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Easy explanation: RV Battery Voltage and State of Charge

| Updated Jan 1, 2024

Here is a short, simple, and non-technical explanation of the different types of RV Battery Voltages ⚡ and their state of charge, or SOC for short.

When it comes to battery voltages and state of charge, even experienced RVers get confused. So, we're going to break it down using layman's terms.

Let's jump right in!

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Battery Voltage vs. State of Charge (SOC)

rv battery
Your RV battery setup. This is a pair of 6-volt batteries in a newish Chevy Roadtrek. 12-volt batteries have six holes, not three.

The battery voltage is what a voltmeter reads, or how many of those four lights are illuminated on your display. The state of charge (SOC) is what the actual storage status of your battery is, 100% being fully charged and 0% being flat dead.

The reason there's not a simple relationship between battery voltage and state of charge is that your RV system is constantly doing different stuff – electricity is coming and going all the time. So, it gets tricky if you're checking the battery system while in use or shortly after using.

There is, however, a simple relationship between battery voltage vs SOC for a battery that's disconnected from all charge sources and loads, and has been sitting an hour or so. This is called the “open-cell” or “resting” voltage of the battery. Of course, you don't want to have to disconnect and rest your battery every time you check the system, but it's a good place to start.

Resting, fully charged 12-volt batteries read at around 12.8-12.9 volts. Counterintuitively, a battery is dead at 12.0 volts (not zero). The minute you apply a load to a battery at 12.0 volts, the voltage plummets and nothing happens. So, a reading of 12.4 volts on a resting battery means it's about 50% charged.

RV Battery Voltage Overview

In general, charging inputs raise the actual voltage on the battery above its resting voltage, and loads (things that drain the battery) lower it below its resting voltage. 

If you crank up your RV generator or plug into shore power, your voltage will go up to 14.0 – 14.5 volts, even when your battery is seriously depleted. If you turn on a big load like the microwave (if you have a big inverter that will power it), your voltage will dive down into the 11.5-11.8 volt range, even on fully charged batteries. 

Again in general, the bigger the charging input or load, the further above or below your resting voltage of your system will be. So, looking at the voltage doesn't give you an accurate reading on how charged your battery is – you have to adjust for loads and charging sources.

hydrometer for your rv battery
This is a hydrometer. The Chinese make millions of them. They'll be glad to sell you one cheaply for your RV battery

An Easy Way to Determine the State Of Charge (SOC) of Your RV Battery

The foolproof way to accurately determine the state of charge is to use a hydrometer – a simple device that measures the density of your battery electrolyte. Fully charged batteries have lots of sulfuric acid in the electrolyte, but depleted ones don't.

The acid changes into lead sulfate on the battery plates as the battery discharges. Since sulfuric acid is heavier than water, fully charged batteries have electrolyte-specific gravity in the 1.255-1.275 range. 50% depleted batteries read 1.175-1.195, and dead batteries read 1.095-1.115. 

A battery hydrometer can be purchased at any auto parts store or on Amazon. In addition to the specific gravity numbers, they'll have color-coded ranges that say good battery, so-so battery, or dead battery.

How to Use a Battery Hydrometer

Now, to use the hydrometer you have to pull out your battery tray, open one of the caps, and suck up some electrolyte into the hydrometer, so you won't be doing this often, or on the road. If you have AGM batteries you won't be doing this, period – you can't get to the electrolyte at all because there are no caps.

For traditionalists, though, you can use the hydrometer to calibrate your voltage display under known load conditions, and you'll know what voltage reading goes with which state of charge.

What Are Known Load Conditions for Your RV Battery?

What do I mean by “known load conditions”? Let's hear from someone who has tested this:

“Well, in my Roadtrek, for instance, the TV is always on, and we usually have a fan going, and one or two inside lights are turned on.  I charge up my batteries, verify that they're charged by making sure the hydrometer reading is in the 1.26 neighborhood, and look at my voltage display when this stuff is running. Three lights and 12.5 volts on the meter. Under this load, that's fully charged. Now I leave the stuff on for a few hours, go back with the hydrometer, and see that specific gravity is down to 1.18 – half discharged. My voltage meter says 11.7 and I have one light on.

Now I know that, under this load of TV plus fan and lights, this is the point that I need to recharge my batteries – don't run your batteries below 50% unless you're in an emergency situation. Now I can put the hydrometer away – I have a voltage number, 11.7, that corresponds to 50% discharged and time to run the generator. Easy.”

This TV is always on - always. Something might happen with the royal family and you wouldn't want to miss it.
“This TV is always on – always. Something might happen with the royal family and you wouldn't want to miss it thus sipping power from your RV battery.”

People have different numbers or different types of batteries and different typical loads, so 11.7 volts on your system may mean more or less than 50% discharged, but the principle is the same – there's a particular voltage that corresponds to different states of charge for your batteries under load.

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RV Lifestyle

Published on 2013-08-23

7 Responses to “Easy explanation: RV Battery Voltage and State of Charge”

August 23, 2013at8:21 pm, Utah_Rookie said:

Very impressive technical article. This is the first time I’ve seen an excellent article on paying attention to the 50% battery charge state and presented in a easy to understand manner.

August 23, 2013at3:09 pm, Judi Darin said:

I remember a blackboard in a structures class that looked just like the one in the photo – flashbacks!

August 23, 2013at10:20 am, Pam Hicks said:

My bad 🙂 thanks

August 23, 2013at9:39 am, Pam Hicks said:

Son of a gun, Campskunk, you are one amazing teacher. This is one more bit of super valuable information that I have now printed out. Thank you, professor!!! Now, if only I could find that link you gave a while back for your sheepskin seat covers……………

August 23, 2013at9:42 am, Campskunk said:

tsk tsk – the BETTER students who WERE taking good notes know that i got my sheepskin seat covers from Driving Comfort here: http://www.drivingcomfort.com/sheepskin-products/custom-fit-sheepskin-seat-cover.cfm

August 23, 2013at9:10 am, Campskunk said:

no, Lisa, sealed batteries just mean you can’t calibrate the 50% discharge point to a known voltage level, not that you can’t have a voltage display. there are many digital voltmeters on the market if you want to improve on the idiot lights. type of battery doesn’t matter for that.

what you COULD do with sealed batteries is get a clamp-on ammeter, figure out the amperage going out of your batteries under load and time it. say you have four batteries with 90 amp-hours apiece at the 20 hour rate – that’s 360 amp-hours, supposedly 18 amps for 20 hours before it’s dead. charge them all the way (shore power overnight), turn things on until you’re drawing 18 amps (about 200-250 watts) and time it – 10 hours is half discharged. your voltage under load after 10 hours will be your 50% discharge level. this will only work with good, new batteries at the 20 hour rate.

August 23, 2013at8:23 am, Lisa said:

Hmmmm…. So with sealed batteries I guess I’m stuck with just idiot lights. I know some folks add meters of some sort but is that possible on these batteries?

Comments are closed.

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