Lithium battery charge profiles

[Vic]

...   So I guess that means it goes from constant current to constant voltage at certain point.[/b]

That sounds like the transition point from BULK to ABSORB to me........??

BUT,  Bulk is not Constant Current,  it (to me) is;  Maximum Available Current,  until the Absorb set-point is reached,  then,  it is Constant Voltage ...  IMO.

Vic

[ClassicCrazy]

...   So I guess that means it goes from constant current to constant voltage at certain point.[/b]

That sounds like the transition point from BULK to ABSORB to me........??

BUT,  Bulk is not Constant Current,  it (to me) is;  Maximum Available Current,  until the Absorb set-point is reached,  then,  it is Constant Voltage ...  IMO.

Vic

And if you were to limit the input or output on Classic  then the power would not be available to loads. Someone asked on another thread recently about a feature that would limit what goes to the battery only - that might do it - so if you set certain number of  amps to battery then it could never exceed that but full array potential could be available for loads.

Larry

[Cniemand]

I purchased the Midnite Solar 200v Classic specifically because I went the Lithium route. Really any controller that allows you to specifically change the voltage set points will be able to work with lithiums.

Lithiums (LiFePo4 in my case) will readily take the amps. I have my charge controller setup to BULK everything into it until it reaches 3.5v per cell (I have 16 cells so once it hits 55v it switches to ABSORD mode) Absorb mode holds it at a constant voltage of 3.5v per cell/ 55v until the current drops to 0.05C or 5amps in my case (100aH cells) which triggers it to go into FLOAT. I have my Float set to maintain them at 3.34v per cell.

Note: Due to not having a WHizbang JR to more accurately have the CC switch from Absorb to Float when it reaches 0.05C due to possibly high background loads I simply program the CC to use 7amps as the ENDING AMPS.

My Off-grid system has been working out perfectly for 2.5 years this way. The cells still remain within 15mV of each other.

[ClassicCrazy]

Thanks for the info on how you setup your Classic for your lifepo4 batteries .

[Cniemand]

No problem, Larry. 

[ClassicCrazy]

With your 100 AH pack - how many AH will you take out before you hit the voltage cutout that you don't want to go below ? No problems running your inverter ?

Larry

[Cniemand]

Without a Whizbang JR for more precise readings of aH in/out I simply guestimate. Rarely do I have the bank fall to 52v before i got to bed and wake up to the sun charging it back up. With the 5120 Watt-hours nominal in the bank, I make a point to use no more than 4 kwH average or 80% for cycle life longevity.

The inverter hasn't any problems. It is an Outback VFX3648. It cuts out 48v. I've only had it shut the system down once in the very beginning. 48v = 3v cells = 100% DOD. Not something you want to do a lot, but they recharged without incident.

I need to purchase or borrow an outback mate in order to change the voltage set points. I would move them to reflect a 90%DOD for system shutdown as that would be a rare occurence but still give you a safe margin. Side note: Also need to find a mate in order to change the charging set points to work with a lithium profile should I ever decide to purchase a backup generator. In the 2.5 years I have managed to not need one.

[ClassicCrazy]

You have me looking at the Calb 100 AH batteries now. I have been thinking of lithium. For now my battery location would get cold in the winter so not sure if that would be too good a thing for lithium or if they need to have temperature compensation setpoints like lead acid do.  Probably better to keep all the cells in series - though I have 24v system and inverter now.
I have an electric bicycle with 48v 10 AH lifepo4 battery on it that has been working great for more than 5 years even though it has been abused in many ways. I have always just relied on the battery charger and bms that came with it to keep it happy. I had problems with cell balance when the charger failed and I was trying to use different charger to do a few cells at a time - bad idea but I learned a lot.  I guess I have about 10,000 miles on that battery pack now though I swapped out a few cells. It is about 60 to 70% original capacity I am guessing.
I really would like to move the solar up to lithium .

[Cniemand]

I'm happy with Lithiums and the CALB grey cells. As far as I know the Calbs do not suffer from capacity loss as much as Pb cells do in colder temps although there still is an effect. The only thing to note is that they make a point of saying you should not charge up cells if they are below freezing (As it can create internal shorts?).

I have our bank sitting indoors below the CC. They do not off-gas thus no longer need to be relegated to a cold "outdoorish" setting. Plus with The smaller form-factor it is fairly easy to hide them inside. The entire dimensions of a 5 kwh bank of 16 cells is 11"x21"x8" @ 150lbs

[ClassicCrazy]

I found some good discussion on bottom balancing lithium batteries here - and some reasons why BMS is not needed.

https://www.solarpaneltalk.com/forum/off-grid-solar/batteries-energy-storage/18809-lithium-top-or-bottom-balance/page2

Larry

[mahendra]

"Depending on the BMS type used, I would think you should still be able to use diversion to use that extra energy.

The thing that would change would be the diversion voltage set point.  For instance, if the BMS doesn't
care what you give it after the battery is full, divert at Absorb.

Or, maybe divert at just slightly over the battery open circuit voltage.

As soon as you start diverting at lower than the battery's resting voltage, it won't be full anymore.

boB"


After reading this thread and specifically this post i would would like to conclude that this setting is ideal for solar/wind/hydro :

"As soon as you start diverting at lower than the battery's resting voltage, it won't be full anymore.

boB"

My reason is because from reading i have come to the understanding that it is not good for lithium to be at  100% they prefer 80-95% approximately.

My question however is what is the most ideal voltage setting for a Midnite classic in relation to the specific lithium.

Am I correct?Please correct me if i am wrong.

Also i understand the once your Midnite Classic has the latest firmware the Aux 2 can be utilized to stop charging lithium which function is this please list by name.

I am also optimistic about lithium since i have a failed cell in my bank and have to think of getting a new bank.It is practically the same cost for me at my location of residence compared to lead acid technology.
Thanks in advance.

[ClassicCrazy]

Aux 2 used with the Whizbang can control the battery to stop charging   with the End Amps setting - when End Amps is setup it will go from Absorb to Float at whatever is in End Amps .

Larry

[mahendra]

point taken i do have a whiz bang.

[mahendra]

is it then logical to make use of this to not achieve 100% soc

[boB]

is it then logical to make use of this to not achieve 100% soc

You might actually be able to use the WB Jr. and Aux 1 SOC% function to operate
a relay or SSR or something when the SOC reaches 95% or whatever ?

boB