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Old 04-03-2016, 22:28   #5026
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

So far I have not had any individual cell drift, BUT I would still stick with the individual cell monitoring because that can give you a heads up on a cell problem. Besides that isn't to costly or difficult to set up anyway.
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Old 05-03-2016, 10:02   #5027
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Without a BMS, how do you monitor the individual cell voltage? Four little displays?

I have four of these on my boat's 12 volt system and eight on my home's 24 volt:


mini led votmeter | eBay

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Old 05-03-2016, 12:19   #5028
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Perfect, thanks to you and Rich. And everyone else!


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Old 05-03-2016, 13:23   #5029
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Why? Well I will say this with love and admiration to my fellow boaters and cruisers, but there are some really ignorant people out there about boat electrical wiring and systems. I quickly saw the HUGE liability I would have in selling and trying to help design everyone's system for them. Every boat is different and trust me...no one wants to pay for designing anything for them.
This is why I stopped directly consulting on LFP builds. Too many folks agreeing to the design, then not following it, then having issues and wanting to lay blame......
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Old 05-03-2016, 13:31   #5030
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

The cellog8 really does a great job. It reads out voltage to .001 and displays highest and lowest cell voltage, and the spread between. With alarms that can be set and all the info it shows for about 20 bucks you can't go wrong. Right now it says I have a 9 milivolt spread between high cell 2 and low cell 4. Much more accurate then four led meters and draws less power.
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Old 05-03-2016, 13:45   #5031
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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The cellog8 really does a great job. It reads out voltage to .001 and displays highest and lowest cell voltage, and the spread between. With alarms that can be set and all the info it shows for about 20 bucks you can't go wrong. Right now it says I have a 9 milivolt spread between high cell 2 and low cell 4. Much more accurate then four led meters and draws less power.
Bingo...we have the same cellog8 and would buy it again and recommend it.
https://www.progressiverc.com/media/CellLog%208M.pdf
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Old 08-03-2016, 14:33   #5032
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I purchased some used Voltronix cells online (really good deal for my first experiment). However, they did not come with interconnect copper straps (busbars?).

I'm having a hard time finding anywhere online to purchase them from. Most sites say that they only let you order the copper straps if you are purchasing cells from them.

Any ideas where to get these? Or do folks just make their own from copper flat stock? (Or flattened copper pipe?)

I suppose I could make short interconnect cables from 4-6 gauge welding cable, but the flat straps sure look nicer.

Thanks.
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Old 08-03-2016, 14:45   #5033
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

This is where I got mine, they have a few different sizes and the are flexible.

EVTV Motor Verks Store: Braided Battery Strap Kit 70MM Centers, Battery Connections, 70mmstrapkit

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Old 09-03-2016, 14:21   #5034
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thank you. Those are exactly what I was looking for.
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Old 09-03-2016, 14:32   #5035
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by SV THIRD DAY View Post
Bingo...we have the same cellog8 and would buy it again and recommend it.
https://www.progressiverc.com/media/CellLog%208M.pdf
Didn't we determine a few thousand posts back that the Cellog8 powers itself from a single cell, and therefore contributes to imbalance?
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Old 09-03-2016, 14:38   #5036
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Didn't we determine a few thousand posts back that the Cellog8 powers itself from a single cell, and therefore contributes to imbalance?
Only if you are using more than 6 inputs.

Do you have a 24V system ?

Even then the difference in current is so small relative to the size of a common house bank cell that it is negligible.
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Old 09-03-2016, 16:16   #5037
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Only if you are using more than 6 inputs.

Do you have a 24V system ?

Even then the difference in current is so small relative to the size of a common house bank cell that it is negligible.
Ah, thanks. Forgot that detail. Yes, I'm 24V. Negligible... perhaps, but I'm still looking for a better solution.
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Old 10-03-2016, 02:36   #5038
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Ah, thanks. Forgot that detail. Yes, I'm 24V. Negligible... perhaps, but I'm still looking for a better solution.
I recall there was an easy hardware mod to the CellLog solving that issue, listed or pointed to in this thread.

Edit: look here: http://gwl-power.tumblr.com/post/199...t-for-celllog8
All it takes is a wire jumper and an optional replacement of one resistor.
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Old 11-03-2016, 22:22   #5039
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Haha I started to note the cellog8 in my notes and realized I already had it from months ago. Sigh. I need to go sailing.


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Old 15-03-2016, 15:23   #5040
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Re: Interesting Observations

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I was recently shipped a $9,000.00 24V LiFePO4 marine battery that had been severely over discharged. The eight 160Ah prismatic cells inside ranged from 0.760V to 0.813V when I received the battery.

Due to these voltages, and an overall pack voltage of just 6.117V, for a 24V nominal pack, I did not want to attempt to charge this as a bank.

I disemboweled the battery & removed the cells and began charging them independently at 2A to 3.0V. Once at 3.0V they were allowed to sit for 24-30+ hours and resting voltage monitored. If the voltages remained stable I continued to charge them to 100%.

