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How to Monitor Your Batteries Voltage

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Just because your voltmeter tells you the battery is full, should you trust it?

Many of us have a house battery in our van or RV that we are charging either off our engines alternator or starting battery, or you have installed solar panels which are charging our  house battery. Hopefully you have a charge controller with a digital display that give you the volts of your battery or you have installed you own voltmeter  to monitor your batteries State of Charge.
Monitoring your State of Charge is critically important to the long term health of your battery bank, because they want to be as close to full as they can be all the time. Every time you draw power out of your battery it is being discharged somewhat and that is called cycling it. If you take your battery down to 50%, or half of it’s capacity, you want to bring the battery back up to 100% full as soon as possible, preferably the next day!  That’s considered one full deep cycle.
The difference between a starting battery and a true Deep Cycle battery is how many cycles they are designed to handle. A starting battery is never supposed to be deep cycled and after 3-7 accidental deep cycles, it will be destroyed and need to be replaced. On the other hand, a quality Deep Cycle battery is designed to intentionally be discharged to half throughout their life and are purpose-built to be able to handle it. Going to 50% is not damaging to them. It’s common for a quality Deep Cycle battery to be able to be deep cycled 500-1000 times.
But, if you are abusive to your batteries, and cycle them down below 50%, you are actually damaging them each time and reducing the number of deep cycles they have. You really want to avoid that!!!
On the other hand, if you only take it down to a quarter, 25% discharge, that’s only a partial cycle and a quality Deep Cycle battery might could do that 1000-3600 times.  A high quality Deep Cycle battery could last 10 years if it’s lightly cycled (25% or less) every day!
A Marine Battery is a hybrid battery, it’s designed to both start engines and to be cycled. However while it’s okay at both jobs, it’s not great at either. I wouldn’t expect to get much more than 100-200 Deep Cycles out of one, and maybe less. Going below 50% will cut that even much more.
As you can see from this, knowing how deeply you are cycling your house battery is very important to getting as long a life as possible out of it. If you lightly cycle (25% discharge) it every day  it will last much longer than if you deeply cycle (50% discharge) it every day. Because Deep Cycle batteries are expensive (from $100-$350 each), and we often have 2-4 of them (totaling $200-$1200) you want to get the longest life possible out of them!
Below is a chart that lists battery voltage and what it means to the State of Charge of your battery:
As this chart makes clear, it’s very important we know at any given moment exactly what the charge of our battery is but that’s not as easy as it seems.  If you’ve ever watched your voltmeter you know it can read as high as 14.4 during the day and and as low as 11.6 when a big load is put on it. Worse, it can seem to change very suddenly by as much as two volts or more. With it jumping around like that, how are we supposed to know exactly what the voltage of the battery really is?

How to Get “Good-Enough” Accuracy

The truth is that total accuracy is complicated, but ball park is pretty easy and close enough to monitor it so we get maximum life out of our battery. Here’s what you have to always remember about the voltage readouts on your batteries:

  1. A battery that’s being charged–power going in–will read falsely high. The bigger the charge going in, or, fuller the battery is, the further the voltage readout is from accurate.
  2. A battery that’s being drawn from–power going out–will read falsely low. And, the bigger the draw going out the more inaccurate the readout is.
  3. A well rested battery–which means hours without any activity  (nothing going in or out) is pretty darn accurate. The longer it’s rested, the more accurate it is. When rested overnight it will be basically correct, after a week it will be dead accurate.

Two extreme examples:

  1. A battery properly being charged at absorption voltage can read 14.8, but stop charging it and it will fairly quickly return to reality which is still about 12.8
  2. If my battery reads 13.2 when I turn on my microwave, the huge draw makes it very wrong, it might read 11.6-11.9. As soon as I turn it off it jumps right back to 12.7 or 12.8.
This is a friends battery bank of 8 golf carts. You better believe he watches the voltage closely so he doesn't have to spend $1000 any more often than he has to!

This is a friends battery bank of 8 golf carts. You better believe he watches the voltage closely so he doesn’t have to spend $1000 any more often than he has to!

