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Old 11-30-2018, 10:59 PM   #1
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mixing battery sizes

I want to set up a new house bank, all new batteries. Due to limited space both height and length I can use gcs and L16s... is there a problem using a couple of L16s and a couple of golf carts. The l 16s will be wired in series as will the the gcs and then they will be paralleled... any issues with different ampacities but all the same type FLAs.
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Old 11-30-2018, 11:04 PM   #2
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I would not do it. Donít think they will charge properly.
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Old 12-01-2018, 12:28 AM   #3
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certainly not ideal

charging will be uneven, longevity will be shorter

better most likely to treat as separate banks like old school, with a switch

but as long as the dissimilarity is only across paralleled connections, not dangerous

ideal is mod the space or select the liwest common denominator model

AGM can lie down
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Old 12-01-2018, 11:16 AM   #4
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Sounds like the OP is another candidate for Calder's "Boat Owners Mechanical and Electrical Manual" or the "The 12 Volt Bible for Boats"

The answer is no, by the way. If charged as one bank, your batteries will die an early death.

Why can't you just use L16's? they have the same footprint as a GC.
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Old 12-01-2018, 11:40 AM   #5
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I am going to disagree with most here. A battery has no idea what it is connected to. It only knows voltage and amps in and out. If all the batts in the bank have the same chemistry and the right charging and discharging profile, it should work fine.

The preference is to have all the batts in the bank the same (size, type, mfr, age), but that is not a good option per the OP. The problem I see here is not that it will not work (it will) but the two types of batt will not age the same. At some point one group will need replacement and the other will still have some life. So he will have to choose to replace all or some and that complicates it further.
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Old 12-01-2018, 11:49 AM   #6
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George has the best answr to this question.
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Old 12-01-2018, 12:33 PM   #7
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I am going to break further with the posters than Ski did and say that I will bet you will see little difference in battery life between the L16s and the GCs. As he points out the batteries don't know what they are connected to. The L16 will see a very, very slightly lower charge voltage due to greater voltage drop in their wiring than the GCs due to the fact that they will pull more current.


But that voltage difference is only very slightly noticeable at high charge rates. Once the batteries get more fully charged and the current decreases, that voltage difference becomes miniscule and irrelevant.


I will bet you can compensate for the voltage drop with the way you wire them. Wire the L16s first in the circuit so they get the maximum voltage and then wire the GCs downstream so they have more wire in the circuit.


But you are just fiddling with trying to make it perfect when almost perfect as psneeld notes below is just as good.


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Old 12-01-2018, 12:34 PM   #8
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My boat is perfect and lives in a perfect world.

I would never be practical when perfect us the best solution.
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Old 12-01-2018, 01:43 PM   #9
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Quote:
Originally Posted by caltexflanc View Post

The answer is no, by the way. If charged as one bank, your batteries will die an early death.



Why?
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Old 12-01-2018, 02:44 PM   #10
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When we bought our boat it had four 6 volt golf cart batteries in parallel with a 4 D battery. The 4 D always died an early death, so we replaced it with two 6 volt golf cart batteries...problem solved.
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Old 12-01-2018, 02:54 PM   #11
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I bet the short life of your 4D was due to using a starting battery for deep cycle service. There are very few if any true deep cycle 4Ds.


It is this sort of thing that perpetuates the mixed battery myth.


Don't get me wrong, I wouldn't mix FLAs with AGMs or Gels. But two different types of new deep cycle FLAs, no problem.


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Old 12-01-2018, 03:07 PM   #12
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Disclaimer: The following post is based on ignorance and stupidity. Be warned!

I have a hard time understanding (not unusual for me) the problem of connecting the batteries in parallel. Letís make it simpler. What the OP is asking is if there is a problem connecting two 12v batteries in parallel of different sizes. In my simple mind Iím just assuming that each pair of L16s and GC2s are one 12v battery.

I will assume that the batteries are of the same chemistry and the manufacturers recommended charging profile are the same. If that is true then Iím not sure it matters if the 2 batteries are of different sizes. The voltage in each battery will stay the same, both during charge and discharge.