Not only had these eight cells been over-discharged (though never hitting 0V) they had also been over-charged.

This bank had been charged at 29.2V or the equivalent of 14.6V for a 12V pack or 3.65VPC. The owner thinks the system held 29.2V for about 4 hours. The case design did a poor job of containing the lower third of the cells and as a result the end cells were bulged a bit down low, not bad but definitely bulged from over charging.

I then decided to begin capacity testing one of the cells. I charged cell #2 to 3.55V and allowed current to taper to 5A. I then applied a 25A constant load at 75F and discharged the cell to 2.5V and counted the Ah's & time.

The first capacity test came in at about 110Ah but I did not record because I was just considering scrapping the cells. Remembering that I had seen recovery of capacity in LFP before, after multiple 100% discharge/recharge events, I decided to push on with Cell #2.

Discharge - 25A @ 75F to 2.5V - Recharge - 40A to 3.55V & 5A

Capacity Test #1 = 110.? Ah (did not write it down forgot tenths)
Capacity Test #2 = 119.8Ah
Capacity Test #3 = 131.4Ah
Capacity Test #4 = 139.6Ah
Capacity Test #5 = 145.2Ah
Capacity Test #6 = 145.9Ah


Once capacity stopped climbing I ended the capacity testing.

It should be noted that even after a complete 100% discharge to 2.5V (note I do not recommend this) the resting voltage of this cell bounced back to 3.019V. This is a clear reason why voltage is a horrible indicator of SOC with LFP.

A fair number of folks, here and elsewhere, have noted diminished capacity under fractional "C" use and I suspect much of this capacity is still inside the cells waiting to be reawakened.

My own cells on my own bank have barely budged in capacity in nearly 800 cycles and are 2009 cells. I do however cycle them deeply before a capacity test. I may repeat this same test strategy on my own bank on the next capacity test.

In this very abused cell I was able to take it from 68.75% of its 160Ah rating all the way back to 91.2% of its as new factory rating. I suspect when new these prismatic cells delivered 110% +/- (many prismatic cells do) but I can't say for sure, so am using 160Ah as a baseline. These cells have been in hard use since 2010 and have March 23, 2010 or an almost 6 year old date code.

I am astounded at how well these abused cells responded to the six 100% discharges followed by compete 100% recharges.

Moral?

If your cells are not meeting your assumed or desired capacity cycle the heck out of them a few times and repeat the test.

I found this quite interesting hopefully others can glean something from it too...
Those are interesting observations, but I wouldn't mix the topic of abused cells with reduced capacity due to fractional C use. Predictably, we already see some who, in the wake of this, start concluding that being sloppy/reckless is in fact ok.

When cells get discharged below 2.0V resting voltage, like the ones you got hold of, the copper substrate of the anode starts dissolving into the electrolyte. It happens gradually over time. When you recharge, no matter how "careful" you are, that copper doesn't return where it originated from. Instead it plates various parts of the cell inside. Specifically, it can form crystals and later cause the cell to short out.

Capacity, on the other hand, has little to do with the copper. It is a matter of lithium moving between the active materials, carbon and iron phosphate. So even if those cells exhibit "some capacity", it doesn't mean they are not copper plated and electrochemically damaged with all the consequences this can have.

Those cells are scrap, because all bets are off now. Working or not, they are a liability and I really can't see the sense in trying to gauge "how much" of a liability in order to keep using them. It is ridiculous considering what happens when things do go wrong.

Overcharging, if severe enough, causes electrolyte breakdown and gas pressure build-up. This causes the casings to swell, but evenly. Bulging low down might have a lot more to do with lack of clamping, vibrations, thermal cycling etc, especially after 6 years in service. The materials inside the cells have sagged.


When it comes to cell voltage vs SOC, 2.5V or 3.0V is a fully discharged cell, there is virtually no capacity to speak of between the two. OCV is the only true indication of the state of charge of a cell and it happens to be easy to read near the top and near the bottom.
Since I personally couldn't care less if my bank is at 45% or 65% (where voltage is too problematic to correlate to SOC), the voltmeter does a great job at telling me if SOC is up or down and I found that it is all I ever need.
You must look at a true OCV curve, not the manufacturer's charge or discharge curve, this one was not recorded in open circuit conditions and it includes the effect of internal resistance.

OCV reads different whether the bank was last charging or discharging.



The curves above were recorded by a university in a lab with fully stabilised voltages, so they waited for ages before taking each reading etc.

At very small C-rates, like often found on marine banks, the current doesn't skew those values much. I find that I use the blue curve to assess charging in the upper range and the orange one to get an idea of how much I have left when the voltage starts dropping (but not while running alternators or heavy loads obviously, because of the effect of internal resistance again).

For example, when I see 13.1V under light discharge, I know I am about 60-65% down and it is highly reliable, much more than any Coulomb counter gizmo that needs constant resetting.
Similarly, when the solar panels are charging gently and the voltage has climbed to 13.4V, I know I am in the 80% SOC.
Last but not least, this is true regardless of capacity: it naturally tracks any capacity fade.
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