Charging efficiency LifePO4

[PNjunction]

I think we're all on the same page, just coming at it from different angles.

I'll distill what I'm saying from a 12v perspective, and without any balancing issues ...

Anything from 13.8v to 14.6v will result in a fully 100% charged battery, the only difference being that at lower voltages, the CV absorb period takes longer to reach .05C, which is the canonical place to stop *absorb* for a fully charged battery.

You can test this, by charging to any voltage and absorbing, then resting the battery for 12 hours, and taking an OCV measurement. 3.38v/cell resting for a fully charged GBS will be the result either way.

The only caution I mention is that when using very low charge currents, if you try to reach 14.6v - you will never get there as the battery will have already absorbed as much as it can.

If you have already absorbed as much as you can, even down to zero amps (although not recommended), if you wait long enough, then YES you may see a sudden rise in voltage after a lengthy stable period due to electrolyte heating, and not charging, since a full charge has already been reached.

Large cells may hide the fact that after you have charged to 100% that now all you are doing is promoting parasitic reactions if you don't remove the charge - even though no current is flowing. Fortunately, most chargers cut off at a certain amount of end-amps, or go to a lower float.

I mention this if someone uses a bench-supply, rather than an automated charger as something to watch out for - perpetual charge once the battery is full - whether that was reached after a slow 13.8v charge-and-absorb, or a fast 14.6v charge and absorb.

From a practical standpoint, most of the serious users try not to go above 14.0v absorb, although bikers who need quick recharges to get back on the road may be very happy with a 14.4v charger / alternator output since time is limited.

In the end, to NOT reach a 100% charge, you just use whatever is convenient for you. Stop at a certain voltage. Don't allow for a full absorb. Basically, just do a load test to find whatever is comfortable for your application capacity wise.

See the problem with trying to pin down an exact voltage with LFP?

[Sunking]

Originally posted by karrak View Post

There is no such thing as a fixed full point at a certain voltage.

I think you're wrong about that.

You are correct Createthis. It is called Open Circuit Voltage or OCV.

As I stated in the previous reply, there are two ways to fully or partially charge a Lithium battery. It is dome exactly like lead acid. Using full charge as an example there are two fully documented ways to do this.

1. Use a CCCV (Constant Current, Constant Voltage) algorithm. You charge ate a Constant Current of 0.3C to 1C until you reach 3.6 to 3.7 volts per cell depending on manufacture. Once 3.6 to 3.7 vpc is reached you hold the voltage at 3.6 to 3.7 vpc until the current tapers to 3% of C. Terminate the charge. Example 3 amps on a 100 amp hour cell. This is the exact same way and voltage you would use to charge a lead acid battery (Bulk-Absorb phase). The only difference is is witth Lead Acid when the current tapers to 3%, the end of the Absorb phase, you lower the voltage to 13.8 volts aka Float Phase.

2. Again with both Lithium and Lead Acid you can fully charge with a Float aka Constant Voltage. Voltages are slightly different. Depending on which lithium battery manufacture your are talking about 3.45 vpc give or take .05 volts. Open circuit voltage of a fully charged lthium is approx 3.43 to 3.5 volts. Depends on manufacture.

Now what you do not want to do is hold a Lithium battery at 100% SOC like you can a Lead Acid. But there is absolutely no problem holding a lithium cell at 80 to 90% SOC OCV. The key to understanding is Open Circuit Voltage and what that voltage is with Lithium. Th eissue with Lithium is the discharge curve is very FLAT from 90% to 10% SOC. Roughly 3.4 to 3.0 volts. or 13.6 to 12 volts on a 4S battery.

[Sunking]

Creathus, with Solar there is absolutely no reason to go to 100% and there is absolutely no problem Floating LFP batteries on a Solar System. You do not have to worry about what SOC the batteries are at as long as they are not 100% or less than 10%.

