Query concerning LiFePO4 charging

Wise masters of the craft, I need guidance, as i am not certain about properly charging my three batteries.  From several sources I discern that best practice means keeping the LifePO batteries between 20 and 80 percent. This is touted as best for battery health and longevity. Is thisาฟ a hard and fast mandate?

But what about the days I charge one to 96 percent?   For example, the charge controller  on this one doesn't stop at 80, so I must manually stop the charging from its solar panel.  This specific system is all in one, so battery, inverter and charge controller are integrated into one box.

Should I let the system operate as designed, or follow the 20- 80 rule?  All comments and opinions welcome

Interesting. I knew that going too low was bad for the battery, but didn't know that charging fully was also likely to shorten the life. I know that getting the last bit of charge in a battery can take a lot longer than the main bit of charge.

Yes, interesting .....but baffling.  Or perhaps I am permanently baffled by life itself.
Are rules for LiFePO so different from all other lithium batteries,  will cycling between 100 and 20 percent , or lower percentage, significantly alter their performance and life span?

Could someone with direct experience with LiFePO explain the true path to battery heaven?

I'm not speaking from experience here, just from the learning via a few videos as I consider jumping to solar for some home use.  The following video you may already have seen, but if not it directly addresses your question.  I think the argumentation might have been done a bit better, but it seems to drive the point home on why charging to 100% for home solar is fine for most users.  

https://www.youtube.com/watch?v=1G8WxY_jcUM

John, thanks, that fellow Will Prowse is one voice among many, but I agree , he is authentic in my book. Watched four or five of his lectures.  

When solar charge or 12v charge from a moving vehicle is in play, I feel confident.  But if I need to choose a 1 or 2 amp trickle charger, for an extra  battery on deck, could I use any that specifies FOR LITHIUM, yet no detail is offered?   The nuance is what I am trying to understand.

For example, I won't use my old charger that is made for automotive batteries.  But would this 1 amp slow charger suffice?  
Screencast to follow

Hi Rico,

The 20% is a hard floor.  More accurately, the bottom 30% or so is an absolutely strict no-go zone as once the cells discharge below that 30%, they won’t charge back up again.  But that’s why you have a BMS.  The BMS gets calibrated so that it shuts the battery off before delving into the no-go zone.

So in a sense, you can discharge to zero, as indicated by the BMS, but technically there is about an extra 30 percent more you can get—you will just never get it back.  But as a sort of best practice, it is recommended that you not discharge below 20% as per BMS, but that is not a hard and fast rule.

As for the 100%, if you want it, go for it!!  But that last 20% or so is the hardest to charge, lowering the charging efficiency.  On a solar system, this might matter if your batteries are a bit oversized for the panels or the sky has been less than perfect.  

Helpful??



Eric

I would suggest that manually disconnecting your controller to stop charging is in general a bad idea.  Either you will forget to disconnect it in time and overcharge, or you will forget to reconnect it and let it drain to minimum.

I think the best course of action is to replace whatever controller you have and purchase a controller specifically programmed to charge Li.  Then just leave it on auto.  You didn't mention anything about what kind/wattage solar panels you have?  Getting a good quality MPPT controller, and wiring your panels in series to raise the incoming voltage is usually the best strategy.

If you can document the Vmp/Voc and Imp/Isc of your panels, and your controller specifications, we can recommend the best wiring combination.  Knowing your extreme winter lows is important, because the Voc of you panels goes up as the winter temperatures go down.  Please get back to us with those numbers.

Eric Hanson wrote:....., you can discharge to zero, as indicated by the BMS, but technically there is about an extra 30 percent more you can get—you will just never get it back.  But as a sort of best practice, it is recommended that you not discharge below 20% as per BMS, but that is not a hard and fast rule.  

I think this is a very important point and therefore request confirmation that I am envisioning this properly.  There is the "real" 0% and then there is the "BMS registered" 0%, which for safety/protective reasons are not the same, correct?  The BMS 'determines' 0% to be when the battery is 'really' at 20% as a means to protect the battery (by shutting down) from over-discharge to the point of damage.  Am I reading this correctly?  My need for clarification is then on the statement "... it is recommended that you not discharge below 20% as per BMS":  Are you saying to not charge below what the BMS is indicating to be 20% or the "real" 20% which the BMS is already pre-set to call "0%"?  Does my question make sense?  

