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When To Charge A Deep Cycle Battery

How to Monitor a Deep Cycle Battery For Charging

To know when a deep cycle or marine battery needs charging you need to know how much charge it has. To accurately check this, you can use a battery monitor.

Battery monitors tell you how much charge a battery has left and sometimes even more.

The ANCEL BA101 Professional 12V Digital Analyzer (click to view on Amazon) can tell you how much charge the battery has, the health, cranking power, voltage, and more.

There are even Bluetooth versions (click to view on Amazon), you just connect to your battery then use the app on your phone to see the charge of the battery.

You can read more about the exact voltages you are looking for later on in this article.

See Also: Best Deep Cycle Battery Chargers Reviewed (12V, 6V, RV, AGM)

Deep cycle battery monitor checking the voltage
A 12 volt battery with no load should read 12.7 volts.

When Does a Deep Cycle Battery Need to Be Charged?

Standard lead acid deep cycle batteries can withstand being discharged to 50% repeatedly and take minimal damage.

If you want your deep cycle batteries to last years with zero damage, you’ll need to charge them before they go below 50% discharge.

That being said, it can be very difficult to stop a battery from going below 50% especially if it’s what’s running the lights in your trailer or RV.

The damage when the battery goes below 50% is minimal and as long as you never let it go below 30% the battery will still hold a decent amount of charge for a long time.

See Also: Best Deep Cycle RV Batteries (AGM, SLA, 12V, 6V)

AGM batteries are better than gel or wet lead-acid deep-cycle batteries with discharging damage and Lithium-ion is even better than AGM.

If you are planning on draining your battery to 20% repeatedly, a deep cycle Lithium-ion battery (click to view article) is going to be the best choice.

One thing to note about lithium-ion batteries for cold weather campers is you should never charge them in temperatures below 40°F.

So if you camp in places that get down to freezing at night and the batteries are outside, make sure it warms up before trying to charge them, otherwise they could get damaged.

Long story short, if you have any kind of lead-acid deep cycle battery, then charge it when it gets to 50%.

If you have a Lithium-ion deep-cycle battery, charge it before it goes below 20% and your batteries will last for years.

RV Life Hack: Equalize RV Batteries That Can’t Hold A Charge

rv deep cycle battery monitor showing a fully charged AGM battery at a full charge
Battery monitor on an AGM deep cycle RV battery showing a full charge voltage.

What Are The Voltages?

12 Volt Flooded & Sealed Lead Acid Batteries

With no load, a fully charged 12 volt deep cycle battery should read 12.7 volts.

This number may be lower if the battery is old and has lost its ability to hold a full charge. It will also be lower if there is something drawing power from the battery.

12.2 volts is a 50% discharge on a standard 12 volt battery.

12 Volt Gel Deep Cycle Battery

Gel batteries are a little different and have a voltage of around 12.85 when fully charged.

When discharged to 50% the voltage should read around 12.35.

12 Volt AGM Deep Cycle Battery

AGM batteries will be around 12.8 to 13.2 volts when fully charged.

When discharged to 50% and AGM battery will have around 12 volts.

6 Volt Deep Cycle Battery

A fully charged 6 volt battery will read around 6.3 to 6.4 volts and only 6 volts when at 50% discharge.

To read more about batteries in general, check out this article here.

Have any questions about monitoring or charging deep cycle batteries? Leave a comment below.

by Jesse
Jesse has always had an interest in camping, technology, and the outdoors. Who knew that growing up in a small town in Sweden with endless forests and lakes would do that to you?

25 thoughts on “When To Charge A Deep Cycle Battery”

    • With no load on a fully charged 12V battery like yours, the voltage will be around 12.7. At 50% the voltage will be around 12.2V. If you’re charging the batteries the voltage will read higher, if there is a load on the batteries they will read lower. To get an accurate measurement you need to take the voltage reading when the battery is rested.

      You can read more about how all that works and the different voltages at different percentages for different battery types in this article here.

  1. Hi I am new to Caravanning and have managed to use it twice now since the lockdown. I have a new AGM 12v leisure battery on the van and When fully charged the display shows about 12.7v. I also have 2 40 watt panels on the roof. I used power very frugally in the evening for a tv using it for a couple of hrs turning the led lights of even though they use very little. Fridge uses gas. The display would show about 12.5v to start and by the end of evening 11.7 at lowest. But according to you it must be dead. And usable power would be .5v (12.8 to 12.3). That doesn’t seem a great deal of usable power. The tec bloke at caravan place said to me not to go below 10.2v as that would reduce its lifespan. He also checked out the battery and charging and said everything was fine. He did say a third 40w panel added would help.

    • Hi Andy,

      With AGM batteries you want to try and stay above a 50% state of charge to keep from damaging your batteries. That’s around 12 volts, I don’t mean that the batteries will instantly die if you go below 12 volts and AGM batteries are pretty tough which means you can take them a little lower than that sometimes and still keep the batteries in good repair.