Another way to look at it, is that many of us use automatic combining relays on our boats that combine our start battery with our house battery during charging. Those are vastly different sized batteries that are being charged in parallel and yet I donít hear about concerns there?

So, despite what Saint Calder has written, can someone explain how paralleling two batteries of unequal capacity creates problems? Iím not talking about age and chemistry, just size.
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Old 12-01-2018, 04:40 PM   #13
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Quote:
Originally Posted by Ski in NC View Post
I am going to disagree with most here. A battery has no idea what it is connected to. It only knows voltage and amps in and out. If all the batts in the bank have the same chemistry and the right charging and discharging profile, it should work fine.

The preference is to have all the batts in the bank the same (size, type, mfr, age), but that is not a good option per the OP. The problem I see here is not that it will not work (it will) but the two types of batt will not age the same. At some point one group will need replacement and the other will still have some life. So he will have to choose to replace all or some and that complicates it further.
Ski, but in a mixed bank they do NOT have the same charging profile. Think about it. That;s why as you mention later, death will be premature.
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Old 12-01-2018, 04:43 PM   #14
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Why?
Read the book. Don't take some stranger's opinion on the Internet for granted.
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Old 12-01-2018, 05:05 PM   #15
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Ski, but in a mixed bank they do NOT have the same charging profile. Think about it. That;s why as you mention later, death will be premature.
If you mean the manufacturer's recommended charging profile, maybe the two battery types don't have the same profile unless they are both Trojans or ???

But what does that matter. The charger probably has an AGM and FLA setting and if you select FLA you get one charging profile, not the one the manufacturer recommends. So what does it matter if the two have slightly different manufacturer recommended charging profiles if the charger isn't going to supply exactly that?

If by charging profile you mean the current that can be pushed through the battery at a given voltage and state of charge, then yes the L16 will generally accept twice as much current as the GC at the same voltage and state of charge. But that is like saying two GCs will accept twice the current as one. What does it matter that one battery is an L16 and the other is a GC.

How about this scenario (reduced to 6V to avoid considering series wiring): You have two GCs connected together in parallel with thick copper busses hooked up to another GC with normal 2/0 cabling.

How is that different than an L16 hooked up to a GC?

More electrons will go to the battery where the impedence is lowest. But as more electrons go to one battery its impedence rises as it reaches a higher SOC and balances with the other one.

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Old 12-01-2018, 05:20 PM   #16
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Quote:
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The problem I see here is not that it will not work (it will) but the two types of batt will not age the same. At some point one group will need replacement and the other will still have some life.
Exactly. Used as a constantly connected Bank, then The Bank will not last as long.

As long as the weakest link.

All components matched, gives the longest and most reliable life.
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Old 12-01-2018, 05:57 PM   #17
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Agreed, matched is optimal. But the weaker cells in the bank may very well be "carried" by the stronger, and very well may last longer than if they formed the entire bank.

I'm going to stand by my assertion unless someone can post something on an engineering level that shows me to be wrong. Aging at different rates is not necessarily a problem, just change batts when they get weak. When that occurs is the topic.
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Old 12-01-2018, 06:13 PM   #18
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That's it..... I am going to rip out all my batteries...and start over.

Going to buy all lab certified (not marine) charging sources, send out my battery wiring and switches to labs to be tested for resistance, then send them to Northrop Grumman to have satellite level
certified connectors added, then hire a factory tech to live abord to make sure the charging profiles are optimal ...etc, etc...

Anything the lab rats want to add to my list since I am giving up on reality boating?
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Old 12-01-2018, 08:55 PM   #19
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My engineering mind likes to take things to the extreme to solve puzzles. ie: put a motorcycle battery in parallel with an 8D and run them for a few years.
Haven't done it, but if I had to bet, I'm on the side of the table with ski.

WHat I have done is a lot of ni-cad work. And, I can tell you with certainty, you must match in a series array!
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Old 12-07-2018, 02:53 PM   #20
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The lesser batteries will bring all other batteries down to their charge capacity. When recharging, the lower capacity batteries will stop the higher capacity batteries from reaching a full charge.
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