Lithium Cells in Float Charge. Will it destroy your battery?

Can you float charge lithium batteries? Should you float charge lithium batteries? What are the best settings for your charge controllers? Will float charge destroy your cells?

I played around with a lot of solar charge controllers trying to understand and find the best absorption and float charge settings. It can be very confusing and it's a bit hard to fit this all in one video. Please let me know if you want to see more details about a specific setting and how this works out in reality. I will break this down and make a separate video about that then...

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[John-xujk]

wow .... literally years of being confused and 20+ minutes clears all the fog !!!
Thank you

[junkerzn]

Just a side note, once you've charged a LiFePO4 (and I think other lithium types too) up, say up to 13.8V or 14.0V and then stop applying voltage, the battery voltage will actually drop down to around 13.6V fairly quickly (well, probably closer to 13.55V). It remains fully-charged, that's the just the natural stable voltage for 4 x fully charged cells. So don't be surprised if you charge the battery up higher that you see it drift back down to that voltage. It just doesn't take a whole lot of current for the voltage to almost instantly drop back to around 13.55V. It can happen fairly quickly (depending on the BMS's vampire draw) even if no load is attached.

So, for absolutely definitely sure, never set the float voltage above around 13.55V... that will cause the charge controller to apply a little current quite often due to the battery wanting to return to its stable voltage... and that is bad for the battery. And, of course, as the battery ages, even 13.55V will cause a constant current. Hence you probably shouldn't set the float above 13.4V or so regardless... and lower values are even safer. Up to a point.

Remember to use the discharge curve, NOT the charging curve, to figure out what float settings you want.

This means that, in fact, if you charge the battery up to 100% and start discharging it with the float set to 13.4V, the battery will still remain at least 90% charged as the charge controller starts matching amps below 13.4V against the load. However, if you discharge the battery sufficiently and the charge controller does not go back into BULK mode, then the battery's charge level will probably sit at just below 90%.

So, to figure out the worst case charge level for the battery, you want to look at the discharge curve and at the voltage point where the charge controller switches back into BULK charging mode. So lets say float is set to 13.20V and the charge controller flips back into BULK mode at 13.10V. Thus, 13.10V on the discharge curve is the worst case charge state that the battery will ever be left in... lets call it 50%. Definitely NOT ideal.

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This creates a conundrum. If you set the float TOO low, the worst case state of charge that the charge controller might leave the battery in will be too low. Things can get iffy below 13.2V, so my recommendation is that the float be set at roughly 13.4V in order to ensure that the charge controller goes into bulk at a reasonable voltage (say, 13.3V, depending on the charge controller). This way if the voltage only drops to 13.31V and the charge controller stays in FLOAT, the battery will still be at around an 80% state of charge.

You can safely set the FLOAT voltage to anything under 13.55V for a new battery, but to deal with battery aging you should consider not setting it higher than 13.45V or so. If you want to be conservative, then use 13.3V or 13.35V. I would not recommend 13.2V for the float because that means the battery could be seriously discharged before the charge-controller decides to go back into BULK mode. The state of charge drops precipitously enough below 13.3V that you just can't depend on setting the float below that voltage.

This is why the Victron's Float is set at 13.30V. Its a good conservative value that will deal with battery aging but isn't too low to cause the battery to be left in too-low a state of charge when it could be charging.

Now Bulk and Absorption are a different matter. In order to charge the battery, voltages in excess of 13.6V are required. 14.0V is a typical target voltage for BULK. Absorption is basically irrelevant... so set the voltage to something inbetween Float and Bulk Target and then set the absorption time to 0. HOWEVER, on some (most?) Victrons, the Absorption *IS* the bulk target voltage. Therefore, you should set Absorption to 14.0V (for roughly 80% charge) and set the absorption time to 0. If the Bulk target (or Absorption, depending) on the Victron is set too low, and the battery is being charged up from a low state of charge, it will probably never reach even 50% charge before the charge controller decides it is done.

Once LiFePO4 reaches the target voltage during charging (Bulk target of 14.0V or so for 80% charge), the charge is done. The battery will remain at 80% charge even as its voltage slowly drops back down to 13.55V, and the percent-charged during discharge will head south from there on down. When you are discharging the battery, you should refer to the discharging voltage curve and not the charging voltage curve.

This target voltage is really what you are comfortable with. Most people use 14.0-14.2V. Use 14.6V only if you want to actually charge the battery to 100% (most people do not as this reduces the life of the battery).

This is my understanding.

-Matt

[LaBamba]

Nice video. Lithium batteries have been around for decades now so there's no excuse for these charge controller companies not to have a lithium battery setting that ELIMINATES all the old lead acid terminology and just uses the correct lithium battery parameters. Then you could simply edit them per your particular lithium battery manufacturer's recommended settings. All this "float", "boost", and "bulk" stuff is useless and misleading.

[bimmerjones]

I have found your observations to be spot on, especially for EPEVER charge controllers, which many think are faulty, or don't work with Lithium batteries. Their default settings are way too high for LIFEPO4, and the gap between boost and float create the challenge exactly as you mentioned. Your video was the most informative and educational one I have seen, and rectified what I thought was an issue with my 3 different EPEVER Charge controllers in my off grid setup.

