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Old 29-01-2013, 15:08   #1651
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Hellosailor did say a ten thousandth of a volt, which was clearly wrong. However, he is certainly right in that reporting voltage to a millivolt is deceptive when pages earlier, in this very thread, we determined that a good Fluke meter only has a true accuracy of 4mV in the 4VDC range.

The repeatability of the measurement is not indicative of stability or any accuracy beyond which the meter can provide; a stable reading of 3.456V could mean it is rock steady at exactly 3.4560000V---or fluctuating wildly between 3.452V and 3.460V.
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Old 29-01-2013, 15:19   #1652
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by diugo View Post
The repeatability of the measurement is not indicative of stability or any accuracy beyond which the meter can provide; a stable reading of 3.456V could mean it is rock steady at exactly 3.4560000V---or fluctuating wildly between 3.452V and 3.460V.
Not quite. The Fluke 179 has a resolution of 0.001V (1 mV) when set to its 6V scale. That means you should be able to detect and measure a change of 1mV between any two readings using the same meter.

It has an accuracy of 0.09% of reading + 2 counts at the same scale, which would equate to 0.005V at 3.5V measured. The accuracy only comes into play when comparing readings between two different instruments (like two Fluke meters) or when comparing against the standard definition of the Volt. When making all measurements with the same meter you should be able to get comparable measurements to within 1mV.
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Old 29-01-2013, 15:27   #1653
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

HS,

Yes, back in the good old days (when I was a design engineer at Fluke) accuracy used to be specified as a fraction of full scale. Things have changed. The Fluke 179 accuracy is specified as 0.09% (not 0.9%) of the reading + 2 counts. On the 6v scale used for a 3.5v reading, a count is 1 millivolt. So the accuracy is (3.5v * 0.09%) + 2mv = 5.15mv. All info is on the Fluke website.

As I mentioned before, the resolution is 1mv. It is safe to assume that over a short period, under identical conditions such as temp & humidity, that a quality instrument like the 179 will produce repeatable results. Most of the time, accuracy is all that matters, but the LiFePO4 balancing experiment is an unusual case where high resolution is actually more important than extreme accuracy.

One of the unanswered questions about these batteries is how much imbalance occurs over time. There seems to be anecdotal evidence both ways but no well controlled experiments. Maine's work is a great start on a proper experiment, but it will take more cycles to get a better picture. High resolution measurements make extrapolation with fewer cycles numerically more sound and allow meaningful results with less effort.

Charlie
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Old 29-01-2013, 15:39   #1654
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by Dsanduril View Post
Not quite. The Fluke 179 has a resolution of 0.001V (1 mV) when set to its 6V scale. That means you should be able to detect and measure a change of 1mV between any two readings using the same meter.
Unfortunately, no. The meter simply does not have the ability to reliably measure a one millivolt difference---whether it's the same reference, the same meter, or different meters. Accuracy always trumps precision.

Only with the more accurate lab-quality devices will you see an accuracy that exceeds the precision, and only then could you reliably detect and measure a difference down to the least significant digit.

I do stand corrected about the 5mV though.
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Old 29-01-2013, 15:53   #1655
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

mmmm here's hoping' new cells will have much less balancing the supplier telling me not to be worried about lower voltage balance.

Not having a workshop or any technical instruments i just checked the multimeter that i have and in the 2v range it's accuracy is quoted at +/- (0.5%+1)
So i guess take three readings and average will be best i can do?

I could try the battery monitor i guess with probes?
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Old 29-01-2013, 16:06   #1656
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mmmm here's hoping' new cells will have much less balancing the supplier telling me not to be worried about lower voltage balance.

Not having a workshop or any technical instruments i just checked the multimeter that i have and in the 2v range it's accuracy is quoted at +/- (0.5%+1)
So i guess take three readings and average will be best i can do?