GBS cells charge at a slightly higher voltage than say CALB. As long as you do not go over 14 volts, you are OK. 13.6 is just fine and when floated is roughly 80%. Your GBS batteries are 100% charged up at 3.5 vpc saturated. By Saturated I mean the cells held at 3..5 vpc (14 volts) until all charge current stops. So you can float at 14 if you so wish without any real problems as long as you draw some power. At 3.4 vpc relax. So set the charger to 13.6, and your Invert er LVD to 12 volts and relax, you are fully protected.

Here is a very good article on FLOAT CHARGING Lithium batteries. Think of Float Charging the exact same way as paralleling any lithium cells. Parallel as many as you want, an dlet them set as long as you want. All current stops. As long as the voltage is less than 100% Open Circuit Voltage, you can float a lithium infinitely. 100% OCV on GBS is 3.48 to 3.50. At 3.4 vpc you are well below 100%.

Createthis is you and even Karrak does not understand there is more then one-way to charge a lithium battery. One thing you do know, which is a good thing, there is absolutely no reason to fully charge a lithium battery. In fact you will double the cycle life by not charging to 100%. Limit to 80% SOC and limit discharge to 90% DOD and you can quadruple cycle life. That is exactly what commercial do to enable them to offer long warranties. Hell they use battery types with no more than 500 cycles.

So how to charge to say 80/90% is the question. How do you do that. Well the answer is simple, there are two good ways to do that.

1. If you use commercial AC Power, you charge at a Constant Current of .3C to 1C until the cell voltage reaches 3.65 volts, and terminate the charge. If you want to go to 100% then you hold 3.65 volts until the charge current tapers to 3% of C to saturate the battery. So if you have a 100 AH cell that would be 3 amps. But that is a terrible way to use Solar. If your system is sized correctly, in summer months you are charged up by noon or mid afternoon and you disconnect the panels and stop charging. Well hell stop and think about that for a moment. You turn off the solar and go on batteries while there is still good daylight left. Pretty stupid huh?

2. There is another way. Use a Constant Voltage method and set the voltage to 80 to 90% SOC OCV aka Open Circuit Voltage. What voltage is that? Depends on who the manufacture is. For GBS around 3.4 to 3.45 vpc. How do you do it? Simple you buy a Charge Controller where you can set the Bulk = Absorb = Float. On a GBS 4S system about 13.6 to 14.0 volts. Initially in the morning hours your controller operate sin Constant Current mode charging your batteries as fast as the panels can charge them. When they reach the st point, say 13,6 volts, the voltage holds and charge current tapers to Zero amps when the batteries Saturate. At 0-Amps is Open Circuit Voltage at 80/90% SOC. The panels do not shut-off. If your equipment demands power after the batteries are charged, the power comes from the panels rather than the batteries assuming the power demand is equal or less than what the panels can supply. You conserve battery power for the night and less cycling, thus increasing battery cycle life. What is not to like? You are mimicking what the pros do like EV manufactures.

To protect your batteries on the discharge side could not be easier. Set your LVD for 12 volts or 3.0 vpc. The danger zone for LFP batteries is 2.5 vpc or 10 volts on a 4S system. At 12 volts you eliminate all risk of over discharge.

[createthis]

There is no such thing as a fixed full point at a certain voltage.

I think you're wrong about that.

[karrak]

I guess I'm a little confused by how loosely everyone is throwing around the term "full" in regard to battery charging. I don't consider 92% or 95% "full". I consider "full" to be 14.4V with all cells top balanced perfectly. This is the absolute maximum capacity of the battery.

But everyone else seems to be loosely defining "full" to mean the point at which the battery starts to accept a lower amp rate of current. This definition of "full" means the battery has a different capacity depending on the charging voltage AND rate. At 0.05C and 13.5V, the battery appears to start accepting a lower rate of charge at about 92% capacity. At 0.05C and 13.8V, it looks like that capacity is higher - 95% perhaps?

There is no such thing as a fixed full point at a certain voltage. From my graph in post #10 you can see that I have defined 100% full as being a charging voltage of 3.6 volts when the charging current has tapered to zero. If I had chosen 4.0 volts/cell I would have pushed a little more charge into the battery so might have around 100.2% of the charge at 3.6 volts. 3.6 volts is a good reference because it is just under the voltage when you start getting Lithium plating on the anode. You should not leave LFP cells at 3.65 volts for extended periods of time. From the graph you can see that at a charge voltage of just under 3.4 volts/cell the battery is around 99% full.

