Easy explanation: RV Battery Voltage and State of Charge

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.  

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 reason there’s not a simple relationship between battery voltage (what a voltmeter reads, or how many of those four idiot lights are illuminated on your display) and state of charge (what the actual storage status of your battery is, 100% being fully charged and 0% being flat dead) is that your RV system is constantly doing different stuff – electricity is coming and going all the time. 

There is a simple relationship between the two for a battery that’s disconnected from all charge sources and loads and has been sitting an hour or so, but you don’t want to do that every time you check the system.  It’s a good place to start, though. This is called the “open-cell” or “resting” voltage of the battery.  Resting fully charged 12-volt batteries are around 12.8-12.9 volts, and flat dead ones are at 12.0 volts, so 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 (drains on the battery) lower it below its resting voltage.  Crank up your Onan or engine generator, or plug into shore power, and your voltage goes up to 14.0 – 14.5 volts, even when your battery is seriously depleted. Turn on a big load like the microwave (if you have a big inverter that will power it), and voltage dives down into the 11.5-11.8 volt range, even on fully charged batteries.  Again in general, the bigger the charging current or load, the further above or below your resting voltage 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) ⚡ In 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, 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 hydrometer can be purchased at any auto parts store for less than $10. In addition to the specific gravity numbers, they’ll have color-coded ranges that say good battery, so-so battery, or dead battery.

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 as the newer Roadtreks do, you won’t be doing this, period – you can’t get to the electrolyte at all because there are no caps. For us 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 Is Known Load Conditions for your RV battery?

What do I mean by “known load conditions”? 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 idiot 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 idiot 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. For those of you still awake, here’s a discussion of this by Surrette, the fanciest battery manufacturer around. 

The voltage figure their table has for 50% discharge is 11.58 volts, so that’s probably using a larger load than mine (they talk about turning all your loads on, which is crazy in my RT), but test your own system and use a load that’s easy to establish – for me it’s VERY easy because the TV is on as soon as my wife wakes up, and stays on until she dozes off and I turn it off.  With this load on your batteries, there’s a foolproof battery voltage that corresponds to 50% discharge. Stay above that voltage, and you and your batteries will live long healthy lives.

 

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7 thoughts on “Easy explanation: RV Battery Voltage and State of Charge”

  1. 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?

  2. 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.

  3. 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……………

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

  5. 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.

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