I could try the battery monitor i guess with probes?
It's plenty accurate enough. If you do the first initial balance at 3.6 volts, it matters little if the actual voltage is 3.58 or 3.62. Beyond that you are only looking at a relative difference between the cells, so if you check one cell 2 times and get the same reading, the comparison to adjacent cells is accurate enough for our purposes.
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Old 29-01-2013, 16:13   #1657
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Yes, there is way too much emphasis on accuracy here. While it's amusing to report that all my cells are within a millivolt of each other, the honest truth is, they're all within 0.1V---which is totally good enough.

My charger charges to 13.9V and floats at 13.3. My alarms sound at 12.8 and 14.0. And my disconnects occur at 12.1 and 14.4.
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Old 29-01-2013, 16:19   #1658
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by Maine Sail View Post
This is not what I am seeing. The cells are still well below the knee at about 3.5V. I am seeing a cell voltage drift that seems only superficial and not egregious. Once you hit it with a load the cells all wind up back within a few mA of each other. Just don't understand why this happens...
the knee starts at 3.4v, that is full for a lithium ferrous cell, everything after that is surface voltage and only useful when charging multiple cells in parallel, it just makes sure all of them are above the full voltage, current will continue to flow to he lower cells till their voltage climbs, indicating they are full.
There are great advantages to multiple cells in parallel compared to single high capacity cells. All these cells are mass produced, a 10% variation in capacity is still within acceptable if you build 110Ah cells and call the 100Ah, the lowest capacity cell still meets spec and no one complains when they get 110Ah out of a 100Ah cell do they. This mass production also creates different internal resistances, tiny, but it's there, if multiple cells are paralleled these differences balance out and a much more stable pack is created.

Here is a Junsi cell logger graph of the last stages of a 720Ah battery recharge from 11.8v, full discharged to fully charged, it's fairly definite where the cells reach full capacity.
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Old 29-01-2013, 17:48   #1659
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Originally Posted by T1 Terry View Post
the knee starts at 3.4v, that is full for a lithium ferrous cell, everything after that is surface voltage and only useful when charging multiple cells in parallel, it just makes sure all of them are above the full voltage, current will continue to flow to he lower cells till their voltage climbs, indicating they are full.
There are great advantages to multiple cells in parallel compared to single high capacity cells. All these cells are mass produced, a 10% variation in capacity is still within acceptable if you build 110Ah cells and call the 100Ah, the lowest capacity cell still meets spec and no one complains when they get 110Ah out of a 100Ah cell do they. This mass production also creates different internal resistances, tiny, but it's there, if multiple cells are paralleled these differences balance out and a much more stable pack is created.

Here is a Junsi cell logger graph of the last stages of a 720Ah battery recharge from 11.8v, full discharged to fully charged, it's fairly definite where the cells reach full capacity.
Terry,

That graph starts at 3.4V? That's a lot more than 11.8, more like 13.1 or 13.2?
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Old 29-01-2013, 20:04   #1660
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by charlie p View Post
HS,

Yes, back in the good old days (when I was a design engineer at Fluke) accuracy used to be specified as a fraction of full scale. Things have changed. The Fluke 179 accuracy is specified as 0.09% (not 0.9%) of the reading + 2 counts. On the 6v scale used for a 3.5v reading, a count is 1 millivolt. So the accuracy is (3.5v * 0.09%) + 2mv = 5.15mv. All info is on the Fluke website.

As I mentioned before, the resolution is 1mv. It is safe to assume that over a short period, under identical conditions such as temp & humidity, that a quality instrument like the 179 will produce repeatable results. Most of the time, accuracy is all that matters, but the LiFePO4 balancing experiment is an unusual case where high resolution is actually more important than extreme accuracy.

One of the unanswered questions about these batteries is how much imbalance occurs over time. There seems to be anecdotal evidence both ways but no well controlled experiments. Maine's work is a great start on a proper experiment, but it will take more cycles to get a better picture. High resolution measurements make extrapolation with fewer cycles numerically more sound and allow meaningful results with less effort.

Charlie
yes to all of the above.

Now onto Mainsail,

This might be more work then you want, but I'm curious if the small percentage voltage becomes more additive once you start using the pack in a typical yacht fashion.

