out of electricity when living in a van off-grid means closely managing the leisure battery bank.

Using more power than the alternator or solar panels supply will eventually lead to flat batteries, no matter how big the battery bank. 

And, over-discharging the battery can shorten its life, leading to costly replacements earlier than necessary.

A battery monitoring system is the best way to keep an eye on the system’s health, so you can adapt your use as and when necessary.

This post covers the challenges of monitoring campervan batteries, what we can and can’t measure, and our top picks of battery monitors to suit your needs.

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Best RV Battery Monitors | Our Top Picks

Victron BMV 712 Battery Monitor

Victron Energy BMV-712 Smart Battery Monitor
  • Built-in Bluetooth Communication
  • Easy Setup
  • Displays current, voltage, used amp hours, time-to-go & more

Renogy 500A Battery Monitor

Renogy 500A Battery Monitor
  • Displays voltage, current, consumed power and battery capacity 
  • Large backlit LCD Display
  • Includes battery shunt holder for easier mounting

AiLi Battery Monitor

AiLi 500A Battery Monitor
  • Monitors state of charge (%), remaining battery capacity, charge and discharge current, and voltage.
  • Best value monitor on the list for the features v price

Innova 3721 Battery Monitor

INNOVA 3721 Battery and Charging System Monitor
  • Limited setup & plugs into your camper’s 12v outlet
  • LCD Display for voltage reading
  • Colour-coded LED lights indicate charge level

Bayite DC Digital Voltmeter & Ammeter

Bayite DC Battery Monitor
  • Displays voltage, current, active power, energy
  • No wires or accessories included
  • Ideal budget option
Build your own camper quick start guide

Why Do We Need a 12v Battery Monitor System?

A battery monitor isn’t an essential component of a campervan electrical system. An off-grid setup works without it.

Suppose you have a small electrical system and don’t rely on it heavily. In that case, you can manage without a fancy battery monitor installed. A simple voltmeter is all you need.

But suppose you want to monitor the leisure battery bank actively and have the opportunity to adapt your electrical use so you can still turn the lights on at night. In that case, you need an easy way to monitor it.

A battery monitor can indicate how much power the battery is receiving, how much you’re using, and how full the batteries are. The Bayite DC Digital Voltmeter & Ammeter is an ideal budget option and provides all this information.

Some battery monitors do a little less. The Innova 3721 only indicates the battery charge level.

Others go a step further, like the Victron battery monitor BMV 712 indicating how many amp-hours remain and historical data. And it includes in-built Bluetooth communication for remote monitoring (within around 25ft of the vehicle).

Armed with the monitor’s information, we can be relatively confident the battery health is ok, and take action if it indicates a problem.

But before choosing the all-singing, all-dancing monitors, keep reading. It’s not all it seems.

The Challenges with Measuring a Battery’s Charge

RV battery monitoring gauge reading high DC voltage

A battery monitor is like a fuel gauge for the electrical system but a lot less accurate.

A fuel gauge measures how much fuel is in a fixed-size tank and may indicate how many miles you have left in the tank.

A battery monitor measuring voltage to indicate charge level isn’t wholly accurate because a battery’s capacity fluctuates depending on environmental conditions like temperature.

The inaccuracy becomes more significant when the battery is in use.

Some battery monitors measure amps used and amps supplied from charging sources. Battery efficiency also changes with environmental factors, how it’s used, and how deeply it’s discharged too. 

So how much of the current supplied is absorbed can’t be accurately measured by the monitor.

The upshot is, while battery monitors indicate the state of the battery, they cannot be 100% accurate always.

What Can A Battery Monitor Measure?

What can a battery monitor do to help us keep our batteries healthy with all this inaccuracy?

Unless you have critical life support systems onboard, an accurate measure isn’t necessary. 

Instead, an indication of their health is acceptable and much better than nothing at all.

A battery monitor can measure some things accurately enough to allow you to interpret the approximate state of the battery’s health.

Measuring & Interpreting Battery Voltage

Digital battery monitor LCD screen

Installing a simple voltmeter like the Bayite is the most simple battery monitoring system. It packs a punch in terms of what it can tell you about your batteries.

It’s all we use in our DIY campervan conversion. It serves us well, and we live in it full-time.

The voltmeter measures the difference in voltage between 2 points and displays it as volts on the readout.

In a campervan or RV, those 2 points are the ground point and the battery positive terminal.

You can interpret the reading to indicate four things:

  1. Estimate of remaining battery
  2. When the battery is in danger of over-discharging
  3. The battery is charging
  4. A potential problem

Estimate of remaining battery

As a battery discharges, its voltage falls. The diagram below provides an estimate of how full a battery is for a given voltage.

The voltage of a lead acid batteries compared to lithium for their charging states

Note this is an estimate only. Different battery types and manufacturers vary slightly, but it’s a good enough reference point.

Note, too, lithium batteries sit at a steady 13v until they’re flat.

With a voltmeter reading of 12.5v on a 200ah AGM battery, it’s about 75% full, so it has an indicative 150ah remaining.

As AGM batteries only offer around 80% of their capacity as usable energy, this reading suggests you have an indicative 120ah of usable energy remaining. 

However, the problem with this reading is when you begin using the battery and charging it.

Let’s say your solar panels are charging the battery. The voltage reading indicates the charging voltage, not the battery voltage.

So when your batteries are charging from the solar panels, it’s not possible to read anything into the battery’s charge state. 

Or you begin charging a laptop. Suppose this is at night to remove any charging current from the example.

The voltage reading drops probably well past the battery’s actual voltage because of internal resistance and Ohm’s Law.

