With powerful, pure sine wave inverters and a plethora of 12V appliances on the market, it is possible to run a whole lot of stuff from your RV battery when off the grid. This is provided you have the battery capacity to do it. When boondocking, it’s important to be able to determine how much battery power your RV has left at any given time.
Many RVs have a display of lights that tell you approximately how full your batteries are. But you may have noticed that the four lights aren’t an incredibly accurate measure of the battery power your RV has left. Below you will find several methods you can use to accurately determine your remaining battery power for both lead-acid and lithium batteries.
All About Lead-Acid Batteries
Lead-acid (LA) batteries are the most common type of battery you will find on an RV. Though your car or truck uses the same type of battery, your RV will most likely have deep-cycle 12V batteries. The difference is the starter battery for your engine is intended to deliver a lot of power very quickly to start your motor. In contrast, deep-cycle batteries are designed to give a steady current or a long period to run your RV lights, furnace fan, etc.
You’ll find three styles of deep-cycle lead-acid batteries in the RV world:
- 12V flooded lead-acid batteries (the kind you have to add water to)
- 12V sealed Absorbent Glass Mat (AGM) batteries
- 6V deep-cycle golf cart batteries (typically flooded type)
Before determining the charge state of your LA batteries, it is a good idea to know what kind of battery you have. If you don’t already know your battery type, this information should be on the battery label.
Depth of Discharge (DoD) of an RV Lead-Acid Battery
Regardless of the type of lead-acid battery you have, the recommended maximum depth of discharge is 50%. In other words, your battery will last the longest when you don’t drain it below 50%. It probably won’t hurt the battery too much if you used 60-70% of the battery capacity every once in a while, but doing so frequently will shorten the battery’s lifespan.
In effect, when determining the remaining battery power of a lead-acid battery, you are trying to determine how close you are to 50% rather than 0%.
How to Use a Multimeter to Determine Battery Power
A multimeter is one of the easiest ways to get an accurate reading of your battery’s current charge state.
Here’s how to use a multimeter to determine your RVs battery power:
- Set the multimeter to the DC setting. Every multimeter is a little different in layout so you may need to refer to the manual of your meter to find the right setting for 12V or 6V batteries. Generally, you will find it near a “V” that has the DC symbol “⎓” next to, or below it (meaning “DC volts”).
- Touch the red probe to the red or positive side of the battery and the black probe to the black or negative side of the battery.
- The meter will display the battery voltage.
- Use the table below to determine your battery power.
Note: You will get the most accurate reading when testing a battery that has been resting (not charged or discharged) for 12-24 hours.
Lead-Acid Battery State-of-Charge Table
How to Use a Specific Gravity Reading To Determine RV Battery Power
A specific gravity test is considered the most accurate way to assess your battery power. It can only be done on a flooded lead-acid battery because AGM batteries are sealed and there is no way to get to the electrolyte solution.
What is a specific gravity test?
The electrolyte solution in a LA battery is made up of sulfuric acid and distilled water. As you use the battery, the acid gets converted into other chemicals and gets used up. When you charge the battery, the process is reversed and those chemicals get turned back into sulphuric acid.
Because the amount of acid mixed with distilled water changes based on the battery’s charge state, so does the density of that liquid. You can measure that density with a hydrometer. The most common type of battery hydrometer looks like a turkey baster with a little float device inside.
One of the easiest to use is an EZRED Battery Hydrometer. You can find them online and in most auto-part stores. The nice thing about the EZRED hydrometer is it’s a temperature compensating hydrometer.
How to Measure The Specific Gravity
- Safety first! Battery acid is very harmful to your skin. Use:
- Protective eyewear
- Thick gloves
- Closed-toe shoes
- Carefully take off all of the covers.
- Squeeze the bulb and carefully insert the battery hydrometer into the cell. Release the bulb to draw fluid into the hydrometer until the battery electrolyte solution fills the hydrometer to the maximum level.
- Read the measured value on the device and write that value down.
