how to monitor RV battery

How to Monitor Batteries – RV, Sailboat & Tiny Houses too

I’m going to be honest, you don’t need a Battery Monitoring Kit, but you will want one.  Your battery is like a fuel tank and a BMK is the fuel gauge.  Operating an RV, Sailboat or Tiny House power system without a BMK is like driving a car around without a properly working fuel gauge.  You’re bound to accidentally run out of juice at some point.

We’re now transitioning from RV to sailboat and busy getting our power system set up in the boat. Our six years of RV’ing have taught us a lot about living off the grid and managing our power consumption (aka monitoring our batteries).  The battery level check button that comes with most RV’s is junk.  Our first three RVs didn’t have a BMK installed (and I am sure we wrecked our batteries because we didn’t).  In fact, I didn’t even know what a Battery Monitoring Kit was just a couple of years ago.  However, once we installed one on the Bounder, now we can’t live without it!

What the heck is a BMK?  How does it work?  Why is it so important to have one on my RV, Sailboat or Tiny House? We’re no experts by any stretch of the imagination, but we did our best to show you our BMK in use and answer those questions in the video below.

Our Gear

Install and Setup

I’m not going to attempt to show or tell you how to set up a BMK (or attach the shunt), I called the manufacturer and their customer service walked me through the step-by-step process, and that’s what I’d recommend you do as well.  I’m writing this article because so many people have asked “how do I know when my batteries are low”,  “how much power can I pull from my batteries”, “what does it mean to manage your power”  “What is a BMK” “Why would I want a BMK?” so I’ll attempt to answer those questions based on our experience.

The Fuel Tank Analogy

The easiest way for us to understand the BMK is to think of it as a fuel gauge.  Consider the batteries in your “fuel tank”.  Your solar panels (or generator) would be your “gas pump”.  The BMK would be the “fuel gauge” because it shows how much “fuel” is left in the “tank”, how much “fuel” is being pumped in, and how much is being consumed.  Most RV’s are built without a BMK which seems ridiculous, would you purchase a car without a fuel gauge?  How do they expect us to know when our batteries need to be charged or “fueled up”?

The Battery Level Button is Junk

Yes, most RV’s come with a little button that you push and it shows an approximate charge level.  This is a trick to make us think we are monitoring our battery level.  Truth is, these meters are pretty much worthless.  They might show 100% when the batteries have a false charge of only 80%.  If you turn on a high draw device like a water kettle or a blow drier, it may show the batteries are instantly at 25%.  How can you truly know when your battery bank reaches the harmful 50% or lower charge when the information this meter is providing is wildly inaccurate.

Biggest BMK Benefit

Every wire that touches the battery bank is wired through the Shunt:  the solar, the inverter, the engine alternator, etc.  The shunt and the BMK work together to give you the information you need to know about your batteries.  The two most important numbers for me are the “SOC (state of charge)” and the “AH In/Out (amp-hours in/out)”.

SOC is the State Of Charge of the battery bank.  I type in my total battery amp-hours, which is 700ah, and the BMK shows 100% SOC only when the battery bank reaches the full 700ah.  Because all the wires pass through the shunt the BMK knows exactly how many amps are going into the battery bank and coming out of the batteries.  Monitoring each amp is the only accurate way to know the exact battery level.

The Amp Math

In our video, we ran several different devices to see how many amps they pulled from the battery bank.  I could get the device wattage from the manufacturer, do a conversion from watts to amps, then come up with an approximate power draw of each device, but we like live testing to see what happens more than calculating the math on paper (we’ll avoid doing math any day).

There are plenty of free Solar Calculators out there, here’s one that we’ve used:

A solar calculator is a good place to start your battery and solar upgrade research, but the numbers are generic and may be different from the devices you actually own.  On the other hand, a BMK will show the exact amp draw of each device and track the total usage throughout the day.  This is why we showed the “real life” numbers in the video from the devices in our RV.  Here are the devices we used along with their approximate amp draws:

  • Cell Phone Charger: 0.2ADC
  • Residential Fridge: 20ADC
  • Hair Dryer: 80ADC
  • Microwave 130ADC  (we don’t actually use a microwave because we have these: Fave Kitchen Gadgets
  • Dyson Hot+Cold
    • 5ADC on Fan High
    • 115ADC on High Heat

For Example

Let’s say on a chilly day of boondocking we run the Dyson heater for one hour (115 amps), the Hair Dryer for 10 minutes (8 amps), the microwave for 10 minutes (13 amps), the fridge is cooling for 20 hours of the day (400 amps) and we charge our phone for 10 hours (2 amps).  Our BMK would show our battery bank as -538 amp hours or 23% SOC.  That is really low.