Check the Batteries When they are Rested

Assuming you sleep at night with everything turned off, in the morning before the sun hits its it, the voltmeter is very accurate. But you might think to yourself, “I need to know more than once a day what the accurate voltage of my battery is.” And of course you’re right, once a day is no where near enough. But by being aware of when it’s wrong and when it’s right, you can learn to compare the two and learn to make a mental correction for it’s inaccuracies.
You can do that by watching it and learning how it works. Over time you can get a pretty good feel for what it really is by knowing what the volt meter says and what’s been going on with it. For most of us, that will be accurate enough.
For example, it’s most important to know your state of charge in the evening after you’ve been using it because that is when you are most at risk of drawing the battery down below the all-important 1/2 full mark at 12.2 volts. If you are close to 12.2, you want to stop using all electricity you possibly can to save your batteries.  Fortunately, there is a fairly easy way to do that. Here’s how:

  1. In the evening just before bedtime, write down the date and the voltage of the battery.
  2. Next, turn everything off so the absolute minimum amount of power is going in or out from the battery, preferably nothing.
  3. In the morning, before the sun hits the panel, write down the voltage.

By comparing the two voltages, you’ll know how far off it is on a typical night and by watching it over a period of time you’ll know what is safe at night and what is not. For example, if your voltmeter reads 12.1 at night before bed, and you check it the next morning when it is rested and it reads 12.3, you know that you can take the battery down to 12.1 at night because that is a false reading, it’s really 12.3. From then on, never let your battery go below 12.1 at night and it will be a happy battery.

Battery Monitors

It is possible to know exactly what your state of charge is by using a battery monitor like a  Bogart Trimetric but you will also have to set up a shunt for it to work. I’ve got to be honest with you and tell you I know nothing about either, because that level of work and information about my batteries simply does not interest me. Rather than spend $200 and a lot of time learning all that, I’d rather just buy another solar panel with that money so I wasn’t always right on the edge of running out–just the opposite, I want to have a system large enough I never have to monitor it closely because it never starts to run down.
My suggestion is you plan your system for the shortest day of the year on a rainy day. If your battery is okay then, it will be ecstatically happy the rest of the time!

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  1. Cae

    I’ve noticed that many of the national forest website now tell your where they have disbursed camp sites. I thought that the national forests allowed disbursed camping within 300 feet of the road over almost all of the forest.

    • Bob

      Cae, you can’t really generalize because the distance from the road varies by every forest. If a Forest is very busy and close to a specific large city or major recreation area, they are generally much more restricted. So you can just expect the Forests near the National Parks and big cities to be very restrictive. One Forest near the Grand Tetons has a 5 day limit for example. Bob

    • Dani

      Hey Bob, I’d be interested in a you tube video series about all things solar…including how to install it into a rig.

      • Bob Farrell

        Bob – You seem like an expert in this area or at least more knowledgeable than myself. I am trying to research what solar system to install (size, etc.) based on my useage needs. Last year at Quartzite, my batteries were drained every morning solely from the heater motor running all night. I had to use the truck to get enough juice to get the generator going to charge the 2 batteries each morning. The batteries are new (685a Marine & RV Deep Cycle). I have a 28′ 5th wheel so I can support a few Batteries. Before I look into installing 2 more 12v batteries, I am wondering if I would be better off by installing 6v batteries instead and then installing the solar to charge them. What are your thoughts on 6v -vs- 12v? Thanks!

  2. Greg

    Nice write up Bob, watched the first solar video you just did and watching the next one (running an AC) tonight. As you probably know, but didn’t mention, specific gravity of the battery is a fool proof measurement of battery state of charge. Also those Bogart Engineering products, specifically the TM-2030 paired with the SC-2030, are a great way to monitor and charge with solar. The TM gives you 5 days of history at the push of the button. No need to write down voltage night and morning to monitor. Spendy products, yes, peace of mind, definitely. Mine is set to push in amps at up to 15.9 volts to really fill the battery bank with those quarters! :^) I check my water the first of the month and usually can go 2 months without needing to add water and then it doesn’t take very much. I am in no way an expert at batteries or solar charging but these products work fantastically in my opinion based on my experience. Your mileage may vary.
    Thanks for all you do. Too bad the summer RTR was cut short but I still had a great time and made great friends. Hope to make the winter RTR.

