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How to Perform Open Voltage Testing on Your RV House Batteries

The last thing you want to happen when you head out on a month-long cross-country vacation is to find yourself with no battery power, or with batteries that quickly go dead after you’ve been driving all day.

Good house batteries should be fully charged after a few hours of driving.

If you think your batteries are not up to snuff, you can measure their voltage to narrow down the source of an electrical problem.

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It’s best to start any troubleshooting efforts at the battery since that’s the source of electricity to your RV when not on shore power or a generator.

In this article I’ll walk you through the steps to check the open voltage on both flooded and valve-regulated lead-acid batteries.

It’s also a good idea to perform regular maintenance on your batteries to lengthen their lifespan. Additional tests, such as the specific gravity test, are outside the scope of this article.

Step 1: Determine the Wiring Layout of Your Batteries

The first step is to determine what type of battery installation you have.

Do you have two 6 volt batteries in series, two 12 volt batteries in parallel, or four 6 volt batteries in series-parallel?

four house batteries in series parallel in a motorhome
Four house batteries in a series parallel configuration

To determine what setup you have, take a look at these sample wiring diagrams provided by Trojan Battery Company.

Safety First:  If these diagrams confuse you, I recommend you take your battery issue to a shop for testing. Many places will do it for free if you buy your batteries there. Making a mistake on reconnecting battery cables is a potentially deadly issue.

Step 2: Get Yourself an Inexpensive Multimeter

The tool you need to check your batteries is called a multimeter.

This tool will allow you to determine where you do or do not have electricity in any circuit.

Not only is it the primary way to check out your charging system, but it can also help you troubleshoot many other kinds of electrical issues.

Multimeters can test any range of voltages, both AC and DC. They can also measure resistance and tell you if a circuit is open. Be sure to read and understand the operator’s manual that comes with your multimeter.

Generally any multimeter in the $20 – $25 range is good enough quality for use in testing your RV batteries and electrical circuits.

Here are a few choices:

Step 3: Check if the Alternator is Charging Your Batteries

To determine if the alternator is charging your batteries, hook the multimeter leads to the positive and negative cables that supply power to the RV.

Check the voltage with the motor off.

It should read somewhere around 12 volts. Start the motor and the voltage should jump up to 13.8 volts.

If it does the charging system is fine and you can move on to testing your house batteries.

Step 4: Check Your House Batteries

This is where it gets a little trickier.

If, as I mentioned at the beginning, you don’t clearly understand how the wiring works, you should take your batteries to the shop for professional service.

To properly test the batteries, each must be tested individually and separate from the battery network.

One battery may be bad while the others work just fine. You can’t tell which is which unless you disconnect the cables and check them apart from each other.

Check the Battery Water Level

If you have flooded batteries with removable caps, you should verify if the water in the battery is at the proper level.

If the water level is low, refill it with distilled water so that the water covers the plates. Use the plastic opening in the battery to observe the height of the water, and fill the battery to its proper level.

Note: Only use distilled water when refilling flooded batteries.

Take a Pre-Charge Reading and Compare With Standard Measurements

Measure the battery with your multimeter in DC mode and the the black and red cables connected to their respective battery terminals.

Use this voltage reading to determine the corresponding “Percentage of Charge” from the table below:

rv battery open circuit voltage
RV battery open circuit voltage readings
Trojan Battery Company

If you determine the battery is between 0% and 70% charged, charge the battery to full capacity.

Charge the Battery Completely and Measure Open Circuit Voltage

Charge each battery as completely as you can.

Overnight should be sufficient. When done, let the battery sit for a couple days, not attached to anything.

Check the voltage after that period and it should be 6.37 V for a 6 V battery or 12.73 V for a 12 V battery.

If the measured voltage after charge is less than these values, the battery has one of two problems:

  1. it was left in a state of discharge too long or
  2. it has a bad cell.

Take the battery in for replacement or further testing.

testing the voltage on house batteries in a motorhome
Testing the voltage on house batteries in a motorhome

Load Test Suspect Batteries

If everything checks out and the batteries still go dead, you need to take them to an auto parts or battery store to have them load tested.

During load testing, the technician will use a machine to apply a heavy load to the battery to determine if the battery requires replacement.

Conclusion

If you determine that a battery isn’t operating properly, don’t be afraid to replace it.

It’s good practice to replace all the batteries in a bank at the same time. If one of them goes bad, it may take the others down with it.

The only time I would say not to replace all of them is in the case where all have recently been replaced and one turns out to be defective shortly after purchase.

Don’t Assume Without Testing

That said, don’t be quick to junk out a bank of batteries before making the effort to slow charge them over a longer period.

A set of gel cell (sealed) batteries need to be charged at a low amperage over a longer period of time. These batteries are designed so that the liquid doesn’t boil off, which can happen when applying a high amperage quick charge.

Safety Is Your First Priority

Understand that when you have a group of two or more batteries you potentially have an explosive portable welder on your hands.

Batteries that have been on a charger have a head of explosive gas built up which escapes slowly through a very small vent hole.

If you momentarily short a battery cable, the arc could cause an explosion.

Always wear eye protection and rubber gloves when working with batteries. Remember, when in doubt, have a professional do the work – it’s money well spent.

You can find more information on specific gravity testing (not covered in this article) and open voltage testing at the Trojan Battery Company website.



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