LiFePo Charge level

papab

I see that Tesla is starting to use LFP batteries.  They have said something a little surprising:

“I’d personally slightly opt for iron pack, as it wants to be charged to 100% whereas nickel prefers ~90%,” Mr. Musk said in the Twitter post

I've been reading about LFP batteries for a while since I just put a 12V/100ah one in my camper van.  Every source I've read recommends NOT charging to 100%.    Now that I've said that I can't readily find a good really reference for either charge level, mostly forums like this one and web blogs like:

https://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/ where he says: "Don’t allow the cells to sit around at 100% SOC after balancing!"

Is there any science behind this?

Wheelman55

The BMS should take care of that issue. 

For instance the Discovery AES 42-48-6650 lithium battery has a capacity of nearly 7,400 watts, however the BMS limits use of that to 6,200 watts. That figures to approximately 85% of total capacity. 

This Discover BMS limits both the upper and lower charge levels. They also recommend storing the batteries at less than 100% SOC. My memory says that they recommend 80% for storing. 

Study up on the specs from your 12 volt battery   The BMS on your battery likely limits capacity like the Discover battery does. 

Horsefly

There is a school of thought that many subscribe to that says when you charge to the full 3.65V per cell, you should not hold the voltage at that level. After disconnecting from the charge source, the voltage will settle to something around 3.4V per cell or a bit more. The cell is still at 100% SoC anyway.

The logic is that it is somehow harmful to hold the cell at 3.65V.  I know it isn't hard to go to too high a voltage for too long, which will cause the cells to bloat, which isn't good. 

I don't really know if 3.65V will hurt eventually, but to me it doesn't matter. If you have more than one cell, even if they are well balanced one of them will reach 3.65V faster than the others. If you have a good BMS, as soon as the first cell hits 3.65V charging will be cut off anyway.

I charge to 3.5V per cell, and hold it there long enough that the tail current drops to something like 1-2A. Unlike lead acid, LiFePO4 cells will drop in charge current very rapidly once they get close to fully charged.

Others charge to 3.45V, but you have to hold it longer. This is the "absorption" phase with most chargers. Again, once it gets close to 100% SoC the current will drop quickly. 

For what it is worth, there are some scientific papers which have documented that LiFePO4 cells are not harmed even if charged to 4V, but I wouldn't do it.

papab

Thanks for the responses.  I'm really looking for science or at least consensus from the manufacturers on charging for good lifetime.   There's a lot of internet lore out there that recommends <100%, and < 3.5V/cell, or cutting off immediately when a cell hits 3.65.  @Horsefly,  I follow what your saying, I've seen that it doesn't take much imbalance in a series for 1 cell to be at 3.65 & the BMS cutsoff charging while other cells are still at 3.5.

What I've found:

Battleborn: 14.4 (3.6), 20-30 minutes absorption time.  They claim you can run them with a 100% discharge for thousands of cycles.  No mention of charging to a lower level for more life.  They recommend 0.5C maximum charge rate for good life.

Relion:
    1-Stage: 0.5C (Recommended) or 1C (Max), terminate at 14 - 14.6.
             This results in 97% SOC
    2-Stage: Same current as 1-stage, Vabs 14-14.6, termination current=5A
             In another place they say 0-6 minutes abs time
             Cold: Can charge down to -20C at 0.05C charge rate !!

https://www.ctechibattery.com/news/How-to-charge-LiFePO4-battery-correctly.html:
    0.3C, 3.65V, 0.1C finish amps. There is a conflicting 0.5C recommendation

My conclusion is that the conservative recommendations on the forums (a lot more on the DIY forum) don't seem justified & 3.65V/cell is probably just fine.  

Other than E Musks twitter post, I can't seem to find anything that says that LFP prefers to be charged to 100% though, and I wonder about what cells they are using on the Teslas that would tolerate fast charging > 1C

Horsefly

papab said:

...

My conclusion is that the conservative recommendations on the forums (a lot more on the DIY forum) don't seem justified & 3.65V/cell is probably just fine.  

Yep, I would tend to agree.  Sometimes what you read on the DIY forum seems like a group of amateur witches, each trying to prove that they have a better potion that uses more ingredients and a better type of eye of newt.  Still, there are some real nuggets of knowledge there and here.

RCinFLA

There are several factors involved with LFP maximum charging side effects.

LFP cells have the most rugged cathode structure of all Li-Ion batteries.  It has a vertical support structure to its polycrystalline lattice that helps prevent cathode structure damage when fully charged.  Most other Li-Ion cathode types do not have this vertical support structure and can be damaged by lattice collapse when fully charged (most of lithium leaves cathode when fully charged, leaving weak structural support to positive cathode).

The greater the cell voltage the greater the likelihood of electrons escaping from graphite negative anode to electrolyte through the protective solid electrolyte interface (SEI layer) during charging.  Electrons should only interact with lithium ions at the graphite neg electrode and LFP cathode pos electrode.  When electrons get into electrolyte it causes it to decompose.  This creates gases (mostly CO2 and CO) that can bloat up the cell but more degrading is the other electrolyte decomposition tar-like components that clog the surfaces of electrodes reducing ability of lithium ions to migrate back and forth to electrodes.

All Li-Ion cell use similar electrolytes with various secondary additives.  The electrolyte accelerates breakdown above 4.3v cell voltage.  For Li-Ion cells with full charge at 4.2v it does not give much safety margin.  With LFP lower cell voltage of 3.65v max charging voltage there is more safety margin for electrolyte.

Bunkysdad

Practicality at some point kicks in…LiFePo4’s do need to be fully charged ‘occasionally’ in order for cell-balancing to take place (and for meter resynchronization, due to cumulative meter drift over time) which last say 20-30 min beyond reaching 100% fully charged…The key here is to not continue the battery charging for an extended period of time - this (over a time) to avoid the possibility of dendrite formation…Beyond the need for occasional cell-balancing, the best practice is that there’s no real need to routinely re-charge to 100% SOC…