    • Bob

      Thanks Greg, really great tips there, I very much appreciate it!! Bob

  3. DesertWillow

    Thanks Bob:
    I was really wondering how many batteries people had and where in their van they have their battery bank. Would love to see more cargo van related electrical articles.

    • Bob

      DesertWillow, my guess is the average is 1-2 batteries, with power users maybe have 4. There really is nothing different about cargo van and passenger vans, they both us solar exactly the same way. Bob

      • Lucy

        Is it possible to charge Lithium batteries using regular solar panels, I mean BIG Lithium batteries.

        • Bob

          Lucy, yes it is, many people are do it. Nut they are very touchy about how they are charged and they are not a plug-play and forget deal. Huge learning curve and if you get it wrong you kill the batteries! Not recommended for beginners. Bob

  4. Joy

    OMG…you guys are sooo techy…..I ‘m just happy if my fan works and my car starts. Other than that ……

    • Bob

      Joy, hopefully you will slowly pick hings up as you got. Bob

  5. Al Christensen

    I have three ways of monitoring my voltage. First, my solar charge controller has a set of lights that gives a very approximate state of charge. (Not that the controller doesn’t know the exact state of charge. It just just doesn’t share that data.) Second, I have a monitor with ten lights that represent a percentage of charge, zero to one hundred percent. Third is a cheap digital voltage meter from the auto parts department that plugs into a 12V outlet wired to my house batteries. It gives a reading from the outlet rather than the batteries, so there’s some voltage loss due to resistance, but it’s safer that it reads a hair low than too high. While none of these three are super accurate, the fact they agree with each other has served my purposes.

    • Bob

      Sounds like a good system Al!! Bob

  6. AZClaimjumper

    Excellent article, Bob; however it makes no reference to lead plate sulfation, which occurs when a battery is unused for a day, week, month. Maintaining battery voltage & keeping the lead plates from sulfating go hand in hand. For this reason, I have a solar panel on my van that provides 1 amp of power & does so at a frequency to keep sulfation from forming on the lead plates as long as light shines on the solar panel. It’s comforting to look at my solar controller & see that green diode glowing because I KNOW my battery is being trickle charged AND the plates are being de-sulfated. BatteryMINDer, which is the system I have installed ONLY maintains a GOOD battery. It won’t charge a dead battery nor charge one that is in poor condition.

    • Bob

      No, there was a LOT that did not come up. The reason so many people can’t comprehend solar or 12 volt electric is because people overwhelm then with too much at once. I assume if I can actually communicate one fundamental truth, we are all far better off than if I overwhelm them with many truths that can’t take in. Bob

  7. Vanholio!

    Good article. But the money metaphor in the video is inspired!

  8. Cindy

    Bob that video was so helpful about amps/volts. Thank you for taking the time to explain it in simple terms for people like me. Every video I see makes me want to get out there! Thank you

    • Bob

      Thank you Cindy! Give in to the urge!!!

  9. Ming

    Nicely done! Both the video and the post. I learn something new all the time.
    Does even the smallest fridge draw 3 amps? How many hours a day does it run on average? It sounds like one needs a minimum of 200AH batteries to run a fridge.

    • Bob

      Thanks Ming, yes, 3 amps is about the lowest you can expect. Lots of variable in how much they run, is it a fridge or freezer, is it winter or summer, did you add extra insulation? I’ve seen it as low as 2 hours a day in the inter up to 10 hours a day in the summer. Yes, as a rule I recommend 200 watts and 200 ah for a fridge. Bob

      • Ming

        Thanks Bob!

