Charging method lifepo4 280AH EVE (float or not.)

[DiyNuke]

Alright so ive been tinkering a bit. ive made an schematic. so far so good. i was planning to use boost converters as the charge controllers.
and set them to the voltage to charge the 16cells to about 70% and when i flip a switch to 90% when i go for an long trip

However these would be constant power supply's then. so charging would be fine but can i leave them on? so that it will have that float voltage all the time with almost no current. or do i need to make an system to switch of the power supply when an full charge is reached/ almost no current is flowing

Ive been searching quite allot but i cant find it so fast. so maybe a link to a difrent thread or something. i would greatly appreciate that.


So core question. Can i leave a float voltage for 24/7?

If you see anything thats not connected right just let me know.
or any other suggestions.

And i am still in the design stage. i bought the cells so they should arrive in 30/45 days

 

[upnorthandpersonal]

When a LiFePO4 cell is full, the charger must disconnect. In other words, you can not leave a voltage on all the time.

One source for this that I consider recommended reading: https://files.ev-power.eu/inc/_auto88/_info/Doc/GWL-Power-Cell-Damage-OverCharge.pdf

 

[John Frum]

Its my understanding that for lifepo4 float voltage <=3.4 volts per cell should not cause undo stress to the battery.

 

[upnorthandpersonal]

Its my understanding that for lifepo4 float voltage <=3.4 volts per cell should not cause undo stress to the battery.

Victron even sets it at 3.525, but I would only do that when your cells actually are in use. If, like OP, you're away for a potentially long time (and I assume nothing is using power), I would not keep them at a high state of charge and have them disconnected from SCC and load.

 

[John Frum]

Victron even sets it at 3.525, but I would only do that when your cells actually are in use. If, like OP, you're away for a potentially long time (and I assume nothing is using power), I would not keep them at a high state of charge and have them disconnected from SCC and load.

Perhaps OP should do one more mode called "storage float" or something like that floats the batteries at 3.2 volts per cell.
To the best of my knowledge the things that hurt a lifepo4 battery by stack rank are.

1. charge below 0 celcius.

2. discharge below ~2.5 volts
2. excess heat
2. charge above ~3.65 volts

3. everything else

As long as the batteries are not too hot and not too cold I think its not a big deal.

 

[gnubie]

Victron's lifepo4 preset, last time I checked, pulls a 24v battery up to 28.4v, and in the absence of a shunt (built in or add on) sit it there for 2 hours before releasing it back to 27v where it holds the battery indefinitely. I can see a point that you don't want long term storage while being held here, if that is the application, disconnect the battery. Self discharge is so low that it'll still be in a good state of charge 3 months later.

Victron doesn't use cheapie cells in its batteries, so that is something to consider, but if their research has shown to them that this is an acceptable thing to do, that's good enough for me given Victron's reputation. I have reduced my own battery's "absorption" (yes yes, no need to bring all that up again please) time to 15 mins because that allows plenty of time for current to drop to a low enough point in my situation.

 

[DiyNuke]

Thanks for the reply guys.

When a LiFePO4 cell is full, the charger must disconnect. In other words, you can not leave a voltage on all the time.

One source for this that I consider recommended reading: https://files.ev-power.eu/inc/_auto88/_info/Doc/GWL-Power-Cell-Damage-OverCharge.pdf

Alright so if i add an timer to the charger. lets say 10h 20h 30h. and after that it will just rely on the capacity then. the coulumb meter should be able to run for almost an year with close to 500Ah 24v (pulls 0.6Ma)

the 70% booster charges with 15A so lets say 23.2H to charge to 70% from 10%
the 90% booster charges with 30A so lets say 15.45h to charge to 90% from 10%
so i should be able to just use an basic timer and not have it floating for too long. and if i know i wont be there for a long time ill just flip the main battery switches. (with an bridge between 1&2 to avoid exessive current when they connect parralel again.)

might do it with an plc since i can get them for an decent price.

And i will not charge it to 3.65v per cell. i dont need to have it full all the time. not worth the reduced cycle life. and yeah its an pretty slow charge then. but its an boat so i probbaly wont be driving it for more than 2 days.

the battery should last 6.6h when pulling 70A (Full throtle) from it which is more than enough to go boating for 2 days.



Correct me if am wrong

 

[DiyNuke]

Okay i just found an good article and now i get it a bit more.

How good is the victron blue smart? if i just buy that if i want to charge it slowly and leave it unatended for a few days and then just disconnected the battery. it should be alright then. just because i dont always trust timers. if it jams it keeps charging and then 1800bucks of batteries is wasted,
instead of 130bucks for an victron

Looking at the charging method from an CTEK charger (lifepo4) it just keeps it at 90%-95% 24/7 so that wouldnt be good at all.

 

[gnubie]

I've never had a Victron do anything unexpected. They are a very reliable unit. You can even set a lower current limit than the model's default (100/50 = 100v max PV, 50 amps) if you want to charge the battery particularly gently. The Victrons charging patterns don't allow for a complete halt once the battery hits the full charge threshold. They will drop back down to 'float' voltage and sit there indefinitely (ie as long as power is applied to the PV input) but the preset lifepo4 float voltage is generally considered to be OK for situations where the battery is not used for a few days. If that voltage is a concern (27v for a 24v battery, 3.375v per cell) a lower float stage voltage could be configured via the smartphone app or Windows program.

