Polk’s Top 7 Tips for Testing your Battery State of Charge

Mark Polk, motorhome  and motor coach maintenance expertBy Mark Polk
RV Education 101

When you put your motorhome in storage the battery(s) lose their charge over time, not to mention the parasitic loads that can drain batteries in storage. You should test the state of charge every month and charge any battery that is at or below an 80 percent state of charge. But just how do you accomplish this? Let’s take a look at my top 7 tips for testing your battery state of charge.

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

1. There are basically three ways to test the condition of your RV batteries.

2. The least accurate of the three methods is using the RV monitor panel. If that is your only means for checking the battery(s), though, 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 motorhome 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 motorhome is not plugged in and turn on a couple of overhead lights to place a small load on the battery.

3. If you have sealed batteries your only choice is to use a voltmeter. Measuring voltage with a voltmeter has its advantages. It 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 less than 80 percent 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 less than 80 percent 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.

4. 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 $10. The electrolyte is a solution of acid and water, so you need to wear safety glasses and gloves and avoid any contact with your skin. 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, you’ll have to charge the battery and let it sit for 12 hours before testing. Fill and drain the hydrometer at least twice in each cell before taking a sample. Take the reading and record it, and then drain it back into the cell. Test all of the cells and replace the vent caps. 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 percent 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.

5. 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.

6. Regardless of whether the batteries are left in the motorhome or removed, you should test the battery state of charge monthly and charge any batteries that are at or below 80 percent. 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.

7. Completely charge the batteries before using the motorhome next spring. If you did remove the batteries remember to connect the positive cables first and then the negative cables when you reinstall them.

Happy camping.

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