Ok, something else just went wrong, so I have the time to write at least part of this. I learned most of what I know about batteries by listening to the people that ran pre-nuclear submarines. They had a simple situation. Either learn how to make the batteries do all that they could - or. (As they say in that service, they would still on patrol.) Lead/Acid batteries were the top of the technology for many years and have only recently been supplanted by new chemistries.
Caution: serious science content follows....
There is an inherent problem with a Lead/Acid (LA)cell. It's entire existence and capability are governed by the density of the acid in the cell. Here in lies the problem. That density (what you read with either a hydrometer or refractometer) can and will vary with temperature and state of charge. Another issue that makes them very difficult to use in parallel is that the density of the electrolyte (acid) in the cell is not universal in any single cell. As a cell is charged or discharged, it will (it is unavoidable) heat the electrolyte. Ego, the terminal voltage of any cell can vary with: the state of charge, the rate of charge or discharge, and the ambient temperature. This heat in the cell is a result of the current flowing through the electrolyte. (Remember this line, it will be important later.) So, in a flooded (any with liquid electrolyte) cell, circulation is essential. During those times that the cell is active, the electrolyte has to circulate and as soon as it does, the cells electrical characteristics will be changing. That is a big piece of the story here. If you think of one cell as three pieces, and idea cell, an series resistance that will kill some of the terminal voltage with load and a leakage resistance that discharges the cell voltage just sitting there. Temperature effects all three.
Now, lets put a multi-cell battery to work.
First we will discharge it. It heats up and circulates the electrolyte and the density of the electrolyte goes down. That's just what it is supposed to do. But, lets put his twin brother (came off the line next) in parallel with him. We load the pair. Cooler one discharges just a little bit less. Now, turn the load off. They try to equalize, but they really can't. The warmer one is a has lower real density, but it also has higher terminal voltage because it is warmer and also has a higher internal resistance as a result of the heating, so it has to cool before anything can happen at all. Now, if that lower density guy is at less than 50% of rated charge, he has started to degrade and when he gets to where he would like to accept some of the charge from the was cooler brother, he can't get all of the difference back because of the sulfur damage to the lead sponge.
These banks will only behave this well if they are consistently cared for and the connections have matching resistance to several milohms.
It is so sensitive that a foot of 2AWG cable and another junction will make a difference. If these two were much more different than twin brothers, this effect is even greater. More about this down the page.
Now, let's exciting.
We have two commonly available 12V batteries in parallel. They are not even very old or tired, but one isn't doing real well. A flake of lead bridges the plates, when that happens the leakage resistance gets low and the cell discharges. Not very fast at first, but as the cell heats, it looses density and the heating gets faster. Then, its terminal voltage goes down. So, that battery voltage goes down, but it is banked with another that has a good solid charge. That means that there is another perfectly good battery to dump power into that failing cell. This starts a cascade of heating and dis/charging that releases both hydrogen and heat. Something has to give......
It does. If you inhabit marinas as I have most of my life, you would recognize this as the start of a battery explosion. How common are these? Not very. In the 300 slip marina where I did most of my work before the depression, there would be about five of these a season. If good battery boxes were strapped down and closed well, the damage would usually be limited.
Now lets play an less exciting hand. This is also more common (like 12~15 a season).
Two 12V batteries in parallel. One is old and going bad but the owner is checking specific gravity an doesn't notice that one is using more water than the other. He fires up the engine to go fishing. He out there idling the engine and trolling, when his fishfinder goes out. This is noticed when the stereo quits, and so does his marine radio. Then the engine studders to a stop and it won't crank. He thinks, "No
problem, I'll just switch batteries." But, he had to switch to the other to start the engine and he didn't, he switched to both. Now they are both dead. So, somehow he gets someone's attention and gets dragged back to a dock. They try to charge both batteries and one won't come up. So, he will replace it, but he does not realize that what was the good battery has been hurt. It will come to get him someday.
Can this be avoided?
To the larger extent, yes. Banks (a 12V - 6 cell battery is a bank) should be isolated as much as possible. The only way I know of to mitigate this issue is to separate and charge each bank individually. That way both, or all, can be brought back to full density without any regard for what his brother wants to do. Hopefully, that way they can never get to be too very different. Some systems that I assembled with this capability have been satisfactory.
Even with all that complexity, the actual capacity of the combined bank was not capacity times the number of batteries in the bank. There was always a loss of 10% at best. Batteries hard wired in parallel will be hard pressed to deliver 75% of the rated capacity. (2ea 100Ahr in parallel can be as little as 150 of the 200Ahr you bought.) Yes, I do have instruments that can measure this, but they are inexpensive or simple to operate.
Now, if that isn't clear, post a question, I will do my best to answer it.
Matt
