12V battery - Group Size U1 or Group 51R

Discussion in 'Generation 1 Prius Discussion' started by AldPixto, Jan 21, 2020.

  1. AldPixto

    AldPixto Junior Member

    Joined:
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    Location:
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    Vehicle:
    2002 Prius
    Background: The original Prius 12V battery had a short life. I think because it was same battery as the Japan domestic model, but it had the additional current draw for the remote keyfob radio and security system. While under warranty my original 12V battery was replaced several times. The last update replaced the small JIS battery connectors with the SAE battery connectors, enabling a larger selection of possible batteries.

    When my last Toyota-sourced 12V battery needed replacement I installed a Group Size U1 battery that was spec'ed by the 1990-2005 Miata. It has all the right features: AGM, vent tubes for trunk mounting, positive terminal in the correct position, and of course it fit with only minor changes to the car's original battery tray and hold-down bracket. Another plus, these Miata AGM batteries were cheap compared with the Toyota battery.

    After 11 years (I get long battery life using a battery tender) it's time for another 12V battery.
    I could of course replace it with another Group U1 battery.
    Doing some measurements it seems I could fabricate a custom battery tray so I can use a Group 51R battery. Group 51R is what the 2004+ Prius uses, it's longer and wider than Group U1 and has roughly 1/3 more Ah capacity.

    I know that the 12V batteries health affects my mileage. The ICE will run longer trying to charge a weak 12V battery.

    My Question: How would a larger Group 51R battery affect my car, compared with the current Group U1 battery? All other things being equal, would my ICE spend more or less time charging a battery with roughly 1/3 more capacity?
     
  2. sam spade 2

    sam spade 2 Senior Member

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    Mar 31, 2018
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    2017 Prius c
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    Four
    No it won't. Until the week before the battery fails completely.
    Trying to stretch your 12 V battery life BEYOND 11 years is just a waste of time and money and worry.
    Stick with what has worked for you before.

    P.S. The car is about 18 years old now......and the battery will be new.
    It likely will out live the total car, regardless of what kind you put in it.
     
  3. Trombone

    Trombone New Member

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    The original 12V battery lasted a long time in my '02, like maybe 8-9 years. I then converted to SAE fittings and have been using DieHard and now Interstate 51R batteries. These have been sealed "no maintenance" batteries. I've never worried about the venting, and despite dire warnings by a particular member of the yahoo!prius site, I'm still here to tell the tale (he thought I'd blow myself up by the hydrogen gassing off. Well, the 6V battery in my '64 Beetle was under the rear seat, so I guess I'm just lucky). It's an easy matter to shim the existing battery tray up about 1" (I use an MDF board cut to dimension), and I replaced the hold-down with a cargo strap. The 12V should have a long service life in the trunk, not subject to engine heat or (partial) exposure to the elements as it would be were it under the hood.
     
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