RV Battery State of Charge – A Geek’s Guide How to Murder Your Battery

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So you’re curious about your RV house batteries’ state of charge?

Prepare to feast from the Tree of Knowledge…

If you’re not thirsty for knowledge, here’s the grab-and-go answer:

A deep-cycle RV lead-acid battery should only be discharged to 40-50% of capacity. This is true whether it’s an AGM or wet battery. A hybrid RV/marine battery should only be discharged to 50-70% of capacity.

Now, if you want to know exactly why these things are, then let’s eat up.

Psst. Before you read this article, I recommend reading my Ultimate FAQ Guide to RV Batteries, particularly all about the question, “Why Is My RV Battery Dead?”

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Table of Contents

Battery Safety First!

Batteries contain immense amounts of energy. An electrical short can weld steel. An arc can blister skin. A wayward circuit or charge can stop your heart

Battery liquids are incredibly acidic and toxic! Batteries can produce explosive, ignitable gases.

When working with or around batteries, always wear eye protection and rubber gloves. If you doubt your abilities (or if someone else doubts you), have a professional do the work. It’s not worth your life.

Basic Battery Science

You, dear Reader, are capable of inventing a lead-acid battery.

The theory isn’t much more complicated than the lemon battery you built in middle-school.

*If you haven’t built a lemon battery, here’s how it works: Beg, borrow, buy or steal a lemon. Insert a copper penny in one end and a galvanized roofing nail in the other. Stick your tongue on the nail head and copper penny at the same time. Feel that tingle in your tongue? Wallah! – you’ve built a battery! And you’re shocking yourself!

  • Psst . . . if you’re reading this content anywhere besides Ask The RV Engineer, it’s been illegally “scraped,” and you’re probably on a spam website. So please be careful! Don’t share any private information, and come back to us at www.askthervengineer.com!

At its essence, a lead-acid battery is nothing more than two plates of lead metal submerged in an acidic liquid.

Here’s a simple mental picture: Imagine a baby bathtub. Place a small dinner plate at one end and another plate at the other. Fill the tub with water. Presto!

Let’s go into a little more detail.

I call this the Battery S’Mores Model.

Grid plates made of lead, a soft metal, are coated in an electrochemically active paste. One plate is called an anode, and the other is a cathode.

(Imagine spreading melted chocolate on a graham cracker).

Two of these lead plates are separated with a rubber sheet called a separator. The separator prevents the plates from touching each other and short-circuiting.

(The separator is the marshmallow between the crackers).

The plates, along with the separator, are submerged in a bath of acidic electrolyte liquid.

(Hmmm … maybe that’s like dunking the S’more in milk? I’ve never heard of anyone doing that, so maybe now’s the time to abandon the S’mores metaphor.)

This electrolyte is sulfuric acid.

Fun fact: Diluted sulfuric acid is a common ingredient in drain cleaners!

The acid allows a chemical reaction to occur between the two plates when an electrical charge occurs. One plate becomes pure lead (Pb), and the other becomes lead dioxide (PbO2).

Electrical charge flows between the plates, and one becomes positively charged while the other becomes negatively charged. That difference in electrical charge is called “voltage.”

When a lead-acid battery is discharged, water and lead sulfate are created. The water dilutes the acidic electrolyte until the battery is recharged.

What we’ve created is called a battery “cell.” Each cell is rated for 2V. So, for a 12V battery, six of these cells are put in the battery case, connected with strips of lead, and the battery case head screwed on top.

And that’s a basic battery.

Deep-Cycle Battery Construction

Dual Battery Bank on Tongue

If you’re not a geek like me, you might want to skip this section. I dive deeper into battery construction, which isn’t necessarily critical to understanding how an RV battery works. However,

The parts of a deep-cycle battery that matter the most to an RV owner are the plates.

While you could just use solid sheet metal lead plates, those don’t work very well. They don’t have enough exposed surface area.

Back in the late 1800s, a French physicist with an awesome beard named Camille Alphonse Faure – not the beard, the man – realized that if he used grids (imagine honeycomb) rather than solid plates, his batteries enjoyed a significant increase in energy storage capacity.

The man was a genius. Even today, most lead-acid batteries are modified versions of Faure’s innovation. Chemical engineers realized that by varying the shape and size of the plates, they could store more energy and also access it faster.

Today, lead plates can be manufactured as grids, spiral coils, perforated tubes, or tubes.

For instance, Optima produces its patented SIX-PACK dual deep-cycle/starting batteries using spiral-wound grid lead plates.

LifeLine uses thick, flat grid lead plates for the ultimate in durable deep-cycle discharging.

So what’s the takeaway for RV owners?

The construction of your battery plates has a huge impact on its performance! A deep-cycle battery needs thick and strong plates for long-term durability and maximum energy capacity.

Compare the robust construction of a true deep-cycle battery to a starter battery, such as what’s underneath the hood of your car.

A car starter battery has thin plates and lots of ‘em! This allows the battery to deliver a huge jolt of energy in a very short amount of time, normally a few seconds.

