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bou2
03-23-2020, 05:37 PM
Does anyone have experience running 2 or more 12v deep cycle RV/marine batteries in parallel to replace my 2 6v and still maintain my 12v supply to my coach? I also have an 80amp solar panel connected to my main "brain" power control system. Thanks, Carl
Chuck v
03-23-2020, 06:38 PM
The short answer is YES but you may not want to do it for capacity reasons.



Many RVs have two deep cycle 12 volt batteries in parallel which works fairly well if both batteries are of the same capacity, model age and manufacturer Never change just one of the pair, always replace both at the same time...) Generally, the 6 volt golf cart batteries are more than twice the amp-hr ratings of similar physical size 12 volt batteries, so two cart batteries in series usually work out ot be more capacity than two 12 volt batteries in parallel. For my small travel trailer that I used dry camping during hunting seasons, this is the reason that I converted from the pair of factory 12 volt batteries to two 6 volt golf cart batteries in series. Furthermore, charging lead acid wet cells in series is much more efficient than charging cells in parallel due to basic chemistry and electrical characteristics...a partially charged cell has a higher internal impedance than a charged one, so the charging current divides unequally to the detriment of getting the charge where it needs to go. This is more of an issue in aged batteries than in new ones due to sulfate buildups, etc. In a series stack like the two cart batteries (and indeed in any single battery where the cells are in series) the current must pass through the series string and the discharged cell with the higher internal impedance will have a larger partial voltage developed across it than a more charged and lower impedance cell -- just what you want to efficiently charge the string!



I always got much longer service from my series golf cart batteries when dry camping than I ever did with the 12 volt units in parallel. I will say that on my 42 foot diesel pusher coach two of the three battery systems were the more common paired 12 volt batteries (the diesel start battery bank and the house battery bank) and the third bank was comprised of golf cart style 6 volt batteries but in a series/parallel configuration of 4 batteries. This latter was done for achieving a very amp-hr capacity within the footprint of moderate weight and sized batteries -- otherwise the use of L16 6 volt batteries would be more common as I had in my off-grid solar home. Since these weigh more than 110 pounds, they are too cumbersome for a vehicle application... https://www.solar-electric.com/trl16vo225ah.html In my off-grid home I used these in a 24 volt stack of 4 of these to power my inverter and local DC loads...once again choosing the series stack configuration for optimum solar and generator charging.

Chuck
bou2
03-24-2020, 10:17 AM
The short answer is YES but you may not want to do it for capacity reasons.



Many RVs have two deep cycle 12 volt batteries in parallel which works fairly well if both batteries are of the same capacity, model age and manufacturer Never change just one of the pair, always replace both at the same time...) Generally, the 6 volt golf cart batteries are more than twice the amp-hr ratings of similar physical size 12 volt batteries, so two cart batteries in series usually work out ot be more capacity than two 12 volt batteries in parallel. For my small travel trailer that I used dry camping during hunting seasons, this is the reason that I converted from the pair of factory 12 volt batteries to two 6 volt golf cart batteries in series. Furthermore, charging lead acid wet cells in series is much more efficient than charging cells in parallel due to basic chemistry and electrical characteristics...a partially charged cell has a higher internal impedance than a charged one, so the charging current divides unequally to the detriment of getting the charge where it needs to go. This is more of an issue in aged batteries than in new ones due to sulfate buildups, etc. In a series stack like the two cart batteries (and indeed in any single battery where the cells are in series) the current must pass through the series string and the discharged cell with the higher internal impedance will have a larger partial voltage developed across it than a more charged and lower impedance cell -- just what you want to efficiently charge the string!



