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Testing your RV Battery State of Charge

November 30, 2009 by Mark Polk · 19 Comments  
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 battery 015

When you put your RV in storage the batteries lose  their charge over time, not to mention the parasitic loads that can drain RV batteries during storage. You should test the state of charge every month 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. An 80% state of charge for a 6-volt battery is 6.25 volts.

 

 

 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 methods is using 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 over head 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 less than 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 6.37 indicate the battery state of charge is less than 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 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 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% 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.

% State

Of Charge

Specific Gravity

Corrected 80 F

Open Circuit

12-Volt

Open Circuit

6-Volt

 

100

1.277

12.73

6.37

90

1.258

12.62

6.31

80

1.238

12.50

6.25

70

1.217

12.37

6.19

60

1.195

12.24

6.12

50

1.172

12.10

6.05

40

1.148

11.96

5.98

30

1.124

11.81

5.91

Discharged

1.120

11.80

5.90

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.

For more in-depth battery information check out our Deep Cycle BatteryDVD

 Happy Camping,

Mark Polk

RV Education 101

RV University

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Comments

19 Responses to “Testing your RV Battery State of Charge”

  1. Thomas Becher on November 30th, 2009 2:50 pm

    Recently I worked on my friends Monaco and we were going to pull the batterys. He had 4 golf cart batterys. I got some Red and Black tywraps and put them on the + and – wires accordingly Red for + Black for – that way you will not make a mistake hooking them back up right. Lots of Ka-Pow if done wrong.

  2. Stephen Oles on November 30th, 2009 3:57 pm

    Question! My 2006 Winnebage Sightseer has been sitting for three months since my last trip. It is parked on a slab, and I have the shore line plugged in to it’s own 30 amp service. the propane shut off valve is closed, but recently, the propame alarm has been going on and off. Some windows are open for air circulation. The monitor is near the door, where the batteries are under the steps. Could the Propane tank suddenly develope a leak? And do I need to get the RV to a repir sevice? Any help would be greatly appreciated.

  3. Mark Polk on November 30th, 2009 5:21 pm

    Stephen,

    This happens to other RVers too. There can be several reasons for the LP gas leak detector alarm going off. If the detector is battery operated the alarm will go off when the battery is getting low. If it’s hard wired and there is a low voltage condition it will go off. Lots of chemicals found in different products like hairspray, cleaning solvents, perfume and more will make the alarm go off unexpectedly.

    Some LP detectors have a reset button (not the mute button) on them. I have been told by others that after resetting it the alarm stopped going off intermittently.

    Check your owners manual to see if there is any information available on your particular LP detector.

    Hopefully with some of these scenarios you will be able to determine the cause. Don’t rule out a LP gas leak too. If you can’t isolate the problem you may want to have it checked. If may be possible that the detector itself is bad, or going bad, and needs to be replaced.

  4. Stripes on November 30th, 2009 6:06 pm

    Early this month I bought my first TT, went on a trip, parked the TT at a storage lot (no power there). I did not remove the batteries. I did other stuff for about a week. I went back to the TT to organize some odds n’ ends, and checked the battery level on the TT’s panel it was at 25% (or whatever one red dot is…”not dead”). I had been planning on disconnecting the batteries anyway, so I did so.

    I’ll be going on a trip in about 3 weeks.

    I’ll be camping at a powered site, and don’t think I will need to use the slide-out or anything other then the signal lights before I get parked up and hook up the power.

    Winter temperatures tend not to go below about 40 here.

    Do I have anything to worry about? Should I go fetch my batteries off the TT and buy a charger?

    After the next trip I plan on disconnecting the batteries when I store my TT. It will probably be a few months before I use it again. Is there anything else I should do?

  5. Testing your Battery State of Charge Acid by about on December 1st, 2009 12:35 am

    [...] the rest here:  Testing your Battery State of Charge By admin | category: acid, acid water | tags: draining-mines, even-though, fossil-fuels, [...]

  6. Mark Polk on December 1st, 2009 7:13 am

    Stripes,

    It’s a good idea to disconnect the battery when you put it in storage, but it also need to be fully charged. The life expectancy of the battery depends on how soon a discharged battery is recharged. The sooner it is recharged the better.

    If a battery is not recharged the sulfate material that attaches to the discharged portions of the plates begins to harden into crystals. Over time this sulfate cannot be converted back into active plate material and the battery is ruined. This happens when a battery remains discharged for an extended period of time. Sulfation is the number one cause of battery failure.

    Make sure the battery has a full charge before you take it to the storage lot. If it will be in storage for months at a time check the battery periodically and charge it as required. For long term storage it might be easier to remove the battery and store it in a cool dry place at your home. You can use a product like the Battery Minder to keep it charged.

  7. bpshand on December 2nd, 2009 8:39 am

    I think there’s a minor typo in the following:
    “A 6-volt battery that is charged should read 6.25 to 6.37 volts. Readings below 6.37 indicate the battery state of charge is less than 80% and the battery needs to be charged.”
    I think it should say “readings below 6.25 indicate the battery state of charge is less than 80%.”