- Continue onto the next cell and repeat until you have measured all the battery cells.
- All the cells should read about the same. If there is .050 or more difference in the specific gravity reading between the highest and lowest cell, you may have a weak or dead battery cell.
- Refer to the specific gravity table to find your current battery power.
Specific Gravity Table
Note: A professional battery hydrometer will give you more pinpointed readings than the ranges on the EZRED model. If you choose to measure your battery power with this method, make sure to get one with a built-in thermometer and follow the directions carefully to arrive at an accurate reading.
RV Lithium-Ion Batteries
Lithium batteries are becoming more popular with RVers—especially those interested in solar systems and boondocking. Like AGM lead-acid batteries, lithium batteries are sealed and maintenance-free. Of course, they are based on using lithium rather than lead in their makeup, and they have several advantages over lead-acid batteries.
There are different types of lithium-ion batteries used in various applications. The most common type of 12V lithium battery found in RVs uses lithium iron phosphate. You will see this marketed using the compound’s chemical formula LiFePO4. This type of lithium battery has proven to be safe, reliable, and longer-lasting than lead-acid batteries. If you are thinking of switching over to LiFePO4 batteries, have a look at Everything You Need to Know About Installing Lithium Batteries in an RV.
Deeper Depth of Discharge
One big advantage LiFePO4 batteries have over lead-acid is it has a much deeper DoD. While lead-acid shouldn’t be discharged below 50%, a lithium battery can be discharged down further without seriously harming the battery. Although experts recommend only discharging it down to 20% regularly to get the longest lifespan.
In practice, that means your 100 amp-hour lead-acid battery only has 50Ah of usable battery power, while just about all 100Ah are usable when using lithium.
So, How Do You Determine How Much Battery Power You Have Left When Using Lithium?
There are no fluids to get into for specific gravity tests when checking lithium batteries, so you will want to use the multimeter method to measure your battery’s charge state via voltage. Touch the red probe to positive and black to negative after setting the multimeter to read DC volts and note the battery’s voltage. Use the table below to determine your remaining battery power.
Lithium Battery State-of-Charge Table
The Easiest Way to Determine How Much Battery Power Your RV Has Left
If at any point while reading this article you thought Hey, I can get a fix on my location within 10ft using satellites and my phone. Hasn’t someone come up with a battery power monitor I can use inside my RV? Then you’ll love this next method. The following are two battery monitors that are much more accurate than the four LED lights on your stock RV panel.
AiLi 500A Battery Monitor
The AiLi 500A Battery monitor is a universal battery monitor that can be used for all types of lead-acid batteries as well as lithium. Unlike using a multimeter, you have to wire and install the monitor or have a professional do it for you. Though once installed, you will be able to see voltage, current, ampere-hours, state of charge (SOC), power time remaining, and power consumption in watts at a glance from the comfort of your RV.
From a price standpoint, the kit costs around $70 on Amazon. However, if you are not an avid DIYer who feels comfortable installing this yourself, you have to add the cost of having it installed to that.
Innova 3721 Battery and Charging System Monitor
The Innova 3721 is great if you are looking for a more plug-and-play method to monitor your battery’s state of charge inside your RV. Most RVs have one of those 12V car ports in them to run things like 12V TVs, heating blankets, and other appliances.
This $20 device plugs into that port and displays the voltage of your batteries just like a multimeter. All you have to do is refer to the voltage tables for your battery type. (If you’re using two 6V batteries wired in a series, the 12V table applies).
Note: If you are in a motorhome, make sure you are plugging this monitor into a plug powered by your house batteries. The 12V car ports located on your dash are most likely powered off your starter battery rather than the one(s) your RV uses.
Easily Downloadable DoD Table For All RV Batteries
Below, you will find all the tables used in this article in one easy-to-reference image. Just right-click on the table and choose “Save Image As” to download it for later use.
Have you ever measured how much battery power your RV batteries have left? How did it go? Share below in the comments.
Levi Henley is a freelance writer and has also been full-time RVing with his wife and pets since 2015.