We decided to follow up the post on our lithium ion battery install with a post on the very important piece of gear that’s so important it was our first RV mod. We love to camp out in the boonies, but we work on our computers ( we’re not retired ) and are techie people. Stable electrical power is important to us.
While we’ve got solar panels making power for us up on the roof and a generator if we need it, a big key to living successfully without running out of power or prematurely killing your batteries is knowing how much power you’re using and how much you’ve got left. This tells you if you can easily make it through the night, a cloudy day, or if it’s time to crank up the generator to keep up the batteries. Once the generator is on, it’s good to know when the charging rate is tapering off, or if the batteries are charged, so we’re not running the generator unnecessarily, saving wear and tear, as well as $$$ for gas, oil and maintenance. Plus, we like the peace and quiet.
Why add a meter?
Almost all RV’s, as they come from the factory, are woefully unprepared to do this accurately. The battery meter that comes standard with an RV is essentially just a voltage meter, and voltage alone is a poor way to measure the battery state of charge ( or SoC ).
Here are a couple of typical RV battery meters.
Why? Because voltage based battery meters don’t work like a gas gauge. Unlike a fuel tank, which, when it has half of its contents remaining is half full, battery voltage only varies about 8% at most between full and empty. Less than that if you do as many people recommend and not drain the batteries below 50%.
Also, the voltage that is measured from a battery can change when the battery is being used or charged, and this change can put the voltage of the battery outside the normal range of the battery meter and lead to very incorrect readings. Here’s the voltage range of a typical 12v battery at rest ( no charge or load on the battery ) :
Most folks don’t like to take their batteries below 50%, though some go as low as 20%, which is the generally agreed point where damage to the battery happens. If you’re using 50% as your floor, that’s a change of just .6V from full to empty. We’ve found that the low voltage below 50% makes the low battery alarm on our inverter sound, so we rarely went below that point.
But when the batteries were charging, either from the generator or now from our solar, which is pretty much any time the sun is shining, that charging pushes the battery voltage above the “full” battery voltage, regardless of the actual charge state of the battery. In fact, normal charging voltages can be as high as 14.6V. Which means that any time the batteries were charging, the meter incorrectly read “full”.
At night, when we had lots of lights on in the RV, and often were running the TV and the stereo, that load on the batteries meant that the voltage was lower than the resting voltage, so according to the chart above, the battery meter often read significantly lower than the true state of charge. Nobody wants to go to bed thinking they’re going to run out of power overnight, so that can be worrying.
Voltage can be a decent way to read the battery’s charge state, but to accurately read the SoC, you should disconnect any load or charge from the battery and let it sit for a half hour or so. The longer it sits, the more accurate the reading is.
You can see the issue. The voltage that you’re looking to measure to tell the battery SoC is just a .6V difference, but charging or using the battery can result the voltage being 2+ volts high or 1V low, making that measurement useless.
Think about it like this: If the gas gauge in your car worked like this, whenever your foot was on the gas, it would read empty. Whenever you were coasting, it would read full, regardless of how much was in the tank. To get it to read accurately, you would need to park the car for a half hour, otherwise it might read as much as a half a tank off of its true level. You might pull over and the gauge would read empty, but a half hour later, it might read that you have a half a tank.
That would be fairly useless, right?
Well, there’s a solution. At least a half dozen companies make meters that work more like the “miles to empty” displays that are common in cars of the last decade or so. Those displays measure the amount of fuel that’s actually coming out of the gas tank, compare it to the speed that you’re going, do a little math and come up with a pretty good estimation of the gas mileage you’re getting and use that to figure out how far you can go. That’s handy.
This was one of the first changes that we made to our RV when we purchased it in 2012. I’ve had a long history of working with battery banks and these kinds of meters, mostly with the Xantrex Link line of meters.
In the 90’s I drove an Electric car while I lived in Key West and it used a Xantrex Link 10 as a “gas gauge”, which was good because the car only had a range of about 30 miles, so you had to know how much ( as a percentage, at least ) it had left. Hey, it was a small island, that was enough. I also helped install them in other homebrew electric cars that I worked on at the time.
