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How to Determine If I Need New House Batteries

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    How to Determine If I Need New House Batteries

    I have a Magnum 3112 Inverter. My house battery bank is four 6 volt Trojan L16E-AC which are 7 years old. Inverter, batteries, and generator all seemed to be working fine this season. Recently, while on anchor I got a fault message on the inverter display panel, Low Battery and the inverter stopped inverting.The display read a State of Charge of 94%. Too high to set off the Low Battery Cut Out of 10.8 v. I ran the generator to charge up to 100%, shut off the generator and with a few minutes the inverter's fault light came on with Low Battery, and SOC was 97%.

    I called tech support at Magnum and was taken through some tests on the inverter. With a multimeter I checked the volts for each battery and they read: 6.66v, 6.22v, 5.50v, 5.95v. I was told when convenient to run a battery balance. Once plugged into power and fully charged I did that and now the batteries are: 7.53v,7.54v, 6.95v, 6.94v.

    I turned off shore power for the inverter to start working and within a couple of minutes I had the fault light for Low Battery.

    When talking to the tech he suggested that the batteries need replacing. I don't want to spend $1200+ to replace batteries and find it not the problem.

    What can I do to be sure the batteries need replacing?
    Jerome Robbins
    Commodore, Fidalgo Yacht Club - 2019
    Anacortes, WA

    2001 Bayliner 4788
    Twin 370 Cummins

    Previously owned:1994 Bayliner Ciera 2855 454 V8-7.4L
    1994 Bayliner Classic 2252 V6-5.2L

    100% State of Charge (SOC) for a 6 volt, flooded lead acid battery is 6.37 volts, but a measurement can be misleading if taken shortly after charging. When a battery is charged it usually reads higher than it's real voltage. This results from the battery not being able to quickly change the lead sulfate to lead plus lead dioxide. The charge happens on the surface of the plates, so you get a higher voltage referred to as, surface charge. To use a voltmeter to determine SOC, I recommend that you put the batteries under load for a few minutes after charging, or disconnect them and let them sit for a day or so. The voltage read after this should be accurate.

    You should also measure the specific gravity of the electrolyte in each cell with a hydrometer. This will point to individual cell issues. Don't forget to correct the reading for the ambient temperature. Here's a good chart:

    1999 3788, Cummins 270 "Freedom"
    2013 Boston Whaler 130 SS
    Anacortes, WA


      At seven years old, the 4 6v batters are probably not holding a load. Replace them. The individual 6v wet cell batteries are not that expensive. Sealed lead acid and AGMs are.


        I have 6 ya wet cells for House bank plus an 8d. Also two 8d as start batteries. They are all wet cells with fill kits to easy water replacement efforts. Had to replace all at the seventh years. Some would hold charge and load but I wanted to do it right to easy worry or concern
        hardly ever use inverter. Occasionally use genset.
        I did loose one of the 8d batteries when th fill kit cell float broke and the battery box flooded enough to corrode the terminal. Replacement was $250 plus labor (I can’t lift and handle the weight of the 8d.

        1990 4588


          I would not go by age alone. I had a high quality, multi stage charger on my previous boat and the 2GC house batteries were still going strong when I sold it after 7 years. A couple of months before selling the boat I actually pulled them and took them to a battery shop for testing and they tested fine.
          1999 3788, Cummins 270 "Freedom"
          2013 Boston Whaler 130 SS
          Anacortes, WA


            7 yrs? replace them. Some get longer...some shorter. Why screw around?Oh, and replace them with good ones like Trojans or Crown.
            1989 26' then 1994 32' now 2001 39'


            • Knothome4788
              Knothome4788 commented
              Editing a comment
              The OP noted he had Trojans now....

            Here’s how to check your house batteries. Simple and easy.

            1. Charge your battery bank until its fully charged. The easiest way to do this is just leave it plugged in inverter/charger running. This voltage should be 13.5 volts or thereabouts depending on the charge profile you set up.

            2. Turn off your charger with all your loads as normal and wait 30 minutes or so. Your bank should read 12.0 volts or thereabout. More if you have very little load. A lead acid battery WILL stabilize at exactly 2.0 volts per cell under moderate loads in just a few minutes.

            3. Give it some time, a few hours. Check your battery voltage. It will read something a bit less than 12.0 volts. Check each individual cell. They should be the same voltage or close.

            4. More time, at least 8 hours total on the batteries. Your bank will now read something less. I have the EXACT same batteries in my 4788 and at this point my batteries are down to 11.75 volts for the string but my boat is a power hog because of my satellite transceiver. Look for differences between cells.

            If you have not made it this far, time to buy new batteries.

            BTW, I made a pretty good living for a couple decades maintaining remote microwave and fiber optic repeater sites, and part of that was maintaining the station battery strings.

            Also from your first post and a battery reading 5.5 volts I am pretty darn sure that battery is TU, but lets test to be sure. At this age, replace them all if you replace a single cell.

