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Dual Battery Setup Question

Discussion in 'Ask All Sailors' started by Northstar1014, May 16, 2018. Add this thread to a FAQ

  1. Northstar1014

    Northstar1014

    Joined Feb 19, 2017
    12 posts, 2 likes
    Cape Dory 27
    US New England
    Hi all,

    I have a question about using a deep cycle and a starting battery as part of the same battery set up. My 27 foot cape dory currently has 1 starting battery and 1 deep cycle, both 12v flooded lead "marine" batteries. They are connected through a 1,2,both selector switch and they are wired together in parallel -- negative to negative. I don't have refrigeration or any major amp draw loads on the batteries other than standard electronics and occasional lighting.

    I understand that it is bad to wire together batteries of different type (e.g. a Gel with a flooded lead acid) but is the current setup with 1 starting and 1 deep cycle battery wired in parallel considered acceptable? If it is potentially detrimental, I will likely replace the starting battery with another deep cycle. Thanks, Ted
     


    dlochner likes this.
  2. dlochner

    dlochner

    Joined Jan 11, 2014
    2,228 posts, 819 likes
    Sabre 362
    113 US Fair Haven, NY
    @Northstar1014 Great question! I've been intending to post a similar question, but just hadn't gotten to it yet. Thanks for posting.

    Over the weekend a new sailor at my marina asked the same question. His setup is one Group 24 FLA battery and one Group 24 AGM battery being charged by a 20 amp West Marine 3 stage charger. He discovered the FLA battery to be overheating, i.e., too hot to touch while his AGM was only 12.2 volts. Aside from other important wiring issues, like no fuses and undersized wire, I'm inclined to believe that the mixing of battery types contributed to the battery overheating.

    My short term advice was to not leave the charger on when he wasn't on the boat and monitoring the batteries, however, I know that is not the best long term solution.

    If MaineSail doesn't respond here, I'd encourage you to repost on the Musing with MaineSail forum.
     


  3. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    5,050 posts, 273 likes
    Alden 50
    US Sarasota, Florida
    The way you describe it, your two batteries are not wired in parallel but rather switched through a battery switch. The effect of parallel connection exists only when you turn the switch to "both".

    Don't do that! Rather, depending on the capability of your charger to serve different battery types (sensing either independently), simply wire the charger outputs to each battery separately.

    Not sure I described that clearly but this is a self-inflicted problem many boaters do not understand.
     


    LloydB and rgranger like this.
  4. rgranger

    rgranger

    Joined Jan 19, 2010
    4,957 posts, 897 likes
    Hunter 26
    US Smith Mountain Lake
    My last sailboat was set up exactly like yours. It is not uncommon to do what you have on your boat. The deep cycle is used to power the cabin lights and electronics and the starter is isolated from the other current draws to ensure that you can get your engine started. The only reason for the deep cycle to be on the dual switch is so that you can use the deep cycle as a backup option to start your engine in the event that your starter batter dies. Your engine should be charging both.

    Typically the starter battery is able to provide better cranking ability for the same size than a deep cycle will provide. However the cranking output is often listed in the specs and you can find deep cycle batteries with significant cranking ability.

    Here is a good article about it.

    https://www.batterysystems.net/deep-cycle-vs-starting-battery/

    I now have a pull-start O.B. so I don't need a starting battery. I now have two deep cycle batteries on my boat.
     


  5. rgranger

    rgranger

    Joined Jan 19, 2010
    4,957 posts, 897 likes
    Hunter 26
    US Smith Mountain Lake
    :plus:
     


  6. Tally Ho

    Tally Ho

    Joined Jan 7, 2011
    1,017 posts, 139 likes
    Oday 322
    US East Chicago, IN
    Right. The negatives will be wired together, but not the positives...they each go to the 1-both-2-off switch.

    I would use the house battery for sailing, s it sounds like you don’t have much of. Power draw, and assuming you are mostly day sailing, should be fine. Keep the starter battery for emergency starting.

    As Don said, ensure your charger has 2 leads to charge each battery independently since they are different.

    I have 2 group 27’s on my O’Day 322 with refrigeration. I day sail mostly, and switch to one battery or the other when away from shore power. Keep the other in case I do something stupid and run down the battery.

    My charger has 2 outputs, so I charge each battery independently (even though they are the same type/age).

