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Old 10-04-2013, 10:14   #2611
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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
Why if I can still get 400Ah's out of a 400Ah battery bank would I want to charge it any higher than 13.7V other than to do an occasional "top balance".....?

The only good reason is faster charging. Higher charging voltage allows higher charging current, that's all.

You don't even need to reach 13.4V if your charging current is low enough---and you have all day to recharge.
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Old 10-04-2013, 10:33   #2612
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I may be wrong on this (wouldn't be the first time) but I think two different standards are used when rating a-hr capacity for batteries. The one we have grown accustomed to is the 20 hour rate (0.05 C) that the LA batteries use. I haven't cycled my cells near as much as Maine Sail, but this is my theory for how the LiFePO4 cells are assigned their a-hr rating. Instead of a 0.05 C discharge, they use a 1.0 C discharge, and instead of discharge to 100%, they discharge to 80% for the rating. These two standards create such a spread that any job that can be done with x a-hr for LA, could be done with 1/2x a-hr for LiFePO4. So if 800 a-hr of LA worked for you, then 400 a-hr of LiFePO4 should do the same.
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Old 10-04-2013, 11:11   #2613
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by deckofficer View Post
So if 800 a-hr of LA worked for you, then 400 a-hr of LiFePO4 should do the same.
Suppose you chose 800Ah as the size of your LA bank, because you knew you may need to periodically draw 400Ah between charges. A 400Ah LFP could handle this---but just barely. What if you needed to draw 401Ah? 800Ah of LA could easily do it. A 400Ah LFP could not.

Another view. Suppose you experienced a total charger failure and your vessel required 100Ah per day just to maintain communication and navigation equipment. A fully charged 800Ah LA bank could provide this for eight days. A fully charged 400Ah LFP, only four.
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Old 10-04-2013, 11:55   #2614
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Doug,

Then let us rate a LiFePO4 as they do a LA. That 400 a-hr at a 20 hour rate would be at least 500 a-hr. Usable a-hr would be 800 X .5 = 400 for the LA, 500 X .8 = 400 for LiFePO4.

Remember, Maine Sail pulled 1 C on his bank and found 25% more than the rating.

Can we agree on that?
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Old 10-04-2013, 12:13   #2615
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Bob, I agree with your comparison for the most part. After all, recall last December, when Balqon was offering 160Ah LFP cells for just $99 each. $99! I seriously considered replacing my ailing 300Ah AGMs with 160Ah LFP for under $400. But the 160Ah's sold out before I could pull the trigger, and I cringed at having to pay $190 for the 260Ah's.

In retrospect, though, the 260Ah's were a much wiser choice, because solar is my only charging source, and in nearly eleven years of fulltime RVing, I've experienced a handful of four or more completely overcast days. Even at a meager 50Ah/day, the 160Ah's would have been disastrous.

When properly sizing a battery bank, one should consider extreme---yet conceivable---operating conditions.
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Old 10-04-2013, 13:03   #2616
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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 only good reason is faster charging. Higher charging voltage allows higher charging current, that's all.

You don't even need to reach 13.4V if your charging current is low enough---and you have all day to recharge.
At 13.6V to 13.8V the bank vastly exceeds what my on-board charge source can supply so no faster charging unless I have the capacity to do so. I doubt many boats out there have the charge capacity, even at 13.6V, to "slow" charging on a Li bank..?
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Old 10-04-2013, 13:11   #2617
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Doug,

Then let us rate a LiFePO4 as they do a LA. That 400 a-hr at a 20 hour rate would be at least 500 a-hr. Usable a-hr would be 800 X .5 = 400 for the LA, 500 X .8 = 400 for LiFePO4.

Remember, Maine Sail pulled 1 C on his bank and found 25% more than the rating.

Can we agree on that?
Actually I pulled .25C/100A and found 25 more Ah's on a 400Ah bank. With a virtually non existent Peukert (I still question this) you get 400Ah, if the bank is 400Ah, whether you draw 1A or 400A.. LA batteries are greatly affected by Peukert, not so much for LiFePO4..

With a 400 Ah LA battery with a Peukert of say 1.30:


If you draw it at 0.01 C / 4A you get 648Ah's
If you draw it at 0.05 C / 20A you get 400Ah's
If you draw it at 1.0 C / 400A you get 163Ah's
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Old 10-04-2013, 18:07   #2618
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I came across a recent Masters Thesis from Sweden on LFP Cells. Specifically focused on formulas to use to determine LFP State of Health (SOH). We will need our engineering friends for much of the thesis, but part of the scope of his thesis was a study of cell cycle life under different conditions. His focus was for hybrid and pure electric vehicles, and I sure wish I could have selected the parameters for the test, but we will have to work with what we have.

