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  1. #1
    Fireside Member 2ACDogs's Avatar
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    Adding 2nd battery question

    We're planning to do some dry camping soon with our 2150RB and it currently has a 1 year old Everstart 27DC with 109 AH capacity. It's going to be in the low 40s to high 30s and based on previous trailers I doubt the battery will last all night running the furnace blower. I have room on the hitch to put a 24DC battery with 101 AH beside the 27. Will this cause any problems with different capacity batteries? I have no problem wiring them properly but I'm no good with theory. Not crazy about running the generator all night and one place we want to go is a primitive BLM campground that has quiet rules. Thanks for the help.
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  2. #2
    Site Team Ynot4me2's Avatar
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    Unfortunately, the batteries have to be the same and preferably the same make, model and age. Is there no way was to squeeze in a 2nd Groupe 27? How about 2 x 6v? Or 2 AGM group 24 if you can't fit 2 x 27?

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    Site Team traveldawg's Avatar
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    Having 2 different brand / age / capacity batteries is not such a good idea. You should always try to get 2 identical batteries and install them in pairs (at the same time too).

    I don't think a 24v battery would help you much since all your circuits & stuff are 12v.
    tn use a different size based on the space you have.

    Maybe consider 2 12v batteries in parallel to get to 200AH.

    I always was partial to 6V batteries in series. Lots of motorhome manufacturers use them for their house battery banks.
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    Rolling Along jjbbrewer's Avatar
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    If you're careful about the initial voltage (charge level) differences, you can do it. Here are a couple of references ..

    https://rvnerds.com/2017/08/07/elect...gescapacities/
    https://m.greenway-battery.com/news/...tion-1282.html

    This is basically the same situation as when you're jump starting a car since more often than not, you're connecting two different battery sizes in parallel.
    Last edited by jjbbrewer; 01-22-2022 at 07:01 PM.
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  5. #5
    Fireside Member 2ACDogs's Avatar
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    I didn't expect opposing answers. Hope this isn't a divisive topic like Chinese tires or towing with a 1/2 ton. For us cost is always a consideration so no new 6Vs or AGMs and another 27 size definitely won't fit the space. I hate to give away a 1 year old 27 to buy 2 type 24s. What bad or dangerous thing would happen if you hook a 109Ah and a 101Ah battery together in parallel?


    jjbbrewer, your 2nd link mentions using diodes to connect batteries with different Ah ratings but no mention of specifics.
    Last edited by 2ACDogs; 01-22-2022 at 07:30 PM.
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  6. #6
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    Quote Originally Posted by 2ACDogs View Post
    I didn't expect opposing answers. Hope this isn't a divisive topic like Chinese tires or towing with a 1/2 ton. For us cost is always a consideration so no new 6Vs or AGMs and another 27 size definitely won't fit the space. I hate to give away a 1 year old 27 to buy 2 type 24s. What bad or dangerous thing would happen if you hook a 109Ah and a 101Ah battery together in parallel?


    jjbbrewer, your 2nd link mentions using diodes to connect batteries with different Ah ratings but no mention of specifics.
    When using two different dc power sources, in your case a 12 vdc and a 24 vdc battery, regardless of the amps, you will be providing a 12 volt system with 24 vdc. Most 12 vdc systems will be "okay" with 24 vdc, they will tend to have a much shorter life span though. Some 12 vdc things won't like it at all, like your refer control and power supply boards.

    In electronics, we use diodes to control the flow and "rate" of the current, I'll come back to that in a second. You will need a step down transformer to convert 24 vdc to 12 vdc. Or a pretty big resistor network, using sand resistors. Both options will generate heat that you will need to dissipate (even in the cold conditions you will be in) or else you will have caused a potential electrical fire point.

    Now, on to the diodes. A diode can reduce the current to a specific value and ensure voltage and current flows in one direction only. The reason to use diodes in a dissimilar power supply (in your case, 12 vdc and 24 vdc) is to make sure the 24 vdc battery doesn't drain the 12 vdc battery. You will end up with a dead 12 vdc battery, that will be difficult to bring back to life.

    The recommendations made about identical batteries might require some explanation. With two different batteries, or two different aged batteries, the older or weaker battery will pull more current from the new battery. Think lowest common denominator. This will shorten the life of the stronger or newer battery, forcing you to replace them much sooner.

    This is true of any battery type, wet lead acid, sealed lead acid, AGM, LiFePO (although this type of battery is an exception to the rule because they last so dang long), lithium, alkaline, etc.

    This is why most devices with alkaline batteries instruct you to replace the batteries at the same time and not mix old with new, or mix different types.

    So, in a nutshell, get the same batteries, at the same time, and install them at the same time, whether that be four 6 vdc batteries in series and parallel or two 12 vdc batteries in parallel.

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  7. #7
    Fireside Member 2ACDogs's Avatar
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    Moonshadow, a type or size 24 battery, not 24V. My hope was to use the existing 1 year old Walmart 27DC 109Ah battery and a new Walmart 24DC 101Ah battery.

    What diodes would I need for a 109Ah and a 101Ah battery to be happy?

    thanks
    Last edited by 2ACDogs; 01-22-2022 at 08:19 PM.
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    Quote Originally Posted by 2ACDogs View Post
    Moonshadow, a type or size 24 battery, not 24V. My hope was to use the existing 1 year old Walmart 27DC 109Ah battery and a new Walmart 24DC 101Ah battery.

    What diodes would I need for a 109Ah and a 101Ah battery to be happy?

    thanks
    Ah, I just read 24DC...

    As for doides, I'll have to do some calculations. What size of wire is going to the converter? What size of wire is between the two batteries? How long is each of those runs?

    All of that above, plays into the type of diodes, and where to place them. Essentially you are going to want specific diodes to match the current of the lowest battery and to prevent that battery from pulling the higher amp battery down. All said and done, I would estimate that you are looking at between $50-$90 in total parts. You will probably also need a crimp tool to crimp lugs on the wires where you will need to cut them to put the diodes. Also, more than likely, as a safety precaution, I would probably design a diode matrix, (for redundancy as well).

    Also, not sure if you know that diodes need to be installed correctly as they only allow current to flow in one direction. If you reverse one, you will get nothing to your system, or you might end up just feeding one battery with the other. I don't know your level of electronic and electrical knowledge, so hence my "warning" in this paragraph.

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  9. #9
    Rolling Along jjbbrewer's Avatar
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    Quote Originally Posted by 2ACDogs View Post
    jjbbrewer, your 2nd link mentions using diodes to connect batteries with different Ah ratings but no mention of specifics.
    For 2 flooded acid deep cycle batteries that are almost identical in size and amp hours, you don't need anything. Just mount them as close as possible, use good cable and connectors.

    Make sure to run the coach positive to one and the coach negative to the other to have them charge evenly.

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  10. #10
    Fireside Member 2ACDogs's Avatar
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    I have no electrical knowledge but I do know diodes only flow one way. This is getting way too complicated, might as well buy 2 identical batteries and put the one I have at the curb with a free sign on it. What happens if I wire the 109 and 101 in parallel and use them?

    thanks jjbbrewer.
    2017 Imagine 2150RB
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