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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #21  
Old 09-05-2006
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Sailingdog,

Correct, as usual. By limiting the concentration of the lead sulphate in solution you limit the possibility of crystalization. Equalization may fource some of the crystals back into solution, but once those crystals form they provide seeds for more crystals to grow. By coating the plates they reduce the batteries capacity by effectively making the surface area of the plates smaller.

This reduction in area will also accelerate plate wear-out by forcing the areas uncoated to provide more lead into solution, reducing plate integraty in that area.

Sometimes I wonder that it works at all.
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  #22  
Old 09-05-2006
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Dave-
" Cell failures are rare compared with connection failures. " Hmmm...and you think there are going to be fewer connections with paralleled batteries than there would be with series cells? Only if you neglect the internal connections in that calculation. Good external conections are not hard to make, or ensure.

One also hopes that even with series cells, no prudent mariner is relying on ONE battery or bank. Even if their house bank is only one primary bank, surely there is a second starter battery which can be used to start the engine and provide power while a primary failure is being looked at.

“Regarding the loop between the two batteries "..."No, this is not tested, but is correct in theory."
I'd love to see you test that out, and see the results. Starting voltages, initial current flow between the two, and changes in that flow over time. Whether it goes down and stops, or goes down, levels, then reverses, etc. Since there is lag in each battery as the electrolyte locally saturates and then evens out again, the reaction will *not* be simple and uniform, each battery should be internally changing chemistry at different rates.
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Old 09-07-2006
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OK, boys and girls, here’s what I propose for the experiment on current between paralleled batteries.

I have two AGM 92 AH batteries, one at 90% capacity and one at 55% capacity. I had finished a load test on them a couple of weeks ago so I know this is empirical data, not theoretical or estimated. Both batteries are Douglas Battery DGS12-100F and are eight years old.

Both batteries have spent the last several days on a charger and are fully charged as defined by the float current (both batteries where drawings less them 100 mA). They will sit disconnected from each other (and anything else) for the next 48 hours. At that point I will measure and record the open cell voltage of each battery.

I will then hard wire the negative terminals together using 8 AWG cable. At the start of the test I will connect a Fluke 8060A 5 ˝ digit (three decimal places, at the 12 V range. It will measure 1/1000 of a volt) DMM in current mode (measuring in the 2A to 1 milliamp range) between both positive terminals. I will record the current every 15 seconds (using a camera as my automated equipment will be in my lab, not my garage). The data point time interval will be modified as the change in current dictates (no use recording a lot of points of the same level. If the slope is flat I will modify the time between photographs, but that time interval will be recorded.) The temperature of the batteries will also be recorded.

Results will be graphically represented in this thread when completed.

This should answer any questions about current flow between paralleled batteries.

Comments? Suggestions? Hellosailor, you asked for this. Now is the time for comment.
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Old 09-07-2006
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I'm waiting with baited breath for the results.
(The Cuban says "no more sardines for breakfast for you")
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Old 09-07-2006
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Go for it Dave!
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Old 09-07-2006
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Looking forward to your results.
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Old 09-07-2006
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Dave, that sounds very logical. Camera, eh? Nice way to kludge a data recorder! I'd also be interested to see what voltage they are at 2-4 days after the start, in comparison to the starting voltage.

I'm unsure if the test is more--or less--reflective of real world conditions by letting them settle for 48 hours after charging. In the real world, they'd be connected during and after charging, so perhaps the comparison would change if they were tested that way as well?

I've got vague memories that one of the equations I never remember (Kirchoff's? Thevinen's?) can be used to model two batteries hooked up just to each other but then again that's theories, and I'm almost afraid to ask how the theoretical internal resistance of a battery varies against reality in the lab.

Wait, let me call my agent, I think I can get Fox and Geraldo to supply armed Pinkerton's to guard it over the weekend, and unveil the results on national TV come Monday. There could be some heavy sponsorhip money in this.
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I figured I would do worse case at initial. If they were connected together they would both start at the same voltage, not much current flow with no potential difference.

I will run the experiment for most of the week. However, I will not be able to get data on Monday and Tuesday, will be out of town for business. However, this will give them more time to settle. I anticipate the current flow will be at a minimum by the end of Saturday, but we’ll see.
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  #29  
Old 09-07-2006
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Dave-
I look forward to the results, no matter what they are.

CardiacPaul-
If you didn't want to further hijack the other thread, why didn't you leave it at "See my comments in..." instead of hijacking it further? I can see why you seem so upset at everything I suggest. Every time I give one or more examples of what's possible, you seize one and rail against it as dogma. There's a big difference between claiming dogma, and suggesting options. I'm only suggesting options.

Your scenarios about Rolls batteries and warranty claims in foreign ports are way too creative for me. If I needed a battery in some foreign port, and wanted it via warranty, I'd look first for local agents. Rolls might have them. If they didn't, I'd ask them to ship the new cell DRY to the nearest US port via Parcel Post or similar economy means, not demand they needlessly ship them as overnight hazmat. Yes, battery acid can be obtained in most places where you find cars and telephones.
But I don't think I'd bother with all that fuss in the first place. I suspect that in any Caribbean port, you'll find there are port industries and fork lifts, and along with them, a convenient local way to buy the one cell you need to replace. You're way too zealous about seizing on one idea and ignoring the options.
Like, insisting parallel batteries must be better simply because a lot of folks were taught to use them. Convenient? Maybe. Cheap? Maybe. But that's like saying automotive alternators are the best ones to use on boats--because so many boatbuilders equip all their new vessels with them.

So let's try to make this clear: If you have a twelve volt battery on your boat, any kind of 12v battery on your boat, you ALREADY HAVE ONE SET OF CELLS IN SERIES. Anything you add to that, adds potential problems. Instead of adding the potential problems, if you simply choose the original six cells for adequate sizing and other constraints, you have the simplest and most reliable 12v battery.

Maybe not the cheapest, certainly not the easiest to obtain. But simplest, most reliable, and with a little perseverance about looking for the options, quite possible the best range of size, fit, weight, installation configurations, and other features.

You can't make one 12V battery, unless you make one SERIES CELL BANK. If you reject the validity of series cells...you're stuck at 2.2v for your ship's power standard. (Or 1.5, or 4.3, etc. whatever chemistry you choose.)
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Old 09-07-2006
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Uh huh! Nice analysis, Hello Sailor! At least as far as it goes...

Now, let's get real world. Quite apart from cost (2-3 times at least) and size (where the hell do I put these tall batteries) and maintenance (12 cable connections to inspect and maintain instead of 2), what happens when you lose a cell during a passage?

You're two days out of LA headed for Hawaii, and one of the cells on your ONLY 12V house battery develops a short. This might be because of bouncing around shook something loose inside, sulphate buildup in the bottom, plate corrosion, etc., etc. In any case, instead of 12.6VDC you now have a 10.4VDC house battery bank. Not enough to run your SSB, or other appliance which needs at least 11-12VDC to operate.

You're many days from shore. You're many more days from obtaining a replacement for the bad cell. Oh, bother!

With a paralleled house battery bank what do you do? You just flip the battery switch from "ALL" to "1" or "2" to isolate the bad battery bank, and carry on as before.

Do you think the sailor with the paralleled battery bank gives a damn that he/she may be losing a tiny percentage of efficiency by paralleling the house battery banks???

Bill
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