Charging efficiency LifePO4

[Sunking]

Alternative doing it your way.

1. Make sure you have the cells perfectly BB
2. Connect in series and charge with solar setting the MNS to 14.4 volts.
3. Monitor charging. as it progresses one cell voltage will be higher than the others.
4. Count Amp Hours as you go.
5. When weak cell shoots up from roughly 3.4 to 3.6 volts it is fully charged.
6. Do your discharge test and measure amp hours.
7. Stop Discharge when weak cell reaches 2.5 volts, or any cell reaches 2.5 volts.
8 Now you at least now what the weak cell capacity is.

[Sunking]

Ok. I will do that. Until I do that, I've disconnected my charge controller.

Before I disconnected the charge controller, I took some measurements:

Victron reads 13.86v.
Fluke 87 at battery terminals reads 13.87v.
Cell 1 reads 3.5v.
Cell 2 reads 3.376v.
Cell 3 reads 3.42v.
Cell 4 reads 3.57v.

You gotta clue. You have 1 cell fully charged, 1 almost fully charged, 1 is close, and one still has room left. Can you tell which ones?

[createthis]

You gotta clue. You have 1 cell fully charged, 1 almost fully charged, 1 is close, and one still has room left. Can you tell which ones?

4, 1, 3, and 2, respectively.

[createthis]

Alternative doing it your way.

1. Make sure you have the cells perfectly BB
2. Connect in series and charge with solar setting the MNS to 14.4 volts.
3. Monitor charging. as it progresses one cell voltage will be higher than the others.
4. Count Amp Hours as you go.
5. When weak cell shoots up from roughly 3.4 to 3.6 volts it is fully charged.
6. Do your discharge test and measure amp hours.
7. Stop Discharge when weak cell reaches 2.5 volts, or any cell reaches 2.5 volts.
8 Now you at least now what the weak cell capacity is.

Probably not possible with existing hardware. Victron is my only AH counting device (other than the pl8) and it refuses to count up unless it has reset to zero at fully charged and counted down first. I'll have to do it on the bench with the pl8.

[karrak]

I'm getting really tired of this, Sunking. I'm following the charging procedure you gave me in the Midnite Classic thread. Now you're telling me it's garbage? Make up your mind.

link to post in question where you tell me to charge at 13.6v for bulk, absorb, and float:
https://www.solarpaneltalk.com/forum...053#post303053

You are correct, Sunking keeps changing the story. His bottom balance then set the whole battery charge cutoff voltage to suit the cell with the lowest capacity is too complex and will not work in reality. The only practical way to make this work is to have a BMS that would stop the charging when the weakest cell is full.

The other major flaw in Sunking's method is that he assumes that the battery will not go out of balance over time. I see he is now hedging his bets and suggests you check the bottom balance now and again. This means having to drain the pack which is a major pain and unnecessary.

These are the reasons that I do not use bottom balancing. I agree with Sunking that reversing the polarity of a LFP cell will destroy it. To stop this happening I have a BMS (for a couple of years it was a cellog8) which will give me an alarm if any of the cell's voltage drops below 2.8 volts. It is that simple.

Willy T tried Sunking's method and had similar problems to you. He now top balances. I don't know of one off-grid system that uses solar to charge a battery that uses bottom balancing. Sunking doesn't do it.

Simon

[karrak]

To run your system with top balance all you have to do is:

1. use your PL8 to charge and balance the battery to 3.6 volts/cell (14.4V)
2. Set your Midnite Classic to charge to 3.45 V/cell (13.8V) with an end current of < C/20
3. Hook up your Cellog8 and set the low alarm voltage to 2.8 volts/cell (11.2V) and the high alarm voltage to 3.6V (14.4V)
4. Optionally, use the alarm output from the cellog8 to disconnect your inverter.
5. Every month or so check the balance of your battery. If it is going out of balance do another balance charge with the PL8

This will protect your battery from being over discharged or overcharged and you will always know how well balanced it is.

Simon

[karrak]

If you want to check the capacity of your battery after you have top balanced it all you have to do is fully charge it, either with your Classic or the PL8 and then discharge it using your PL8 until the first cell reaches 2.8 volt. It is that simple.

[Sunking]

Willy T tried Sunking's method and had similar problems to you. He now top balances. I don't know of one off-grid system that uses solar to charge a battery that uses bottom balancing. Sunking doesn't do it.

Simon you are one big liar. Willy never tried anything and you are wrong about me.

[createthis]

FWIW, I'm seeing only 60ah go in when I charge from 2.5V to 3.35V in a cell. Seems like charging to 13.35v cuts out about 40ah capacity.

[Sunking]

FWIW, I'm seeing only 60ah go in when I charge from 2.5V to 3.6V in a cell.

How are you terminating the charge?

If you terminate when the current tapers to 2 to 4 amps @ 3.6 volts is 100% SOC. In other words saturated.

OTOH if you terminate as soon as a cell voltage reaches 3.6 volts, then yeah it depends on how fast you are charging. Charging fast about 60 to 80%, if charging slower than say C/4 80 to 90%

With the lower end Chi-Coms you hit a wall on how fast they can be charged. Sure you can charge at 3C rate, but that still takes 1 hour plus to fully charge. The hump or hill depending on which Ch-Com we are talking about is around C/4 to C/2. All has to do with Internal Resistance, Chi-Coms are high, and GBS is the highest.

