Simplified lifepo4 charging and care

[Ampster]

I think earlier in this thread dendrite growth was discussed in the context of too much time spent at the top. Too much time even at low Amperage can abuse some cells. Apparently abuse doesn't just happen at the bottom. I don't have better answers than are already contained in this thread.

[robeyw]

I only mentioned LiCoO2 because I received many very seriously over discharged, none swelled many recovered to useful capacity, I have seen much regarding over discharging them (which means going below 3V) , and the issue is said to apply to various chemistries

I can only offer anecdotal evidence of three or four cases where I have inadvertently over discharged Lithium cells of various chemistries and the cells have swollen, lost capacity and been rendered useless.

I so far as I can see, lithium dendrite growth only occurs on lithium metal. As far as I know, to get lithium metal in a lithium ion battery, it has to be very badly abused. My question regarding LFP cells only applies to cells operated within the manufacturer's spec (above 2.5 volts) but below 3.15 volts and without high charging rate.

[Ampster]

What does the charge discharge curve look like for LiCoO2? This thread is about LiFePo cells and the curves you showed are not for LiCoO2 either.
if your cells have been over discharged they have already deteriorated and probably have their own charge/discharge curve unique to each cell. There is no simple way to answer your question. Do you have a logging charger discharger that you can use to test the capacity of each cell? That is the only way to know. You should be very careful doing this and have some way of monitoring temperature as you charge and discharge. Are you doing this in a safe place where an overheating cell won't burn down your house?
The shorted cells are a result of dendrite growth and I don't know of any way to reverse that.

[robeyw]

One question would be how low a SOC? If the manufacturer recommends not going below 2.5 V, I would assume 2.9 is fine rather than the 3.15V. Then how long does it take? The published ageing data I cited does not show the effect. I have many LiCoO2 cells that have lost most of their capacity some thru normal ageing and many that I received in very overly discharged condition due to the drain of their BMS. None are swelled though some are shorted presumably due to copper plating but that should not occur above the recommended minimum voltage.

[Ampster]

I was looking at this tread from the beginning and would like sum support or refutation regarding the claims of PNjunction 08-23-2014 and 08-26-2014 where he says Don't over-discharge. .............

So, where is the evidence that a low SOC is in any way harmful?

If you look at the slope of the discharge curve you can see that it gets steep very quickly. At low SOC dendrites can grow more quickly and they can destroy a Lithium battery quickly. I can only offer anecdotal evidence of three or four cases where I have inadvertently over discharged Lithium cells of various chemistries and the cells have swollen, lost capacity and been rendered useless. . I had read the research and the literature long before I saw @PNjunction 's posts and experienced what he described.

However, I would not take the word of an anonymous poster on a forum on the Internet, Try it on a few cells that you don't care about and see for yourself if you don't want to believe me or @PNjunction. The best empirical evidence is the one we experience first hand.

[robeyw]

I was looking at this tread from the beginning and would like sum support or refutation regarding the claims of PNjunction 08-23-2014 and 08-26-2014 where he says Don't over-discharge. Set your alarm for a pack voltage of 12.8v under load. Disconnect them if they reach 12.6 to 12.7v under load. (3.15 to 3.175v per cell). This will be approximately 80% DOD. . . What happens if you go below 80% DOD?
If you do find yourself going well below 80% DOD a few things happen.
If not taken too far, say down to 90-95%, you'll be cutting cycle life.
If you leave it in a heavily discharged state long enough, the battery starts to eat itself, and this may or may not be accompanied by gas formation swelling. So you need to get to a recharge asap.
HOWEVER, if you value your cells, in a heavily discharged state, you'll want to limit the charge current to no more than 0.01C until the cell terminal voltages reach 3.2v. THEN you may apply the normal charge current, which is usually limited to about 1C max on most large prismatics.