PNJunction, your anecdote "DO NOT ABSORB TO ZERO AMPS!" talks about the use case where the charging voltage, 3.8V is higher than the cell's 100% SOC voltage, 3.6V. Of course it's possible to overcharge in this case and achieve thermal runaway.

If you limit the voltage of an LFP cell to 3.8 V you will not get any thermal runaway. The charge current will taper to zero and the the cell will just sit there. If you leave 3.8V on the cell for an extended period you will degrade its performance. Breakdown of the electrolyte that generates enough heat and gas to cause immediate physical damage to the cell does not happen till around 4.5V.

What I'm not clear on is if it is possible to overcharge the same cell if the charging voltage is set to 3.375V, which is well below the cell's 3.6V 100% SOC voltage. And if it is possible, how long does it take?

Charging an LFP battery cell to 3.375 cannot overcharge it. The problem with charging to such a low voltage is that how full the cell will be at the end of charge is very dependent on the end current. When I first installed my battery I used to charge to 3.375 volts per cell with an charge finish current of C/20. I was finding that the end SOC could vary by around 20% depending on the power coming from my solar panels. I now charge to 3.45 V/cell with an end current of C/50 then drop back to a float voltage of 3.35 V/cell

Simon

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

[createthis]

I made it to the middle of Page 11 in that ADVRider thread, then I lost interest as it got into cranking and other stuff unrelated to charging. Pages 9 and 10 were full of good info though.

I guess I'm a little confused by how loosely everyone is throwing around the term "full" in regard to battery charging. I don't consider 92% or 95% "full". I consider "full" to be 14.4V with all cells top balanced perfectly. This is the absolute maximum capacity of the battery.

But everyone else seems to be loosely defining "full" to mean the point at which the battery starts to accept a lower amp rate of current. This definition of "full" means the battery has a different capacity depending on the charging voltage AND rate. At 0.05C and 13.5V, the battery appears to start accepting a lower rate of charge at about 92% capacity. At 0.05C and 13.8V, it looks like that capacity is higher - 95% perhaps?

PNJunction, your anecdote "DO NOT ABSORB TO ZERO AMPS!" talks about the use case where the charging voltage, 3.8V is higher than the cell's 100% SOC voltage, 3.6V. Of course it's possible to overcharge in this case and achieve thermal runaway.

What I'm not clear on is if it is possible to overcharge the same cell if the charging voltage is set to 3.375V, which is well below the cell's 3.6V 100% SOC voltage. And if it is possible, how long does it take?

[createthis]

I have graphed SOC v Cell Voltage from two sources. The data for the blue line comes from here http://advrider.com/index.php?thread...#post-18206897 and the data for the yellow line comes from some tests I have performed recently.

Thanks for this link. Missed it the first time around. Delving in now. Good data.

[createthis]

I have graphed SOC v Cell Voltage from two sources. The data for the blue line comes from here http://advrider.com/index.php?thread...#post-18206897 and the data for the yellow line comes from some tests I have performed recently.

Thanks for this link. Delving into this data now. Very helpful. Sorry, I didn't see it earlier.

[PNjunction]

I've read an LFP cell is fully charged at 3.6V.

That is a generalization, best used to let those think that LFP can be used as a drop in for a lead acid charger running a typical 14.4v CV. It can be done, but there is more to the story.

Here's what I don't understand:
- I don't understand why you write "3.45v fully charged", above... is that even possible?
- Is it possible to raise an LFP cell to 3.6V using a float voltage of 3.4V? Won't the charge controller cease to float at some point?

At this stage I think I am over complicating things. A "full" charge and the voltage it represents changes depending on what charge current you are using, and how much time is allowed for an absorb.

