lead acid batteries in parallell?

I have seen this a few times now and have always felt that it is a bad idea because if one cell in one of the batteries is worn out more than
the others then you will always be discharging one battery with the other.
The first time I saw this was on tractor starting batteries, and recently on a scissorlift. How common are parallell battery arrangements and is this something worth worrying about?
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Frank White wrote:

Parallel batteries are very common on cruising boats (usually under 50 feet in length). Faulty cells as you describe are uncommon except toward the end of the useful life of the batteries.
The general rule for paralleling batteries is that the batteries should be of the same make, type, size, and age.
Good luck.
Chuck
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I've seen 48 VDC battery plants in telephone offices with 3 and 4 different strings in parallel. They might be described as relatively similar in type, somewhat similar in size, and very different in age and make.
The point of similarity is necessarily the voltage ratings, which essentially means they all must be able to operate with the same charge voltage, the same float voltage, the same cutoff voltages, etc. etc.
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Floyd L. Davidson wrote:

Unless one has a shorted cell there should be no problem in parallel. My Diesel Truck has parallel batteries.
It appears to me that if they are not the same size the one with the larger capacity will supply most of the current. Since the voltage will always be equal they should discharge to the same state of charge even though they are different size.
When being charged they will charge according to their size, the voltage determining their state of charge.
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Rich256 wrote:

Just to make sure we're all on the same wavelength, I agree that there will probably be no explosions or meltdowns if one connects two different batteries (of the same voltage) in parallel.
However, if for example one of these is a conventional lead-acid type and the other is an AGM type, or if one is new and the other is near the end of its life (i.e., with high internal resistance) then connecting them in parallel will not be nearly as satisfactory as had both batteries been similar. Can it be done? Sure. We've all done it when necessary. But is it a good design practice? I don't think so.
If the OP were technically able to determine that the relevant characteristics of the batteries being considered make them good candidates for paralleling, then what was the purpose of the original post?
Telephone company batteries tend to differ from automotive type batteries. They are usually monitored closely and systematically by trained personnel.
Chuck
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US Navy submarines have been using batteries in parallel for decades...same aH, rating, no problem !!

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BULL!!! Want to tell me were on a submarine you think the lead acid batteries are in parallel?? Battle-lanterns (6V primary cells that are never recharged) ??
Nuc boats only have one ships battery and it's over a hundred cells in series. Diesel boats (of which the US Navy has zero) had two ships batteries. Yes, those could be connected in parallel, but only during discharge, they charge separately (various connections could be set up from the manuevering room)
Parallel operation can work well for small batteries like your typical marine engine starting application. But it isn't the best choice. One battery can go bad and 'hog' all the charging current. Many marine applications include a switch to allow switching from one battery, to the other, both, or neither (nice to be able to disconnect the batteries when working in the engine compartment)
daestrom (former Chief Electrican's Mate, USNavy Submarine Service)
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Should work fine as long as you use the proper isolation system. Here's one link of many that explains it in the event you don't already know of such devices.. http://www.thejump.net/fishingarticles/Ship-Shape.htm Good luck....Ross
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