I came away with 2 observations:

1) Temperature is not a huge factor in cycle life 23 degrees C versus 35 degrees C showed if anything warmer was better. That's really good!

2) Shallow cycles increase not only the number of cycles, but the lifetime usable capacity. I eyeballed it at about 40%, but YMMV.

The cycles were done at very different discharge rates, all hugely higher than our application. There has to be a typo in the description of Cycle D.

Here is the link to the 150 page thesis:

http://komar.bitcheese.net/files/JensGroot.pdf
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Old 10-04-2013, 22:12   #2619
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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
Actually I pulled .25C/100A and found 25 more Ah's on a 400Ah bank. With a virtually non existent Peukert (I still question this)
Your test provided one data point of T1=4.25h and I1=100A. The cell is spec'd by Winston to pull 400Ah at 0.5C, which means another data point is T2=2h and I2=200A. The Peukert exponent is therefore n = (logT1 - logT2) / (logI2 - logI1) = 1.09.

Obviously this is just an approximation, as the spec is unlikely to represent a random cell. For a more accurate value, you'd need to conduct two discharge tests under identical starting and ending conditions.
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Old 11-04-2013, 06:06   #2620
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by ebaugh View Post
I came across a recent Masters Thesis from Sweden on LFP Cells. Specifically focused on formulas to use to determine LFP State of Health (SOH). We will need our engineering friends for much of the thesis, but part of the scope of his thesis was a study of cell cycle life under different conditions. His focus was for hybrid and pure electric vehicles, and I sure wish I could have selected the parameters for the test, but we will have to work with what we have.

I came away with 2 observations:

1) Temperature is not a huge factor in cycle life 23 degrees C versus 35 degrees C showed if anything warmer was better. That's really good!

2) Shallow cycles increase not only the number of cycles, but the lifetime usable capacity. I eyeballed it at about 40%, but YMMV.

The cycles were done at very different discharge rates, all hugely higher than our application. There has to be a typo in the description of Cycle D.

Here is the link to the 150 page thesis:

http://komar.bitcheese.net/files/JensGroot.pdf


Am I reading this right ?
For instance cycle A is between 50.0-22.6%SOC=27.4%C/mini cycle
so is it 100/27.4 cycles/1C cycle ? 3.6 ?
If so, then 8000 1C cycles*3.6mini cycles = 28800mini cycles ?

This would contradict what GOBOATINGNOW has been saying, but would be very good news for my application.

Perhaps I am not understanding, I did not read the whole thesis.
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Old 11-04-2013, 10:35   #2621
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Quote:
Originally Posted by NahanniV View Post

Am I reading this right ?
For instance cycle A is between 50.0-22.6%SOC=27.4%C/mini cycle
so is it 100/27.4 cycles/1C cycle ? 3.6 ?
If so, then 8000 1C cycles*3.6mini cycles = 28800mini cycles ?

This would contradict what GOBOATINGNOW has been saying, but would be very good news for my application.

Perhaps I am not understanding, I did not read the whole thesis.
I had to go back and reread the details again, but I think you are onto something. Which would drastically change my original 40% improvement estimate for shallow cycles in the original post. It would change to at least a 400% improvement. He says:

"According to this definition, one capacity throughput corresponds to one full discharge and one full charge cycle."

So the left of the first chart is "remaining cell capacity" and the bottom could be read as "total 1C rate capacity consumed".

I attached a chart showing a 4000 second slice of Cycle A (top) and Cycle B (bottom). I could not figure out how he calculated "total capacity consumed", it could be a running total (probably) or the difference between max and min of each 4000 second cycle. So I don't know how to translate to "mini cycles". But either way, the cell charged and discharged far more power in shorter cycles.

Im assuming the max and min values shown in the table included with the first chart were for the life of the test, and this profile comparing A to B is "typical" since the max mins are close but not the same.
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Old 11-04-2013, 18:29   #2622
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There is certainly now more evidence to suggest that Li batteries should not be exposed to high depths of discharge and that in fact regular recharging should be carried out to ensure that the battery should not be exposed to deep discharges. Note that this is purely in relation to extending battery life.

This is not the same as the issues I mentioned such as load sharing when an impressed voltage is maintained or why Li shouldn't be floated. But I accept that recent work on Li seems to be disproving the mini cycles issue ( ie that mini cycles count as full cycles )

Its worth noting that most of this seems to aimed at Li Ferrous cells which already have a vastly superior cycle life to other Li technologies.

Very interesting paper

Dave
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Old 11-04-2013, 19:09   #2623
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There is certainly now more evidence to suggest that Li batteries should not be exposed to high depths of discharge and that in fact regular recharging should be carried out to ensure that the battery should not be exposed to deep discharges. Note that this is purely in relation to extending battery life.