      10.2v on a 12V AGM battery is pretty much a dead battery, you never want to fully discharge an AGM battery because that will damage it right away.

      If it were me I would try and keep the battery from going below 12v as much as possible. It’s ok to take it all the way down to 11.7 which is about 30% capacity sometimes but if you do it every day it will damage the battery even faster.

    • yeah. I dont let my 800aH bank of AGMs drop lower than 12.4v. I know that Im not using their full “safe” potential but these batteries are expensive so I want them to last as long as possible. AGMs that get discharged down to 10 and 11v on a regular basis arent gonna last long. If you can afford it, dont even bother with lead acid based batteries. Just go LiPo from the start. Even if you cant afford it. Lipo’s are much cheaper in the long run because of superior energy density, usable energy, and double to triple life span.

    • If you listened to that “tech bloke” you mentioned, and have been frequently discharging your battery to 10.2 to 11.7, Id wager that youre already having issues with that battery. The advice given by this article about safe discharge parameters for AGMs and other lead acid batteries is quite accurate. As far as those parameters making it “look like” there isnt much usable energy to be safely pulled from your battery, thats because those parameters measure just the voltage level of the battery. What really matters is the current(amps) a battery can provide but amps remaining in a battery arent as easy to measure as the voltage of a battery. I’ll explain; Lets say that your battery is a 100aH battery. To make that battery last as long as possible you should never use more than 50% of its capacity-which for a 100aH battery is 50 amps. Well, depleting 50amps of a fully charged 100aH AGM battery will drop the voltage of that battery from around 12.7v to around 12v. So, although a voltage drop of 0.7v seems fairly insignificant, it reflects that half the batteries available energy reserves have been used up. Also, when setting up solar panels to charge a bank of batteries, the general rule of thumb is a 2 to 1 ratio. Meaning, you should use panels capable of producing twice as many watts as you have amp hours of battery storage. For example, you really need 200watts of solar panels to charge a 100aH battery. The two 40 watt panels you are using will never keep up with the charging demands of a regularly used 100aH AGM battery. Even if you add another 40watt panel, you’ll still struggle to keep a regularly used 100aH AGM battery charged. You might get by if you add two more 40watt panels to what you now have. Theres a lot to know about this stuff but its not difficult to learn.

  2. I think people new to battery youse take the under load voltage rather than at rest or off load which is .1-.2volts higher so there battery is not as discharged as it may at first appear.Just my thoughts on this post as under load and at rest is not often mentioned.All that said I killed my 1st leisure battery in a year by taking it way to low in the winter just a few times.Now with a 120w solar panel and two 110amp AGM Battery’s they rarely go below 12.3v even in the winter.

  3. I have a 545Ah battery bank of 5- 109 AH Everstart 27dc Walmart batteries (under 1 year warranty), and
    4- 12v 150AH Trojan 1270 batteries in another 600AH bank that wont hold a charge.
    I have 4-250 watt 24volt solar panels in series and parallel running at 60 volts rms, charged with a 75 amp MPPT charge controller @ 12vdc.
    The problem is that both battery banks have been ran bellow 10 volts, due to leaving my inverter on 24/7 for the last 7 years, to run a 80 watt 120volt fridge, I have left the system unattended for several months at a time, and have neglected to top off the Trojan water levels too many times.
    The question is: can I repair the Trojan bank by dumping the acid, cutting open the battery and repairing whatever has failed; or are they just junk?

    • First of all, after 7yrs of service, those Trojans are near or beyond the end of their expected life span anyway, abuse or no abuse. Secondly, youd just make a mess by cutting them open to try and fix them and thats about it. Theres nothing inside them to fix, really. After 7yrs service and abuse, their problem is going to be severely sulfated plates and increadably high internal resistances and surely even a short or two. None of that is user fixable. If they dont hold a charge, theyre junk.

  4. if they are nomrall deep cycle batteries you can add destiled water and not acid. deep cycle batteries that are floded use destiled water from what ive gathered nothing else. but you can always call the manufacturer and ask.

    • The short answer is yes. A 200 amp hour lithium battery is going to give you a good amount of power.

      The long answer is it depends on things like how much power your pellet stove uses, how many watts of solar you will have, and how often you will be using the battery.

    • in addition to what Jenni said, you’ll also want to keep the battery well away from the stove or any other significant source of heat. High temp is a major killer of lithium batteries and the combo of heat and lithium batteries can easily cause fires.

  5. I keep trying to find the (50%) voltage with a load. All say (resting) voltage. if I’m using them I just have to guess, or turn my stuff off and let the batteries “rest” beford I check to see if they’re at 50 % voltage.