For now, lets just say that at a low charge rate that is already starting at a low .05C, then 3.45v would be the max you would want to go. If you were hammering it at say an incredible 1C charge rate, then to achieve a full charge then 3.6v would be your CV, and you'd allow the absorb end-current to dwindle down to .05C and no more and stop.

Is it a problem that I have three thousandths of a volt difference between cells? Are they going out of balance, or is the difference in voltage an indicator of cell weakness/strength? For example... is the 3.346 cell my weakest cell and the 3.343 cell my strongest cell?

You are doing fine. The difference may not be a true imbalance at all, but a mere slight difference in each cell having slightly different manufactured capacity, and also internal resistance.

Just be sure you don't mix methods - you said you were bottom-balanced, so don't be frightened by some imbalance "at the top". As long as any individual cell does not exceed 3.6v, and other cells are reasonably close (which yours are at this stage), you should be good. Your trigger to stop should be based on the weakest cell, that is the *first* one to achieve the level of charge you desire.

Run some load tests - you bottom balanced and hopefully you have an LVD in place so you don't actually hit that.

[PNjunction]

I've read an LFP cell is fully charged at 3.6V.

That's a sweeping generalization. Mostly done to satisfy the "drop in" mentality of being able to use a lead-acid charger set up for 14.4v to function properly with LFP.

It REALLY depends on your charge current, and how long you let it absorb.

Ie, you can achieve a full charge at these extremes:

.05C charge current to 3.45v and stopping.
1C charge current to 3.6v and absorbing at 3.6V to .05C

What happens is that the cell voltage can vary whether one is charging fast or slow, or in fact charging or discharging. SOC based on voltage differs under these circumstances. Charts are ballpark. A measured load-test will tell all.

There is an actual formula for it, but the gist is that most don't NEED a 100% charge all the time. 98.5% is good enough.

In other news, I checked my cell voltages just now (3am) after manually cutting the power to the charge controller:

3.345
3.343
3.344
3.346

series total: 13.378V

Measure each one again in about 12 hours. Compare to a generalized chart. Get a generalized SOC based on voltage. That might be good enough for your purposes.

Instead of a chart, how about this meter from Optimate:

https://www.amazon.com/Optimate-TS-127-charge-lead-acid-lithium/dp/B00DRPRZ9E/ref=sr_1_1?ie=UTF8&qid=1465805541&sr=8-1&keywords=Optimate+ts

Works well enough for my non-lab purposes.

Is it a problem that I have three thousandths of a volt difference between cells? Are they going out of balance, or is the difference in voltage an indicator of cell weakness/strength? For example... is the 3.346 cell my weakest cell and the 3.343 cell my strongest cell?

Not really a problem since you are bottom balanced. Where it may start to differ more significantly is when you set your CV for 3.45v and try again - which the trigger for stopping would be when the FIRST cell gets to 3.45v, assuming you actually need near a 100% charge.

I think you'll find we are making more out of this than necessary. The truth is, if one plays the voltage game, and tries to line up the cell voltages to be perfectly exact, they may end up taking too long to do so, negating the benefits of the voltage micro-management in the first place!

You are very close and conservatively minded so I don't think you'll have many problems.

[createthis]

DO NOT ABSORB TO ZERO AMPS! .05C at most!

I've read an LFP cell is fully charged at 3.6V.

Here's what I don't understand:
- I don't understand why you write "3.45v fully charged", above... is that even possible?
- Is it possible to raise an LFP cell to 3.6V using a float voltage of 3.4V? Won't the charge controller cease to float at some point?

In other news, I checked my cell voltages just now (3am) after manually cutting the power to the charge controller:

3.345
3.343
3.344
3.346

series total: 13.378V

Is it a problem that I have three thousandths of a volt difference between cells? Are they going out of balance, or is the difference in voltage an indicator of cell weakness/strength? For example... is the 3.346 cell my weakest cell and the 3.343 cell my strongest cell?

[PNjunction]

CY does a great job of educating the riders over on ADVrider. My LFP experience actually started with Shorai's.

While charts are ok, just remember that they are dealing with primarily high-rate LFP energy cells, (A123, Shorai, Antigravity, EarthX etc), while we with large prismatics like GBS, Winstons etc are primarily lower rate power cells.

