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 first need to know how much charge it has. To accurately check this, you can use a battery monitor.

Battery monitors can 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 are made to withstand being discharged to 50% over and over again and take minimal damage.

If you want your deep cycle batteries to last for years to come with zero damage then charging them before they go below 50% discharge is the optimal way to go.

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 be able to hold a decent amount of charge for a long time.

See Also: Best Deep Cycle RV Batteries (AGM, SLA, 12V, 6V) 2021

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

If you are planning on draining your battery to 20% over and over again getting 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 your batteries are stored 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 to come.

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?

7 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.

  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?

  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.


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