Charging values

Buckworth · Aug 19, 2020

OK, my batteries arrived (3.2v 280ah lifep04).

Just confirming the rate and intensity they should be balanced at On a YouTube video the dude says put them in parallel, set the charger to 3.6v and crank up the amps until there's no current going into them, and this will mean they are balanced. Is this correct? Why not 3.2v?

Also, I have a configurable charger that I will run from a generator (if the bank gets too low at night). Should the bulk and float charges be set to 3.6v?

Thank you very much.

snoobler · Aug 19, 2020

3.2V is the nominal voltage. This can be thought of as an "average" voltage used for power and energy calculations.

tictag · Aug 20, 2020

Buckworth said:
and this will mean they are balanced. Is this correct?

Yes...and no. Yes, technically this is how to top balance your cells (though I would personally use 3.65V) but you never really know whether the cells actually stay balanced until you discharge them together - their voltages should stay pretty much identical i.e. within thousandths of a volt, across the entire discharge cycle.

Buckworth said:
Why not 3.2v?

Because 3.2V will not charge them.

Buckworth said:
Should the bulk and float charges be set to 3.6v?

No.

Bulk/Absorption Voltage = 3.65V per cell (to 100% SoC).
Float voltage = 3.3V per cell or slightly higher (e.g. 3.35V!) if you find the battery draining under loads that should be being powered by your SCC/charger. The higher your Float voltage above the cell's nominal voltage, the more damage will be done to the battery. This is not the case for lead-acid batteries.

It's a balancing act, enough Float voltage to power your DC loads from your SCC/charger, but not so much to cause any premature ageing of the battery.

Some people will just avoid Float charging altogether (lithium ion does not need, nor does it like, float charging), but I prefer to find that balance where the battery stays fully charged ready for the evening, or are full charged going into a poor weather spell.

You must make sure your charger does not go into Equalisation mode - this is strictly for lead-acid batteries (and then only occasionally) and will undoubtedly over-charge your battery or cause the BMS within it to shut-down. Most chargers will allow you to turn this off, or at least configure the equalisation charge time to 0 hours. If you can set the equalisation voltage, set this to your Float voltage (belt'n'braces).

Buckworth · Aug 21, 2020

tictag said:
Yes...and no. Yes, technically this is how to top balance your cells (though I would personally use 3.65V) but you never really know whether the cells actually stay balanced until you discharge them together - their voltages should stay pretty much identical i.e. within thousandths of a volt, across the entire discharge cycle.

Because 3.2V will not charge them.

No.

Bulk/Absorption Voltage = 3.65V per cell (to 100% SoC).
Float voltage = 3.3V per cell or slightly higher (e.g. 3.35V!) if you find the battery draining under loads that should be being powered by your SCC/charger. The higher your Float voltage above the cell's nominal voltage, the more damage will be done to the battery. This is not the case for lead-acid batteries.

It's a balancing act, enough Float voltage to power your DC loads from your SCC/charger, but not so much to cause any premature ageing of the battery.

Some people will just avoid Float charging altogether (lithium ion does not need, nor does it like, float charging), but I prefer to find that balance where the battery stays fully charged ready for the evening, or are full charged going into a poor weather spell.

You must make sure your charger does not go into Equalisation mode - this is strictly for lead-acid batteries (and then only occasionally) and will undoubtedly over-charge your battery or cause the BMS within it to shut-down. Most chargers will allow you to turn this off, or at least configure the equalisation charge time to 0 hours. If you can set the equalisation voltage, set this to your Float voltage (belt'n'braces).

Thank you VERY much for the detailed reply. I am doing a lot of reading and finding conflicted or very complicated explanations...which is understandable.

If I am understanding what you are saying, since I have a 4 cell 12v system I should set my bulk charge to 4x3.65v (14.6v), my float charge to 4x3.35v (13.4v), and disable equalization. I was reading that lifep04 can be safely overcharged at 4.2v per cell (16.8v for a 4 cell system?). Is this a thing?

I forgot to mention I have a solar charge controller that has a lithium setting...so these charging values would be for nighttime or dark weather charging from a generator.

Thanks again.

RCinFLA · Aug 21, 2020

Because LiFePO4 batteries have such a low equivalent resistance and shallow slope to discharge curve the only two points of high confidence of knowing state of charge is fully charged or fully discharged voltage.

High confidence point is 3.65v where charging current has dropped to low level is 100% state of charge.

The accuracy of reading will not be good enough for mid range state of charge determination for a typical consumer grade (cheap) DVM, . LiFePO4 cell has less than 50 mV difference in open circuit voltage between 50% and 90% state of charge. 50mV at 3.325v is a difference of 1.5%

It is true if using the same DVM, at nearly the same voltage, at the same instant in time the relative accuracy is better but you have to let the battery rest and stabilize for 30-60 mins after any charging adjustments. If batteries are at different temps then rest time should be longer to allow temp to stabilize.

The safest way to speed up the balancing is to use regular inverter charger until highest voltage cell reaches 3.4v then parallel top balance from there to 3.65v. You have to be sure to watch all cells looking for 3.4v approach which is the risk. The rate of cell voltage rise above 3.4v will be exponential.

tictag · Aug 21, 2020

Buckworth said:
If I am understanding what you are saying, since I have a 4 cell 12v system I should set my bulk charge to 4x3.65v (14.6v), my float charge to 4x3.35v (13.4v), and disable equalization.

Nailed it!

Buckworth said:
I was reading that lifep04 can be safely overcharged at 4.2v per cell (16.8v for a 4 cell system?). Is this a thing?

No.

Buckworth said:
I forgot to mention I have a solar charge controller that has a lithium setting...so these charging values would be for nighttime or dark weather charging from a generator.

Doesn't really matter what the source of the charging is, the batteries don't 'know' or 'care' about this. These are generic parameters for the battery chemistry and should be used whether you're charging from solar, genset, wind, nuclear reactor...

snoobler · Aug 21, 2020

tictag said:
Nailed it!

No.

Doesn't really matter what the source of the charging is, the batteries don't 'know' or 'care' about this. These are generic parameters for the battery chemistry and should be used whether you're charging from solar, genset, wind, nuclear reactor...

Yes it is. Mostly. LFP chemistry can take up to 4.2V, but there is negligible capacity gain above ~3.6V. It's also used as a fast charge target, i.e., "bulk" peak charge current to 4.2V and omit "absorp" charging.

Is it good practice for maximum cell cycle life? No. Will the occasional jaunt to 4.2V measurably damage cells? No.

tictag · Aug 21, 2020

snoobler said:
Yes it is. Mostly.

You are, of course correct, but I was tailoring my response for a non-expert in lithium-ion battery management (as I presume OP is). Nothing good comes from charging a LiFePO4 to 4.2V, yes there are caveats to that in that it is technically possible, but I thought the simple answer was best in this case.

snoobler · Aug 21, 2020

I feel compelled to point it out because so many folks seem to think these LFP cells are dainty little things that can be damaged if they go slightly out of published spec.

Charging values

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