[dennisbosworth]

I am glad I found you, here on YOUTUBE. I just am in the process of switching my solar system from lead acid to lithium and I am finding the charging algorithms to be completely confusing - that is, until I found your channel. Now, things are starting to make sense! Thank you for sharing your knowledge.

[bathfun]

My goodness, I actually understand this now.

Thank you.

The frog is fast becoming a star!

[Nadeeja]

You're a life saver Andy. I just finished wiring up my DIY battery pack and solar inverter and was so confused on what to set for bulk and float charge values. There's no absorption option in my inverter and this is exactly what I needed. Its all working smooth as butter now. Thank you so much. Love from SL <3

[sharonpointon]

Thank you, thank you, thank you. From a women who lives in a caravan and knows nothing about LifePo4 batteries, this has really helped me to understand how they work. Good to know that they don't have to go into float. 😁

[lebogangmogashoa]

I've been struggling for 3 days with charger settings for my lithium battery. Found useless contradictory info all over the internet, until I stumbled on this video. Great advice. Looks like I have now set my charger correctly. Only if I had seen this video sooner

[benediktschluter]

Best video I've seen on this topic.

[FloryJohann]

You took years of mysteries away in just a few minutes and showed how it is working.

[jeffbowers]

Excellent video explaining why float shouldnt or cant be used with lithium.
You made it more clear than my battery manufacturer and solar componant supplier, both of whom did ok but not as well as you. Thanks for the effort.
Glad to see you escaped from that tiny box.

[steve]

Another great video. You explained it well. I tend to think of the float voltage as mainly keeping the cells at a pretty high state of charge, and allowing you to run loads off the solar power when you have good sun. As you said, if you use either no-float (my CC does provide a 2-stage with no-float) or a really low float, you are missing out on using the sun when you have it.

[capecoaster]

Thank you very much for addressing the setting parameters relative to the chemistry of the battery. That is the fundamental feature that need to be look at first , before embarking on the charge/discharge of batteries.

[kuhrd]

What you are talking about is not really self-discharge. You are talking about the surface charge going away which is a product of battery internal resistance. A fully charged flooded lead-acid battery is fully charged at 12.8V and some AGM Lead Acid batteries are fully charged at 13.0V. So the float voltage for most charge controllers will be slightly above that to keep the voltage high enough to prevent sulphation while also keeping the voltage low enough to reduce water loss. Most quality lead-acid batteries will lose about 5% per month due to self-discharge. So a 280Ah FLA or AGM battery will have a self-discharge rate of about 20mA @ 12.8-13V.

When you charge most lithium chemistries they will still have a small amount of surface charge but they have lower internal resistance and also a lower self-discharge rate at about 0.5-2% per month. The self-discharge rate of your 280Ah cells will be about 2-8mA. Your lithium batteries will still draw power to match their self-discharge rate at whatever voltage you charge them to but your meter is likely not accurate enough to show the current at that rate. The reason you don't float charge your lithium batteries with the same settings as lead-acid is that the charge curves are different enough that you can use a differential instead and prevent the lithium battery from staying at a high charge state when you are trying to get the maximum number of cycles out of the cells. You typically try to cycle most lithium battery chemistries between 10% and 90% state of charge to get the maximum cycle life while also taking into account the trade-off with calendar aging.

[spyke]

Late to the party here, but here is a perspective you may have missed, and that is charge current... So I want my batteries to charge to 3.4V, however when I set my charger to this voltage as we approach this voltage the current in the battery will start to drop, eventually reaching 0A at 3.4V when the battery is fully charged. This will take a long time. So I set my boost charge to 13.8V so that I can make maximum use of available charging power and keep the current high even at the desired 13.4V. Yes it means I am "over charging" my batteries past my target for a few hours, but at least I charge as fast as possible. Then when we switch to float charge, my loads will bleed off the excess charge and we will then reach my desired 13.4V and solar will kick in an supply the load. This way I get my batteries charged as fast as I can to take advantage of available sun.

[lifepolicy]

Now revisiting your video two years later: on my Victron the Lifepo settings are bulk 14, 2V and float 13, 5V. That makes perfect sense because like you said in another video the voltage from a fully charged battery drops to 13, 5V after a while no matter what if you charge it with 14.2V or 13.8V (13.5 is 100% from my mfg's handbook). When the charger hits float, you must restart it to start a new cycle.

[genxgamerdad]

Wow, glad YT recommended your video, this cleared up a lot. Recently on a forum where people adamantly said not to float Li batteries. Even a representative from a certain inverter/charger company said Li should not be floated. As a solar newbie, I was confused by this. Looks like Float for lead and Li are two different things entirely. My battery manufacture even recommends a float voltage for their Li batteries. I set my batteries up per my manufactures recommendation including float, and all seems fine.

[immrnoidall]

I'm not trying to brown nose here but, seriously, you should get some kind of kindness to humanity award, lol, for your educational generosity. Just this one video has a classroom size of 127 k so far. I believe when teaching, there is nothing better than real world lessons.

[chasethames]

Whenever I’m reading anything regarding my solar system or in solar forums it’s Andy’s voice in my head and if it’s not for you then I’m sorry we can’t be friends 😊