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Thread: Batteries for Dual Battery systems?

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    Default Batteries for Dual Battery systems?

    I'm in the process of installing a dual battery system or having it installed as the case may be. I've done part of the work already. Some still to do. I have been told several different things about battery choices and want to know if anyone, anywhere has some knowledge and experience that can enlighten me. The things I've been told seem to be counter to each other in some cases.

    First: I need to use "exactly" the same battery in both positions in the system.

    Second: The batteries don't need to be the same battery.

    Third: Batteries need to be of the same type and have the same charge rate so the alternator can keep them up to speed, but don't need to be the same exact size or make.

    I'm going to use a Optima 34 series in the second or auxillary position. But their optima battery for the standard position on my FJ Cruiser is a larger and heavier Group 27. Since both would be yellow tops, they would be of the same type. I can't seem to find out from anywhere whether they have the same charge rate. The group 27 has a heck of a lot larger capacity and is the battery Optima recommends for my application as a primary.

    Hopefully someone will have an answer.

  2. #2
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    goodtimes is offline Expedition Portal Moderator Expedition Poseur
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    What are you separating them with?

    My 2 starting batteries (diesel) are identical - old school flooded, non-maintenance free, off brand batteries. My house battery is a Odyssey AGM.

    There is a National Luna charge controller separating them - and I haven't had any problems with them being different sizes & types.
    "Life is meant to be enjoyed, not endured" ~someone

  3. #3
    I've got an Odyssey PC1700 and an Optima yellow top in my dual batt. setup. Separated with an IBS charge controller. No issues whatsoever with the batteries being different.

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    Meh on all of that. I've got a Yellow top and a Red top optima in my rig. Sitting right next to each other under the hood, wired directly in parallel. They are the same size battery but that is it. Otherwise, not a single problem.

    Oh and its been like that for 10 years so yes, it works.

    Only thing I've heard is to avoid the blue top optimas. Apparently the cause spontaneous vehicle combustion. Obviously this is anti-productive.

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    Thanks for the replies. Odyssey and Optima are basically the same technology and "might" have similar charge rates. I personally think two Optima batteries of different sizes shouldn't be a problem. But it's interesting to see what people are using. The Blue top Optima by the way are actually intended for Marine application as in water not military. I don't think they are a good choice for a vehicle that has such high charge rates off a constant charge alternator system which boats don't have, but what do I know. That could be why they have had problems in the past.

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    I have an Optima Yellow top in my FJC as a second battery, and the standard sealed lead acid battery that Toyota installs as the primary battery. I have the dual battery kit from Dirty Parts, and everything works fine.

    When people hard wire two 12v batteries in parallel in an attempt to increase the AH capacity it's standard advice that both batteries be the same brand/model, and in fact it's best if they have the same date code. Even then, I've found that configuration doesn't give me the performance I expect.

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    Quote Originally Posted by cnynrat View Post
    I have an Optima Yellow top in my FJC as a second battery, and the standard sealed lead acid battery that Toyota installs as the primary battery. I have the dual battery kit from Dirty Parts, and everything works fine.

    When people hard wire two 12v batteries in parallel in an attempt to increase the AH capacity it's standard advice that both batteries be the same brand/model, and in fact it's best if they have the same date code. Even then, I've found that configuration doesn't give me the performance I expect.
    Interesting. Dirty Parts are the people who told me that they didn't have to be the same.

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    For the Odyssey batteries, do some research. They make the Sears high end batteries. Same thing as the Odyssey, just got a different color and has a different sticker on it.

    Could save you some $$$ to get the same thing.

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    Quote Originally Posted by LR Max View Post
    For the Odyssey batteries, do some research. They make the Sears high end batteries. Same thing as the Odyssey, just got a different color and has a different sticker on it.

    Could save you some $$$ to get the same thing.
    I already knew that but there are two reasons I won't do that. First I have been a Sears victim many times on many different items and they will never get another penny of my money. Second, my spare battery tray is made for an Optima battery and I already have it. I have nothing against Odyssey batterys other than that.

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    Quote Originally Posted by flintsghost View Post
    Interesting. Dirty Parts are the people who told me that they didn't have to be the same.
    And just to be clear, they do not need to be the same in this application.

    What some people in the RV world do (and I guess some off-road trailers as well), is take two batteries and wire them in parallel to try to double the available capacity (amp-hours). Note that there is typically no switch in this configuration. The discharge behavior isn't always what you expect from that configuration, particularly if the resistance of the two batteries differs from one another. Hence the recommendation in this type of setup to use same brand/model/date code - it's an attempt to try to ensure the two batteries have the same resistance.

    In a switched dual battery set up in a vehicle it doesn't seem to be a problem. I think that's because by and large the only time your batteries are connected in parallel is when they are charging (because the vehicle is running and the solenoid is closed). You might get some charging anomalies, for example one battery may not charge as quickly or fully as the other, but as I said I haven't seen a problem with two very disimilar batteries. I've had my dual battery kit installed for about 4 years now.

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