      • william Smith

        Hi Bob, I have a small frig and it draws 1.6 amps. The small motor that turns it on has a cam with notches and it runs and stops then starts again can’t remember how long it will run or stop.Wonder how many people know that you can lay a frig on its side for hauling only if the tubes on the compressor are pointing up and if down the oil will collect in the tubes and big loss for cooling and you will think you frig is shot or call a service tech. Thank you Bob for helping with the info. Have you heard of ZAPP Battery works? They should be producing life time batteries in 5 or 6 months you can drain them and fully charge them in 30 minutes. Have a Very Merry Christmas and a Happy New Year Bill in Texas

  10. Brenda

    Hi Bob
    nothing to say about batteries, at this time but was wondering what you think about popup tent campers in the desert with high winds. do you think the winds would tear up the canvas? because at this moment that might be my only way to leave the city life and join the tribe. I would love to hear your thoughts on this subject. thank you

    • Bob

      Brenda, I’m not a fan for that reason. Not only will the winds damage the fabric, it will make you very cold. Plus popping up and down the camper all the time gets old. But, if it’s all you can afford I’d say go for it! If you get a good deal you can always sell it and get what you paid for it and start looking for something else. Bob

  11. Patrick Parish

    Hi Bob, I have been retired for 5 years (67 years old) and have wanted to try camping for an extended stay since retiring. So even though I may not ever because my wife doesn’t want to, I am enjoying hearing about what you and others are doing. We do camp but only a few days at a time. We started with a tent then a popup and now a 21 ft travel trailer. I also like the videos/articles on batteries and solar power. I worked at my dad’s automotive electrical shop and then opened an automotive air conditioning and radiator shop for 30 years. Then worked for a neighbor doing commercial electric. Have really been interested in solar and wind power. In your experience, what few companies excel in making good solar panels and equipment for a reasonable price?

    • Bob

      Patrick, thank you for your very kind words! To be honest, solar panels have become a commodity and it’s hard to find a bad one. They all come with 20-25 year power production warranties and it would be very unusual for one to fail early. Some are better made than others, for example Kyocera is a better panel and actually quite cheap per watt. But, buying on lowest price will be fine even an off-brand.
      Solar controllers are a different story. You can spend much more for a controller with bands like Blue Sky, Morningstar, Midnight Solar and others, but you aren’t necessarily buying more quality (although they are far better and will probably last longer) you’re buying
      1) Made in America and backed by an American. If you call with a problem, you’ll talk to somebody in English who KNOWS the answer.
      2) Much more and very important features like being able to set the absorption voltage and time it runs.
      3) Much higher quality
      4) Better warranty and much better support. If it’s their fault, they will usually make it right.
      However, for most people you are better off with a reasonably priced kit and for that I wholeheartedly recommend Renogy. I know numerous people with them and they all have been happy with them. More important, if they weren’t happy, Renogy made it right.
      For most people, buy a Renogy kit.

      • Tracy

        Hi Bob i need several of the renogy kits. Where can i purchase them?

    • william Smith

      Midnite solar makes the best controler
      Waiting for the Zapp Battery company to start producing batteries as they say they are lifetime, very little maintanance can you can drain them and fully charge in 30 minutes but believe it would depend on you system to do that.

  12. AZClaimjumper aka Bill Cole

    At the RTR-F, a guy with a user name of Wada shows an Inductive amp meter. I’ve been trying to find something about whatever it was he was showing.
    I have an electrical problem & want an inductive meter that tells me how much current is flowing through the wire.

  13. Howard Klutterbuck

    Bob, may I suggest that you add “VoltMinder Digital Volt Meter w/ 6 Foot Cord for 12V Batteries” to your amazon store.
    I bought my first one about 15 years ago from the inventor. It is is a digital volt meter with an alarm on it. I set mine to alert me at 12.4 volts, but you can set it where ever you like. It has saved my bacon several times, when I was alerted to a low battery problem while I slept. It woke me in time to start the truck and charge the batteries back or leave the truck running until I could get somewhere to make repairs. Mine was plugged into a lighter socket in the sleeping area, so I could also monitor the voltage while watching TV for extended periods. Great product from a good man. Also it’s made in America!
    Thanks for all your great information

  14. Annette Wigge

    Since I no nothing about electrical & battery set ups who do I go to so they can do it?

