Help figuring out LiFePo4 charge settings for Classic 150

[ScottTENN]

I'll be honest.   I'm scared.  Scared to damage expensive batteries and the really bad fire.  Joke but not a joke.
Been doing this for several years now, from wet to SLA to now LiFePo4.

What I'd like is a review of my configuration based on the battery specs attached.  If someone would be so kind that would be fantastic.
I picked these specs because of another forum user stating almost identical specs.

24v system  (2 batteries in series)  2000w (1000w stationary roof -S facing, 1000w adjustable pitch -SSE facing)

Absorb 27.7v (15 minutes)
Equalized (Auto unchecked)
Float 27.6v
Limits :
  Min Volts 22
  Max Volts 28.8
Compensate Equalize Voltage (unchecked) but set at -0.5 at 25c
Battery Status Meter 99% eff%, 300Ah, 0%/c
MPPT Mode Sweep Interval 3 (min)
Ending Amps 15 (Use Whiz Bang Jr.)
Rebulk Volts 26.4

Today was my first day with this setup.  What I saw as a long float with high amps to start.  First hour was in the mid 30s for amps.  I'm 3 hours into float and 27.6v at ~6amp.  Takes 4 just to run the inverter.  My nightly draw is small for the moment and the batteries are charged before high noon.  So I have yet to see full panel production for a recharge cycle.  (If I run some devices I can see the panels compensate.)

Thanks for any input.

[mobywile]

Not an expert here, but I have been living with LFPs for 2 years now.  Love 'em.
Did you check with the manufacturer about series connections?  My information is to NEVER connect LFPs in series.  The BMS can't perform correctly, I believe.
Your numbers look OK, but you will need to tweak them as time passes.  I have my Absorb set at 28.0v for 6 minutes.  Float at 27.4v.
I would be interested whose batteries you purchased.  I have 6 of the Simpliphi 3.8, 24v in parallel.

Moby

[ScottTENN]

Manufacturer states that you can have 4 max in series.

What makes you look to change over time?  My big scare is the under and overcharge.  With a Pb voltage and hydrometer can give you a visible clues as to the batteries state.  LP, not so much.  Voltage is the same even at a deep discharge.

What do you look for to know when to tweak?

[Steve_S]

EndAmps / TailCurrent is 0.5xAH Rating of the largest pack.  24V/280AH = 15A for example.
I've doc'd a LOT of LFP info already in the Lithium Sub-Forum. 
Your settings are close to mine and I run 1190AH / 30.4kWh of LFP.

All of that is in the Lithium SubForum.

FYI.  LFP can take 1C Charge Rate and 0.5C Discharge Rate. 
For a 24V/100AH that translates as 100A Max Charge CC to 90% then CV (Float) Constant Voltage Variable Amp to finish to 100%.   For Discharge it means a Max of 50A per battery pack in Parallel.

There are few limits with Batteries in PARALLEL as Parallel only adds AH of Capacity and no voltage variation.  SERIES is another issue as each BMS has to be capable of it, Series increases Voltage not Store Capacity and many BMS' are not capable or are limited due to the FETS used in the BMS'.

NOTE:  Battery Packs in Parallel will share & divide Load & Charge relative to each battery packs capacity and SOC.

In Lithium Land, we use SOC not DOD and view C-Rates slightly differently, the terminology differences that everyone MUST be aware of... it's not Apples to Apples.

BTW:  I am presently chaging with Solar & Genset pushing 175A at my bank, no problems...  LFP also charges FASTER than Lead, so be prepapered for that - takes some getting used to.

Hope it helps, Good Luck.

[ScottTENN]

Steve_S, it was your post on http://midniteftp.com/forum/index.php?topic=5690.0 that I based mine off of with small tweaks based on what I understood from my batteries manual.

I'll look for the Lithium sub-forum.

[Steve_S]

I do a lot on DIYSolarForum and have many docs, guides and whatnot on there and sometimes I forget what I've posted on here and elsewhere.  I am now in process of wrapping up several things and prepping everything for my departure, so I will be churning out one or two more docs to complete things and then I am done.  Long story short, I am terminal and will not see the end of 2022.

[boB]

I would think that if kept independent and the BMS is any good, that you should be able to add almost as many Li batteries in series as you want.

No tapping in-between BMS's and no communications to the outside world because of voltage limits.

When a manufacturer of Li with BMS says not to series too many or none at all, that is most likely because their BMS balancing techniques are too weak.

boB

[australsolarier]

bob,
it has something to do with the max voltage of the mosfets.

[ClassicCrazy]

bob,
it has something to do with the max voltage of the mosfets.

yeah I just watched a video by Will Prowse where he tore apart some lithium 12v and he mentioned how one brand you can series them, but another you can't .
He also finds different brands have some sloppy builds where they put the high temperture sensor in wrong place , and some of them didn't install a low temp cutoff sensor even though the bms had a place to put one on.  He puts the sensor in some ice water and sees if it cuts off charging . Thing is you wouldn't know how they are built - he has to cut the tops off and take them apart . So at least you get some indication on the particular brands he does tear downs on.
Seems like most good deal lithium packs are sold out these days. Kind of wish I had been one of the people who took advantage of a 48v 100 ah server rack lithium for $1500 or whatever they were going for. The ones I looked at recently online are about $1000 more these days if they have them.
Maybe that is now the more realistic price .
Larry

[boB]

bob,
it has something to do with the max voltage of the mosfets.

True.  May not be certain instant death though.

The only problem would be when one pair of battery back-to-back FETs turn OFF and there is too high of voltage across that pair of MOSFETs ratings from the remaining batteries that are still on.

As long as the FETs do not turn off, you're OK.  That can't be guaranteed of course.

One way to fix that would be for the batteries to be placed in series to use only ONE pair of MOSFETs that are rated for high enough voltage OR to have all of the FETs turn off at the same time.  The system would have to be designed for that so that isn't necessarily a good fix either.

Could also have an external voltage sense that, when too high, shorts out that series bank or offers better protection for the internal FET pair.   Maybe a new product idea for someone ?

boB

[Steve_S]

This may help.
BMS MOSFETs Explained
https://diysolarforum.com/threads/bms-mosfets-explained.29318/

[australsolarier]

bob, i agree with you there.
there are many of those 12V lifepo4 packs with built in BMS's on the market. some of them specify you can put 4 in series. so that is the easiest to buy. i suppose they are more for the beginner hobbyists.
i myself settled on the Batrium BMS, i can see in the computer room what each cell does, particularly in absorption.
(in the small 12V system though i hooked up a few 12V lifepo4 batteries in parallel with the built in BMS. this is sort of neat, you just hook it up and forget about it.)

[australsolarier]

steve_s
thanks for that link. that thread is well worth reading. does not explain anything about putting the BMS's in series though.