Charging LifePO4 parameters (not same questions as the other title)

[harpo]

Ok, I have read a number of discussions hear on charging parameters for Lithium Iron Phosphate but none really answer my questions. Hopefully, someone can succinctly answer them.
First, as I understand it, LifePO4 batteries should be charged at a constant voltage. No need for the three stage charging I have done with AGM. Is this correct?

Secondly, if that is the case and I want to charge my 12v LifePO4 battery pack to 90%, what voltage should my charging systems by set at? First the AC charger (if I can get a charger to charge at a constant voltage only) and secondly, my MPPT charge controller.

It seems universal that if one stays in a discharge range between 10 and 90% of SOC that battery life will be optimized. How is this best accomplished using both methods of charging indicated above?

Thanks in advance. Any links on the subject appreciated.

 

[MrNatural22]

@harpo if you would like to delete your post push the report bottom on the bottom left of your post and ask one of the mods to delete it.
?

 

[snoobler]

First, no. Constant current with optional constant voltage. A 3 phase (bulk/absorp/float) charge cycle is actually quite suitable, but not necessary.

Second, depends on charge current and capacity or "C" rate.

The best way to satisfy your desire is to use a coulomb counting battery monitor, start at fully charged, discharge with typical loads to 10% - there you go. Use that voltage. In the absence of test data, 2.9-3.0V/cell is a popular target and works for many cases as it's around where the knee starts

The upper knee can be elusive and may never occur until at near 100% SoC with low charge currents. Recharge the battery via the typical method and current you anticipate always using. Once your battery monitor hits 90%, use that voltage.

On float, be advised that the battery will still slowly charge at 3.4V/cell until it is 95% full, so if you don't want charge during float, you'll need to lower the float to 3.3V/cell.

 

[MikeT357]

I charge my individual LiFePo4 cells with a buck converter. Before I hook up the buck converter I make sure to adjust its max voltage. 3.6v if you want a 100% charge. 3.25v if you want a 80% charge. Once hooked up I adjust the max current to 2 amps. The buck converter automatically shuts off when the preset max voltage is reached
When using: I have a boost converter that raises the output voltage to either 6v or 12v. The boost converter shuts off when the battery voltage drops below 2v.

so, I use max voltage and constant current. Max voltage is the must critical. Anything over the recommend max charge voltage will ruin the battery.
If you run, say, 4 cells in series, the max voltage is 3.6v X 4.

 

[harpo]

First, no. Constant current with optional constant voltage. A 3 phase (bulk/absorp/float) charge cycle is actually quite suitable, but not necessary.

Second, depends on charge current and capacity or "C" rate.

The best way to satisfy your desire is to use a coulomb counting battery monitor, start at fully charged, discharge with typical loads to 10% - there you go. Use that voltage. In the absence of test data, 2.9-3.0V/cell is a popular target and works for many cases as it's around where the knee starts

The upper knee can be elusive and may never occur until at near 100% SoC with low charge currents. Recharge the battery via the typical method and current you anticipate always using. Once your battery monitor hits 90%, use that voltage.

On float, be advised that the battery will still slowly charge at 3.4V/cell until it is 95% full, so if you don't want charge during float, you'll need to lower the float to 3.3V/cell.

So I am getting a bit confused. If I have four 3.2v cells to make a 12v battery and I want to charge them with both solar and/or external charger what are my parameters? I thought one would look at the data sheet and for a 12v setup the recommendation charging voltage is 14.6. So I am thinking "constant voltage." But, you are saying constant Current NOT constant voltage.
In my experience with chargers and charge controllers one sets the charging voltage and the current is modulated by the charging device. Are saying to throw "X" amps at whatever voltage when you say charge with constant current?? (I doubt this, I just don't quite understand).

What type of charger would I purchase to do the charging on LifePO4 batteries. I presently have AGM's and using a cotek 35A charger when my solar panels don't provide enough charge.

