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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 09-05-2006
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btrayfors, there's no need to wake up at 3AM to manage battery banks if you've been "managing" the power system in the first place. If I'm racing or cruising 24x7, we discuss power management, engine runs, and charging cycles in advance, with an eye towards switching banks optimally and not running the engine when an offwatch might prefer to sleep.

Criticizing the poster rather than contributing to the technical discussion, serves no purpose here. Dave and I can appreciate the concept of discussion without diverting into personal quests or attacks, you're welcome to join us on that level.

Yes, I have redesigned charging systems to match the components, and yes, I have managed energy budgets. Sometimes compromising "whats right" with "what we have to do" and "what we can afford".

But I've yet to hear any reason in favor of parallel batteries, any reason that makes them in any way advantageous to separate batteries, beyond "Well it's cheap and easy to get them that way."

Cheap may be a good reason, but that doesn't make it optimum design practice unless "cheap" is your goal.
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Old 09-05-2006
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Hey, lighten up!

I'll be happy to join you on any level you want, but only if you bother to read the posts. We've given you at least three perfectly good reasons why paralleling batteries for the house bank often makes sense, but you've chosen to ignore them.

B.
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Old 09-05-2006
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B, I've read the posts. And as I've said, the "discharge depth" argument is fallacious, it has no bearing here. The only other firm and clear argument about parallel batteries has been that they are cheaply built using readily available parts, i.e. the batteries from WalMart or Sam's Club.

And I've conceded that cheap sometimes is good. But aside from "cheap"...there have been no other arguments made that indicate anything *certain* about any advantage of parallel batteries.

At best, Dave has said that depending on various factors, in certain cricumstances, there may not be any disadvantage to their performance--and that's ignoring the extra failure modes that are possible.

I'm saying that any configuration which provides zero definite advantages, and doubles my chances of a failure, is a bad idea. Cheap batteries in parallel may be convenient--but I've yet to see any reason to think they're any cheaper, either. (Battery pricing being a very variable thing.)
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Old 09-05-2006
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OK, let's try again...one more time.

A series/parallel setup of house batteries on a (larger) cruising sailboat may be advantageous because:

1. lower "depth of discharge" in real world usage (far from a fallacious argument);
2. lower acquisition cost;
3. smaller size...will fit where other options won't;
4. easier on your back, assuming it's you who have to schlep the batteries;
5. better availability worldwide...a key factor for cruising sailors;
6. faster charging, lower run times for engine and/or generator; and
7. endorsement by experienced sailor-engineers such as Calder and Verry.

Providing that the installation is a good one, the potential "downside risks" of paralleling batteries are few and, in my experience and reading, very remote. This is particularly true because the "paralleled battery installation" itself, with reasonable care and attention to battery maintenance, is likely to reduce the downside risk of, e.g., a shorted cell by reducing the degree of sulphation and by maintaining a higher state of battery health.

A final factor...then I'm done. You can never discount the human factor, and the ability of a tired or careless or ignorant sailor to inadvertently murder his batteries. By keeping the system simple: one house battery bank only, one voltage measurement only, one charging rate only, etc..... you significantly reduce the chance of this happening.

Bill
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BTW Hellosailor, I’ve been meaning to ask. A single point of failure for a connection will render the single bank as you advocate useless. A parrelled bank has a greater chance of being at lease partially usable, if at a lower capacity.

“I'd been told that's not the case, and that is the problem. They WILL equalize, yes. And then as each battery is different, and has a different rate of self-discharge (among other things caused by the physical changes and sulphation happening differently in each battery), as soon as they have equalized they begin to drift apart and the current loop between them starts to flow again. This process does not stop, it continues and it drags them down.”

Agreed, each battery will have a different open cell voltage and they will drift apart if they are not connected. This drift will cause a small current to flow until the equilibrium voltage of both batteries are the same. This will NOT however drag the batteries down. What is removed from one battery will be stored in the other. Discounting self discharge, the amount of charge will remain constant. The current will be very small and eventually stop. The total charge capacity of the battery bank will not change.

And just to clear it up, full discharge is whatever you define it as. The more and deeper you discharge your batteries the faster they will loose capacity. I routinely discharge batteries at work to 1.75V/cell (10.5V) to get full capacity out of the battery. (I say routinely, by that I mean during each discharge cycle, maybe ˝ dozen times per year.) However, being cheep and not wanting to replace my batteries all that often, I also use the 50% discharge point as fully discharged, about 11.25V (1.88V/cell) under load.
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Old 09-05-2006
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????

Quote:
The depth-of-discharge (DOD) argument holds no electrolyte here,
I am assuming that it is true that: "from an electro-chemical standpoint, a lead acid cell's life time is extended by maintaining it as close as possible to a fully charged state."

If that is true then why is the "DOD" not to be considered? I have read the posts (albiet quickly) and am not clear where that notion was disputed and laid to rest.

Last edited by sailandoar; 09-05-2006 at 06:11 PM.
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Sailandroar-
"If that is true then why is the "DOD" not to be considered? "
Because, the DOD argument has nothing to do with how the batteries are ganged up. You can run your batteries to any DOD regardless of the series/parallel/alternating configuration. Bozo had red hair and big shoes, Bozo was a clown. Therefore everyone with red hair and big shoes is a clown? No, that's invalid logic. Same thing with the DOD argument. Invalid and fallacious since how deeply you cycle your batteries is a decision that can be made independent of how you configure (gang) them.

Bill-
OK, let me address why I disagree, one more time:
1. lower "depth of discharge" in real world usage (far from a fallacious argument);
A: Yes, fallacious and invalid. As above, you can cycle the batteries to any depth, regardless of configuration. That's not debatable, that's fact.
2. lower acquisition cost;
A: Still unproven. As I've said, yes, you an easily get cheap batteries from a bulk store. That doens't mean buying industrial batteries from an industrial source is any more expensive.
3. smaller size...will fit where other options won't;
A: False again. Watt-hour for watt-hour, the battery bank size will not differ at all. And I can fit six 2.2v cells into a wide variety of spaces where your pair of 12V cells simply can't go. If anything, the advantage here is for 2.2v cells in series.
4. easier on your back, assuming it's you who have to schlep the batteries;
A: False again, same as #3. The 2.2v cells are available in a much wider range of sizes/weights.
5. better availability worldwide...a key factor for cruising sailors;
A: False again. Anywhere there is shipping and freight, there are industrial battery suppliers supplying the 2.2v cells for fork lifts. You may not see them on the main shopping street or by the upscale marinas--but they are there until you get way off in the boonies, at which point you are looking at "emergency repairs" not scheduled battery replacement. Since these batteries can be expected to have a 10-15 year life, there's no reason to worry about "Oh wow, I need 500AH of batteries RIGHT NOW!" for any purpose besides emergency repairs.
6. faster charging, lower run times for engine and/or generator;
A: Totally false. Charging speed will depend on the system chemistry (AGM versus wet) and the alternator capacity, and the battery capacity. Given the same battery capacity, chemistry, and a sufficient alternator output to match that, the charging time will be the same for either configuration.
and
7. endorsement by experienced sailor-engineers such as Calder and Verry.
A: Specious logic. Yes, it is pleasing to say that "someone said it is good" but we're looking for facts not opinions here. What Dave and Calder have said is that you can, under some circumstances, get away with it. They have not, to my knowledge, made any point that parallel battery installations are in any way BETTER. Again, aside from the fact that you can build them with WalMart batteries.

So out of your seven points we have:
1-Fallacious
2-Unproven
3,4,5,6-Simply false.
7-Specious

And your only point made is the one I've conceded all along, that you can certainly use cheap stuff from WalMart real easily in parallel banks.


Dave-
"A single point of failure for a connection will render the single bank as you advocate useless. A parrelled bank has a greater chance of being at lease partially usable, if at a lower capacity." I think you've got that backwards.
A connection failure can be remedied with a wrench. A CELL FAILURE is the only thing that would take down the series battery bank. The odds of that same cell failure taking down your parallel bank are double or higher. IF you catch the failure in time, sure, you can split the bank and run on half of it. But you can also do that by using two banks. Unless your bank sizing and alternator are matched/mismatched in certain ranges, that's going to be a questionable point anyway. If you have a 500AH parallel battery bank, you'll need a 100AH continuous rated alternator to charge it in minimal time. Vary the alternator size, vary the bank size...and you may find that running two alternate banks provides a better answer--plus that redundancy.


Regarding the loop between the two batteries "The current will be very small and eventually stop. " This is the first and only time I've ever heard that. Have you measured this? With wet or AGM cells? What were the specifics?
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Sailandoar,

There are several wear-out mechanisms in a lead acid battery. The one the limited depth of discharge addresses is plate integrity. Basically, the discharge chemical reaction within a battery dissolves the lead off the anode and puts it into solution as lead sulphate. The cathode reaction dissolves the lead oxide of the cathode and puts it into solution as lead sulphate and water. Charging the battery drives the reaction in reverse and hypothetically replates the lead and lead oxide back on their respective plates. The deeper the discharge the more of the plates are put into solution and the plates become porous and spongy. As the material replates, it does not fill in all of the pores. There is also some tendency for the lead and lead-oxide to plate to impurities and precipitate to the bottom of the cell. Occasionally dendrils of lead can grow between the plates and short the cell. The precipitate of lead and lead oxide can build up in the bottom and short the plates as well.

By limiting the depth of discharge the integrity of the plates remain higher. The amount of lead in solution is limited so the amount of precipitate and dendril growth is limited.

This reaction is taking place all the time in a battery, whether or not the battery has a load connected. Charging the battery at the float voltage level forces the chemical reaction back as rapidly as it naturally occurs without a load (self discharge). This is why batteries don’t last forever, even with a float charge.
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One other point on depth of discharge. If batteries are deeply discharged, they stand a much higher chance of having the plates sulfate. The lead sulfate may recrystallize into a dense coarse-grained form which has high resistivity. This will often inhibits recharging or discharging and effectively reduces the capacity of the battery. By limiting the depth of discharge, you can help prevent plate sulfation.

BTW, the plates will also sulfate if they are exposed to air....so checking the electrolyte levels is fairly important.
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Hellosailor,

“A connection failure can be remedied with a wrench. A CELL FAILURE is the only thing that would take down the series battery bank.”

Yes but while you found the wrench, and then located the problem your system will be without power. No lights, no autopilot, no GPS, no radio. Not a chance I would want to take. A cell failure in a series bank would reduce the bank voltage, not necessarily take the bank offline. Cell failures, when they occur, tend to fail short. The only open condition that I have observed during a cell failure is when the battery explodes.

“The odds of that same cell failure taking down your parallel bank are double or higher.”

Cell failures are cell failures. The rates of failure should be the same no matter which configuration you use. A cell failure in a parrelled bank will take that battery out, not the others. The equilibrium voltage would be reduced by one cell, about 2V and that one battery would stop discharging into the load. Now before you start, the remaining battery bank will not discharge into the battery with the bad cell, the cell voltages of the remaining cells of the bad battery will be higher then the discharge or load voltage of the remaining batteries.

Cell failures are rare compared with connection failures. Cell failures also happen gradually with the exception of mechanical damage or catastrophic explosions during overcharging. Connection failures, especially in a harsh environment like a marine environment are much more commen. Thermal expansion and loosening, corrosion, vibration all can cause failure of connections. One connection in series with your battery string that fails removes the string from the circuit. While one connection can bring down a paralleled bank, its more likely to remove part of the bank from the circuit rather then the entire string.

“Regarding the loop between the two batteries "The current will be very small and eventually stop. " This is the first and only time I've ever heard that. Have you measured this? With wet or AGM cells? What were the specifics?”

No, this is not tested, but is correct in theory. The theory is sound for all lead acid batteries. If you want I will test this during the weekend. I just happen to have two 92 AH batteries in my garage, one to recycle and one to install in the boat. The test will be simple and I have the equipment.
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