Adding such a device provides important information about battery condition and power usage.
By Mark Quasius, F333630
Batteries are the heart of any RV. They provide power for lighting, fans, water pumps, and other 12-volt accessories. Eventually, though, batteries discharge. How long they hold a charge depends on how much they’ve been used and their condition. Boondocking, in particular, places demands on batteries and requires a bit of power management to ensure they retain their charge long enough to allow plenty of use between recharge cycles. And power management requires power monitoring.
Voltmeters are often used to determine a battery’s charge level. Ideally, a 12-volt battery should read 12.6 volts when fully charged. However, that’s the reading for a battery at rest. RV batteries rarely are at rest.
Discrepancies in voltage are common, because something always is drawing power, or the battery is connected to a converter or charger that is recharging the battery. As a result, a voltmeter can’t be used to determine the charge level unless the system is shut down and the batteries are allowed to rest for 15 minutes. That’s fine if you are taking the time to diagnose the charge level or condition of the batteries, but it’s not very practical when you’re using the RV and wish to take a quick glance at how the batteries are doing.
To determine the batteries’ true state of charge (SOC) — the percentage of charge remaining — a battery monitoring system is needed. The system monitors the amp-hours removed from a battery and the amp-hours returned to it during a charge cycle. It then computes a true SOC and displays that information, as well as voltage, amp-hours used, and more.
Standalone units, such as Bogart Engineering’s TriMetric Battery Monitor, Xantrex’s LinkLITE, and the now discontinued Link series from Heart Interface (now merged with Xantrex) are available. Each unit’s display device must be mounted in a panel where it can be seen easily.
I chose Magnum Energy’s ME-BMK Battery Monitor Kit. It utilizes an existing remote display from a Magnum inverter, so adding another display was not necessary.
All battery monitors require a precision resistor — a shunt — to be installed in the negative battery cable. Current going into or out of the battery passes through the shunt. The shunt acts like a transformer and sends a small signal over lighter-gauge wiring to the battery monitor.
My coach is equipped with two Magnum inverters, so I had to choose which display panel to use. The main sense module of the BMK connects to a Magnum inverter via a four-conductor, telephone-style cable with RJ-11 modular connectors. I mounted the BMK sense module on the wall of a basement compartment that is immediately adjacent to the bay that contains the coach batteries. I then ran a cable (which came with the BMK) from the BMK to one of my inverters. I plugged one end of the cable into the green network port on the inverter and the other end into the BMK sense module.
The next step was to install the shunt. On a typical battery bank, you simply remove the ground cable from the chassis ground stud location, connect the cable to one end of the shunt, and add a short jumper cable from the other end of the shunt to the ground stud. This ensures that all power flowing in or out of the battery bank passes through the shunt, where the amp-hours can be counted by the battery monitor.
In my case, the motorhome has eight deep-cycle 6-volt AGM batteries and two large Magnum inverter/chargers. Separate negative cables ran from each inverter to each group of four batteries. Each group of batteries was connected by a cable to a common ground stud on the chassis. This was a good wiring scheme but required a bit of remodeling so that all of the current would pass through the shunt.
First, I removed the inverter ground cables from the negative posts on each battery bank and attached them to the chassis ground stud. I then connected a short jumper cable from the ground stud to one end of the battery shunt, and I attached both negative battery cables to the other end of the shunt. However, the short threads on the shunt didn’t allow for two large cables to be stacked together, so I mounted an isolated stud to the back wall of the battery compartment and connected the two negative cables to that stud. I then added another short jumper cable and connected that between the stud and the shunt.
I mounted the shunt on the back wall of the battery compartment where there was plenty of room to place the cables and add the additional isolated stud. I then made my jumper cables using 4/0 welding cable, which is flexible and capable of handling large current flows without loss of voltage. A heavy-duty cable crimp tool was used to compress the copper terminal ends onto the cable. (Soldering high-capacity cables is not recommended; large current flows can overheat the connector and the solder can melt, causing the connection to fail.) I also slid some heat-shrink tubing over the connection and used a heat gun to constrict it. The tubing is filled with an adhesive/sealant and will prevent corrosion on the back side of the terminal connector. I sprayed battery terminal sealant to coat the studs and further protect them from corrosion.
Next, the sense module was fitted with a four-conductor connector plug. The lighter-gauge orange and blue wires were run to the shunt and connected to its smaller terminals. The twisted-pair wires prevent magnetic interference, which would affect the signal, so it was important to keep them twisted as long as possible. The red and black wires were run to 12-volt positive battery power and to ground, respectively. This supplies power and a ground connection to operate the sense unit.
All that was left was to program the BMK via the inverter’s remote display panel. The remote display is self-aware, so you simply access the setup menu and activate it. Next, enter the amp-hour size of the battery bank, which in my case is 880 amp-hours. The system can be set at “charge efficiency,” but in most cases you can leave it set at “auto” and let the BMK calculate the charging efficiency automatically. I left mine at “auto.”
Once everything was installed and the menu settings were specified, it was time to wait. The battery monitor kit may need up to 24 hours to analyze the usage and determine when the batteries are at 100 percent state of charge. In my case, it only took a few hours, because my batteries were already being charged in float mode.
The display shows that the batteries are fully charged when three criteria are met. First, the charging voltage has stabilized over a period of time. Second, the charging current has decreased to a low percentage of the amp-hour capacity — typically less than 2 percent. Third, the amp-hours that were removed from the battery are within 1 percent of being fully returned. Once those conditions are met, the system monitors all incoming and outgoing traffic and displays a true SOC at all times.
By toggling through the display, you also can view the real-time DC amps that are going into or out of the battery; the amp-hours returned to the battery during the last recharge cycle; the amp-hours removed from the battery (resettable); the total amp-hours removed since the sense module was first connected, which can serve as a battery service life indicator; and the lowest and highest battery voltage since the last reset.
The system was easy to install. With a Magnum inverter, wiring does not have to be run into the coach. Just plug the system into the inverter and use the existing inverter-to-remote cable to pass any information to the display.
If you do not have a Magnum inverter, you can install one of the other brand units that are self-contained. The only difference is that the display will have to be mounted someplace in the coach, and a cable will have to be fed to it. The installation of the shunt is common to all battery monitors and the information displayed by the monitors is very similar.
If you do a fair amount of dry camping, you’ll be able to make informed decisions to help with power management, plus you’ll have a better handle on the condition of your batteries and how they are aging.
Bogart Engineering (TriMetric)
19020 Two Bar Road
Boulder Creek, CA 95006
Magnum Energy Inc.
2211 W. Casino Road
Everett, WA 98204
3700 Gilmore Way
Burnaby, BC V5G 4M1