Most people limit the useful lifespan of their batteries. Once installed they never pay attention to their batteries again until they fail. Once they fail the typical RVer scratches his/her head wondering why they failed so quickly and promptly purchases new batteries without ever getting to the bottom of the problem. If you are like me when I purchased my first travel trailer, then you probably replace your batteries each spring. I used to think this was a normal process for RV owners until I started researching the upgrades I wanted to make to my RV and I started to learn how batteries worked. 

If you plan to invest in new batteries, then it is extremely important that you follow a couple of rules to protect your investment. After all, I spent over $1000 on the AGM batteries for my new 5th wheel. If I had to replace them every spring like I did on my old trailer, (which had 2 standard deep cycle flooded acid batteries that I purchased from SAMs), I would go broke. So here are some of the things you need to understand: 

  1. Don’t let your batteries become discharged by more than 50% at any time. Once they are discharged by 20%-50%, then recharge them immediately. If they are discharged or drained by more than 50% they will start to become damaged and most likely will never function at 100% again.
  2. Don’t use the built in charger in your RV. Disable it or disconnect it if you can and purchase a true 3 stage charger for your RV batteries. This may not apply to some higher end RVs that come with a true 3 stage charger, however, the majority of travel trailer, 5th wheel and RV owners will need to deal with the problem of sub par chargers in their RV.

Let’s discuss these items a little more… 

Don’t Let your Batteries Become Discharged by More Than 50%: 

As a general rule it is not good to let your batteries become drained by more than 50%. As I stated before, once they reach the 50% threshold it is time to recharge them. Doing so will ensure that you maintain the optimal lifespan of your batteries. Taken care of properly, it is not unheard of for gel or AGM batteries to last 8 years or more. If you don’t follow this rule, you will damage your batteries and they will never be 100% again. Similarly, it is important to understand that a 12 Volt battery, although called a 12 Volt battery, actually gives off slightly more Voltage at full charge. A 12 Volt battery is actually made up of six individual battery cells that provide approximately 2.12-2.15 Volts each. That means that a fully charged 12V battery actually gives off 12.72V to 12.9V or more.  

So the question is, how do you know when your batteries are discharge by 50%. The answer is through monitoring the “state of charge” of your batteries, which is determined by monitoring the amount of Voltage they are giving off. Although this sounds like a pain, it is really not that bad. In my case I have two ways to monitor the Voltage of my batteries. Since I have installed an inverter and solar panels with a solar charger, they both came with remote panels that I have installed inside of my RV that display the current Voltage of the battery bank. Therefore, I can monitor them easily at all times. If you don’t plan on installing solar panels or an inverter, then you can use a digital Voltmeter or purchase a battery monitoring device. In some cases you can even purchase battery monitoring devices with a remote panel that you can install inside the cabin of your RV for constant monitoring much like the ones that came with my solar charger and inverter. 

Solar & Inverter Remote monitoring panel

The 2 remote monitoring panels that I use to determine the Voltage of my battery bank.

In theory, determining your batteries true state of charge is difficult. Technically, you cannot determine the true state of charge for your batteries when they are under load (meaning that any device is drawing power from them). Determining the true state of charge requires that your batteries be at a resting Voltage (meaning that they have been sitting for at least 24 hours without being either discharged or charged). You also need to know the specific gravity of the acid solution inside of your batteries as well as the temperature where the batteries are currently located (which could be different if they are inside an RV cabin or space as opposed to being mounted on the outside such as on the tongue of your RV in a cheap battery box). In reality, most people are not going to go to these lengths to determine their state of charge. Additionally, if you are using gel or AGM type batteries that are sealed, there is no way to even open the battery and use a hydrometer to determine what the specific gravity of the acid solution is like you could with older flooded acid batteries that have caps on top. Therefore, in order to determine the state of charge, you are going to have to use an educated guess and common sense based on what you do know. 