I would take 100% discharged to be where the open circuit voltage is 2.5 volts. Looking at large prismatic cells from Shenzhen QH Technology, they have a cycle life chart, charging and discharging at 1C showing 3000 cycles to 20% loss of capacity when cycling 100% of capacity and 4500 cycles when cycling to 80% of capacity so the total charge cycled over the life is the same. I received some of these shipped surface mail all at 2.9 volts and they held that voltage for a month as I was working out how to use them. They have visible foil over pressure protection and there is no evidence that any are puffed up. I recently found a chart from powertechsystems for what they call a new and improved cell with the same end points as the QH cells but showing much better cycle life at lower discharge rates. With such a long cycle life, calendar aging surely dominates. The chart is attached for reference. A paper Calendar Aging of Lithium-Ion Batteries, Journal of The Electrochemical Society, 163 (9) A1872-A1880 (2016) looked at aging as a function of SOC for 3 cell types including LFP and in every case the slowest ageing was when held fully discharged. Their chart is also attached for reference.

So, where is the evidence that a low SOC is in any way harmful?
Cycle-Life.gifCalandarAgeing.gif

[Sunking]

If you don't want to float your LFP battery all you have to do is set the float voltage low as smily3 has said.

I haven't seen any hard evidence that says that floating LFP batteries is bad for them. This information from A123 http://liionbms.com/pdf/a123/charging.pdf seems to indicate that floating LFP batteries is not bad for them. Google "Is floating LFP batteries bad for them?" for more information. I would be interested to see any evidence that backs up the statement that floating LFP batteries is bad.

Simon

WTF have I been trying to pound into your thick skull for a year?

There is absolutely no difference between Floating a Lithium at something less than 100%, and two cells in parallel in any EV or laptop.

[karrak]

The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4, but there are very few charge controllers designed to not float a LifePO4 battery.

If you don't want to float your LFP battery all you have to do is set the float voltage low as smily3 has said.

I haven't seen any hard evidence that says that floating LFP batteries is bad for them. This information from A123 http://liionbms.com/pdf/a123/charging.pdf seems to indicate that floating LFP batteries is not bad for them. Google "Is floating LFP batteries bad for them?" for more information. I would be interested to see any evidence that backs up the statement that floating LFP batteries is bad.

Simon

[PNjunction]

Hmm, would it work to have the float voltage be something really low, like 11.5? Basically, get charged up, then the voltage drops off to basically nothing?

Yes, that works if one is using a charge controller designed for lead acid. Two rules: NO temperature-compensation, so disable that. And, IF your controller drops back to 13.6v or less in float, then you won't run into issues.

I know what Sunking is saying, but I'll still reference the term "absorb", as being 13.8v or higher, and when either a timer or current monitor trips, it falls back to 13.6v float - since we're shoehorning a Pb controller with an LFP battery.

Ideally, no float at all, BUT if you have a sudden parasitic drain / minor ground short happen, that 13.6v float, might save the day - or at least prevent the bank from hitting the low-voltage disconnect.

One just chooses which direction they want to take according to their needs.

[Sunking]

The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4,

Who told you that silliness?

A lithium battery charger is a Float Charger silly. Most Charge Controllers that will allow you to set voltages via keyboard down to 00.0x volts will work just fine. The Genasun controller is a float charger you set to 14 to 14.2 volts for LFP.

The problem is you do not grasp how batteries charge. FLA and LFP use the exact same charge algorithm. Only the voltages change slightly to protect the innocent. A FLOAT CHARGER is a CC/CV charger. They mean the same thing. Every charge controller made is a CC/CV charger. Only thing you have to change is the voltage FLA = 14.4 to 15 volts, LFP 13.6 to 14.2 volts. All you need is a controller you can set the voltage down to 13.6 volts up to 14.2 volts in at least .1 volt increments, .01 even better.

The only real difference is if you want to go to 100% with Lithium, you will need a way to terminate or turn off the solar controller when current tapers to 3/33 at 3.6 vpc. Genasun just floats below 100% at 14 to 14.2 volts. 14.6 is for 100% saturated to C/33.

[smily03]

Hmm, would it work to have the float voltage be something really low, like 11.5? Basically, get charged up, then the voltage drops off to basically nothing?

[createthis]

The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4, but there are very few charge controllers designed to not float a LifePO4 battery. The dinky little Genasun controllers and the Victron 100/30 100/50 series are the only ones I can think of, and I'm really just guessing because they have LifePO4 charging profiles (I have no idea how those profiles work under the hood). If you use the wrong charge controller, you need to wire a relay between the charger and the battery and shut it down when a certain voltage is reached, or you need to monitor it while charging, which is a bad idea IMO because humans make mistakes.

