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Prius Auxillary Battery Not Maintenance Free

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by mortweaver, Feb 7, 2010.

  1. mortweaver

    mortweaver Junior Member

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    2004 Prius
    Toyota claims the auxillary battery is maintenance free since it is a sealed lead acid battery. Not true! The battery is vented to exterior atmosphere. With over 200,000 miles of Toyota hybrid experience, I suggest removal of battery annually to check each cell for elctrolyte level. With a normal charge system voltage level of approx. 14 volts, there will be a small loss of water vapor through the vent line due to normal charge-discharge cycles over time. When the electrolyte level gets well below the top of cell plates the battery efficiency drops off and may even go "open circuit" where it will not accept a charge. Ignore instructions on battery "Do not add water". Peel off lable on top of battery and pry translucent plastic plate off. By lifting small rubber caps on each cell one can then determine if distilled water should be added. Approx 3/8 - 1/2 inch above top of cell plates is adeqate. When replacing translucet plate, snap into recess area so it is flush with battery top. Toyota denies that any battery maintenance is required. The vent line is a safety requirement to avoid possibility of a faulty charging system over-charging a sealed battery (not used in Prius models) and exploding it in passenger compartment. Unlike other models using true sealed lead acid batteries, the Prius uses old technology which requires at least minimum annual inspection depending on vehicle use and climate.
    Dave Weaver (Prius 200,000 + miles)
     
    #1 mortweaver, Feb 7, 2010
  2. jayman

    jayman Senior Member

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    My 2007 FJ Cruiser is the same, it came with a Panasonic battery that had stickers like "Maintenance Free" and the like

    Last summer, peeled them off to discover each cell has a cap that can be unscrewed. Did so and the electrolyte level was barely above the cell

    I had to add distilled water to each cell, and also consider this a yearly maintenance task

    My FJ can sit for long periods, up to 2 weeks at a time. I also have it plugged into a VDC Electronics Battery Minder, to keep the battery at full charge and desulfated
     
    #2 jayman, Feb 7, 2010
  3. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Green Valley, AZ
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    2015 Prius
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    Two
    There's confusion over this point because Prius is supposed to be equipped with AGM batteries that do not routinely require the addition of water. However, it seems that some Prius were shipped with liquid acid batteries, contrary to Toyota specs.

    If your car battery is AGM and you remove the vent for inspection, you should see fuzzy white mats separating the lead plates. In that case, you definitely do not want to add water covering the plates as suggested by the OP, as that will kill the battery. If the battery is not holding a charge, you could try adding an ounce or two of distilled water to each cell as this may result in temporary rejuvenation of the battery.

    Regarding "Maintenance Free", my impression is that this is mostly a marketing ploy to sell batteries. If you can easily open up a liquid acid battery, whether Maintenance Free or not, and periodically add distilled water, go for it.
     
    #3 Patrick Wong, Feb 7, 2010
  4. bruceha_2000

    bruceha_2000 Senior Member

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    Vehicle:
    2018 Prius Prime
    Model:
    Prime Advanced
    I found this to be true. When my 12V died overnight Fall of 2008, NO warning and it would not hold a charge, I ordered the Optima and while I waited for it to arrive, I secured the 12V deep cycle boat battery into the 'basement' and connected it to the battery clamps (I know this sounds bad, but it was a secure connection and the battery couldn't move). At the suggestion of someone here (could have even been Patrick, I don't remember :unsure: ) I opened up the 'maintenance free' battery and added water. Sure enough, it would then hold a charge. I was quite surprised to find caps under that yellow sticker. I've not used the battery for anything but if the 12V dies in the 2006, you can bet the first thing I do is pull the caps and see if it needs a little liquid to bring it back for the few days it will take for an Optima to come in the 'mail' :).
     
    #4 bruceha_2000, Feb 10, 2010
  5. Green_Is_Good

    Green_Is_Good New Member

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    I just refilled my battery with distilled water. It's a pain compared to a battery under the hood. BUT it was very satisfying to see the charger charging the battery. Saved myself $150.
     
    #5 Green_Is_Good, Apr 7, 2010
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