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Don’t Guess, Test your Battery State of Charge

November 8, 2010 by Mark Polk · 7 Comments  
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RV batteries

 The only way to know the condition of your RV battery(s) is to test the state of charge. All batteries will lose a charge over time, not to mention the parasitic loads that can drain RV batteries in storage. You should test the battery state of charge periodically and charge any battery that is at or below an 80% state of charge.

 Note: An 80% state of charge for a 12-volt battery is 12.5 volts, and 6.25 volts for a 6-volt battery.

 Testing the battery state of charge is not difficult to do. There are basically three ways to test the condition of your RV batteries.

1)      You can use the monitor panel in the RV.

2)      You can measure the voltage with a digital voltmeter.

3)      You can test the specific gravity with a hydrometer.

 The least accurate of the three testing methods is the RV monitor panel, but if this is your only means for checking the battery(s) it will give you a general idea of the condition. When you check the condition of your battery using the monitor panel make sure the RV is not plugged in to shore power, if it is you will get a false fully charged reading. To get a more accurate reading of the battery’s condition check the monitor panel when the RV is not plugged in and turn a couple of overhead lights on to place a small load on the battery.

 Measuring voltage with a voltmeter has its advantages. If you have sealed batteries your only choice is to use a voltmeter, and measuring voltage can give you a quick picture of the batteries depth of discharge, so you know when they need to be recharged. To measure the voltage you need a good digital voltmeter. Set the meter on DC voltage and connect the red lead to the positive terminal and the black lead to the negative terminal.  A 12-volt battery that is charged should read 12.5 to 12.7 volts. Readings less than 12.5 indicate the battery state of charge is below 80% and the battery needs to be charged.  A 6-volt battery that is charged should read 6.25 to 6.37 volts. Readings below 6.25 indicate the battery state of charge is below 80% and the battery needs to be charged.

 Note: To get an accurate reading the battery should not be tested if it has been charged or discharged in the last 12 hours and preferably 24 hours.

 The preferred method for testing the battery’s state of charge is to check the specific gravity reading of each cell. You can purchase a hydrometer at an auto parts store for about ten dollars. The electrolyte in the battery cells is a solution of acid and water so you need to wear safety glasses and gloves and avoid any contact with your skin. The first step is to remove the vent caps and check the electrolyte levels. There has to be enough in the cells for the hydrometer to pick up a sample. If you have to add any water prior to testing you will have to charge the battery and let it sit for 12 hours before testing the condition of the battery.

 Next, fill and drain the hydrometer at least twice in each cell before taking a sample. Take the reading and record it and drain the electrolyte back into the cell you are testing. Test all of the cells and replace the vent caps when you are finished. Specific gravity readings for a charged battery should read between 1.235 and 1.277. Specific gravity readings below 1.235 indicate the battery state of charge is less than 80% and the battery needs to be charged.  If there is a .050 or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell in the battery.

 Note: If your hydrometer does not compensate for temperature you must correct the readings to 80 degrees F. Add .004 for every 10 degrees above 80 degrees F and subtract .004 for every 10 degrees below 80 degrees F.

 If you put your RV in long term storage it’s a good idea to remove the batteries and put them in storage too. This is quite simple to do. When you remove a battery always remember to remove the negative terminal first and then the positive terminal. Label the battery cables so you remember how to make the connections next spring. Clean the batteries with a 50/50 mixture of baking soda and water if necessary. Check the electrolyte level and add distilled water if necessary.  Test the battery state of charge and charge any batteries that are at or below 80%. A discharged or partially charged battery will freeze much faster than a charged battery. Store the batteries in a cool dry place but not where they could freeze. Completely charge the batteries before re-installing them next spring. When you reinstall the battery connect the positive cables first and then the negative cables.

 Knowing the battery state of charge and re-charging a discharged battery in a timely manner will extend the life of the battery.

Caution: RV batteries can be dangerous to work around. If you are not familiar with batteries or if you do not feel comfortable working around batteries have your battery maintenance performed by an authorized service center.

For more information on deep cycle batteries be sure to check out our Deep Cycle Batteries Care & Maintenance DVD or our Deep Cycle Battery Care & Maintenance E-Book

Happy Camping,

Mark J. Polk

RV Education 101

RV University

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Comments

7 Responses to “Don’t Guess, Test your Battery State of Charge”

  1. Skip Kelly on November 8th, 2010 6:53 pm

    Hey Mark…..
    Good advice about the batteries. Here are a couple more tips on battery storage.
    Never store wet cell batteries directly on a concrete floor for any length of time, as they will discharge at a much faster rate. Use a piece of scrap plywood or an old floor mat under them for insulation.
    Always store batteries well below eye level to prevent serious eye contamination.
    Even the dry particles on top of a battery contain acid.
    Skip Kelly
    Alturas, Ca.

  2. Mike on November 8th, 2010 7:02 pm

    I’m surprised you don’t know about battery/coolant refractometers (about $50).

  3. butterbean carpenter on November 8th, 2010 7:04 pm

    Howdy Mark,
    Good article!! Now, what about re-charging batteries left too long without checking? How low can they go without being ruined??

  4. Mark Polk on November 9th, 2010 6:53 am

    Mike,

    We used refractometers in the military to test antifreeze and electrolyte. If you test on a regular basis or use it frequently you can’t beat it. They are easy to use and eliminate the need for temperature compensation.

    The main reason for not mentioning it in the article is the cost. A refractometer can cost from $50 to over $100 vs. a hydrometer for a few bucks. If you don’t test batteries or antifreeze on a regular basis it may not be worth spending the additional money.

    Butterbean,

    We’ll talk about re-charging next week.

  5. Geoffrey Pruett on November 9th, 2010 11:07 am

    Also do not overspend on the digital voltmeter, for measuring battery voltage the difference in accuracy between the $3.95 Harbor Freight unit and the one sold at auto parts stores at $30.00 plus is that you will feel worse if you destroy the expensive one. After 50 plus years as a electronics technician and electrician the lesson is have one at hand, not spend for a fancy toy. The only upgrade that makes any sense around a vehicle is the soft case and larger display on the ones around $20 if you watch the sales. A set of test leads with insulated clips completes the electrical test set for any vehicle. Do not leave home without them!

  6. kellie on November 9th, 2010 6:12 pm

    Wouldn’t buying a $50 tool to save a $50 battery be counter productive?

  7. Keith on November 10th, 2010 8:32 pm

    I use to work in a garage and the standard test for a battery was to apply a load test with a carbon pile. You would crank the carbon pile down to draw 3 times the amp hour rating for 15 seconds and if the voltage dropped below 9.6 volts you had a defective battery. I believe that test is still valid.
    Keith

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