I'm not even sure how that would work, because charging to 13.8v you'll hit 13.8v, but the resting voltage will drop you back down to 13.6v or lower, so it's like you need a high voltage cutoff combined with a state machine that only turns the system back on once it has dropped below a certain voltage. If you do that, you lose the ability to power things directly from the solar panels too.

[karrak]

Now for the Dan's and Karrak's out there, you need all the help you can get in the form of automation and need a Nanny Device to control your life. Karrak needs it for his commission, and Dan needs it because no one can think for themselves.

There might be an imaginary universe made up by Sunking or maybe a parallel universe where NASA uses LFP batteries in its spacecraft, where LFP batteries don't go out of balance, where LFP batteries only last two and a half to five years, where charging an LFP battery to 100% will give you half the lifespan compared to charging it to 90%, where there is a 10% difference between charging and floating an LFP cell at 3.4 V/cell and 3.45 V/cell and where I make a commission out of selling cheap Chinese electronic goods. In reality, none of this is true.

As for automation, I admit I am lazy. I would rather type in the Internet address of my or my friend's battery monitor on my phone or computer to check up on our batteries rather than grovel around on the floor in the shed with a multimeter or in the case of my friend's system, drive 150km and then have to measure sixteen voltages after having to take the plastic shield off his battery. I also like science, so I am getting logged data that will inform me how well LFP batteries work rather than just relying on hearsay.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

Well said and is what I have said many times in different ways. It comes from the Pb mentality one must fully charge a battery which is nonsense with respect to Lithium Ion batteries. It f you are building a commercial product you have absolutely no choice as a manufacture other than to use automated charging from a product liability POV and Ignorant Joe Public. Not to mention all the up-sale that comes with it.

As I have said on many occasions before, charging an LFP battery to around 99+% has not been shown to significantly reduce its lifespan. If you have an off grid system that is relying on solar power to charge it you want to have your battery as full as possible at all times there is sunshine to tide you over the cloudy days. Not much different to making sure that your car is full of petrol if you are driving somewhere and don't know how far it is to the next petrol station.

FWIW I think you can expand that to include 24 volt or 8S, and to some extent 48 volt 16S. At 12 or 24 volts of a conscious person with heartbeat and knowledge of a lithium battery can easily determine the battery health with just a glance of the pack voltage. You only have a usable range 12 to 14 volts on a 4S system and will darn well notice a 3 volt error. If you see something below 12 volts will get your attention real dang fast. A simple LVD built into an inverter, and strategic charge regimen is all one really needs to protect his/her battery. All it takes is some knowledge and understanding.

The three volts you talk about is to avoid a disaster when you are discharging an LFP battery. When charging the margin is only one volt. 4.5 volts on any cell will severely damage it. Leaving an LFP cell at or greater that 3.65V or less than 2.5V for any length of time will damage it. This gives you a margin of error of only 0.2 volts when charging and only 0.5 volts when discharging.

Simon

[PNjunction]

Erm - cool. I think the major problem is that there is so much info out there, a newcomer doesn't know who to believe, nor have the lifetime or stomach to do stuff like I do like read 3000-message long threads still active 5 years later.

I think it would have to be an example project to be the most instructive. For example, how to use say an iCharger like my 306B to do your build / balance / test. And how when using a 1S connection like I do for both individual initial cell charging, and later pack charging, how FAST is actually *slower* than Charge, if you do the math right.

Now THERE is a gadget which would draw some good attraction (likewise a PL8 or similar) for us in that demographic. Just be aware that the canned presets are not always the best!

I just dont have the words how useful these higher-quality hobby chargers are to get your battery in condition to prepare for an eventual solar charge.

I just wish something like this was a sticky or something - maybe complete with a screen-grab(s). Once it is visualized, then the light-bulb goes on.

I just get too tongue-tied to do a proper writeup about it. I was asked once by an organization to do it, but declined because I KNOW I'm not a good technical writer.