Here is how I manage my batteries. I currently use six – 6 Volt Lifeline AGM batteries that are wired in series and parallel (more on this in 6 Volt vs. 12 Volt batteries). Lifeline, the manufacturer of my batteries, publishes a basic chart that tells me what the state of charge of my batteries are based on the Voltage.

State of Charge Voltage
100% 12.80 or greater
75% 12.55
50% 12.20
25% 11.75
0% 10.50

I know that I never want to let my batteries run below 50%. So in my case I never want to let my batteries run below 12.20 Volts. Using the Voltage meters that are built into my inverter and solar charger remote panels, I monitor the batteries regularly and I generally use 12.50 Volts as my cutoff point. So if for instance I am watching TV, which is running off the inverter and drawing power from the batteries, and I notice that the batteries have reached a Voltage of 12.50, I will either turn off the TV or crank up the generator so that I can continue to watch TV (although I have never had to do this). 

Why do I stop at 12.50 Volts when I can run them down to 12.20 Volts before they are 50% discharged? The reason is because of phantom loads. Remember, there are other items in my RV that are using continual 12 Volt power such as the radio memory, the CO2 detector, and the refrigerator electronics. I need to make sure that I still have enough power for these items to run until morning when the sun comes up and starts to recharge the batteries via the solar panels. In reality, I could probably lower the point at which I stop to 12.30 Volts. The reason for this goes back to what we discussed before about how to truly determine the state of charge (making sure the battery is at a resting state, know the specific gravity and temperature, etc.). Since I am drawing power from the batteries at the same time that I am monitoring them with the Voltmeter, I am not actually seeing the true state of charge. In fact, what I often see after I turn off all the items I am using (except the phantom loads, which I cannot turn off) is that within an hour after the batteries have “rested” a bit, the Voltage will actually go up again. I prefer, however, to error on the side of caution. 

The state of charge charge that I supplied above does not apply to all 12 Volt batteries. Flooded acid, AGM and gel cell batteries are going to have different state of charge specifications. Similarly, batteries from different manufactures are going to have different state of charge specifications. It is all based on their construction. Therefore, make sure to get the exact state of charge specifications for the batteries that you purchase from the manufacturer in order to accurately determine where your 50% state of charge cutoff is. Just to give an example of the differences in state of charge for different batteries, following are some specs that I found on the web while doing research. Neither of these charts was associated with any type of specific battery and look how different they are then the actual manufacturer specs for my batteries above. This is why it is important to get the exact specs from your battery manufacturer.

State of Charge 

Standard Deep Cycle 12V Battery (example 1) 

Standard Deep Cycle 12V Battery (example 2) 

100% 12.7 12.70
90% 12.5 12.58
80% 12.42 12.46
70% 12.32 12.36
60% 12.20 12.28
50% 12.06 12.20
40% 11.9 12.12
30% 11.75 12.04
20% 11.58 11.98
10% 11.31 11.94
0% 10.5 11.90

Disable or Disconnect the Built in Charger in Your RV: 

As many people are aware, when your RV, trailer, or 5th wheel is plugged into an electrical outlet (or the generator is running) the batteries are being recharged. This is accomplished through the combination converter/charger that is built into your RV. The converter/charger is converting 120 Volt AC power from the power grid or generator into 12 Volt DC electricity to recharge the batteries. What most RVers don’t realize is that most stock chargers that are built into RVs will ruin their batteries.  The reason is because most built in RV battery chargers are not true 3-stage battery chargers and they will either undercharge or overcharge the batteries in your RV, which will quickly ruin them. 

In order to charge a battery properly it is usually done in three stages with a bulk charge, acceptance charge and float charge. The bulk charge is the first stage. This is where the battery is charged to approximately 80-90% of its full charge. The acceptance charge (sometimes called absorption charge) is the second stage and it is here that the maximum Voltage is applied, which gradually tapers off as the internal resistance of the battery increases during charging. The float charge is the third stage and is basically a trickle charge that is meant to keep the battery topped off. 