This is not the same as the issues I mentioned such as load sharing when an impressed voltage is maintained or why Li shouldn't be floated. But I accept that recent work on Li seems to be disproving the mini cycles issue ( ie that mini cycles count as full cycles )

Its worth noting that most of this seems to aimed at Li Ferrous cells which already have a vastly superior cycle life to other Li technologies.

Very interesting paper

Dave
Yes this is strictly LFP not other Li chems. But what we don't know...was it charging to 100%, or discharging to around 10% that had the biggest impact on cycle life? 25 to 50% was good, but was it the top or bottom or something else that caused the reduction in the full cycle testing? Need more testing....
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Old 25-04-2013, 21:52   #2624
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Well, I is back from my tripping around. i feel my whole vacation was save by having a lithium ferrous house battery. 20 mins into the trip the alternator failed for the first time, actually it chewed the belt to bits. The constant high load of LI battery charging is not only hard on alternators, it's very hard on the drive belts as well. For the first 4 days of my trip the battery powered everything and was finally dragged down to fully discharged, then never got over 20% SOC for the first 2 weeks. We were woken in the early hrs some mornings by the cell logger alarm signalling a cell had dropped to 2.85v, turn everything off and wait for the sun to come up. Finally had a chance to fully charge the 360ah battery, well, at least till a cell reach 3.6v and sounded the alarm again. I hadn't had a chance to balance the battery before we left, the 2 replacement cells for the ones I cooked arrived a day or so before we left, simply condition charged and slapped in. 2 of the cell that were in the 4 cell over charged group seemed to have survived so I slapped them into service as well. 28 days with everything running off the house batteries, starter, vehicle electrics and house power, including the electric blanket at night, no point in suffering if you don't have to eh :lol: The trick became to find a spot to plug in at the end of every 3 days, recharge overnight, back on the road to free camps with no power. We could have used a generator instead of plugging in, but we didn't have one.

A few things we found,
it's better to limit the alternator max current to around 60% of it's capacity, easier on the drive belts and on the alternator itself, slipping drive belts add a lot of heat into the alternator rotor and they don't live long when you do that. Methods of limiting that output are varied, mine was to use an inverter and mains charger, unfortunately I had already stuffed the alternator by that stage.
Don't try to reduce current by reducing cable conductor area, the copper cooks, goes brittle and snaps inside the insulator.
Very few charger are designed to output 100% of their capability for long periods without additional cooling like a second fan
Limiting total battery voltage as a form of charging control only works if the cells remain balanced, monitoring cell voltages is the path to long battery life, a cell can still go over 3.6v yet the terminal voltage remain below 13.8v.
Charging with a constant voltage of 13.8v is very slow to get the last 10% or more in, fine if you have plenty of time, a higher charge voltage is required if time is limited but you need to get near fully charged before you stop the charge cycle.
There is still an absorption stage if the charging current is high compared to the battery capacity, above around 0.1CA appears to be the limit for skipping an absorption stage, but absorption should be less voltage or charge rate to reduce the speed of the high cell going over voltage, even as low as 0.01CA will still eventually have the high cell going over voltage, it must be controlled by monitoring cell voltage, not over all terminal voltage.
It appears these cells die from heat, the 2 that died were brand new but in the centre of the 4 cell pack in parallel, the 2 outer cells are still performing fine, I put that down to them being on the outside and able to dissipate their heat better

As far as the float charging debate, my 720Ah house bank has been float charged by the solar regulator every time they reach fully charged, often by mid day, for the last 700 cycles, they are still performing as well as the day they were installed, there would have been many thousands of mini cycle during that period of time but no sign of any ill effects.

We could not have achieved what we did using lead acid batteries, we regularly used 100% of the Li batteries capacity, not just down to 0% SOC, but down to 2.85v per cell, genuinely full discharged. They bounced back every time. Yet to see what the long term damage was, but in an emergency situation, life expectancy of the batteries would be the last thing on my mind, I'm guessing it would be the same at sea. What you want is the 12v required to power the equipment, a battery that drops to 10v or less for the last part of it's capacity delivery is as good as useless, so capacity can really only be measure down to a useful voltage under load. My batteries were still delivering 20 amps @ 12v or more when the 2.85v alarm went off for a low cell, I chose to turn the load off, the cells were still giving their all, in a true emergency no doubt you would let the battery suffer if it meant keeping yourself alive.

T1 Terry
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Old 25-04-2013, 22:37   #2625
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

A lot of good info Terry, some struck home. On a MH trip to Baja I had just increased my LA battery bank (this was 25 years ago, before LiFePO4) from 200 a-hr to 800 a-hr. Had run the bank pretty low during a week on the beach and figured it would get charged during a 300 mile drive. The charge lead coming from the alternator through the dash and back to the bank wasn't heavy enough and the insulation caught fire with smoke pouring out of the dash.
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