Finally, just a point which may be relevant irrespective of battery type:  Does a charger determine that the battery is at 100% by (a) zero amps of current flowing from charger to battery, (b) a certain voltage being reached and triggering shut-off/float stage, or (c) some other parameter that I'm not thinking of??  Maybe it's more complicated than this depending on battery chemistry and charger design?  

Again, so glad to have seasoned experts here to help with our real-time conundrums.  Just amazing to me the advancement in battery/solar technology since I first saw a panel in the 1970s and the idea was just getting its wings.

Much obliged for all contributions to the discussion,  folks.  I still read a variety of opinions on this, and I might be learning more a out chemistry specific data that drives a choice of charging details
I believe one issue is not amperage, etc.
Rather it depends on charger style, specifically the CC / CV choice, it seems to fit a LFP battery exactly (?)

Opinions please

Another post from a year back that is legit, I think

Yes, I understand my LFP is not a Li ion, but the charging profile should be same

Eric and Michael, I misspoke, or should have mentioned upfront that my project involves an all in one unit, which includes battery, BMS, and charge controller.  I can't swap out parts of that setup.
Since the LFP battery is small, only 245wH , i desire keeping another battery set to charge during daylight hours.  I bought the 12v external battery on sale , but want to ensure it will perform adequately
My unit specs, at the mo I use a 100w panel for charging.  

Now I seek to charge the 12v. Using DC will require q separate controller of course. So I want to use a quality charger, or smart charger hooked to my AC output.  

After more reading last night I think this charger will be a safe bet, saw it mentioned more than once with praise. Same brand I believe that T. Rubino just purchased also

Rico Loma wrote:

Wise masters of the craft, I need guidance, as i am not certain about properly charging my three batteries.  From several sources I discern that best practice means keeping the LifePO batteries between 20 and 80 percent. This is touted as best for battery health and longevity. Is thisาฟ a hard and fast mandate?

But what about the days I charge one to 96 percent?   For example, the charge controller  on this one doesn't stop at 80, so I must manually stop the charging from its solar panel.  This specific system is all in one, so battery, inverter and charge controller are integrated into one box.

Should I let the system operate as designed, or follow the 20- 80 rule?  All comments and opinions welcome

You would have to provide a make and model for your batteries. Every BMS is slightly different. Usually "100%" is just a value they assign to indicate full without damage to the battery. Some BMS just monitor others actively block power input/output. So the model matters. If its a unusual unsupported one then it gets harder.

David , I wanted to send this again, please refer to specs on my all in one unit. Internal battery is very small.   My only question at present is how best to chargethe battery shown here, it is 100Ah 12v battery,  LFP chemistry.  Would you advise using AC power for a smart charger designed for LFP battery?  If not, 0lease explain.

Rico Loma wrote:Yes, I understand my LFP is not a Li ion, but the charging profile should be same

Hi Rico,
I'll raise my hand here and note that it ain't necessarily so.

[Edit, links to images were bad]

The first set of curves below is of an nickel-manganese-cobalt / graphite battery (typical higher-energy lithium formulation)  from a paper here.

and the second set is from this webpage. (There's nothing special about the papers per se, they just had good charge curves.)

Note that the slopes of two families of curve are pretty different. The state of charge of a battery on a charger is estimated by its voltage and how much current is flowing into the cell at that voltage (the C rate). The charger needs to know (or be told) what the right curve is, unless it has logic internally to figure it out.

Notice also that the voltage levels are very different, but this is at the level of the cell. At the level of the pack, it's a little more forgiving, since a "12V" battery with LFP will have 4 series cells, but an NMC "12V" battery will have 3 series. They still have different desired top-end voltages, but it's not so dramatic as at the single cell level.

I see that you're looking at a Victron charger, and I think that is a good choice. Be sure to do the math on which one you want, Victron offers more flavors than Baskin' Robbins. LFP is a great battery choice, and almost all home power system products are going to be built with that as an option for the battery bank. Reputable brands with good documentation will keep you out of trouble (when properly applied).  

To your original question, I suggest you follow the 80/20 rule, and your battery manual should show the voltages recommended to do that.  You posted a photo of the LFP battery, is that indeed your unit? It probably came with a short brochure, or if for some reason it didn't, pretty much any 12V LFP 100Ah manual will tell you the same thing, like pg7 of this one.. The state of charge limits are built into the recommended min and max voltages.