  15. Wayne

    I have a DC Outlet on the exterior of my van can I plug a solar panel plug into it to recharge the house battery it has a 15 amp fuse in it thanks

  16. Laura J Phillips

    I love your videos, I follow Carolyn too. I’m planning on doing full time RV, there is a lot I have to do to clear out my mobile, but have been watching and learning. I’m am especially interested in a Solar Generator, I’ve seen UTube vids on them they built there own. Is there somewhere they can be bought already made, or can you do a vid that explains it,I think a lot of people would be interested, I’d like to use it for the air conditioner and be totally off the gas generator! Thoughts Possabilities ?? Thank You

  17. Laura J Phillips

    I’m interested in getting a totally solar generator can they be bought already made? I want to power the air conditioner. When I do transition I will need to live at Slab City year round. Thank You

  18. Pete

    Hi Bob,
    This will be both introductory, and information sharing.
    I’ve been researching RV solar for a little while, now, as the better half and I are planning on the Class A life (Did the van by the river thing as a young musician, I want it more comfy than that, now) and one of the things the industrious and resourceful solar-seeker can do to increase the capacity of their system by spending that Renogy money in China. Yep, China.
    Yeah, yeah, buy American, American workers, blah blah blah. American corporations have screwed over a lot more American workers than anyone else, both by outsourcing and by importing talent from many other countries (Carrier, anyone?). To displace American workers. I was forcibly retired from Silicon Valley five years ago (before I turned 60), and I’m no longer influenced by such nonsense. I do what’s best for my family and I don’t care about the rest. Besides, where do you think those Renogy panels come from? Pennsylvania? /conversation
    So, buying solar from China. First thing to realize, is your gateway into the world of Chinese solar. It’s sorta like ebay but not really, though they are the closest Chinese analog, Some companies won’t sell to individuals, others will. Some will sell *some* things to individuals (meaning you don’t have to buy 10,000 panels). It often depends on the discretion of the vendor. Guangdong ProStar Energy (located in Guangzhou) and Shenzhen Lithpower Technology, obviously in Shenzen, will both sell to individuals.
    Bob, these companies are unbelievable! Prostar has several gigawatt solar farms going in the Middle East, Africa, India and of course, China, itself.
    Unreal. Yet they will sell a couple panels to a moronic goober like me.
    I can put together a system that will run everything in the bus (the musician in me wants it to be a tour bus) and barely break a sweat. Those two panels (400w total; 36vdc, MPPT charge controller, 3000/6000 inverter; the wife needs her hair dryer) will cost me a LOT less than a grand, and the batteries will add a grans. For TWO lithium-ion batteries. So, for the cost of improved system components (PWM controllers will never touch my system, I want ALL the juice) I’ll have a solar system that should meet my needs, indefinitely. If we trash the batteries the night before (love li-ion), the rising sun brings new juice to our world.
    I like it and I can tell PG&E (and every other electric utility in the nation) to buzz it. I can also charge the batteries from the alternator of the rig’s engine. Honestly, Bob, I’m not convinced I need a generator and I certainly don’t want an Onan. I’ll sell whatever’s in there and have more room.
    Better a Honda. Even if it’s not totally up to the task, it can take a load off the batteries without sounding like a lawn mower. Battle Axe (meant with great affection) can dry her hair
    You can ping me here (I promise to check) and anyone who wants more info, just ask me. I can even give you contact names.

    • Michelle Dubuc

      Hi Pete
      I just bought an 87 Cobra Van American motor home and I am interested in the kind of solar your talking about that you use in the above post. Where can I have that done or where can I go to get what I need to do it? I’m a military veteran so I am very handy and can do myself with the right information.

  19. Military Battery Reconditioning System Review

    very informative article, thank you

  20. Cathy E.

    I would like to get in touch with the guy who installs the Renogy solar on RVs. I am not happy with the job done on my first small battery, suitcase solar, ect. I want to know if he is ever in the KC, Mo area to install flex panels ect.

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