  • Lithium – Because we have lithium batteries we can drain our batteries this low (we can go as low as 10%) but we prefer to keep them 30% and above.
  • Lead Acid or AGM – Led Acid or AGM batteries should NOT be drained below 50% as it drastically reduces the life of the battery and its ability to hold a charge.

Now let’s add solar to the mix!  Let’s imagine it’s a sunny day and we’re bringing in 50 amps from 10 am to 6 pm with our solar kit.  8 hours of charging at this rate would put 400 amps into our battery bank.  With that solar power added into the usage mentioned above, our BMK would read -138 amps or 80% SOC.  That’s a lot better than 23% SOC without solar, but it’s still not 100%.

What does all this information really mean at the end of our day?  We need to conserve more power.  With these details we know we if we keep using this much energy we’ll continue to discharge our batteries at a rate of 20% each day, meaning we’ll be forced to run the generator which is something we don’t like to do while wild camping.  Instead, we’ll put on warmer clothes so we don’t need to run the heater and cook on the propane stove instead of the microwave, this small change will get our battery bank back up near 100%, which is where we like it!

As I mentioned before you can do a lot of this math without a BMK.  You can manually track your usage throughout the day and can use the built-in battery meter to help give you an idea of your battery consumption.  For us, having the BMK to accurately monitor our battery levels is the best way to manage our use and keep our batteries from unsafe discharge levels.

We didn’t have a BMK the first several years on the road and we survived, but our batteries did not!  I remember hitting ZERO on our Vesta while camping in the Mono Lake National Forest as a freezing winter storm rolled in off the nearby mountains.  Our batteries were completely dead, we couldn’t run the heat, we couldn’t start the RV, we just had to bundle up and snuggle until daylight when our solar could bring in some juice to start the coach and we could head to a lower elevation.


Famous for my "how-not-to" videos, and typically the man behind the camera, sometimes I’m forced to be here in the “spotlight”. When you see my face you’re probably reading something more technical than adventurous, but either way I do my best to tell it like it is and infuse my opinions into the commentary…after all this is a blog and not MSN.

Comments (39)

  • Doug Crombie

    Hi Nikki and Jason. I love all your videos. In your battery monitoring video, you had a Magnum BMK. On Curiosity you have a Xantrex. Do you have a preference for one over the other?

  • Dave B.

    The “amps” you list for device consumption over time are actually “amp-hrs”. For example, you note that the fridge consumes 20 “ADC” (I presume you mean amps, direct current) and that if it runs for 29 hrs, it consumes 400 Amps. That is incorrect. It consumes at a *rate* of 20 Amps but consumes 400 Amp-hrs as an *amount* of energy. For your fuel tank metaphor, Amps are like gallons/hr (a rate) and Amp-hrs are lije gallons. I’m afraid this error could be confusing a lot of people.

  • Margo

    I want solar and am trying to educate myself on everything power. I am tired of jumping my rv with my toad and not knowing what power is left in my batteries. I hate that I have to run my Onan 5500 for who knows how long to what charge and burning that gas and the fumes and air pollution that could kill me if I fall asleep which also means no tv to fall asleep too, which could be nice when the nightlife is so loud outside my open windows. Did you say AC…. wow so how is all of this done on batteries and not AC.

  • gary kolenich

    We are hitting the road for a year adventure and your blog is awesome. We are going off the grid and was wondering how many watts of solar panels do I need to keep our 6v batteries charged. I’m looking at a Goal Zero Maintainer 10 Trickle Charger – Solar Powered. Saw it on etrailer.

  • Vicki

    Hi Jason and Nikki!
    Just wanted to say that I’m really enjoying your blog! I came across it as I’ve been planning on buying a catamaran and sailing the Caribbean too! I had a piece of land in maui that I shipped an Airstream Trailer to. I put a solar system and composting toilet on the property! I’ve learned alot as time went on and wonder if you are using an MPPT controller? I found the efficiency went way up with that addition. I really like your shading and parallel vs. series tests and recommendations for the lithium batteries. I used wet cell as they were way less expensive, but had nothing but problems, especially because I didn’t have the BMK! I also wonder if you’ve had, on the boat, any corrosion problems. My property in Maui was on the ocean and both the composting toilet fan and the MPPT controller got damaged quickly due to corrosion. The fans were cheap to replace, but the controller is not! Anyway, thanks for your info and maybe I’ll see you out there soon!

