Charging efficiency LifePO4

[createthis]

The yellow line is a charging voltage, a resting voltage of 3.35 V/cell is more like 98%-99%

I was looking at the blue line. Isn't that resting voltage?

[karrak]

I was expecting the Midnite Classic to enter resting state, but now I'm wondering if it will continue to float with 0 amps input or something.

I don't have a Midnite Classic but I would have thought that if you have programmed a float voltage of 13.6 volts that it will keep the battery voltage at 13.6 volts as long as the sun is up and the current going into the battery will taper off to nearly zero. With the current at zero your battery will be around 99% full.

Simon

[karrak]

I was looking at the blue line. Isn't that resting voltage?

There are not enough data points on the blue line. We need some more data points between 90% and 100% to get a more accurate figure at the voltages you are looking at. We also haven't been told how they performed the test so we don't know exactly what their "resting voltage" really is.

[createthis]

I don't have a Midnite Classic but I would have thought that if you have programmed a float voltage of 13.6 volts that it will keep the battery voltage at 13.6 volts as long as the sun is up and the current going into the battery will taper off to nearly zero. With the current at zero your battery will be around 99% full.

Simon

The Midnite Classic did indeed enter a resting state and stayed there as the voltage fell below 13.6v toward it's final resting voltage. I didn't let it sit long enough to find the final resting voltage. I plugged in my inverter and charged my laptop at that point. The Midnite Classic kicked back in and supplied all of the inverter's power via solar. I'm mostly pleased with how this is working. However:

I'm disturbed that you keep telling me my battery is 99% full. That wasn't my intention with this. I'm worried that will mess up my bottom balance.

I did my homework (or tried to) before this. In this post, PNJunction indicates that a charging voltage of 3.45v per cell (13.8v) will yield a 90% SOC: https://www.solarpaneltalk.com/forum...138#post302138

By using a 13.6V charging voltage I was hoping to even be a little more conservative than that.

[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.

[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]

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]

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.

[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]

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.

[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.

[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.

[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 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?

[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