  6. If i have a 105ah calsium deep cycle battery and i have to charge it…
    105ah ÷ 20amp charger = 5 hours charge time.. but that is for when it is fully depleted. If i want to charge it every time it is only 50% depleted should i then only charge it for 2.5 hours then?

    • If you’re using a smart charger, the charge speed should slow down the closer the battery gets to full so it doesn’t damage it. It might take more like 3 to 3.5 hours to fully charge your battery from 50% charge.

  7. I think my system may not be performing correctly and but I have no idea how to check it. I have 600ah worth of Renogy AGM batteries and 800w of solar on the roof. The only thing I am running is a chest fridge designed to be DC and low voltage (iceco VL65). During the day my batteries will say 100% by 11am even on cloudy days but by 7pm (5:30 sunset). I’m already normally down to less than 70% with just the fridge turned on.
    If I want to turn my TV on to watch a movie then it brings it down to ~50% every single night.
    I can’t run any lights or any electronics of any kind or it kills it even faster. Heck, I can’t even charge my phone after dark.
    I feel like 800w and 600ah should be performing better. Any thoughts?

    P.S. right now it is 7:30pm and I’m at 68% with 12.5v.

    • Hey Matt. The general rule of thumb for solar is to have twice as many watts in solar panels as you have Ah battery storage. In your case, you should have at least 1200 watts of solar panels to keep 600aH of battery storage adequately charged. My system consists of 2500watts solar panels and 900aH of battery storage. Thats a bit overkill on the solar panel side but leaves me room to add another battery or two to the system in the future. With more watts in panels than i need for my storage bank, my storage bank is fully charged by 10am and stays that way all day, even though Im running a fridge, my furnace, a computer and projector, or whatever else. Its because i have enough juice coming in from my 2500watts of panels to not only run whatever i want to run, but also keep the battery bank topped off at the same time.

    • PS: As I said, it sounds like you dont have enough juice coming from your panels to power your stuff AND keep your batteries charged at the same time. So your batteries slowly lose ground throughout the day. You definitely need more panels. I would also suggest upping your battery bank by 100aH. OR if you can afford it, switch to LiPo. Youd get nearly twice as much useable power from 600aH of LiPo batteries as you do 600aH AGM’s. You could use 80 to 90 percent of the LiPo’s 600 amp hours while only 50% of the AGM’s 600 amp hours. Its a massive upgrade. LiPo’s also charge extremely quickly in comparison to lead acid based batteries. So they wouldnt take any longer to come up from 80 or 90% discharge than it takes for AGMs to come back from 50% discharge. Probably less time. And LiPos actually like to be discharged(to a point). They last more years if you keep them in a constant cycle of use and charge. They do not like to sit at 100% charge like lead acids do. Anyway, start with 400watts more in solar panels and go from there. i think you’ll notice a nice change.

  8. How about a question? What setting on the Noco Genius 5 should I use for deep-cycle batteries? I can’t find that in the user instructions even though they say it’s suitable for deep cycle.

    • Hi Ken,

      It doesn’t matter if it’s a deep cycle or a starter battery, it’s mostly about the voltage and what the battery is made of. It your deep cycle battery is a standard flooded lead acid you will use the first 12 volt mode.

      If it’s AGM use the 2nd AGM mode, and the 3rd mode is for lithium. The 6 volt mode works for any kind of flooded lead acid 6 volt battery.

  9. This is the best site I’ve come across to see rv battery information that I might someday actually be able to understand 😁. Having said that, I have an rv, a 150 w solar panel (with a 70 amp mppt charge controller), two 12 volt lead acid deep cycle batteries, and a 3000 watt generator. When I’m at camp I have the generator on for a couple of hours in the morning and then about three hours in the evening. The only draw during the day otherwise is the CO2 alarm and the fridge (which is actually running on propane), and maybe the radio occasionally. In the spring or fall, the furnace may kick on in the early morning hours and I’m surprised that my batteries don’t last through to when I put the generator on again in the morning. Is it likely that I’ve done something wrong to have ruined them? I thought that between the generator on morning and night, and then the solar panel during the day, that there’d be plenty of charge happening to replenish them 🤷‍♀️. Any suggestions would be appreciated.

  10. This article and comments are awesome! Thanks everyone who answers. I’ve got a brand new Pow-R-Surg deep cycle battery running a 12v 15 amp pump to move water up to a tank up a hill. I run it for about an hour and find that the voltage reading drops to 11.7. When I hook it up to my charger to re-charge, the charger estimates that it is at 70% charge. According to comments here and elsewhere, a battery at 70% charge should still have a voltage reading over 12v. Anyone have any guesses about what is going on?

    • Hi Greg,

      Are you taking the voltage reading while the pump is running or directly after? The voltage could be dropping due to the load, then going up again after resting for a few minutes.


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