    Cheers,

    Greg
     


    LloydB and rgranger like this.
  7. Northstar1014

    Northstar1014

    Joined Feb 19, 2017
    12 posts, 2 likes
    Cape Dory 27
    US New England
    Thanks to all, very helpful

    To clarify, I don't have a battery charger on board, so charging is accomplished only via the output from the alternator. I do have 2 red cables coming from the engine to the battery positives, so I will need to verify that they are BOTH connected to the alternator and therefore able to charge both batteries independently.

    Question: If I go out for a long day of motoring and have the selector on the BOTH setting (which, after reading above, I likely will not do), my understanding is that the alternator would be topping off both batteries simultaneously. In this case, would the fact that 1 battery is a Starting and the other a deep cycle be detrimental to either battery? --Ted
     


  8. dlochner

    dlochner

    Joined Jan 11, 2014
    2,228 posts, 819 likes
    Sabre 362
    113 US Fair Haven, NY
    If both batteries are similar, i.e., both are Flooded Lead Acid or both AGMs, there should be little or no problem. It is not ideal, but not a huge deal because both batteries will have similar charging profiles.

    If one is an AGM and one an FLA then the charging profiles will be different and that may cause an issue.

    One place to look for more information is Maine Sail's site, MarineHowTo.com. Here's a link to the battery section: https://marinehowto.com/category/electrical/batteries/
     


  9. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    1,202 posts, 210 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    Check where those two red cables go. Hopefully they don't connect directly to each other, or they're connecting the batteries regardless of the switch setting.
    The best thing for you to do if one battery meets your needs (you can sail as long as you want without using more than 50%) is probably to have the alternator charge the house battery directly. Then add an echo charger to keep the starting battery topped off. My other concern would be whether you're ever really getting the batteries fully recharged just using the engine. That usually is harder than you'd expect.
     


  10. Don S/V ILLusion

    Don S/V ILLusion

    Joined Sep 25, 2008
    5,050 posts, 273 likes
    Alden 50
    US Sarasota, Florida
    The best answer is "maybe".

    The reasons are varied but foremost is that your alternator can't sense two batteries. If one is fully charged and the other not, the alternator 'sees' the lower voltage state which causes it to 'do what alternators do' causing over-charging the fully charged one.

    Also, being different capacity batteries also confuses an alternator since it doesn't differentiate.

    If that isn't enough, batteries such as yours have different internal resistance. Not good to combine them with a switch while charging.
     


    LloydB likes this.
  11. Benny17441

    Benny17441

    Joined May 24, 2004
    5,451 posts, 325 likes
    CC 30
    US South Florida
    Marine flooded batteries are not that expensive and quite forgiving. To connect in parallel a starting battery and a deep cycle battery might be similar to connecting a brand new battery to a 3 year old one. In any instance minimize the time when you set the battery switch to "Both" and you will be fine. When you are ready to get new batteries get two new, deep discharge batteries as deep discharge batteries are quite capable of starting the small auxiliary diesel engines used in most boats. You will also double the house capacity. For redundancy and equal utilization of the batteries you can alternate their use with every outing or if needed use the two batteries connected to provide a larger bank and provide redundancy with an auxiliary starting , portable battery pack in case both batteries are simultaneously discharged. The good thing about inexpensive flooded batteries is that if one fails on a trip you can always get off the boat and find a replacement almost anywhere or at any hour.
     


  12. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,686 posts, 517 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    It is all about the charging voltages. In most cases your "start" and "deep cycle" 12V batteries have the same or very, very close charge voltage requirements eg: 14.6V & 14.7V. The far bigger worry would be your alternators regulation voltage and is it correct for either bank? Most times they are not. If it is you're golden...

    Placing them in parallel, during charging, is standard practice across multiple industries including marine, RV, heavy equipment, rescue, fire, and big rigs. It is not going to cause issues so long as teh charging voltages are close to what the battery maker wants to see.

    Just don't forget to switch off of BOTH when you're done charging and you'll be fine using BOTH/ALL/COMBINE position for charging from the alternator..
     


  13. Northstar1014

    Northstar1014

    Joined Feb 19, 2017
    12 posts, 2 likes
    Cape Dory 27
    US New England
    Fantastic, thanks. I will leave my setup as is then, and plan to use the deep cycle battery 90% of the time (using BOTH only to charge). This will leaving the starting battery fresh as a backup. I did notice that my setup has a black cable connecting the negative to negative of my batteries, which is missing from your diagrams on page 1 of your post 1/BOTH/2/OFF Switches Thoughts & Musings. Is this black negative-to-negative connection appropriate and necessary in my setup? Best, Ted
     



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