  • Nikki,

    We have a small T@B trailer with outside galley. Our 80 watt Zamp portable system is often shaded by trees and can’t keep our two 6volt batteries charged. Do we just need more panels?

  • Breena Greek

    What kind of lithium batteries do you all use? I know that you are on your boat now, but we are just getting started with out rv adventure! I plan to go with the Go power pack, and lithium batteries maybe, but there are so many out there I am not sure how to sift through them. Thanks!

  • mark goldsmith

    Quick ? How does the Magnum BMK w/remote work on a boat when you have multiple battery banks. Our boat has a starting, house and bow thruster batteries.

    • Joe the computer guy

      There is the same issue with the RV. It has the house and chassis banks. The remote (ARC50) gets hooked up to the inverter/charger. The bmk is hooked up to one inverter/charger and in theory to one back There is a shunt (measures the current to ground) hooked up to the main bank. So the main bank is what is monitored. Depending on your setup and system the different banks can be connected together via solenoids but that is going to be setup specific. You setup the system by telling it the size of the battery bank. Then it know what the SOC is based on how much you have taken out/put in in AH. Magnum is great to work with if you ever need to call them and ask questions.

  • Bruce

    Just found you blog series. Very interesting comparing your rig with ours. Our rig is very small and we do not have a lot of space. We are Australian, but have a NorthStar TC700 slide-on camper (purchased 8 years ago in Oz, but made in the USA). All four wheels are driven, and we can go pretty much anywhere most other 4WD vehicles can go.
    Our TC700 did not come with any metering for the battery, and we suffered as a result. It was not long before I installed a Voltmeter and Ammeter. since then the metering has gone through a number of upgrades, the latest of which works very well. It is a Cycle Analyst (details on the web), and displays V, A, W and Ah, with a backlight. When the 180 Ah LFP battery is fully charged (potential above ~13.8 V) just reset the Cycle Analyst, and the Ah reading tells how much charge you have drawn and how close the battery is to going flat. You have to re-zero every few days, because the battery is only 95% efficient, so you never get out quite what you have put into it. This is an excellent low cost solution to battery monitoring.

  • David Brandi

    Hi Jason, we live in Georgia and are interested in getting a complete solar system Like the one you have. I have 2 questions: 1, if we change out our motorcoach can we take the solor with us? And number 2, who could we get to do the install and how much would it cost?. We love your blog so keep making those great videos. We appreciate you! Oh by the way we have a 2004 Infiniti four winds 35 foot motorcoach with a gas engine

  • Lisa Fechtler

    I just paid $300 for 2 chassis batteries for my Monaco Diplomat. That was in February, since then I have had to charge them a half a dozen times just to get my RV to start. I turn the switch off to them to thinking this would keep them from going dead. NOT working. Any ideas.?
    Thank you

    • You should grab a battery tester from the auto store and check to see if your batteries are bad. Bad batteries won’t hold a charge.

  • William

    What type of electrical power system will you use on your boat? (Solar, gas generator, water generator, wind generator, etc.)

  • David Gilmore

    Thanks Jason and Nikki,
    We’ve been following you for several years and really appreciate your time and effort in producing such great videos. We are heading in the opposite direction…sailboat(s) to rv(s)…closing in on our own adventure. Lived on sailboat several years ago in Hong Kong and cruised Pacific Northwest on our own boat and on Oceanlight2 with Tom and Jenn…(great folks providing eco tourism opportunities from April to October every year). Your upbeat and informative blog is really refreshing. You’ll appreciate all the hard work you are putting into learning your new craft…sailing…as it can be pretty exciting at times especially when mother nature provides you with her version of entertainment…Safe travels and thanks again…looking forward to following your progress…
    Dave and Anita

