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16s Lifepo4 48v Bulk/Float settings

drbytes

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Oct 21, 2021
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Hi

I'd like to know what the correct charging / cutoff settings are for a 16s 304AH Lifepo4 battery bank. I currently have this setup :

1642623533005.png

I have no idea what to enter for float charge and bulk charge, I did read that floating is a bad idea with Lifepo4 and therefore I decided to enter 16 times the max safe voltage of a single cell; 58.4v.
For the bulk I found some information that it should be around 3.55v per cell, so 56.8v.

Am I on the right track with this ?

PS: I got most of the info out of this article : https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/
 
I have no idea what to enter for float charge and bulk charge, I did read that floating is a bad idea with Lifepo4 and therefore I decided to enter 16 times the max safe voltage of a single cell; 58.4v.

I don't understand this logic at all. That logic would be more appropriate for bulk/absorption. Float is to specify the voltage needed to "float" the batteries at 100% charge and is derived from lead-acid terminology.

If you're using solar, you must have a float voltage specified, or the solar will allow the battery to power the loads.

If you're not going to use the battery for anything after charging, no float is needed.

For the bulk I found some information that it should be around 3.55v per cell, so 56.8v.

Am I on the right track with this ?

PS: I got most of the info out of this article : https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

3.55V/cell is good for generator charging.
If charging with grid/solar where you're not in a hurry, 3.45V/cell is less stressful to cells but increases the charge time and will get to about 98% full.

Float 3.4V/cell
 
I think I need to understand what float actually means. Thx for the advice, I'm going to use your recommendation and school myself. :)
 
If I set float to 3.4 per cell, the Growatt stops charging the battery and won't go past 3.4v/cell
So my initial thought of setting it as high as possible, for this inverter, seems to be the way to go.
 
If I set float to 3.4 per cell, the Growatt stops charging the battery and won't go past 3.4v/cell
So my initial thought of setting it as high as possible, for this inverter, seems to be the way to go.
Why do you want the charge voltage to go as high as possible? The higher the charge voltage, the lower the cycle life. My 16S packs float at 54.5V.
 
Because I'm possibly misinformed, so could be temporary ignorance.
However, one thing I am sure of is that I do not want to baby this battery, it's been bought to store as much as possible. Everywhere I look it says the safe operation window of a cell is between 2.75 and 3.65v.
From that I deduce I have a cut off at the low end of 44v with a maximum of 58.4v.

If I stop charging at 54.5 on a sunny day I'm leaving a lot of watts on the table that would I need to pull from the grid later at night.

I'm just following the specifications from the manufacturer, really.

I suppose I would have to figure out how much AH are actually in between 54.5 and 58.4. If it is negligible and seriously lowers the pack life I would then ofcourse float at 54.5v.
 
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Because I'm possibly misinformed, so could be temporary ignorance.
However, one thing I am sure of is that I do not want to baby this battery, it's been bought to store as much as possible. Everywhere I look it says the safe operation window of a cell is between 2.75 and 3.65v.
From that I deduce I have a cut off at the low end of 44v with a maximum of 58.4v.

If I stop charging at 54.5 on a sunny day I'm leaving a lot of watts on the table that would I need to pull from the grid later at night.

I'm just following the specifications from the manufacturer, really.

I suppose I would have to figure out how much AH are actually in between 54.5 and 58.4. If it is negligible and seriously lowers the pack life I would then ofcourse float at 54.5v.
If you aren't trying to maximize cycle life then bulk charging to 3.65V per cell is fine (assuming your single cell BMS overvoltage cut off isn't much higher than that). I definitely wouldn't float that high though unless you're in a situation where the float charge will never be for very long.
 
@niktak11 : The plan is to capture as much solar to power the loads at night. With the stipulation that all solar power goes into powering the loads first, only charge battery second. So at maximum it would float 6 hrs a day, mid summer, for about 2 months in the year where I live (Belgium).

@smoothJoey : No, I don't want to abuse it. I want it to perform optimally and I was under the impression that I was doing that by staying under 3.65v.
 
@smoothJoey : No, I don't want to abuse it. I want it to perform optimally and I was under the impression that I was doing that by staying under 3.65v.
Charging:
Its generally necessary to charge to 3.45 volts per cell or higher to get an lfp battery full.
Charger with higher voltages just gets it full faster.
As in more time in constant current and less time in constant voltage.
In order to maintain the top balance its generally required to regularly charge into the high knee, especially for commodity cells that are popular on this forum.
But you don't want your BMS to trip on cell overvoltage during normal operation, that is abuse of the BMS.
If your battery can be charged to 3.65 volts per cell without any cell exceeding 3.65 volts you have a perfectly balanced pack and its not abuse to charge that high.
But... that doesn't happen in the real world with any cells I've ever heard of.

Discharging:
You also don't want your BMS to trip on cell undervoltage, that is also abuse of the BMS.
There is ~15% of the battery capacity in the low knee.
The further you discharge into the low knee the more stress you place on the battery.
The cells datasheet says you can discharge as low as 2.5 volts per cell but the reality is you can only discharge until the weakest cell hits 2.5 volts per cell.
The weakest cell dictates the capacity of the battery.
Since the cells are top balanced for our application one cell typically nosedives before the others.
That one cell that is approaching 2.5 volts before the others is getting stress that the other cells are not getting.
Its already the weakest cell and the added stress is making it even weaker.
The other problem is the lower you discharge that cell the more likely it is to drift and mess up your top balance.

Summary:
Charging into the high knee is a necessary stressor.
Discharging the weak cell into the low knee should be avoided if possible.
To find out where the weak cell nosedives discharge your pack at a representative load and observe where the voltage drops off.
 
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This is excellent information!
For 8S :) But I get it now.. you charge to 3.45 to get really close to the packs true capacity without hitting the cells too hard, once a month they get charged to 3.65 to top balance.

I now need to monitor which cell is my weakest link and cut off just above that.
Thx!
 
This is excellent information!
For 8S :) But I get it now.. you charge to 3.45 to get really close to the packs true capacity without hitting the cells too hard, once a month they get charged to 3.65 to top balance.
I will update to make it generic.
I suggest 3.55 nightly if your BMS doesn't trip.
If it does titrate between 3.45 and 3.55.
The balance should improve if the bms doesn't trip.
Almost all diy packs will trip the bms if charged to 3.65 so I don't recommend that.
 
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If I set float to 3.4 per cell, the Growatt stops charging the battery and won't go past 3.4v/cell
So my initial thought of setting it as high as possible, for this inverter, seems to be the way to go.

You didn't mention you had a growatt. Those settings only apply for solar charging on a Growatt.

The Growatt AC charger will not do bulk/absorption/float. It is a simple ON/OFF based on settings 12 and 13. It will being charging when the voltage drops below setting 12 and stop charging the instant it hits setting 13.

If you are trying to fully charge/float on a Growatt with AC current, you can't. Set 13 to 3.55V/cell and you'll get close to fully charged.
 

pls. check the above youtube link.
Mr. Andy - off-grid garage clearly explained about lifepo4 battery charge setting..............
thanks to Uncle Andy :love:
 
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