Unfortunately, those thin plates will literally collapse if the battery is deep-discharged. Metal or paste can flake off the plates like falling leaves off a tree.

So battery manufacturers have all come up with all sorts of ways to design new plate geometries, plate alloys, and separators.

Check out this video for some ideas:

You could spend the rest of your life – and many battery engineers do – just researching and analyzing battery plate and separator shapes.

We won’t. But now that you know more about battery construction, let’s get into what you really care about: Your battery state of charge.

What Is an RV Battery State of Charge?

Check out this helpful graphic below.

RV Battery State of Chart Image

Basically, your battery state of charge is like how fuel your gas tank is. At 0% state of charge, your battery doesn’t run. At 100% state of charge, it’s topped off.

*There’s actually more to it than this! A “dead” battery could mean three or four different things! Again, I recommend reading my Ultimate FAQ Guide to RV Batteries, particularly all about the question, “Why Is My RV Battery Dead?”

State of charge doesn’t exactly tell you how much power you have left.

You see, RV lead-acid house batteries shouldn’t be discharged to less than 50% of capacity. So a 50% state of charge would actually mean that, even though your battery is still half full, you shouldn’t use it anymore.

Rate of Discharge

If Usain Bolt can run the 100-meter dash in 9.63 seconds, can he run the 50-meter dash twice as fast?

Of course not! But for some reason, many people think their batteries can.

Here’s the ugly truth: A battery’s Ah rating decreases the faster you use it.

Think you can drain that 100Ah battery in just X hrs and still get 100 amps? Think again!

For deep-cycle lead-acid batteries, the standard discharge time period is 20 hours. So if a battery has a 100Ah rating, that actually means 100Ah @ 20-hr discharge time. Do a little math, and you get a 5A/hr discharge rate.

Here’s the good news! If you discharge slower than the standard time, you’ll get more energy out of your battery! You could get 105, maybe 110 Ah!

Bad news: If you discharge faster than the standard time, you’ll get less energy.

This is indicated by the C-rating. As I said, a C20 rating means the battery is completely discharged to 0% state of charge in 20 hours at a constant rate.

  • SLOWER: 100 hours
  • MEDIUM: 20 hrs
  • FAST: C10
  • VERY FAST: C5
  • SUPER FAST: C3

“What about C2 or C1?” you wonder.

Well, deep-cycle batteries are not designed to be discharged at super fast rates. This has to do with the battery design (plates and all that).

Technically, C-ratings are also based on temperature, but let’s not get into that yet.

How to Measure Battery State of Charge?

Simple Rule: As you charge your battery, its voltage increases. As it discharges, voltage decreases.

By measuring the voltage, you know how much charge remains in the battery.

Knowing a battery’s state of charge answers questions like, “Is my RV battery dead?” and “How do I know if my battery is fully charged?”

Hydrometer

A battery hydrometer is used to test the state of charge of a battery cell.

It measures the density of the liquid electrolyte by measuring its specific gravity. You see, the concentration of sulfuric acid in water is greater in a fully charged battery than a dead one.

When fully charged, a typical flooded 12V battery has an electrolyte specific gravity of 1.280.

When discharged, a typical flooded 12V battery has an electrolyte specific gravity of 1.140.

If you don’t understand any of what you just read, then dare I suggest a hyrdometer isn’t for you. It’s one of the most accurate methods to measure state of charge, but it’s not one you should perform often. It requires direct interaction with diluted sulfuric acid, which is dangerous.

And obviously, if you have a sealed “maintenance-free” battery like an AGM, then you can’t use a hydrometer anyway.

Open-Circuit Voltage

Measuring your battery’s open-circuit voltage is easy!

You just measure the voltage across the battery terminals.

To properly test the open-circuit voltage of each battery in a battery bank, the battery must be isolated from the system! Remove the battery cables and check each battery separately.

  • The battery must not be connected to any load!
  • The battery should be allowed to stabilize for at least six hours.

This is done using a voltmeter/multimeter.

Don’t get scared by the name! A multimeter is just an electrical measuring tool that can measure voltage plus other things like amp draw (A) and resistance (Ω).

If you don’t have a multimeter, get one. Spend at least $20. Choose one that can measure DC voltage, AC voltage, resistance and amperage draw.

If this is your first multimeter, here’s a good YouTube video on how to use one. The OE instruction manuals are a difficult pill to swallow.

Battery Monitor

The easiest solution to measure your battery state of charge is to use a battery monitor. Many RVs come with battery monitors pre-installed. Solar charge controllers commonly double as battery monitors.

More advanced battery monitors offer the following features.

  • Battery charge level
  • Incoming/outgoing amperage draw
  • Ampere-hours (capacity)

Some monitors will track your charging cycles and health history of your battery as well!

Visit my Changing Gears Gear Store to see some recommendations for battery monitors, and keep your battery healthy!

Ross

RV engineer by day, intrepid blogger by night (and occasionally weekends). This website is all about how RVs work, and sometimes why they don't. Bookmark pages that you find helpful, and join my email list for exclusive monthly awesomeness.