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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #61  
Old 09-15-2006
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btrayfors will become famous soon enough btrayfors will become famous soon enough
Understood. And, you're right, the amperage doesn't vary a lot as voltage declines through the usuable range.

Here are some "real world" measurements from my ongoing pulsator tests. These were measured on a 2-battery T-105 bank with a load consisting of three 12V bulbs totalling 130 watts (rated).

VOLTS AMPS AH
13.11 10.36 0.00
12.48 9.92 5.10
12.40 9.88 27.20
12.28 9.83 49.30
11.92 9.67 102.80
11.52 9.50 148.40
11.14 9.34 174.20
11.04 9.28 178.80
10.88 9.20 183.40
8.69 8.20 190.20

As you can see, the amps only varied between 9.2 and 9.9 with voltages from 10.9 to 12.5.

Bill
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Oops...forgot the KWH column. Here's the whole thing:

VOLTS-AMPS -- AH -- KWH
13.11 - 10.36 - 00.00 (began load)
12.48 - 9.92 - 005.10 - 0.06
12.40 - 9.88 - 027.20 - 0.33
12.28 - 9.83 - 049.30 - 0.60
11.92 - 9.67 - 102.80 - 1.23
11.52 - 9.50 - 148.40 - 1.76
11.14 - 9.34 - 174.20 - 2.04
11.04 - 9.28 - 178.80 - 2.09
10.88 - 9.20 - 183.40 - 2.14
08.69 - 8.20 - 190.20 - 2.21

Bill
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Old 09-15-2006
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Dave/Bill...thanks for the excellent work and discussion. Most helpful!
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Hey, it's fun. Hope it's useful, too, to dispell some wayward notions :-))

Here's a last note re: paralleling batteries...

Question:

Given the same house load (frig, lights, nav gear, radios, etc.)...

And given an installation consisting of two banks of two T-105 batteries rated at 225AH for a 20-hour load, with the banks wired through an A-Both-B-Off switch ...

Which situation will provide more total amp hours before the batteries are depleted (drawn down to 10.5V or whatever cutoff point you like)...

1. running them as two separate banks, one after the other; or
2. running them as one large combined bank?

If you've followed this thread closely, by now you should know that the answer is #2, because you'll be putting a relatively smaller load (actually, half the load) on each of the batteries in the combined bank, and we know that the smaller the load the more AH can be drawn from a battery.

Yet another reason to combine 'em :-))

Bill
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Old 09-15-2006
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Good point Bill. One still has to wonder:

What is the percent incidence of failure in batteries, from a shorted cell?

Because one shorted cell in one of two separate batteries, just kills one battery. One shorted cell in a paralleled set, will cause that 275A "short" between them that I mentioned come time ago. And leave you with no spare battery, unless you blew a primary fuse. Or, started a battery fire.

I get the feeling that's the kind of embarrassing question ("Hi, how many of your batteries short out and fail?") that no battery maker will disclose to the public.
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Cell failure through shorting, in my experience, is extremely rare on a reasonably well maintained battery setup. I have personally NEVER seen one.

I have seen "weak" cells, and "bad" cells, but no real shorted cell that I can remember. It is, of course, possible (but so is it theoretically possible to vote in an honest and competent government....we just never seem to do it :-))

All house batteries should be fused at the maximum load expected. I believe my fuses are 125A...can't remember exactly..they're sized to be able to take a full charge from the 110A alternator or the 120A battery charger. So, in the VERY UNLIKELY EVENT of a 275A short, the fuse(s) would blow instantly.

I admit I haven't thought in any detailed way about the actual behavior of a shorted cell. Maybe it would just act like a short circuit between the two adjacent cells -- same as a strap -- and therefore would just not contribute its 2.14 volts to the whole???

Bill
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Bill, what's so hard about voting in an honest government? I usually vote five or six times in any election with no problem. Heck, I even know dead people who find it no problem to vote.

" Maybe it would just act like a short circuit between the two adjacent cells -- same as a strap -- and therefore would just not contribute " Yeah, but then you've got a 10 volt battery hooked up to a 12 volt battery, and that just CAN'T be good. Like the guy trying to waterski on the back of a Roman Trireme, he's not just "not contributing".
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Old 10-02-2006
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I’m Back…

This and some other questions have caused me to dive into my collection of papers on battery technology. I have found out some interesting items;

1. AGM and Gel (sealed) batteries fail open most of the time (>80%). If you have a series only string, look out!
2. Wet cells fail shorted most of the time. This is due primarily to plate growth and corrosion.
3. Battery system reliability increases with paralleled batteries, dependent on a whole bunch of variables, but typically 10% improvement with each paralleled string. (88% for 1 to 98% for two to 99.5% for 3, etc.) This is true for wet and VRLA cells.
4. Some paralleled systems have been used up to 16 different strings all on the same bus.
5. Different capacity batteries that have the same chemistry can be paralleled with no degradation in performance or operational life.

And last, but not least, the current of between the two batteries under test (I left it going while I was away) after 2 weeks 1 day is 0.0000005 amps (0.5 micro-amps).

Citations for these conclusions are available, just ask.
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Good information, Dave.

Nice to have you back.

Bill
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Old 10-02-2006
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Thanks Dave...
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