 

[John Frum]

They will drop back down to 'float' voltage and sit there indefinitely (ie as long as power is applied to the PV input) but the preset lifepo4 float voltage is generally considered to be OK for situations where the battery is not used for a few days.

From memory, Battleborn says you can leave their batteries full with a charge voltage of 14.6 for a "couple" of weeks.

 

[DiyNuke]

ah okay. well then ill buy the small victron charger.

And i bought 2 timers which i can set from 1h-270h so i can let it charge for 20h and then stop it and then just leave the system. and then maybe a timer which turns on the victron once a month. for a day then i am in full control. and since the battery doesnt mind being at 50% at all.

ill get rid of the 70% and 90% circuit then since it cant really be done only using voltage so ill just use the 2 boosters with 15A each.
working togheter with the temperature cutoff which also switches of the victron. otherwise it could start charging in the winter if i dont add an 40$ temp sensor


Why 2 timers? its chinese so if i put them in series and 1 fails the other one isnt as likely to fail at the same time. so a bit of redundancy
overthinking it? maybe a bit but i just love all those small details. (which can fail. but its an boat so i gotta do something xD)

 

[DiyNuke]

From memory, Battleborn says you can leave their batteries full with a charge voltage of 14.6 for a "couple" of weeks.

but its kind of wierd to think that the cells can last so much longer just by not storing it at full capacity. and yet most chargers keep them almost at 100% besides the mastervolt victron and some other chargers then. (manualy set lower)

 

[John Frum]

but its kind of wierd to think that the cells can last so much longer just by not storing it at full capacity. and yet most chargers keep them almost at 100% besides the mastervolt victron and some other chargers then. (manualy set lower)

Quantify "so much longer" and who says?

 

[John Frum]

BTW 3.4 per is about 95% SOC.

How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries –...

By: Rob Beckers You have just sold your first-born into slavery, remortgaged the house, and bought yourself a lithium-ion battery! Now you want to know how to take care of your precious new purchase: How to best charge lithium-iron batteries, how to discharge them, and how to get the maximum...

www.solacity.com

 

[Ampster]

BTW 3.4 per is about 95% SOC.

How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries –...

By: Rob Beckers You have just sold your first-born into slavery, remortgaged the house, and bought yourself a lithium-ion battery! Now you want to know how to take care of your precious new purchase: How to best charge lithium-iron batteries, how to discharge them, and how to get the maximum...

www.solacity.com

That is the charging voltage, correct. They will still settle at 3.30 to 3.35.

 

[MisterSandals]

They will still settle at 3.30 to 3.35.

I think the OP should find where HIS batteries/cells settle and put the float just below that. Otherwise your cells would charge to full and then within minutes settle and charge back up to full again.

This could happen dozens of times a day. Nobody talks about micro-cycling but I'm pretty sure it cannot be good.

My Victron 100/30 does not "hold at float". When voltage drops to float value, it goes into bulk, then absorb and when full, coasts in the float stage.

 

[gnubie]

My Victron 100/50 bluesolar unit running the latest firmware behaves as below, but the same behaviour seen with all prior versions that have ever been installed on it:

- morning, battery is in some state of discharge, charger goes to bulk mode once sufficient light is available and stays there until battery reaches the target voltage (28.4v)
- switches to 'absorption' stage once target voltage reached, for my configuration hold here for 15 mins, then fall back to float
- once in float it stays and holds 27v with as much current as the solar array can provide to support any load on the system, even in the face of minor discharges it holds in float mode and returns to 27v if there was a slight drop, however should the load be too much for the watts available from the array for too long voltage will fall to the rebulk setting
- as soon as rebulk voltage is reached it will change back to bulk mode, commence pulling the battery back up to the target voltage (28.4) and upon reaching that return to float mode without lingering at 28.4v.

I guess it comes down to how one describes "hold at float", but given the behaviour of the unit once float stage is reached the description seems pretty apt to me.

*edits to improve wording to avoid misinterpretation of what I've posted*

 

[MisterSandals]

I guess it comes down to how one describes "hold at float",

To me, "hold at float" would mean maintaining the float voltage.
Restarting a complete charge cycle until fully charged is NOT what i would EVER call "hold at float".

But, i do not control the wording.

 

[gnubie]

I can make a video or put up a web cam (done that already for something else) if you'd like to see my Victron charger in action, but mine definitely carries out the behaviour I described and that is quite well described "hold at float". What else could one call

- once in float it stays and holds 27v with as much current as the solar array can provide to support any load on the system, even in the face of minor discharges it holds in float mode and returns to 27v if there was a slight drop, however should the load be too much for the watts available from the array for too long voltage will fall to the rebulk setting

that?

From my simple on screen monitor. It has been holding float mode and voltage for 23 minutes so far today, and it will stay there until array power no longer permits it to stay there (26.98 is not sufficiently lower to trigger rebulk, and look at that, it's bobbed up to 27v again as I type all the while staying in float mode)

 

[MisterSandals]

the behaviour I described and that is quite well described "hold at float".

I think we are in agreement. :*)