Caveat: there are several different min voltages noted, and sizing that is a function of the discharge rate you want. For instance, if you set 12V as a "safe" minimum, you might nuisance trip when you run a chopsaw on a cloudy day (high C rate pulls voltage down, even though charge is OK). Many systems have inverter logic to adapt to this, like requiring a duration-below-limit before tripping. Anyway, that's not exactly to the point, but it worth keeping in mind. The charger side does not care much about this.

Good luck!
Mark

Rico Loma wrote:David , I wanted to send this again, please refer to specs on my all in one unit. Internal battery is very small.   My only question at present is how best to chargethe battery shown here, it is 100Ah 12v battery,  LFP chemistry.  Would you advise using AC power for a smart charger designed for LFP battery?  If not, 0lease explain.

you would need to pull up the specs on the secondary battery not the all in one unit. It will tell you the charge voltage and you program your victron for that voltage.

Again, I think this is a good choice if using AC power

Victron energy  Blue Smart IP67, 12v 25 amp battery charger
Here is one claim from website, looking for opinions on this idea please

I have that charger, and as you say, it is an AC-source charger. If that is what you need, it's a great tool. I use it as a bench maintenance charger.

If you want a solar-driven charger, that is not the unit for you. I believe Mr. Rubino has a solar charge controller from Victron (as you noted), which is the sun-to-battery device.

What are you trying to do with it?

Sorry I didn't see posts, THANKs for opinions.  This helps a lot. My one spell of using off grid solar was over 30 years ago, I am clearly a dinosaur.  Learning as I go but must make this work in a remote site off grid. I have two weeks or less to decide, purchase, and install.   Here are specs on my stand alone batteries

Mark, you have been an oracle, appreciate the help
Here are important specs on battery BMS from company website....GreenOEpower

Hi Rico,
Thanks, I think I am beginning to put the pieces together, but a quick sketch of your overall system and design intent would go a long way...

Those BMS notes are 100% internal to that battery. You will never interact with the BMS that is buried in it. It's telling you that it is doing right by the LFP cells. However, the internal BMS of a sealed battery like that is better used as a backstop to serious system design errors. It's not your charge controller, it's not your disconnect, it's not doing anything except getting the manufacturer out of liability (and keeping the strings of 4 series cells inside balanced, which is super important).

Your interaction with that battery will be through the top terminals, and the question is how do you plan to take charge in and out, and control that?

Best,
Mark

Mark you are right, I won't depend on internal bms of the battery.  I am now, unless persuaded to use my existing DC output instead , will use my Ecoflow unit's AC output.
This will power the smart charger for external LFP battery

I have thus far only considered AC power for using a smart Victron AC charger for the LFP battery.  I haven't read or heard any evidence to use a DC output from the Ecoflow,  rather than my current idea.  Yes, the inverter has to jump in to power the AC charger.  But my question remains, if not this method,  I am all ears, what is a better safer method? I have used the ecoflow AC and a standard low tech charger to fully charge a lead acid battery.  No problems there.  Please advise on my next step, if you can

I am not adequately describing my bare bones system, here is how simple this iteration will be.  
Again, this is only to charge 18v tools batteries and one phone.  Appreciation for the guidance
Coffee stains and all,,,,,,

Hi Rico,
Thanks! That is a perfect sketch to begin with. What are the 2 larger batteries doing? As drawn, all is well with your plan, and using an AC-powered charger to maintain the big batteries is fine in principal... but what for?

Also, looking at the back of your EcoFlow, I see 300W max AC output. That Victron 12V 25A charger would draw more than that (12V*25A = 300W, and you are never 100% efficient, and you are always charging over 12V...) Do make sure the charger you select will not overdraw your EcoFlow.

If you are hoping the EcoFlow can use the external battery in parallel with its internal battery, that seems to be a questionable hope. Maybe I am missing the point of the larger battery?

Best,
Mark

Thanks for your patience and attention to detail, I did post the wrong charger, I intended to use same company but 7A version.  I will not get close to the 300w limit while charging.  