Take the load off the battery, and the voltage will settle. 

For an accurate voltage reading, a battery needs to sit unused and not being charged for around 3 hours. It’s not a realistic prospect when living in a campervan.

When the battery is in danger of being over-discharged

Over-discharging is especially bad for leisure batteries’ health and shortens their lives drastically if done too often.

Lithium batteries are unaffected by being fully discharged, but lead-acid don’t fair so well.

Check on the battery specification for the manufacturer’s advised discharge level. Then use the voltmeter reading to monitor how close to this level you’re getting.

If getting too close, you can take remedial action to cut back on use, protecting the battery.

The battery is charging

Someone using a mobile Bluetooth app to remotely monitor the batteries

When the battery is charging, either from shore power, from the engine with a split charge relay or battery-to-battery charger, or the solar system, the voltmeter displays the charging voltage, usually around 14.1v.

There’s no other reason for the reading to be so high, so you can use it to indicate the batteries are charging.

There’s more information about charging profiles in this article about charging leisure batteries.

A potential problem

The voltmeter can indicate a potential problem with the electrical system.

For example, you might expect the batteries to be charging with the engine running or the sun shining. The reading should be around 14.1v.

If not, it’s a good indication you have a problem somewhere, either with the voltmeter itself or with the charging method.

Equally, if the voltage reading is extremely low, it indicates the battery may be losing a lot of current.

Perhaps your load is too high, or there’s a faulty appliance. Either way, it’s time to start troubleshooting before you kill the battery bank.

Measuring & Interpreting Current Readings

Installing ammeters is another method to monitor aspects of the electrical system.

Reading current use in real-time is useful for battery monitoring and early warning of problems.

The only way to measure current with any accuracy is to pass it through a sensor. 

Ohm’s Law states Amps = Voltage ÷ Resistance

An ammeter consists of a shunt and a sensor.

The shunt is installed on the negative wire, introducing a known resistance, which leads to a tiny voltage drop.

The sensor measures the voltage drop between one side of the shunt and the other, using Ohm’s Law to work out the current.

Though the shunt increases the current drawn, it’s by such a small amount that it shouldn’t affect the fuses needed. It’s worth checking, though.

There’s a few places within an RV electrical system ammeters could play a part:

  • To measure the total (net) amp use, considering the charge coming from solar or the engine and any load on the system.
  • To measure current drawn by individual or combinations of components.
  • To measure current provided from charging sources. Note, though, the current supplied isn’t the same as the current absorbed by the battery. A good MPPT solar charge controller will indicate how much current the solar panels are producing.

Hall effect sensors are a less accurate but less intrusive way to measure amps. They work by measuring the magnetic field around the wire.

We don’t recommend them because they’re so inaccurate and vary widely depending on environmental factors like wire thickness.

What to Look for When Buying an RV Battery Monitor

Choosing the best RV battery monitor system for you is as much about satisfying your sense of curiosity as anything.

Remember, whatever information the monitor provides is only indicative.

For the most accurate reading, especially on battery voltage, use the battery manufacturer’s recommend monitoring system.

Here’s a list of things to look out for when buying a battery monitor for your electrical system:

  • The prices of battery monitoring systems vary a LOT. Cheap and cheerful voltmeters can cost less than $10, while some high-end monitors can cost hundreds. How much you’re willing to pay to monitor your battery bank is up to you.
  • Mounting styles differ too. Many monitors fit flush for a more aesthetically pleasing finish, but others protrude. It might be necessary for you, depending on where you install it.
  • The current draw varies a little, and if living in your camper full-time, so small it’s hardly worth considering. But if you want to monitor your batteries while the camper is in storage, bear in mind that the monitor does draw current, so it will deplete the battery over time. If you prepare your RV for winter storage, you may want to switch the entire electrical system off to avoid this.
  • Accessories & fittings | Check if all the fittings necessary are included with the monitor of your choice.
  • Ease of installation is important, too, as is access to installation instructions unique to the device.

Battery Monitor Wiring Diagram

The diagram below is an example of how to wire a battery monitor. Some devices may have different installation instructions, so always follow the manufacturer’s instructions.

RV battery monitoring wiring diagram

How to Read RV Battery Monitor Panel

High-end battery monitors provide user-friendly displays. The Victron BMV 712’s mobile app doesn’t get much more straightforward.

For monitors without an app, reading the panel is usually a simple matter of pressing a button and rotating through the display.

Read the user manual in detail for the specifics of your monitor.

For simple voltmeters, refer to the diagram above for your battery’s indicative charging state based on the voltage reading.

Whatever RV battery monitoring system you install, you’ll quickly get used to what normal, healthy readings look like for your set up once living in the camper.

The measurements may not be completely accurate, but they’re good enough to provide an early warning sign if things aren’t working correctly and allow you to adapt usage if running low on power.

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6 Best RV Battery Monitors _ A Buying & Installation Guide

Check out our introduction to RV battery monitoring systems for a brief overview.

Gary Molloy

Friday 28th of May 2021

I have a monitor for the engine battery and it only works within 10ft of the campervan.are there any what connect greater distances?my storage is 10 mile away and up until now I have to be next to my van for the Bluetooth to work.thanks.

Angela and Graham

Friday 28th of May 2021

Hi Gary, The Bluetooth enabled battery monitors only work within the range of the Bluetooth. For truly remote monitoring, you'd need a full BMS system. Victron do a range of products for this exact purpose but it's more specialised than simply monitoring - we'll get round to writing a post on it one day! Hope this helps. Graham