I always got much longer service from my series golf cart batteries when dry camping than I ever did with the 12 volt units in parallel. I will say that on my 42 foot diesel pusher coach two of the three battery systems were the more common paired 12 volt batteries (the diesel start battery bank and the house battery bank) and the third bank was comprised of golf cart style 6 volt batteries but in a series/parallel configuration of 4 batteries. This latter was done for achieving a very amp-hr capacity within the footprint of moderate weight and sized batteries -- otherwise the use of L16 6 volt batteries would be more common as I had in my off-grid solar home. Since these weigh more than 110 pounds, they are too cumbersome for a vehicle application... https://www.solar-electric.com/trl16vo225ah.html In my off-grid home I used these in a 24 volt stack of 4 of these to power my inverter and local DC loads...once again choosing the series stack configuration for optimum solar and generator charging.

Chuck

Great answer Chuck. You address some very important considerations:
o battery matching
o battery age
o anticipated use re:off grid camping
o anticipated capacity need

In my case, all 12v are brand new and marine deep cycle. So, I think unless otherwise swaded, I'll go with my current plan as the amp-hr capacity bank will be much deeper than provided by my previous 6v series configuration.

Thank you again,
Carl
Chuck v
03-24-2020, 11:26 AM
Carl,


I am surprised that you found a pair of12 volt deep cycle batteries in the normal footprints (group 27, 31, etc...) that exceed the capacity of golf cart batteries... Most 6 volt batteries have an amp hour rating of 215 to 230 amp-hours, whereas the marine 12 volt deep cycle ones have only about 80 or 95 amp-hours each, giving a parallel capacity of 190 at the most optimistic -- still short of a good cart battery. What are the actual batteries you are selecting, and what brand/model of cart battery are you replacing. You sound pretty knowledgeable, so I am presuming you are not confusing cold cranking amps of peak short term ability with the steady power delivery of amp-hour ratings...


Chuck


here is a good read on this topic:
Why use Golf Cart batteries over a Marine Deep cycle or Auto battery?

We all know that the lead acid batteries are chemical storage containers and can exhibit different characteristic depending on how they are constructed. With the 12 volt car starting battery, there are twice as many thin plates in comparison to the 6 volt golf cart batteries. The starting battery has much more plate exposure resulting in it being able to give up a lot of electrons in short bursts. On the other side, heavy charge-discharge cycles for prolonged periods of time can distort and damage the thin, delicate plates. Furthermore, the positive plates will corrode much faster than a true deep cycle battery - which results in a very short lifespan.

6 volt golf cart batteries, with its thicker plates, are able to endure long periods of deep discharging and recharging with less effect on its life span.

A product that can be tempting for a first-time battery buyer is the Group 24 or 27 RV or Marine deep cycle battery. These can be found at auto parts stores, department stores and discount warehouses. This particular battery comes up in discussions often enough to know that people are taking serious look at one of these for their house system. They are 12 volt batteries and it doesn't matter what "new" technology they advertise, they are still a cross between a starting and deep cycle design.


If there is a reference to Cold Cranking Amps (CCA), it is most likely one of these dual purpose batteries and should be avoided.


To explain it in cost effective detail, let's look at how many ampere hours (AH) you can draw out of a Group 27, 12 volt dual purpose battery, versus a typical GC-2 golf cart 6 volt battery. As an example we will use a comparison of the West Marine brand to avoid mixing different brands and pricing structures.

The Group 27 battery cost $120, has an advertised discharge life cycle of 200 and is rated at an AH capacity of 80. The GC-2 costs $190, life cycle of 700 and is rated at an AH capacity of 215. We will use 2 Group 27 batteries connected in parallel to maintain 12 volts and provide a 160 AH capacity. Two GC-2's will be hooked in series to provide 12 volts and a 215 AH capacity.



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*The depth of discharge is not specified for either battery so the assumed 80% DOD is only for comparison purposes.

According to the spec's provided, the more expensive GC-2's will actually cost a third that of the cheaper Group 27. As far as comparing various brands, the plate thickness will be one of the top determining factors that you can use as a general guide. The only problem you will encounter is this number is not typically published in the spec's for the GC-2 type of battery.
Chuck v
03-24-2020, 11:38 AM
Having some issues attaching the table in formatted style...so trying it as an image: