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Old 12-03-2013, 17:18   #2341
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I can assure you that I have done far more studying on impedance than you have or ever will. I reread your hilarious joke a few pages back about engineers. With your mail-order "electronics" degree, you may be more fun at parties---but please leave the real science to others.

First off, the purely reactive transients you describe are infinitesimal in comparison a large battery's internal resistance. More to the point, do external reactive devices (like motors) in a circuit influence the battery's internal resistance? Of course not. Reactive devices only influence the reactive components of the impedance---resistance is not affected.

I get the distinct impression that you have hooked up an impedance meter to a boat battery and then observed wildly varying impedance while the boat was in operation. Thus immediately concluding that the internal resistance of a battery varies wildly. I guess this is why we engineers make the big bucks.

I'll tell you what. If you promise never to mention impedance again in this thread, we can have a more intelligent discourse on how internal resistance might actually change---and to what extent it would do so in the small time it takes a fuse to blow.
You seem to excel at ad hominem attacks. Other then that I see little science. Being an electronics engineer for 40 years working on a variety of projects from military through aerospace etc including high power electronics and various charging systems , well its just a mail order degree funny , I had to actually attend for 4 years , not to mention the masters , but hey what about it.

I like to use the term impedance as its more correct and reminds you that there other components at work.


Battery internal resistance is not a constant value , saying a battery has X milliohms is an over simplification. A battery is a perfect voltage source ( which has zero resistance) in series with a resistance that has a relatively complex relationship. That's a basic first order model , but a very simple one , better models model resistance to match impulse loads, load dumping etc.

As I said an evaluation of first and second order battery models will reveal all this.


Mainsails record of practical achievements stand by them selves. You seem to just quote specs and ratings without any real understanding of what's actually going on in a short circuit that is battery derived. That's not to dispute some basic tenants of what has been said about interrupt ability, but like lots of things its just not about the sticker value.

Before you continue perhaps you might put a scope on a battery short and have a look at the various waveforms.

Dave
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Old 12-03-2013, 17:39   #2342
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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No heat (well very little) and no bulging, so I probably didn't need to band them. Rather safe than sorry.
Maybe I'm missing something but, why would you ever want to band them together. It seems to me packing these cells close together while it makes for a nice tight package but, allows the batteries on the inside to heat up more then then ones on the outside. Also they might try to expand expand and possibly short internally. Wouldn't it be better to separate the batteries slightly to allow heat to dissipate more? I separated the AGM's in my 48 volt pack and also used dri deck underneath each one for added ventilation. Why would you not do that with LifePO4 technology too?
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Old 12-03-2013, 17:41   #2343
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Maybe I'm missing something but, why would you ever want to band them together. It seems to me packing these cells close together while it makes for a nice tight package but, allows the batteries on the inside to heat up more then then ones on the outside. Also they might try to expand expand and possibly short internally. Wouldn't it be better to separate the batteries slightly to allow heat to dissipate more? I separated the AGM's in my 48 volt pack and also used dri deck underneath each one for added ventilation. Why would you not do that with LifePO4 technology too?
But there's is little heat in Li tech under normal circumstances , they are not like LAs in that respect. Then you have the added complexity of rigging high current interconnects over longer distances.

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Old 12-03-2013, 19:04   #2344
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Originally Posted by mbianka View Post

Maybe I'm missing something but, why would you ever want to band them together. It seems to me packing these cells close together while it makes for a nice tight package but, allows the batteries on the inside to heat up more then then ones on the outside. Also they might try to expand expand and possibly short internally. Wouldn't it be better to separate the batteries slightly to allow heat to dissipate more? I separated the AGM's in my 48 volt pack and also used dri deck underneath each one for added ventilation. Why would you not do that with LifePO4 technology too?
I can only quote what Ive read...so keep that in mind. Fast charging or discharging leads to heat or something, which can cause the individual pouches (cells?) contained within the square format prismatic battery to bulge. The strapping inhibits the bulging and keeps the cells happy in these conditions. The basic chemistry is more tolerant of temperature (on the high side) than our traditional LA/AGM. So heat alone, is not as big a factor.

Mine are banded per the manufacturers recommendation. In my case, there was no warranty without banding. But for a practical manner, short of a failure on delivery, I would not count on a long term warranty with any manufacturer. But the suggestion does imply how seriously they believe it should be done.

Bob
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Old 12-03-2013, 20:33   #2345
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I just read Jack Rickard's latest blog post. Skip the first part about Brian, unless you follow it often. But later he delves into both LiFePO4, plus an analysis of the Boeing problem like only Jack can. Good reading....:this is a guy with the most practical experience with LiFePO4 in the US that is willing to talk about it. See:

http://blog.evtv.me/2013/03/if-i-onl...-a-10-lb-sack/
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Old 12-03-2013, 20:59   #2346

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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

His bottom line is that the Boeing charging design is defective and that the batteries are being constantly overcharged. I find it hard to believe the Boeing would do that.

He's also counting the number of hours on the aircraft, and ignoring the fact that the Japanese regularly and repeatedly had replaced the original batteries as a routine maintenance procedure.

Boeing's BMS system allowed a dangerous and continuous overcharge of the batteries? And all the agencies that have been testing the systems, including Yuasa, who know how to charge their own batteries, missed this?

Nah.

Gremlins. Check the video showing the white smoke billowing out of the aircraft, if you look closely in the smoke you can see one laughing, he's the one that spat molten metal at the firefighter.