I sure do wish there was a definitive "book" on LifePO4 cell chemistry so someone like myself could find answers to the many questions we have when attempting to convert from lead acid to LifePO4. Seems like everyone using this technology has different ideas on charging/discharging. It is very difficult for us trying to figure this all out. Right here on this Forum there is very little agreement on many aspects of LifePO4 cells. Charging/discharging is just one example. Using the entire capacity of the battery vs. using only the 10-90% window for battery longevity is another. There is no agreement on this of whether only using part of the battery's capacity increases cycle life. There are other disagreements and theories. Some appear to have logic behind them others it seems to be personal preference but very STRONG personal preferences.

Then there is the whole grade A vs. grade B debate. It appears there really is NO way to tell what you are going to receive when you order for instance 280Ah cells from China. The only way seems to be to test what you get when they arrive. Some here have gone so far as to say one should order a couple more than what you need because it is likely one or two of the shipped cells will likely be significantly lower than the rest of the shipment. WHAT? What kind of marketplace is this? Order stated grade A cells and one may or may not get grade A cells-----you'll just have to test them to see what you get. SURPRISE,,,,,,,,,,,a couple cells exceed the data sheet stats but most do not. And, seemingly, this is accepted because it seems everyone just accepts that's the way it is. Am I the only one that thinks this is like a bad movie and it just keeps playing over and over again?

 

[snoobler]

You may not understand the concept of constant voltage charging. If you're charging and the voltage isn't constant (it's rising), you're not charging at constant voltage. You are applying a constant current to the battery as the voltage rises. Once the voltage hits the programmed peak voltage, it transitions to constant voltage charging and lowers the current to maintain the constant voltage. The image below depicts the charge profile for a lead acid battery, which is conceptually the same for LFP:


The difference with LFP is the first stage is the longest, stage 2 is optional because the battery is mostly charged after stage 1 and stage 3 is completely unnecessary as LFP doesn't self-discharge to any significant degree.

However, in cyclic applications, stage 3 is useful because it nails down the voltage the SCC will attempt to maintain when the batteries are full and the solar is powering the loads.

Think of LFP as FLA/AGMs, except they can be at any state of charge without degradation. They do not need to be fully charged every day like FLA/AGM and can last much longer. LFP are good drop-in replacements for non-starter FLA/AGM. They work well with FLA/AGM chargers provided the chargers are compatible with the voltage requirements. Provided a charger bulk/absorbs at less than 14.6V and floats at less than 13.6V, it is safe to use with LFP.

It can be that simple. There are nuances. Folks get obsessed with what amount to fine details. They want to know EXACTLY how to get their battery to operate only within a 10-90% range or whatever. There are too many system variables to determine that outside of the system with any reliability.

It can be as simple or as difficult as you want.

Grade A vs. Grade B.

Sorry. I'm convinced no one is getting grade A unless they're paying a premium for them. I've tested 4 of 9 of my cells from Amy, and none meet the data sheet capacity. By definition, those are Grade B cells. They are otherwise very high quality and well performing grade B cells NEARLY meeting the capacity, and for the price, they are an incredible value. BTW, I'm one who recommends buying extra cells. Shit happens. It's how you exert more control over the situation. After waiting two months to get your cells, do you really want to wait another two months if one is damaged?

Lastly, you sound like the type that isn't okay unless it's zero risk or there are any un-ticked boxes. You should probably just buy some Battleborns and be done with it. It will cost you 6X as much for the same capacity, but it will remove all those details and all the uncertainty for you.

 

[Reed Cole]

People accept the downsides of buying cells direct from China in small amounts because the cost is so much less, not because they are happy with the downsides. Like I wouldn't be living in my van if housing were more affordable.

 

[harpo]

Thank

You may not understand the concept of constant voltage charging. If you're charging and the voltage isn't constant (it's rising), you're not charging at constant voltage. You are applying a constant current to the battery as the voltage rises. Once the voltage hits the programmed peak voltage, it transitions to constant voltage charging and lowers the current to maintain the constant voltage. The image below depicts the charge profile for a lead acid battery, which is conceptually the same for LFP:

View attachment 32656
The difference with LFP is the first stage is the longest, stage 2 is optional because the battery is mostly charged after stage 1 and stage 3 is completely unnecessary as LFP doesn't self-discharge to any significant degree.

