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We are finding that batteries 1 & 2 from the diagram below, consistently use more water than 3 & 4, by about 1/3rd. The whole bank is nearly 3 years old, and though not as vibrant as when new, they hold their charge well and seem to accept a charge well.
Any thoughts?
Service & equalize the batts and move the neg take off to batt #3.... After you have them serviced and equalized beg borrow or steal a bench top power supply with adjustable voltage control. Start at 13.2V with all battery caps off and bring the voltage up .1V at each step. Using a flash light you want to see all batts beginning to gas at the identical voltage. Cells gassing earlier than others are likely contaminated or beyond useful life. Put a sticker next to these cells to monitor them..
 

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I am going to explain way IMHO (not IMHO, but fact. Sorry I can not scientifically explain, so I will use IMHO and you make your own decision,) paralleling batteries is riddled with complications and prone to early battery failure. Charging such a battery system is also a challenge.

I am going to use this reference to back up my info... http://www.nyc-arecs.org/batt1.pdf

Battery School | Batteriesnorthwest.com | Advantages and Disadvantages of using two 12 volt batteries connected in parallel or two 6 volt batteries connected in series.

What these guides do tell you, but do not spell out for you, I will do. First all battery cables have resistance. The longer the cable the more resistance. Just A Little Corrosion Causes Big Voltage Drops as well, acerbating the issue. So any battery that has a cable that is longer or corroded will have more resistance than a battery with cables in better condition. That means that the battery or bank of batteries with bad or longer cables will draw less amps, this giving an uneven draw from each bank.

Now assuming both your battery banks at perfectly identical, and cables are not an issue, almost impossible to have both with the exact state of charge and condition...all the time. The bank or battery with the lower voltage will be the governing output voltage. If one bank is 12.5 V and the other is 12.3 V the voltage of the two in parallel combined will be 12.3 V. Is is an issue with charging system.

And that brings us to your issue. IMHO, in order to charge this system correctly will be to isolate each bank and charge separately. If you are trying to charge both banks with one charging source...Now I am not up on the new chargers and know nothing of the new technology. But on my boat the charger charges until the voltage is reaches a preset point. If the battery system is reading 12.3 V combined, even if one bank if full, the charger will continue to charge both banks, cooking the bank that has a full charge and may never bring the lacking bank up to full charge. That is the extreme, most likely the bank will come up to snuff and charger will shut off, but at the cost of damaging by over charging. Over charging side effect is the boiling off of the water. Also when over charge a battery bank you heat that bank causing increased corrosion of the plates and reduces battery life making the two banks even more out of sink.

I inherited a five, 12v parallel battery system, and could never get it right. The added hydrogen produced from the over charging causes all kinds of issues with corrosion and hardening of rubber and shortening of battery life. And is an explosive gas! Since I went with two 6V 375AH in series, I have had no issues...period! Series is simple, Parallel is riddled with issues unless you have a perfect setup. And there is no such thing as perfect. You might start out with as close to perfect as you can, but this will change with use.
Yes that would be opinion. Unfortunately not well backed up in a real world situation over 100's of thousands of boats, heavy equipment, off-grid and industry all using parallel configurations. I have a friend with an off-grid system with 12 6V batteries now at year 11. They are series / parallel for a 12V bank..

If properly wired, parallel batteries can last 10+ years with zero issues. I have a set of GEL batteries in parallel going into its 15th year this spring. If parallel was the death of them well I suppose that would have happened oh, maybe, 12 years ago..;) They are still in about as perfect balance as can be expected at 15 YEARS!!! I have seen piles of series only banks, most all of them, die well before this parallel GEL bank... Unlike most boaters, I have the tools to measure these imbalances. These GEL's lasted 15 years because they are GEL, properly charged and because they were wired properly to begin with..

Batteries wired in parallel, even incorrectly wired, will all be at the same voltage except for short durations where voltage drop is an issue at higher currents. Take the high load away and they all wind up back at the identical voltage very quickly. Proper wiring can help minimize any voltage differences. The statements about parallel batteries at differing voltages and the lower voltage being seen by the charge source is not even in-line with Mr. Ohm's law....

Improper wiring of parallel or series parallel banks does and can create imbalances and varying "ages" between batteries but even this does not happen overnight and usually takes years.

I have many banks that have exceeded 5+ years, even with horrible wiring, because everything else was done well. I also have many series only banks that are dead at a few years because the owner simply murdered them..

The problem on many cruising boats is there is simply no option other than to parallel, if you need the capacity. In a perfect world I always try for series only, but we don't live in a perfect world. The bank I have going in this week is 675 Ah's and the only way to get that to fit is series/parallel.. Next week is a 440Ah bank and again the only way to get it to fit is series / parallel...I fully expect both of these banks to easily go 7-10+ years.

Making batteries last comes down to many, many, many factors just one of which is how the batteries are wired...
 
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