As such, any charts based upon those high-rate cells will be in the ballpark, but not truly exact from a voltage standpoint.

Thus, a GBS cell when fully charged, and allowed to rest for 12 hours, will measure out to about 3.38v resting.

[PNjunction]

I have graphed SOC v Cell Voltage from two sources. The data for the blue line comes from here http://advrider.com/index.php?thread...#post-18206897 and the data for the yellow line comes from some tests I have performed recently.

I started my LFP career with Shorai's, and commend CY for not losing his mind educating the riders about it for so many years.

Still, their application is primarily a $$ starting application, and as such cells like Shorai, Antigravity, EarthX, A123 are high-rate energy cells, unlike the GBS cells, which are power cells. As such, when playing the resting-voltage game to determine full charge, the charts will not be exact among the different manufacturers.

Thus, a GBS rested for 12 hours will be around 3.38v when fully charged. Canonically, that is near when .05C absorb current is reached between 3.45 and 3.6v CV. Still that is ballpark, as a faster charge will net you less of a full charge if you do an actual capacity test. Perhaps a negligable difference for casual use, but it is there.

CY actually has a large prismatic 20ah LFP batt similar to what I ended up with, and was successfully able to start his bike with it, but the main emphasis was still on the high-rate starter LFP's (and the warnings about using them in low-rate applications like radios and heated gloves which catch unwary users running them for long periods of time!)

My tests involved setting Bulk=Absorb=Float to the same voltage at the start of the day and taking an SOC reading when the charge current had dropped to around zero.

Stop doing that. .05C end current at most.

[PNjunction]

DO NOT ABSORB TO ZERO AMPS! .05C at most!

I see talk already here about doing this. I've already done this for you, but if you want to see parasitic reactions in your face, do the following: (I actually do NOT recommend doing this unless you are willing to kill cells for science like I do)

1) Charge at .05C or less current.
2) Set your CV voltage high, like 3.8v.
3) Use a small sized cell to cut down on the time to see the parasitic reaction and not be fooled by a large cell that hides the phenomenon.

Here we go..

Cell goes though the bulk stage of charging, and starts to naturally taper off current. Even though your CV is set for 3.8v, since you are charging at .05C (or less even), you will never reach it - by the time you *start* to drop below .05C charge current naturally, the cell voltage will be about 3.47 - maybe 3.5v.

But is there life after "absorb to zero" ? There certainly is, but not from what you expect!

Right - while basically fully charged now, the cell stabilizes at around 3.45v or so, and current takes awhile to actually drop to zero amps.

But wait- there's more! Let it sit like that, basically floating at 3.45v fully charged. Grab an adult beverage, a lawnchair, and a multimeter that beeps on voltage changes.

With my 20ah cells, after about 30 minutes of being totally stable (3.45v and no current), suddenly the voltage started to skyrocket - and NO CURRENT flowing! Yikes!

What is happening is you are no longer charging, but have transitioned into the parasitic state - in this case it is usually called electrolyte heating, which causes the terminal voltage to rapidly rise with no current flowing. A big change chemically, and it was an education in chemistry, not electronics at this point to witness that.

In other words, it was fascinating to see a change in voltage with no change in current - not even the presence of current! THAT taught me an important lesson about LFP, and made me wonder how many balancing boards are actually kicking off on parasitic reactions, and not charging itself!

[PNjunction]

Using 13.6v as your charging voltage is nicely conservative, BUT from a solar standpoint if autonomy or the desire to truly fully charge your battery is the goal, and you are cyclic in operation then 13.8v is a better choice - even if you never actually reach it!

The reason is that when you set yourself up conservatively with the CV stage, your absorb will be very slow once you reach it. This may not be desirable if your day starts out pretty cloudy and you are nearing the end of your insolation period. Setting it for 13.8v CV instead of 13.6v, will help allow the panel/battery combo to kick ass quicker when the sun does peek it's head out between the clouds.

It's up to you of course - this is only the plan I follow for my own comfort.