            KEVIN SANDERS
            4788 LISAS WAY - SEWARD ALASKA
            where are we right now​​​​​​???​



              Have them tested, it's free. Also, check the prices on Sam's Club or Costco. Hell, I have Wal Mart batteries older than yours that have not failed. Stupid question of the day, did you check the water ?


                Make your life simple. Get 2 8D's for your house system and pay a few bucks to have them installed. They are about $250.00 Canadian each and I pay 40 bucks to have the old ones pulled and the new ones dropped in place while I enjoy a cocktail. I usually get 5 to 6 years out of them.



                  Originally posted by iceclimber View Post
                  Have them tested, it's free. ...
                  Taking out four 100 Lb of batteries to get them tested is not fun.
                  1999 3788, Cummins 270 "Freedom"
                  2013 Boston Whaler 130 SS
                  Anacortes, WA


                    Check each cell for water...
                    Look at the area around each cell cap for moisture...especially while charging...
                    Get a good hydrometer and test each cell...this will check the chemistry of each cell...

                    Equally important is to check the voltage at the input to the inverter to make sure all connections are solid...

                    1988 3888
                    Twin Cummins 6BT's 210hp
                    Onan 8KW


                      A specific gravity hydrometer should be the first test you make. It will tell you "exactly" the state of each cell.

                      Forget everything else you have been told.
                      Midnight Star
                      1996 3587
                      Twin diesels, Hino 250's
                      Ladysmith, BC
                      History: 1996 - 2655, 2001 - 2855, 1984 - 3270


                      • capnken
                        capnken commented
                        Editing a comment
                        I agree. and if you have a load tester (I got one at Harbor Freight) you can also collect some info about load/battery state. PS...I would stick with the 6 volt batteries, your array offers more cycle reserve due to more plate area for house batteries. For start, 8D would be good choice. PSS....I have a friend that uses Optima batteries for start,...I will stick with my heavy 8D batteries for start though. Another thought: new solid state multibank chargers offer charge cycles with pulse charging to help de-sulfate plates, float charging, etc., even settings for seasonal charging, so that can extend battery life significantly. anyway, Good luck.

                      I would agree with the comments about how to check for bad cells, etc. One thing that I didn't see in your post was what was the voltage across the battery bank with the inverter showing "low battery" If in fact the voltage had dropped to cause the fault then there are bad batteries in the group. Unless there was low water in some that was the cause, I would say time to replace the bank. Never good in my mind to try to weed out the current bad ones - the others will fail in the near future.

                      Also, from the Trojan info on this battery, a voltage of 5.75 equals 10 percent charge - as you have one that reads 5.5 volts it must be a problem
                      Ron O'Blenis
                      B 38 175 Hinos 1989
                      Completed Great Loop


                        Ok assuming the battery bank is a FLA deep cycle type, there is obviously at least one bad battery cell in your bank. After checking that the fluid level is above the plates on all cells, the method I would choose to determine this is thus:

                        1) Completely charge the whole bank by charging at bulk (14.5 or so volts DC) until you get to the point where the batteries are only taking in .1 to .15 C (Where C is the rated discharge at the 20hr rating of the battery bank). For a 600 AH bank this would be 3 to 5 A's (600AH / 20 Hr * 0.1). This will be 100% charged.

                        2) Let the batteries stand for a period of 24hrs with NO load and then use a hydrometer and check the specific gravity of each cell. They ALL should be close to 1.265. Recheck and add distilled water to the proper level for all cells (this should be to the recommended level for your batteries in general this is just below the vent slots of the cell). If ANY significant variation then do a "equalising charge at 15.5V (or whatever your battery manufacturer recommends for such). Any cell that varies by more than than 0.030 then the battery is probably done.
                        3) After the equalization charge, check the specific gravity of each cell. If any are out more than the 0.030 then you know at least one battery is bad.
                        4) Assuming all cells are now fairly close Apply a load equal to the 20hr rated load. For a 600AH bank that would be 30A DC (600AH divided by 20hrs = 30A) after 10 hrs remove the load and let stand un loaded for 10 hrs then check the specific gravity again. IF all the cells are reading close to 1.210 then you still have some life left in the bank. If not then maybe it's time to replace the whole bank.

                        Now I will admit that IF there are no major differences between the measured specific gravity measurements of the cells the battery bank could still be used but the available capacity would have to be adjusted accordingly.

                        This load test should be conducted at least once yearly to determine the true condition of the battery bank.

                        RB Cooper

                        A few notes:

                        a) I only do the load test to the 50% level since I don't like taking my batteries below that but many experts recommend taking the bank all the way down to the 0.0% level i.e. running the test for the full 20 hrs.
                        b) new batteries usually require quite a few cycles of discharge before they get to the full capacity load. Check with your manufacturer for details.
                        c) Coulomb counter type battery monitors should be adjusted for the new determined available AH discharge. So if your down to 1.20 or so after 10 hours your 600AH capacity is really about 20% less so 480AH your new 50% discharge is 240AH available.