  • illya

    The website looks different, is it something i did or did you guys make things bigger? I decided on a vmax 125 AH battery for the cargo conversion. No real plan on how to recharge it yet but i forgot i have to power the water pump also. I was trying to avoid running 12v lines from my tow vehicle to the trailer since ill already have 120v ran.
    Today im getting my ac unit which will be all inside and vented thru a dryer style vent and AC fan that comes on when the unit powers on. The condensation will go down into a pvc pipe with rough filter media to trap the water and bring in moist cool air like a swamp cooler for added cooling. I really didnt want the back of a window unit sticking out, so ghetto lol. Ill take pics of my build on this and get them up somewhere. Also using a composting toilet 🙂
    You guys have so much great info on this site! Thank you

    Thanks, illya

  • Jim

    Yikes, just a quick FYI regarding something you demonstrated in your video. No offense, especially since you mentioned that you rarely use a microwave, but you should really NEVER run a microwave oven with nothing in it – I was always taught that the magnatron tube could get destroyed in short order from the microwaves bouncing back into it. Use something even as small as a cup of water as something to absorb the microwaves. Now if it was a convection oven too and you used it ONLY on convection for your test, then you’re only using the heat section and should be safe – it’s just when you use “microwaves” with nothing in it that it could destroy itself. Luckily you only did that for a VERY short time (we’ve probably all done it by accident at some point) and most likely were lucky enough to not damage it. Other than that, great information from you guys as always! Thanks! 🙂

    • Jim

      PS: reviewed video and noticed that you did say you probably shouldn’t run it for very long without anything in it. Missed that the first time, so just a friendly FYI that I would still recommend not running any microwave without something in it to absorb the microwaves regardless of how short a time it’s run. And sorry for veering off the main topic – just wouldn’t want a lot of people to maybe try the same test without something in their ovens too. 🙂

  • Thanks for a great post and video. I have just installed a solar panel on my motorhome and I am about to change the leisure batteries. The idea of a BMK is appealing. Trying to work out all the usage from manufacturer’s ratings, applying the maths and estimating the usage time is both complicated and only a best guess calculation. This will allow you to see what you are using, the state of your batteries and the recharging effect of your solar panel in real time, cutting out the complicated equations and best guess estimates.It is fair to say though that in Europe we try not to be as power hungry, especially with inverter use. My current project is to try to convert as much as possible to draw directly from 12v DC power. Laptop chargers, camera battery charges and even a 12v lead to use my electric razor are all available. 12v DC to 5v USB ports for all phone and tablet charging. It is silly to use an inverter to ramp up the 12v voltage to 110 (230v here in the UK) and then converting it back down to 5v USB – a huge draw and wasted power. This just leaves the other half’s hairdryer and occasional coffee from the coffee maker running through the inverter. Both short time usages. I am still keen to get a battery monitoring kit so I can always see how things are performing at a glance and determining if I will need extra solar panels or battery storage capacity. Thanks again.

  • Christopher E.

    Your battery and BMK resemble battery care on my all-electric Nissan Leaf, complete with the obsession! Have you considered converting your boat motor to electric? Probably the last thing on your mind (“let’s just get under sail already!”), but maybe something to research in years to come. I’ve been reading about sailboat owners who swear by their electric motors. Carry a gas generator for emergency charging.

  • Pete

    Is your towbar still available? If so, do you know how it can be shipped? My memory is that it is too heavy for UPS.

  • illya

    Great video and very informative 🙂
    I am still debating if i will need a battery in my cargo trailer conversion, i would love one of those kits for my main setup, I’m sure i would probably crash or at least run off the road checking it constantly lol. I would like to know how to accurately test a battery to know exactly how many AH it really has, instead of just going by the rated specs, I bet that electronics guy carlson on YT could tell me how. You guys pushed me the little extra i needed to getting my solar and i am so glad i did, it is a lifesaver for sure. My 6 batteries are always 100% SOC overnight now and that will certainly help their lifespan. I wrote down the 12v items ill be using in my trailer camper and it isn’t much at all, biggest thing is the maxx fan, so i may not even need a battery but having one may just save me one day also. Tough decision. Thanks for another awesome video ! Looking forward to you guys getting settled on the boat and let the adventures begin.

    Thanks, illya

  • Roger Brekas

    Great video again. Since watching your videos, I’ve purchased a digital battery monitor that plugs into my power port and I never draw my batteries down below 50%. You’re great teachers. Thank you.