Most of the converter/chargers built into RVs are single stage chargers that are meant to provide a bulk charge only. As a result, these chargers supply a bulk charge at all times, which will eventually cook your batteries if left plugged in indefinitely. The reason these types of chargers are used is because they are cheap (do you see a theme here). Some higher end RVs may have a separate converter and three stage charger, however, chances are slim. Therefore, if you plan to upgrade your RV batteries to gel or AGMs and you want to protect your investment, then I highly suggest you upgrade your charger also.  This is especially important since most gel and AGM batteries need different bulk, acceptance and float charge Voltage levels. If you purchase a quality 3 stage charger, then you should have the ability to change the charge levels based on the type of batteries you are using. Here is how I have handled this situation in my new 5th wheel and my old travel trailer in order to give you some examples or ideas on how you can handle this problem in your RV. 

In my old travel trailer I had two standard flooded acid deep cycle batteries that I purchased at SAMs club. Once I learned about the issues associated with most built in RV chargers, I made some simple changes that really helped maintain the life of my batteries. First, I went out and purchased a true three stage charger that was capable of charging two batteries simultaneously. It was a marine charger that was meant for tough conditions and could be mounted outside and get wet. I mounted it to the front of my travel trailer near the two batteries that were mounted in plastic battery boxes on the tongue of the trailer. This charger had its own power cord.  I then purchased a marine battery switch that was capable of handling two batteries. This switch allowed me to control the power connection from the batteries to the trailer. I could choose to run the trailer off of battery #1, battery #2, both batteries simultaneously, or turn them off altogether. As we discussed earlier, the normal operation of the trailer was to charge the batteries through the converter/charger built into trailer any time the trailer was plugged in. Therefore, to prevent the trailer from charging the batteries with the cheap built in converter/charger when I was plugged in, I would go outside and turn the batteries off via the marine switch so they were no longer connected to the trailer charger. In order to maintain the batteries state of charge, I would then plug the true three stage charger into the trailer outlet (located outside) and voila, the batteries were being charged by a true tree stage charger that maintained them properly. When I stored my trailer, I would also turn the switch off (so that the batteries were not being sucked dry slowly through phantom loads) and I would plug in the three stage charger to maintain the batteries. 

In my new 5th wheel things are a little different. First, I disabled the charger/converter completely.  I did this by simply turning it off at the AC circuit breaker (I could still enable it any time by simply turning the breaker back on). My AGM batteries are charged in two ways: through the solar panels, which are connected to the batteries through a solar charger, or through the inverter, which is powered by AC power when the trailer is plugged in to an outlet and acts as a battery charger when AC power is present. Both of these devices have true three stage chargers built in that enable the end user to set the correct bulk, acceptance and float charges based on the type of batteries that are being used (look for the other articles on this site that discuss my inverter and solar setup for more details).  Additionally, the trailer came with an internal switch that allows me to cut the 12V power off from the batteries to the trailer. Therefore, when I store my trailer, I am able to turn off all 12V power sources so as not to drain the batteries. Technically however, I could leave the switch powered on while in storage since my batteries are being recharged daily by the solar panels. 

If you plan to store your RV for a long time, make sure you have some kind of plan to maintain the batteries and recharge them occasionally. While in storage, turn off all items that use 12 Volt power. If you do not have a way to disconnect the batteries from the 12V system, then pull the 12V fuses.  Remember, there are items such as the radio memory and CO2 detector that are drawing 12V power at all times. You can check to see if anything is drawing power from your batteries with a quality ammeter. Additionally, batteries will naturally discharge over time even without anything drawing power from them. If you don’t have a true three stage charger that you can leave plugged in all the time, then use the internal RV charger overnight once a month or more often if necessary to maintain the batteries and assure they are not discharged below 50% of capacity. Another option is to use a small solar panel and charger, which can be purchased from almost any automotive store and is relatively cheap. 

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