Meaning that we don't have 100 amp discharges, but more like 1 here 4 there then maybe 30, back down to 20...etc? All with differing load times.

I can see where just after full charging, and varying cell voltages could in essiance be brought inline with a single 100 amp discharge, but what if we get into quintessential lower voltage discharges?

Lloyd
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Old 29-01-2013, 20:05   #1661
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I think you will find the graph starts at 3.34v. The graph shown is an hr long, it takes a bit more than an hr to charge a 720Ah battery, even for lithium ferrous, the charge rate is 10 amps roughly, very little current into a 720ah battery, the bit at the end of charging still gave this result.

Here is graph of a 100Ah Winston LYP 12v battery being charged with a constant current 40 amps set to cut out at 15v, there is a tapered charge reduction as the cells reach fully charged, unfortunately I didn't have anything that recorded the charge current at the time.
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Old 29-01-2013, 20:10   #1662
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

http://www.nfpa.org/assets/files/pdf/research/rflithiumionbatterieshazard.pdf

Some more information to read thru.

Makes using smaller cylinder cells with smaller enery in parallel look better than larger cells with more energy. Also the steel cases sounds better than plastic. I hope we are keeping a eye on what could happen if a cell does catch fire. The Boeing investigation is still on going & all 50 B787's are not flying. That is only a 50 AH cell & if one faulty cell caused the fire in both cases then the future for Li-Ion is in doubt.

Regards Bill
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Old 29-01-2013, 20:28   #1663
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
Originally Posted by bill good View Post
http://www.nfpa.org/assets/files/pdf/research/rflithiumionbatterieshazard.pdf

Some more information to read thru.

Makes using smaller cylinder cells with smaller enery in parallel look better than larger cells with more energy. Also the steel cases sounds better than plastic. I hope we are keeping a eye on what could happen if a cell does catch fire. The Boeing investigation is still on going & all 50 B787's are not flying. That is only a 50 AH cell & if one faulty cell caused the fire in both cases then the future for Li-Ion is in doubt.

Regards Bill
Once we have a thermal runaway...no matter the Li-type cell all we can do is wait for the energy to stop. Firefighting is to the surrounding flammables.

Containment of fire spreed is always number one in firefighting.

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Old 29-01-2013, 20:37   #1664
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
Originally Posted by bill good View Post
http://www.nfpa.org/assets/files/pdf/research/rflithiumionbatterieshazard.pdf

Some more information to read thru.

Makes using smaller cylinder cells with smaller enery in parallel look better than larger cells with more energy. Also the steel cases sounds better than plastic. I hope we are keeping a eye on what could happen if a cell does catch fire. The Boeing investigation is still on going & all 50 B787's are not flying. That is only a 50 AH cell & if one faulty cell caused the fire in both cases then the future for Li-Ion is in doubt.

Regards Bill
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Old 29-01-2013, 21:13   #1665
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Quote:
Originally Posted by bill good View Post
http://www.nfpa.org/assets/files/pdf/research/rflithiumionbatterieshazard.pdf

Some more information to read thru.

Makes using smaller cylinder cells with smaller enery in parallel look better than larger cells with more energy. Also the steel cases sounds better than plastic. I hope we are keeping a eye on what could happen if a cell does catch fire. The Boeing investigation is still on going & all 50 B787's are not flying. That is only a 50 AH cell & if one faulty cell caused the fire in both cases then the future for Li-Ion is in doubt.

Regards Bill
That sounds like "Because the Hindenburg caught fire all balloons should be banned or at least in serious doubt.
The Boeing batteries were lithium cobalt oxide, a known thermal runaway battery http://www.gsyuasa-lp.com/aviation-lithium-batteries no one is suggesting to use these cells for house batteries. The nonsense the press is going on with at the moment is quite laughable for anyone who knows anything about these type of batteries, they a prattling on with nonsense and gullible people are taking it as genuine information, now it becomes scary. When people start believing the rubbish they hear on the 6 o'clock news we are in real trouble.

T1 Terry
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