Simplified lifepo4 charging and care

[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

[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.

[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?

[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.

[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.

[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

[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.

[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

[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]

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]

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]

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]

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.

[maple flats]

After using my old Lead Acid Trojan battery bank for just under 11 years, they needed replacing. I reviewed my options and went with 16 LiFePo4 cells, 100AH to form a 48V (nominal) bank. To review my set up, I am grid tied with battery back up just for 1 critical circuit. (I am a maple syrup producer and 1 piece of equipment, which is very expensive must never freeze). Grid tied and net metered.
I am now just finishing my build and connection for this "back up system". The main heat for the well insulated room to heat that equipment is a propane furnace, the back up is for electric heat if the propane fails and the grid goes down. It's pretty much a second back up.
I should be ready to activate the new bank soon.
Being a 48V bank I have the charge set to stop at 53V, and on the low end it will be at 50V. This short variation should give it 3000-5000 cycles, I won't live that long (I'm now 73). If the power goes out and the furnace quits I will have enough power to run 1 night, I then have a diesel generator that can recharge the bank. On the bank, I have a 24V heat pad to keep the bank above 37F and it is all in an insulated enclosure, that heat pad is powered by the bank thru a DC-DC converter 48V to 24V. I have that because this type of battery can not be charged if under freezing temps.
My inverter is an XW6048, now set to sell power by net metering when the bank is fully charged. I will again contact XW tech support to get the setting s needed to sell with LIFePo4 batteries. It will not be the same as it is now.

[Ampster]

53 Volts for 16 LFP cells is indeed conservative. By my calculation that is 3.31 which is below the resting voltage of my cells when I charged them to 3.45. No worries though, you have a well thought out system. Are you using a BMS? What brand of LFPs are you using?