And forgive me , looking back at my posts this is not a great explanation

"Since the LFP battery is small, only 245wH , i desire keeping another battery set to charge during daylight hours"
That is my 12v external LFP, with quadruple the power in storage.  I can charge it during peak daylight hours. The house gets 300 sunny days per year and rarely drops below 2 C in winter. When I am using tools and lights at 3 am, for example, I would potentially need to use this BACKUP battery feeding juice to my ecoflow unit via DC charge port.  I have the approved cables.  No, I never envisioned any other way to use the external LFP.

To reiterate,  this is for a restoration project on a1930 house, secluded and without power.  For my purposes this plan works better than having a temp pole installed by the local power company.  I need to go with a bare bones setup.  This plan seems the most economical and efficient,  in these circumstances.

I am aiming this post specifically to John who earlier asked about “true” 0% and “BMS” 0%.  But I want to add in that this is a fantastic thread, chocked full of great, detailed information.  All of this discussion is very helpful to someone jumping in on one of these or similar projects but is a novice and needs a starting point.



Ok, on to John’s question.

So a LiFePo4 battery can be safely operated between 100% charge and about 30% charge.  You can check me on the actual, more precise low point setting, but it is something in the low 30’s%.  Below 30%, the LiFePo4 will continue to discharge all the way to zero but the battery will not accept a new charge, essentially ruining it from a functional standpoint.

So from this point on, the new zero percent will be indicated by the BMS which stops the battery from discharging any further, which I suppose we can call BMS ZERO.  The 20% you refer to is actually on top of BMS ZERO.  Don’t read that as 50%, because it is a percentage of the remaining 70%, and not additive to the 50%.  Math not being my thing, I don’t know what this point is compared to TRUE ZERO, but it is a pretty large portion of what the battery could store if that darn lower 30% issue were not there.  Nonetheless, the LiFePo4 battery still has a substantial amount of charge left.

Discharging below BMS20 is not catastrophic, it is more like a best practice to ensure the longest life from your battery.  Also, charging up to 100% is not so much dangerous as it is unnecessary.

I hope that this clears up some confusion and adds to the overall discussion.



Eric

Thank you kindly Eric,  your voice is always a welcome sound, for off grid and many other topics

Eric Hanson wrote:

I hope that this clears up some confusion and adds to the overall discussion.

Eric

Clear and perfect, Eric.....Thank you!  

HI Rico,
https://www.amazon.com/GREENOE-12V-100Ah-LiFePO4-Rechargeable/dp/B0DP6WNN96?th=1

Here is a link to the company's amazon page. It lists a charge voltage of 14.6 volts. It seems to be made as a direct drop in for an automative battery. I believe you should be good with a charger that you can adjust to 14.6 volts. I might choose 14.4 volts as a safety measure and experiment with it from there. As to how to use it in conjunction with your other unit... I would probably use the larger battery to feed power into the DC in of your all in one unit and power loads from the all in one. It should deplete the stand alone lithium battery to keep itself charged which will shut itself off when it reaches a low voltage state. Power yur loads from the all in one. Use the adjusted vicron ac to dc charger to top off the stand alone lithium battery at home. That is how I would do it. You would need a separate dc charge controller if you want to feed solar into the stand alone lithium battery.  The sketch you showed above would only work if you were charging the lithium battery remotely before using it on site but would not help you increase your run time when you are working remotely.
Cheers,  David

"It lists a charge voltage of 14.6 volts. It seems to be made as a direct drop in for an automative battery. I believe you should be good with a charger that you can adjust to 14.6 volts"  David, thanks for the opinions.

Ok, I thought auto batteries are high on cranking amps, never thought of 100A as ideal for starting a car.  Not to worry though, this is my GA dry run, I will buy equivalent battery new  near the project, it is over 2000 miles away, that is what I am trying to reiterate,  really remote.  

From my post yesterday:
"Since the LFP battery is small, only 245wH , i desire keeping another battery set to charge during daylight hours"
That is my 12v external LFP, with quadruple the power in storage.  I can charge it during peak daylight hours. (From the Ecoflow&&&&&)
The house gets 300 sunny days per year and rarely drops below 2 C in winter. When I am using tools and lights at 3 am, for example, I would potentially need to use this BACKUP battery feeding juice to my ecoflow unit via DC charge port.  I have the approved cables.  No, I never envisioned any other way to use the external LFP"

I am glad you agree that I only feed the external battery from an AC source. And like i pointed out, i will feed DC power back to the Ecoflow when needed  its 1024Wh feeding the internal battery which is 245Wh.  But I won't be working down the lane from my house.  This is why I am testing everything at home, in advance of my journey.  Today external fed the internal battery smoothly and efficiently

Breath of fall on this fine Sabbath, our  UGA Bulldogs won a rough game in overtime and the sunny  yet cool uplands are beckoning.  My new  low amp charger is doing a great job, drawing fewer watts than I guessed. Many ways to adjust for different charging scenarios.