Here’s a picture of my car from that time:
When my brother set up his RV in 2002 for off-grid living and full time travel, he added a Link 10. He upgraded his batteries and added a second Link 10 in 2004 so he could monitor both of his battery banks. Follow the links for the details.
Xantrex hasn’t stood still with their design. Their latest meter is the LinkPro, which has an LCD display, a “fuel gauge” style bar graph at the top of the display and can even give an estimate on how long your systems will run before you reach “empty”.
Adding a meter to our RV.
We understood when we purchased our 2003 Holiday Rambler Traveler in 2012 that we would have to replace the batteries soon. We wanted to keep our battery investment for as long as we could, so along with the new batteries we installed a battery watering system ( the batteries are in a difficult to reach location ) and purchased a Xantrex LinkPRO to monitor the charge state and energy use.
I found a spot to install it near the battery compartment, in what we use as a shoe cubby. In addition to the meter head ( shown above ) there is also a device called a shunt that measures the current flowing through the battery(s). The shunt needs to be wired into the large negative wire that connects the battery(s) to the ground of the RV. Typically, you will need to add a short length of cable to go from where you decide to mount the shunt to the battery, and any wiring that used to go to the negative wiring of the battery needs to go to the other side of the shunt. The shunt can handle 500A, so it’s ready for any RV 12v load, including inverters to 3000W.
The shunt ( bare ):
Here is ours, installed ( yes, it’s “backwards” but those are just labels, one side isn’t really any different than the other…):
The thin wires go the LinkPro, the heavy gauge wire goes to the negative pole of the battery ( left ) and the wire on the right goes to ground all the 12v loads in the entire RV.
The whole endeavor took me a few hours: make a 2 foot cable to extend the negative wire, install the shunt in the battery box and run some wire to where I cut a 2 1/16th hole to mount the meter head.
There are a few settings you need to adjust for your batteries, like the Amp-Hours of your battery bank, and the depth of discharge you want the meter to consider “empty” for your system. We set ours to the recommended 50% SoC, knowing we could go a bit lower if we needed to without damaging the batteries.
Our first trip with the meter came a few months later. Having good data was addictive, and we watched the meter constantly, learning about how our 12v electrical system worked, and from a practical standpoint, how much energy it stored.
We discovered that we could use the meter to figure out how much power individual items used in the RV, by looking at the amp display before and after turning a particular item on and figuring the difference. We discovered that the incandescent globe bulbs in our bathroom vanity fixtures were real energy hogs, using about 5 amps ( 5 amps x 12 volts = 60 watts ). We replaced them with LED’s that use about 6 watts. We did the same thing for the fluorescent lights that were used throughout the coach. At night, we only use <20 watts to light the whole coach most of the time.
We were thrilled to find that we could relatively easily go two nights without going below 50% charge. We didn’t have solar yet, so we ran our generator to recharge the batteries, but the LinkPRO told us when the charge rate was tapering off and we could turn off the generator, saving wear and fuel. We knew that if we brought the SoC up to 80%, it would hold us for another day before we got back down to 50%.
One of the cool screens the LinkPRO can cycle through is one that shows how long your batteries will last at the current rate of discharge, so you know if it’s evening, you’ve got the lights on, and it shows 12 hours, you’ll make it until morning.
Another benefit that showed up soon was that we were able to easily figure out how much power we use on a daily basis, which gave us a good data to use to figure out how much solar we would need to replenish the batteries.
Here’s the LinkPro showing our solar system bringing in 26.2A, during testing right after the solar install.
We spent a lot of time watching that meter early on, but we began to understand how our electrical system worked and how much power we used. Now, a quick glance at the meter tells us how we’re doing. We’ve had the meter for a little over four years now, and it has been invaluable to keeping us and our batteries happy.
