There are numerous charts out there on what voltage levels are acceptable, or at what point you can expect your batteries to start being damaged.  Damage may not be in the form of “it doesn’t work”.  It may be in the form of degradation in performance. 

Also this is in general terms.  For those batteries in the $100 to $200 range.  “Flooded” types, and “AGM” types.

It can’t be stressed enough.  Get your info from your manufacturer on the specific battery you have or intend to use.  This message will be repeated several times.  

If someone is making claims you can discharge your batteries lower than the charts below.  It is strongly suggested you contact your battery manufacture about the specific battery you intend to purchase.  Then know and understand the specs.  Go only off the manufacturers recommendations.

We are only touching on common Flooded Deep Cycle Marine, and common Deep Cycle AGM.  We aren’t talking about other options such as Lithium. 

Of course, those other options are most likely nowhere near the price point we are talking either.

Bear in mind there is always changes to technology.  There are always those willing to find an expensive alternative.  It is possible there are batteries out there that may not follow the general rules?

The caution is:  If you are getting your info off a Facebook forum or the like.  More often than not you will be getting erroneous information.  It often sounds good, but often told by someone with incorrect assumptions…..or the famous “It works for me” mentality. 

Again, speak to tech support at your battery manufacturer for true and valid info.

While you can certainly discharge your batteries below 50%….and yes your loads (devices) will still function.  Taking your batteries down into the realm of 50% to 20%.  Most everyone with any practical experience will recommend you don’t. 

Many even adhere to the 60% cut-off rule.  Flooded or AGM.   The recommendation is only for your own wallet.  It will reduce you having failures at inconvenient times.

If you don’t mind replacing batteries more often, or having no power when you need it.  Then throw caution to the wind and ignore all of this.

While the “State of Charts” may vary. Even between “Flooded” or “AGM”. The variations are so small they are nearly insignificant.

It is wise to heed voltage levels and use the 12.0V voltage as the point you want to stop draining your battery, and making sure you fully recharge at that point.

Unknown to many that are not familiar with 12V batteries, a fully charged battery can read anywhere from 12.6V to 13.8V.  Perhaps even more?  12V Chargers output more than 12.0V.

The general rule of thumb is: You can siphon off your batteries stored energy down to the point the batteries are at about 50%.  Called your “Usable Amount”.

Many people that want longer life out of their batteries are vigilant and never let them discharge below 60%. In generalities, for our price point, this holds true whether you are running off “Flooded” or “AGM”.   12.0V is close enough to be considered universal for both types.

Those telling you that you can discharge “Flooded or AGM” to 0-40%, etc.  They are most likely leading you down the wrong path.  Again, ask your manufacturer!

While batteries will certainly continue to work beyond a 50% discharge, damage can start. You may not even realize it.

Damage can come in the form of never again achieving a peak re-charge, discharging faster under load, and loosing their ability to hold a charge for the period a normal “good” battery might have.   Thus your battery storage capacity may never again be at peak performance.

Some times damage is immediately apparent. The battery will not charge up at all, a cell goes bad, etc.  Sometimes damage is not so evident.   Sometimes the battery may even test good on some testers.  Erroneously testing good.  

If your battery storage capacity is marginal for your actual use.  You may quickly realize the batteries have been affected. 

If your battery capacity is over engineered for your actual use, you may not notice.  Even though it may be degraded.

If you have more advanced metering equipment you may be able to accurately read your voltage, simply by checking a monitoring meter. This may require more expensive meters, shunts, and so on.

A rudimentary metering system that gives generic or vague information like “drained”, “weak”, “Full”.  Without the actual voltage read-out to at least the tenths position.  Only knowing vague information is not close to being optimal for you to know what is happening.

Another method that is not built into your RV system is a technique of testing your battery with a VOM (Volt Ohm Meter).   More for troubleshoot in the event you trashed your batteries.  This is called testing “Resting Voltage”.

To read “Resting Voltage“, Then apply the reading to an appropriate chart for your specific battery type.  It may not be exact.  But should be really close.  It may not be really convenient to use this method though.

Make sure the battery was completely charged first.  Then “Resting Voltage” readings are taken when all “Loads” and Charging Sources” are disconnected off the battery.  Basically, remove all connections off your battery.  Like you are going to remove the battery.  

The battery is then allowed to “rest” and equalize, or stabilize, prior to taking a voltage reading.

Depending on whom you ask.  The person might tell that you have to allow the battery to rest anywhere from 5 minutes to an hour or more.  The longer this rest period is.  The more accurately this will identify a marginal battery.  15 minutes is often used.  

The point of all of this being. You need to monitor your battery. Even a cheap meter that gives you a close guess-t- mate, is better than nothing.   Insure you are maintaining its charge level at 50% or more at all times. Preferably 60% or more.  (Flooded and AGM’s)

If you are not using a meter built into your RV system.  Then you notice lights are going dim, DC motors (like a fan) start running slower, your Propane detector starts beeping, or items simply stop working. You have most likely gone into the realm of potential damage.  Beyond the 50%.

While on the subject of batteries.  When connecting more than one battery together, make sure they are “like” batteries.  Meaning the same type, same size capacity, and some will even go as far to say the same age.  For example, don’t mix Flooded and AGM.

Further, charger outputs must match the type of battery being charged.  Don’t use an AGM charger on Flooded, or vice versa.  Or two dissimilar type batteries on a common charging source.