For lithium Chi-Coms the Voltage Set Point you select like 3.6 will be reached sooner than one would like depending on charge rate. The faster you arrive at Set Point, the longer it takes to saturate (charge current tapers to 0)Charge at C/2 you reach 3.6 volts but only gets you 60^ and another hour to Saturate to 100%. Charge at C/4 and you reach 3.6 volts @ say 90% SOC and a short 15 minutes to Saturate. Charge at C/20and you are at 100% as soon as you reach 3.6 volts.

It is all about resistance.

[karrak]

FWIW, I'm seeing only 60ah go in when I charge from 2.5V to 3.6V in a cell. Seems like charging to 13.4v cuts out about 40ah capacity.

Did you mean charging from 2.5V to 3.35V? Depending on the charging current I could well believe 13.4 volts will charge to an SOC of only 60%.

Could you give some more details of how you discharged and charged the battery.

Simon

[createthis]

Did you mean charging from 2.5V to 3.35V? Depending on the charging current I could well believe 13.4 volts will charge to an SOC of only 60%.

Could you give some more details of how you discharged and charged the battery.

Simon

gah. I did. I've got so many voltages floating around in my head they just leap out of my fingers by themselves, I guess. I'm using a Powerlab 8 v2 for these tests, and I've got nice pretty graphs saved for every test I've run this time.

Charging to 3.6v (visible high knee in the graph) gives 108ah, but I only get 103ah ( 95% ) out when discharging. Charging to 3.35v gives 61ah, and I get 59.55ah ( 97% ) out when discharging. I'm currently trying a charge to 3.4v. I expect to get most of the 108ah this time (and hopefully no high knee, or very little), according to this graph (from this page: http://www.powerstream.com/lithium-p...ge-voltage.htm ):

I understand why Sunking recommended 3.35v for floating now. There seems to be some debate on the internet as to whether or not float is harmful at this voltage. Having performed more research, the overall opinion seems to be that float is indeed bad at all voltages, but maybe it matters less at the 50% charged range because battery cycle life is so long at this range. My understanding is that floating a LifePO4 causes micro cycles in the mah range (charge, then discharge, repeat very quickly), and the LifePO4 chemistry sees these as real cycles, reducing the life of the battery each time.

I don't know why, but previously I was under the assumption that floating was bad because it would overcharge the battery. That doesn't appear to be the problem at all, so at least I cleared up one misconception.

Unfortunately, if you don't float, you can't really preserve your battery's charge and pull most of your load from the solar panels. That's a frustrating trade-off for solar.

I have to wonder how Genasun's MPPT charge controllers and Victron's 100/30 charge controllers handle this situation when they're programmed with a LifePO4 profile. I don't see any mention of it in their user manuals.

[Sunking]

I understand why Sunking recommended 3.35v for floating now. There seems to be some debate on the internet as to whether or not float is harmful at this voltage.

You see I know what I am talking about.

There is absolutely no problem Floating Lithium Batteries. Every commercial EV and Laptop floats lithium batteries. If you stop and think about it, you would figure it out all by yourself. Every EV and Laptop use parallel/series connections. So ask yourself what happens if we take two batteries and put them in parallel? You have a Float Charger. Only one trick you have to remember; DO NOT FLOAT AT 100% SOC. Some commercial Lithium Battery Chargers are Float and never turn off.

I have to wonder how Genasun's MPPT charge controllers and Victron's 100/30 charge controllers handle this situation when they're programmed with a LifePO4 profile. I don't see any mention of it in their user manuals.

There is nothing special or complicated. about Genasun or Victron controllers. They are less complicated than those made for Pb batteries. Lithium only requires 1-Stage, whereas Pb require up to 4-Stages.

They are very simple Float Chargers with only 1 single voltage setting. You set them to 14.0 to 14.2 volts and forget about it. When the batteries saturate, if there is a demand by load equipment for current, and the panels can supply it, the current comes from the panels. Otherwise from the batteries like any battery system with any battery type. You keep getting stuck there is something different about LFP charging/discharging when in fact there is nothing different about them you need to concern yourself with.

As I told you several weeks ago, if you want to nick pick, you have to find the right voltage set point.

[createthis]

I'm ignoring your last post Sunking, because you seem to be contradicting yourself. You state "You keep getting stuck there is something different about LFP charging/discharging when in fact there is nothing different about them you need to concern yourself with." right after you state "DO NOT FLOAT AT 100% SOC."

Reading the Genasun specs for the 10A 140W lithium charger, it reads like it floats at 12.5v, but that might be a typo. I'm not sure.

The Victron user manual for the 100/50 mppt is a little more specific. Absorption 14.2v, float 13.5v.

[createthis]

Asked Victron about this via email:

Hello,
I'm thinking about purchasing your 100/50 MPPT for use with a 100ah 12v LifePO4 battery.

I've read that LifePO4 batteries should not be floated, but I see in your user manual that this charge controller floats at 13.5v. Is this a constant float voltage at 100% SOC? Won't that reduce battery life?

Thanks,

Got this response:

No we do not think , that this will reduce the battery life and that is why we reduce the voltage absorption and float with lithium,