For those who choose to camp off the grid it can be nerve-wracking trying to figure out how much juice (amp-hours) is left in the batteries, especially when you’re relying on them to power essential equipment like your furnace on a cold night. Most trailers and motor homes come from the factory with deep cycle lead acid batteries and include a basic gauge that tries to tell you whether the battery is charging, full or low based on its voltage. While voltage can be used to indicate the charge level (state of charge) of a battery, in order to get an accurate reading the battery needs to be at rest (not charging or discharging) for at least four hours. That’s not easy to do when your camping and need to use the battery. Measuring the specific gravity of the batteries’ electrolyte is accurate, but who wants to get out a hydrometer and pull a sample of battery acid at 10:00pm to check how full the batteries are? Also, specific gravity measurements are not possible with sealed or lithium batteries.
Another challenge for the stock voltage battery gauge occurs if you’ve upgraded to lithium batteries. Lithium batteries operate at higher voltages than lead-acid batteries and have a much flatter discharge profile meaning there’s not much voltage difference between a full lithium battery and one that’s 90% discharge. The stock gauge will always show charging with the lithium batteries until the battery get below 5 or 10%.
Enter the battery monitor. Battery monitors measure the flow of electricity into and out of your batteries and based on their capacity (in amp-hours) provide a fairly accurate estimate of the amount of charge remaining. This is the same method used by your phone or laptop to estimate it’s charge level. It’s not perfect but it’s probably the best you’re gonna get.
How Battery Monitoring Systems Work
In order to measure the flow of current into and out of the battery bank a device called a shunt is installed in the line between the battery negative and the RV ground. The shunt must be sized to carry the maximum current expected from the batteries. The most common rating for shunts is 500 amps, which should be more than enough for most RVs, but be aware that there are battery monitors that are rated as low as 30 amps or less. Make sure you read the spec’s before you buy.
The shunt is the place where the measurement takes place, but since it’s usually located in an out of the way location in most cases the battery monitoring system will have a remote display that connects to the shunt. There are some newer units where the shunt has Bluetooth® capability and your phone or tablet becomes the display for both configuration and readouts.
The display will calculate the flow into and out of the battery and can show real-time current flow in our out of the batteries, % full or hours at the current usage rate until empty among other things. Some can be programmed to sound an alarm when the batteries go below a programmed charge level.
Do You Need a Battery Monitoring System?
That depends. If you almost exclusively camp where you have electric hookups then probably not, but as you spend more time camping off-grid monitoring becomes more important. Only you can make the call. If you find you’re spending 5 or 10 nights a year unhooked and you’re concerned with your batteries’ charge levels, maybe it’s worth the time and expense to install one.
We frequently camp without any hookups in state parks, national forests and national parks and as part of our upgrade to lithium batteries added a Victron BMV-700 monitor a few years back. It’s given us a much better understanding of the status of our battery bank and some peace of mind. We’ve found it to be well worth the cost. It’s a lot easier going to sleep at night knowing you’ve got 150 amp hours left and are using about 60 a day then just guessing.
Popular RV Battery Management Systems
There are quite a few brands of battery monitors in the market. Here are a few of the most popular ones in the RV world:
Victron
If you read much about solar power and batteries relating to RVs you’ll probably run across more people using Victron battery monitors than any other brand. Their BMV-700 series monitors are quite popular. Some of them have Bluetooth® capability built in and several can also monitor a second battery such as a motorhome’s starter battery. They also have a product called the Smart Shunt that uses Bluetooth® to connect to your phone or tablet, eliminating the need for mounting a separate remote display.
Renogy
Renogy has a 500 amp battery monitor with shunt, the RBM500-US, as part of their offering of batteries, chargers and solar panels. If you have Renogy Smart lithium batteries they also have a monitoring screen that displays information from the batteries’ built-in BMSs.
Xantrex
Probably better known for its inverters and chargers, Xantrex offers its LinkPro (10,000 amp) and LinkLite (1,000 amp) battery monitoring systems. Both are capable of monitor a second battery, allowing the same unit to monitor both the motorhome’s house batteries and starter battery.
Bogart Engineering
Bogart Engineering has been making its Trimetric battery monitors for more than 35 years. The latest in that series is the shunt-based TM-2030, which is compatible with all battery types.
How Hard is BMS Installation & Setup?
It’s not rocket science, but you do need to realize that battery banks contain a lot of energy and that there is risk of electrocution resulting in injury or death, fire or other damage to your RV and its systems. If you’ve done electrical work in the past and are comfortable with disconnecting and re-connecting battery banks, make sure you read and understand all installation instructions and warnings from the manufacturer and follow them. If you’re not sure, hire a pro. It’s not worth risking injury for the few hundred dollars you might save.
Whatever model BMS you choose, the shunt will need to be installed in the negative battery cable, between the battery bank and RV ground. If the system has a remote display, that also needs to be installed along with running a wire from it to the shunt.
Configuring the system is pretty straightforward. You’ll need to tell the monitor how big your battery bank is in amp-hours along with a number of device-specific parameters that let the monitor know how your charging system and batteries are set up. There are likely dozens of parameters, but in many cases default values will be fine. Consult the user’s manual for help in customizing them.
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