    • Nik

      Just a heads up, if it plugs into the power port, then all it can do is measure voltage. Unfortunately Voltage is an incredibly inaccurate measurement for how full your batteries are. If you are charging, it will read too high, if you are running something big like a microwave it will read WAY too low. True BMKs measure amps in and out (by measuring voltage drop a giant resistor called a shunt) and comparing that to the size of your battery bank. If you aren’t doing all of those things, your digital meter is just a fancy (and not very helpful) volt meter.

      • Ross Williams

        I think it is important to understand that even most amp counting BMK’s are not really accurate. Over time those inaccuracies can lead you to believe you have a lot more available power than you actually do have. They need to be re-calibratedd regularly.

        For lithium batteries, voltage only really provides useful information at the extremes. It can tell you when you battery is fully charged and it can tell you when the battery is getting very low, but it won’t change much in between. But voltage can tell you if your battery is really fully charged and warn you if your battery is getting low even if the amp count BMK is telling you something different.

        With AGM batteries, voltage can give you a rough idea of how much power you have left. To be truly accurate it needs to be rested with no load for 12-24 hours. That isn’t practical. But absent major loads, like the ones you describe, it will still give you a rough measure to tell you if you are in the 50% discharge range when an AGM battery should be recharged anyway. If you are consistently getting close to 80% where you have to shut off the electricity, then you need a larger battery bank.

        Monitoring your batteries can become addictive. What you really NEED is actionable information and that depends on both your system and how you use it.

  • Mike

    been learning how to live by shadowing you guys with my R.V. now Im ready to go get my Catamaran, if you ever up the N.E river at the top of the Chesapeake Bay, you go ahead and throw a line on our dock and stay awhile

  • Michele

    Just in case anyone is trying to follow your math to calculate their own usage:

    >> the Hair Dryer for 10 minutes (8 amps), the microwave for 10 minutes (13 amps),

    The hair dryer s/b 13 amps (80/6) & the microwave s/b 22 amps (130/6)

    It’s 10 minutes (1/6th of an hour), not 10%. 😉

    Looking forward to watching your new adventures.
    I wish you fair winds and following seas.

    • RobertM

      I noticed that too. Instead of doing the divide by 6, I just guesstimate by halving the values. 80ADC=1 hour; 40=half hour; 20=15 minutes

  • Gordon Medley

    That was some good info, thanks.

  • Bri

    We don’t have nearly as many devices on our sailboat (it’s a little Catalina 22 day sailer) but my husband is basically glued to the charge controller and monitor. Every time we’re at the boat, he checks it pre and post sail. We’ve found that a 20W solar panel is enough to keep the boat fully charged while charging phones, running the stereo for several hours and even powering a laptop or small tools for working on the boat- dremel, hot knife etc. We’re certainly not going to plug in a microwave any time soon, but it works really well for our boat.
    I’m a relatively new reader, but I’ve enjoyed catching up on your adventures, keep up the great work!! And if you ever land in Oregon we’d love to buy you a beverage. Safe journeys!

  • I. Walls

    Hey Guys, love the videos. Glad you finally went wet. I’ve been on the hook for a few years now. Just watched the BMK video and just one thought. Everything you tested went through the inverter, which in and of itself uses tons of power. If everything you used were actual DC I think you would save many many amps from not having to use the battery to ‘invert’ the DC battery amps to ‘ac’ power. That’s been a big plus for me and many of my friends. On the boat I use solar and wind. Staying off the inverter saves a lot of battery. Keep up the good work. Entertaining and informative. Good luck with the lessons and the new life.

  • Larry Odom

    Excellent information in an easily understood fashion. Thanks!

  • Paul de Muinck

    We suffered a dead battery scenario our first camping trip and invested in series connected 6V Golf Cart batteries and the Xantrex Link10 battery monitor. Our next trailer will have one as well although the Link10 is replaced with an LCD model now.

    We really like the Xantrex units since they have a display on them as well as Serial output for downloading data.

  • John Puccetti

    You two are just plain fun to watch. But most of the information could have been in one five minute video two years ago. Battery technology and discharge is the core of everything to your information. Your travels is what is interesting.

  • John S.

    “Because all the wires pass through the shunt the BMK knows exactly how many amps are going into the battery bank and coming out of the batteries.”
    Really? That caught my eye so I had a look on the Magnum Energy website and it now makes more sense.

    Great little video (love the intro) and the GoPower link will be very helpful.

  • Thom Colby - 888-391-5245

    Hey you guys, thanks for the new post about “power”. Hope you’re enjoying the holiday weekend. All the best,


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