However, in cyclic applications, stage 3 is useful because it nails down the voltage the SCC will attempt to maintain when the batteries are full and the solar is powering the loads.

Think of LFP as FLA/AGMs, except they can be at any state of charge without degradation. They do not need to be fully charged every day like FLA/AGM and can last much longer. LFP are good drop-in replacements for non-starter FLA/AGM. They work well with FLA/AGM chargers provided the chargers are compatible with the voltage requirements. Provided a charger bulk/absorbs at less than 14.6V and floats at less than 13.6V, it is safe to use with LFP.

It can be that simple. There are nuances. Folks get obsessed with what amount to fine details. They want to know EXACTLY how to get their battery to operate only within a 10-90% range or whatever. There are too many system variables to determine that outside of the system with any reliability.

It can be as simple or as difficult as you want.

Grade A vs. Grade B.

Sorry. I'm convinced no one is getting grade A unless they're paying a premium for them. I've tested 4 of 9 of my cells from Amy, and none meet the data sheet capacity. By definition, those are Grade B cells. They are otherwise very high quality and well performing grade B cells NEARLY meeting the capacity, and for the price, they are an incredible value. BTW, I'm one who recommends buying extra cells. Shit happens. It's how you exert more control over the situation. After waiting two months to get your cells, do you really want to wait another two months if one is damaged?

Lastly, you sound like the type that isn't okay unless it's zero risk or there are any un-ticked boxes. You should probably just buy some Battleborns and be done with it. It will cost you 6X as much for the same capacity, but it will remove all those details and all the uncertainty for you.

Thank you for the post and simplifying some things that many tend to make very complicated.
I have read this post several times trying to absorb it all but still have some questions: Back to constant voltage/constant current, right now I have set my CC as well as my Cotek battery charger (via AC current) at a bulk and absorption voltage of 14.4. Now, for instance, when I use the supplemental Cotek charger the voltage pops right to 14.4 and the amps are at first 30 or so (35amp charger) and declines as the battery becomes charged. Now is this constant voltage charging or isn't it? Seems to me it is as the voltage remains the same and amps change as the battery accepts a charge. I don't know, somewhere this is not sinking in for me.

Next, from what I can gather, there should be no Float voltage for LifePO4 batteries but most chargers have this feature and one can't disable it. Some chargers let you alter the float voltage, some don't. If left to float couldn't one conceivably overcharge a LifePO4 battery if there is no load placed on the battery soon after charging? One can turn off a charger to eliminate or lessen the float time but CC will just keep throwing the float voltage to the battery primarily because, I suppose, most all chargers on the market were designed for lead acid technology. So then what? Especially, if there will be no load on the battery for perhaps days?

 

[snoobler]

Charge current, wire gauge, length and connection quality also influence voltage. If you have thin, long or loose/poor connections, there will be a voltage drop between the charger and the battery where the charger measures a higher voltage than the battery sees. If the 14.4V is reported by the charger that may be the case. If it's measured at the battery, then that's not the case.

Your stage 1, constant current or bulk is very short. You then describe phase 2, constant voltage or absorption. State of charge also influences voltage. If your battery is already at a high state of charge, 80%+, what you describe is fairly normal for FLA/AGM though bulk should last a little longer than you describe. This leads me to believe that you are experiencing significant voltage drop between the charger and the battery.

LFP will not behave the same timing-wise. They will stay in the constant current phase for much longer. They will stay under 14V for a very long time provided the connection quality is good.

Float is not needed, but it is beneficial in these systems. Float also serves as the floor below which the charge controller will not let the battery go. If you disable float, the battery voltage will drop under load, and the SCC will not provide solar power until the battery drops to the re-bulk voltage potentially discharging your battery when you would prefer the solar to be providing that power and keeping your battery floated. Re-bulk triggers the whole bulk/absorption cycle again.

If you want the system to supply solar power to the loads and keep the batteries at the "full" charge you've chosen, and if you do not want to charge the batteries during float, select 13.2-13.4V for float. If you wish to allow very slow charging that may allow the battery to fill to 95%, select 13.6V.