  8. D. STILB on December 4th, 2009 1:48 pm

    What is the proper way to connect a battery maintainer to two [2] 6 volt house batteries that are connected as 12 volt? Thanks

  9. bpshand on December 7th, 2009 2:20 pm

    To D. Stilb:

    I’m pretty sure you connect the negative of the maintainer to the outgoing negative of one battery and the positive of the maintainer to the outgoing positive of the other battery.

  10. Bob on January 15th, 2010 5:40 pm

    Hi Mark,

    I have a 2007 Winnebago Access 31C on a Chevy Workhorse chasis with a 6L V-8. I store it but use it often. When not in regular use I usually (once a week) go down and run the engine and generator to get the pressures up and recharge the batteries etc. I had to go out of town recently and when I came back the engine battery was dead (what I expected) and the house batteries were fully charged (also what I expected as they have a disconnect switch). Anyway this is the first time the engine battery was dead so I push and held the emergency start button and NOTHING happened. I thought this button was to connect the house batteries when the engine battery was dead? I made sure the disconnect switch on the house batties was on so there was power, still nothing. My question is what good is the switch if you can’t get power when you need it? All the batteries are new except the engine battery which is stock as the motorhome is only 2 years old. I purchased it new. Any suggestions would be appreciated. Thanks.

  11. GMAs on January 16th, 2010 4:19 am

    I wish someone would check with the battery experts. If you are going to put the RV into storage a good idea is to remove the bats as they will self discharge normally even if disconected. Removing them helps but that is not the final answer.

    One will find that they will not only self discharge through time but also will do a process called sulfation. Here the batteries even if kept in charge will sulfate. The only way to keep this from happeing or becoming worse is to have a charger that has a built in DE-SULFINATOR. this then keeps them in top shape and ready for use again.

    Testing your battery with a volt meter is not a good test alone. One needs to not only have the volt meter but also a load bank so that the batteries can be tested for voltage under a load as spec’ed by the manufacture. Only then can one find out the real cap of the bat.

    And don’t be so quick to say they are good when you put them on a charger and they drop in current quickly. this is also a sign that the plates have sulfated and you are only developing a surface charge …the battery will not really be charge’n.

    for more info on battery sulfation and its damage one can google the internet or ask a good battery expert. Load bank testing can be done free at most of the autozone stores.

  12. Biff on January 23rd, 2010 3:32 pm

    The best thing you can do for your batteries is to get an intelligent, multistage, level maintainer; such as Battery Minder.
    I use them on virtually every battery we have, except the “everyday driven” Odyssey.
    A total of 8 in all (boat, tractor, mower, etc.) No more yearly pain-in-the-budget.
    My 1999 F250 diesel batteries were replaced for the 1st time this fall; 11 years.
    These devices were introduced to me by a Travel Trailer ad several years ago.
    The very nice gal at “dragon fly designs” on eBay has been our provider.
    Here is her Link:
    http://cgi.ebay.com/ebaymotors/12-Volt-BatteryMINDer-Charger_W0QQcmdZViewItemQQcategoryZ6755QQihZ018QQitemZ280229541824QQrdZ1QQsspagenameZWDVW

  13. Biff on January 23rd, 2010 3:44 pm

    AND; an adequate Battery Load tester can be found here:

    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93784

    It is often on “coupon” for $19.99

    P.S. The Battery MInder we suggested earlier has a automatic Desulfication cycle as part of its logic also.
    Enjoy

  14. marie aviano on January 24th, 2010 7:38 am

    Mark: I have a class a coach in storage in NY (got pretty cold here near 0 degrees) I took my brand new coach batteries, (3 deep cycle) BUT I put them inside my coach!! (someone told me that would be okay!!). Are they no good now? or could I put a battery minder on them or just take them to Autozone and have them checked?

  15. welrdelr on January 24th, 2010 8:42 pm

    I have had my share of battery problems with trailers and motor homes and most of the time it turned out that the battery was no longer any good. I don’t know if it is me or the batteries but I buy batteries for the engine compartment with three year guarantees and they are serviced regularly and they only last about 30 months … I just sold a 1999 truck that had four batteries in it since it was new and I only had to pay for one.(save your bill of sale). That was a good idea about the Red and Black tie off on the positive and negative connections on the two 6 volt batteries.

  16. Boondocking Made Easy -> The Basics | Wheeling It on March 8th, 2011 8:38 am

    [...] typical 12V wet-cell batteries, 12.7V is fully charged, while 12.2V is ~50% charge). Learn how to check the voltage, and before the charge gets too low (ideally before it gets below 50%), make sure to charge them [...]

  17. Dave on June 5th, 2011 12:41 pm

    I have 4 six volt battery for my RV house. One battery erode below the positive post and I need to replace that one. The batteries are about 3 years old . should I replace them all now?

  18. Dave on June 5th, 2011 12:46 pm

    I have four 6 volt batteries in my RV . 1 is corrouded at the post . The batteries are 3 years old should I replace them all now?

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