In my opinion, everyone should have one of these, or something similar. If you don’t have solar, it will help you figure out how much you need and save you money on generator runtime, while giving you a few more quiet hours. If you do have solar, it will give you solid data on how much power you’re generating. Either way, it may extend the lifespan of your batteries by keeping you from draining them too deeply.
One of the other features of the LinkPRO is the ability to drive an auto generator start, so if your batteries get down to a preset level, your generator starts up automatically ( within set parameters, like quiet hours where the generator will not start ) to bring your batteries up to a different preset level. We haven’t found the need for that yet, but it’s nice to have the option.
There are several other companies that make similar items. The images below are Amazon Affiliate links. If you found this article useful, please consider using these links to purchase your hardware. You won’t pay more, but we’ll receive a small commission. Just click on the picture to be taken to Amazon.
First, here’s the LinkPro.
Here are some alternatives, and my opinions on them.
The Bogart Engineering Trimetric TM-2030-RV
This has been available a long time and has a good reputation, though it has been criticized ( with good reason, I think ) for lacking the ability to utilize a Peukert value, which is an equation to predict how batteries deliver less power with higher currents. This could make the Trimetric less accurate than the other meters here, but still worlds better than a typical voltage only meter. I prefer the other meters here, but if you get a good deal, or you use their solar charge controller, this could be a good value. The Trimetric comes in versions that can be mounted through a surface, or with a surface mount case, as shown in the image below.
I’ve set one of these up, and I found it a little more challenging than the LinkPRO to program. While I feel it’s a good meter, it’s a little larger and doesn’t give a “time to empty” number. Pricing is good, though. If you’re on a budget, this might be the one for you. There is a flush mount version, but the RV version includes a surface mount box. You may need to order the Shunt separately, so confirm before you order.
Victron BMV-700 series
A great meter. There are several models, some capable of checking voltage on a second battery bank, but not full power measurements. This is the 700, for one bank. I have recommended this meter to several folks and they’ve all been very happy with it. The BMV can trigger an auto generator start and there is even a bluetooth module you can add that will let you monitor your battery state from a smartphone app.
The Magnum BMK ( Battery Monitoring Kit ) is an add-on shunt and measurement device for their ME-ARC control panels, typically used with their inverter/chargers and solar controllers. If you’ve got one of their inverters in your RV, check to see if you’ve got the BMK. If you don’t, you should get this. It’s a great add-on, allowing you to accurately measure the in/out power and SoC. Frankly, I think any RV manufacturer that installs a Magnum inverter without this is doing their customers wrong.
However, if you’re already set up with another brand of components, I don’t think this is a well priced choice, since you have to buy both the BMK and the ME-ARC panel. Magnum also makes an auto generator start.
The Smartgauge is different than the rest of the meters here, in that it does not use a shunt. It only needs two wires to the batteries, monitoring their voltage with a precision algorithm that manages to accurately measure the power remaining in your batteries. They also adjust as the batteries age and lose capacity, something that I don’t believe any of the other meters do.
I know I said at the beginning of this article that voltage only monitors were inaccurate, but this would seem to be the exception to the rule. While I have not used one, I’ve read an in-depth review and believe that the methodology used is solid. They’re about $100 more than a shunt based meter and are only capable of reading lead acid batteries ( no lithium yet…), but if that works for you, not having to install a shunt makes installation a breeze. The only downside I can see is that while the battery SoC is accurate, you get no information on how much power stuff is using, which we found pretty important to learning how to conserve energy.
Blue Sea M2 SoC Meter
Intended for boats, the M2 faceplate is water resistant to IP66 standards. Capable of measuring voltage from three different battery banks and SoC for one, the M2 works with batteries to 48V and shunts to 5000A, though a 500A shunt is included. The M2 doesn’t appear to have a configurable Peukert value and while there are a bunch of lead-acid battery types you can select, Lithium Ion isn’t among the choices. However, since the software is user upgradable, so it’s possible to add features in the future. I like the OLED display that shows multiple values at once, but the price is a little higher than most of the other meters listed, about $270.
