Side note - if one properly installs fuses on each string that is added in parallel, hopefully that would at least help protect against the case of a single dead battery causing the parallel string to do a massive discharge.
John
From: "aweekdaysailor@yahoo.com [electricboats]" <electricboats@yahoogroups.com>
To: electricboats@yahoogroups.com
Sent: Thursday, April 28, 2016 4:45 PM
Subject: [Electric Boats] Batteries wired in parallel (and series...)
[I didn't want to hijack the other thread on series...]
Hi Folks, I am formerly active member and have been away a long time, but I still have the boat (Aja, 30' 1978 Hunter) and she is still electric (but I caved and installed an outboard...I am on SF bay after all...)
So...I noticed someone who sounded quite sensible/knowledgeable recently posted concerns about wiring batteries in parallel. The issue being (if I understood correctly) that a short in one cell will potentially cause high amp discharge from the remaining paired battery(ies) into the shorted unit potentially causing explosion, fire, acid, screaming...
That had me a bit concerned since I have a "buddy" string setup. 8 x 12v deep-cycle (sorta..) batteries, a pair wired in parallel and then the paired "buddies" connected for 48V. This has served me quite well (going on 10 years now) and I'm getting very good battery life in general. I occasionally re-balance the batteries using a high-amp automotive tester and pair weak with good, etc. (if really weak I head to Walmart for another...)
I do independently charge the "buddies" so I am not using a 48V charger.
However, in counter to the concern, I did find this article Parallel Batteries which seems to dismiss the concern.
So...what say the gang? Dangerous? Or (within reason..) Safe? To save you a little reading, I've quoted the relevant section below:
"Curiously, (or perhaps not so,) reference 1 does not list plate shorts as a failure mechanism of deep cycle batteries. Cell shorts are a significant failure in starting and lighting batteries, but not in deep cycle batteries which use better plate separators. Cell shorts are possible, however, and as reference 2 points out on pages 30-31, cell shorts can occur when a cell has been allowed to selfdischarge too deeply and the lead sulfate become soluable enough to diffuse through the separator. A dendrite is then formed on recharge which conducts most of the current through the cell.
Cells which short rarely do so with a low resistance. A cell is made up of alternating negative and positive plates having an intervening separator. Shorts between adjacent plates, are usually a dendrite or a local failure of the separator. It's fortunate that cell shorts are not normally low resistance. Consider a 200 Amp hour cell. Suppose that the cell developed a short that allowed 100 Amps to flow. At that current, a 2 Volt cell would be producing 200 Watts of power across the short. A 200 Ah cell could support 100 Amps of current for about 30 minutes. During that 30 minutes, in excess of 340 Btu of heat would be generated. There is 25-40 pounds of active material in a 200 Ah cell, which will rise to the boiling temperature of water in about 10 minutes. The cell probably ceases to be a battery before boiling is reached. Assuming that it doesn't, we would have enough Btu left, (210), to boil about 1/2 cup of electrolyte. That's a lot of steam!
Obviously, if cells did short with a low resistance, batteries would be much more hazardous than they are. We'd have to mount batteries in special cases that could deal with the heat and acid steam. And given such dangers, there would be regulatory limits on the size of cells that could be manufactured. To get any reasonable capacity a large number of small cells would have to be paralleled. Hmmm.
As mentioned, a cell rarely, if ever, shorts with a low resistance. Some people would have us believe that a shorted cell in one battery of a multi-battery bank will cause a fire. The argument goes that the remaining batteries will rapidly discharge through the cells of the defective battery. Such huge discharge, the story continues, will create an inferno with batteries boiling like a Boston tea kettle, spewing hell infinitely in all directions. Battery cases will blow gaping holes in their sides leaving a rather untidy mess for the maid. It makes us want to wear our rain gear every time we inspect our batteries to find out if any unauthorized cold fusion experiments are occurring."
[also...within a couple of months I may have some interesting news relevant to the industry..teaser...]
-Keith
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Posted by: oak <oak_box@yahoo.com>
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