Thanks for corrections and comments,  will leave town in a few days, please know I will carefully apply my newfound skills, ciao, rico

I use my BMS to set the charge limit as well as the discharge limit. I also made or "built" my batteries from cells and have smart programable BMS's with ACTIVE balancing. Active balancing means balancing is happing ALL THE TIME. Standard BMS only balance at full charge. My upper voltage for charging should be right around 53.7 volts with resting voltage between 53.1 and 53.2 and my low voltage limit is 51.2.

Dang Larry, I was just wondering about your take on things.  You always have knowledge that comes from real life work.  

Sounds like custom battery building is in my future, that sounds like the true path. But I'm preparing for an odd project , far from home, it is work but also a bit of adventure
Used my smart charger yesterday and it specifies  THE CC CV method when LFP is the battery.  It is pumping out 14.6V when set for LFP, and it can be adjusted for LA or AGM as well. I think I am in good shape. Thanks for the reply

One question popped up yesterday, could I use a 450 watt 12v solar panel for the charging of my all in one unit, instead of 100w at 12v? One electrician told me that was possible but he is new to diy solar installation

Larry you brought up another question
If I weekly or monthly charge to full capacity, to equalize or balance the cells, is that ok? Do I need to switch to BMS that is active, even though this is not a complex , whole house system?  Your opinion will help me, thanks

Rico Loma wrote:Dang Larry, I was just wondering about your take on things.  You always have knowledge that comes from real life work.  

Sounds like custom battery building is in my future, that sounds like the true path. But I'm preparing for an odd project , far from home, it is work but also a bit of adventure
Used my smart charger yesterday and it specifies  THE CC CV method when LFP is the battery.  It is pumping out 14.6V when set for LFP, and it can be adjusted for LA or AGM as well. I think I am in good shape. Thanks for the reply

One question popped up yesterday, could I use a 450 watt 12v solar panel for the charging of my all in one unit, instead of 100w at 12v? One electrician told me that was possible but he is new to diy solar installation

The wattage of the panels is not that significant it's the combined voltage and amperage that is. You need to know what the maximum PV voltage of your system is and stay below that and also stay below the maximum amperage. I use stand alone parts because that's what was available 15+ years ago when I bought. My system can handle 150 volts at 100 amps. Ideally you want the voltage as close to max as you can get with out going over for the best charging, especially in cold weather when voltage spikes can occur. The amperage isn't as critical as long as you have enough to do what's needed but stay below the maximum.

Yes balancing the cells as you describe should be fine once a month or once every couple months as long as they aren't getting to far apart.

I built my batteries from parts due to the cost savings at the time.
Having an active balancer wastes a little power but it also keeps the cells happy. Is it necessary? Probably not but it should give me the most possible life from my parts. My goal is as much power as possible for as long as possible for as little $$$$$ as possible!

Since I happened to have my phone with me when I went out to the building where the batteries are I checked the balance. Across a 16s2p system consisting of 32) Eve 280AH cells the voltage difference is 0.002 yes two thousands of a volt. Max voltage 53.8 active charging voltage, I probably need to lower this as I am still going higher than my 53.15 (80%SOC) resting goal. With a 51.5 (30%SOC) low voltage cutoff. Since hooking up this set of batteries and getting the initial charge I'm almost positive the state of charge has never gone over 90% or below 65% . These cells take a lot more charge from the exact same system on cloudy days. Something to do with the chemistry is my best guess as to why.

Something else I just thought about. The building mentioned (6X8 or 64sqft) above we've always called that the battery building. Back then the old lead acid batteries took up to 1/2 of the building (32sqft). As battery tech got better that shrank with the latest set of LifePO4 cells I'm using roughly 1X4 (4sqft)  total!!!

The weight difference is even greater!