NASA Clipper BM-1 and BM-2
Another Marine meter, made in England, the BM-1 and 2 are fairly simple meters with capabilities to 100A ( BM-1 ) and 200A ( BM-2 ) which may be too low for some RV’s with large inverters or chargers. They’re available in 12v and 24v versions ( the other meters here generally auto-adjust to the system voltage ) and can monitor up to a 600Ah ( 999Ah for the BM-2 ) battery bank, which may be too small for some larger RV’s. The BM-1 is also available in a surface mount “compact” version if you want to mount it somewhere that you can’t drill a large hole for the back of a meter. NASA says the BM-1 and 2 account for Peukert, but the peukert value does not appear configurable, nor are there settings for different types of batteries, so the Clipper meters would not be good with Lithium batteries. A bluetooth enabled version of the BM-1 ( with no display, just a bluetooth app for your phone ) is also available.
If you can live with the current and capacity limitations, or you want a meter with no visible display, NASA might have what you need, though availability of these meters seems limited in the USA.
This is an add-on accessory for users of Blue Sky Energy’s IPN networked solar controllers, like the 3024iL . The IPN network is used to allow up to eight of their controllers to work together to charge a single battery bank, all acting as one larger controller and the ProRemote can monitor and control all of them. It also uses a 500A shunt to track battery SoC. Reading the manual, I can find no mention of a peukert value or integration with their algorithm, though it has temperature compensation.
If you’ve got a Blue Sky Energy controller ( or more than one ) this would be a good choice, but IMHO there are better, less expensive meters available if you’re using a different brand.
Midnight Solar Whizbang Jr.
The Whizbang Jr. is an add-on accessory only for users of the Midnight Solar MPPT controllers. It works with the Midnight Solar Classic ( high voltage and current ) and KID ( waterproof for marine use ) controllers. I don’t see a lot of these in RV’s, but if you have one and the MNGP ( Midnight Graphics Panel ) the Whizbang Jr. will add better battery monitoring. You may have to upgrade your firmware in both the controller and the panel. The Whizbang Jr. makes no mention of a Peukert setting, nor of lithium battery compatibility.
Ebay $32 Power Meter
This is a very inexpensive Chinese made power meter that measures amps out of your battery, and amps back in. Unlike most of the other meters here, it does not have a setting to reset it to zero, rather, it counts back down and won’t go negative. It does not appear to have any Peukert settings, but it will read voltage and amperage, as well as remaining battery Amp Hours and SoC in percent. Accuracy probably won’t be as good as the other meters here ( but might not be that much worse in some situations ), but you can’t beat the price.
While you may save a hundred dollars or more by going with one of these, you need to be aware of a few things: First, the manual is sparse and there won’t be a lot of support. Second, the meter is NOT CALIBRATED. From what I understand from someone that’s used one of these, it’s basically set at zero and has no idea what 12v and 100A are. So you need to know what you’re doing, and have devices ( or friends with devices ) that can measure voltage and current accurately to set the meter. You need to have a good voltage source and a way to drive the meter with a known current. Without this step, it’s worthless. Please read the manual linked above and understand what you’re getting into prior to purchasing.
The image below links to a current eBay sale, which may end soon. If it has, you may want to try searching for some of these terms. “DC Programmable 0-200V 0-500A Voltage AMP Ah Power Combo Meter Battery Monitor” to find an auction. Also, there are a lot of different versions of this meter on eBay, some with different current capacities, some without shunts. Make sure you know what you’re buying.
In my opinion, this meter is cheap, but may not be a good value. Any of the other meters above is a better meter. If you’re willing to live with the pre-use calibration with no support, and want to save a few bucks, give it a shot. Let me know how it goes.
That’s the list of meters I know of. If you know of ones I missed, please email me at firstname.lastname@example.org and let me know, or leave a message in the comments.
So, there you have it. Battery meters are a smart investment for everyone because they put you in control and they save you money in the long run by protecting your batteries and saving wear & tear on your generator.