I don't know how the guy fried his Apple laptop, but I've had laptops running on AC for the better part of 4-5 YEARS without battery damage.
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Old 12-03-2013, 21:30   #2347
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I'm relating this from memory, but I think the battery on the 787 had never been changed. Pack or cells, based on the report.
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Old 12-03-2013, 21:51   #2348
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Battery internal resistance is not a constant value , saying a battery has X milliohms is an over simplification. As I said an evaluation of first and second order battery models will reveal all this.
And that's an overcomplication. The internal resistance of a cell is a very basic quantity, which can be easily measured by any patient sailor with a multimeter. Constantly insinuating that it constantly varies does not help to promote its understanding.


Quote:
You seem to just quote specs and ratings without any real understanding of what's actually going on in a short circuit that is battery derived. That's not to dispute some basic tenants of what has been said about interrupt ability, but like lots of things its just not about the sticker value.

Before you continue perhaps you might put a scope on a battery short and have a look at the various waveforms.
I don't need an oscilloscope to understand what happens. I hate to break it to you again, but it's just Ohm's Law.
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Old 12-03-2013, 22:44   #2349
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I don't have the reference handy here, but the internal resistance (impedance) of a LFP cell is anything but simple. The expression I recall had at least a dozen terms in its arithmetic. Nasty. An ohmmeter will only report the apparent resistance under the one narrow condition being measured.

One problem with LFP technology is that the behavior is complex enough to be non-intuitive. This is a problem for the layperson until the "best practices" are better defined. Kinda like SSB antennas...

Apparently even Boeing and Yuasa got it wrong. That would be a hundred engineers and thousands of discussions. Lucky for us we have a simpler application, however.
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Old 13-03-2013, 03:50   #2350
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks daddle, at least those that look at this issue in any depth realise that ESRs for imperfect voltage sources is anything but just a "number"

Quote:
I don't need an oscilloscope to understand what happens. I hate to break it to you again, but it's just Ohm's Law.
Yes thats true in any one instant in time, ESR has a particular value, but depending on the battery type usage, load characteristics , ESR can vary, in some chemistries quite considerably. Hence the ESR at a particular current may not in fact be the ESR at say short circuit current values, Li-ion has a characteristic that simply put allows the ions to detach rapidly ( deintercalation) hence the ability to provide short circuit currents often at a rate greater then the ESR would suggest for normal current delivery. Its one of the great strengths ( and weaknesses) of the technology.

The reaction of disconnect devices and hence their interrupt capacity is a function of many factors, one of them is the nature of the voltage and current rise ( or fall) times under short or near short circuit conditions. There are other factors at play as well, including the presence of any inductive or capacitive elements etc etc.

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Old 13-03-2013, 09:00   #2351
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

There's a big difference in a brief short circuit that simply blows a fuse, and a protracted short caused by a tool across a terminal or a cell puncture or internal failure.

If it takes only 0.01 second to blow a 300-amp fuse, that's just 3 amp-seconds---a mere drop in the ocean for a boat battery. Not even enough time to heat up a wire, much less a battery. I don't plan on testing this personally on my sole Class T fuse, but I would speculate that battery voltage would barely dip. Internal resistance would also not be affected. So Ohm's Law could accurately gauge peak current I=E/R, as voltage can only decrease, and the resistance of the fuse rapidly increases from near zero to infinity as it opens.

A crowbar short, on the other hand, would exhibit a huge voltage drop across the terminals. Inside the battery, the various components---the electrolyte, cathode layer (mostly FePO4 under full charge), anode layer, aluminum cathode, and copper anode---would all rapidly heat, probably in that order of decreasing resistivity. Certainly, cell internal resistance would likely change under this extreme scenario. The big question is, would it increase or decrease?
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Old 13-03-2013, 09:31   #2352
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I don't have the reference handy here, but the internal resistance (impedance) of a LFP cell is anything but simple. The expression I recall had at least a dozen terms in its arithmetic. Nasty. An ohmmeter will only report the apparent resistance under the one narrow condition being measured.
One cannot use an ohmmeter to measure the internal resistance of a cell.

Ignoring that, I concede that there are apparent reactive elements---capacitance and inductance---present even in simple cells. These pose a major problem when one tries to use an impedance meter to determine the actual internal resistance. It is the fundamental reason why an impedance test at 1kHz often yields a different impedance than one at 10kHz. What is the correct frequency, Kenneth?

The problem is determining the cell's apparent capacitance and inductance---quantities certainly not found on any datasheet, and likely unique to every cell.

Way, way easier---and much more practical---to simply use Ohm's Law in most analyses of a battery under normal DC operation.
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Old 13-03-2013, 09:43   #2353
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Ok i've watched the technical to and fro here and appreciate you guy's love the intricate side but really isn't this a bit pedantic?

Not having a go at all at anyone but the thread is for those using Lithium as house banks not the science, first principals, the fundamentals etc...

Cannot similar argumentative technical discussion be made of currently available battery chemistry?
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Old 13-03-2013, 09:52   #2354
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I'm with you on this Lagoon4us. I'll monitor with readouts and audible alarms individual cell voltages to insure balance and no cell going to low during discharge or too high during charge. Fuse the bank like I would for any bank, and be done with it.
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Old 13-03-2013, 09:56   #2355
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Cheers Bob, i mean it with respect totally but we have to get on with it, my batteries arrive soon, the solar is in place the cables arrive with crimpers and crimps on friday....... There's 750 watts wanting to get moving!!!
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