How much Cell Voltage drift is acceptable for LiFePO4?

[NochiLife]

Yes, I can understand not wanting to "float" Li-cells for a long time. My MPPT configuration is currently set to 3.53vpc (56.5v) "absorption" (which is a timed cycle), then 3.50vpc (56.0v) "float." ("Equalize" disabled.) Overnight, the bank falls to around 3.31vpc (53v). I previously had the "float" voltage lower (55.5v), and observed zero MPPT current when it switched to "float" mode for quite a few minutes. Is this a good (or bad) configuration? Suggestions?

[Ampster]

OK, but then I have an issue. Initially when setting up the charging settings for my LiFePo4 bank, I had the MPPT drop a volt from "Absorption" to "Float." Problem was, when that happened, the MPPT would go straight to zero current output, and all the loads would run off the battery until its voltage fell down to the new low (5 minutes later). If the voltage was "too high", I would expect it to fall if removed...?

If I understand the earlier post by PNjunction you don't want to be floating Lithium cells for long term any way. It sounds like your settings are cutting off current at a set voltage. If that is what you want then leave it alone. I am not as familiar with the Lead Acid charging terminology and I am not sure you even need absorption with Lithium. It is often a timed cycle and maybe that is what you are observing. To make it simple I have set my integrated charge controller to bulk and a refloat voltage slightly below the bulk max voltage. That way I get the benefit of some charging from the sun until the end of the day. I have loads on the system and when thu sun goes down my batteries are topped off. There is not much risk of extended float charging in that scenerio.

[NochiLife]

Just be careful since like many, one can get obsessed with voltage alone and forget about *time*.

In other words, you can charge a cell to a conservative 3.45v, but if left on charge long enough - even with the voltage limited to 3.45v, you will end up over-charging and damaging the battery. The only reason people don't notice at first is because it takes much longer to achieve full charge at 3.45v vs a higher 3.6v.

From a cell standpoint, *current* decides when you are done, (as long as you use the minimum CV setting of 3.45v) not battery voltage. Typically .005C current. Anything more, and you are oxidizing it.

So - can you overcharge a 12v drop-in if your CV is set to a conservative 14.0 volts? YES, given enough time. This voltage is low enough to allow some leeway if one is cycling it daily so you'll never reach full charge anyway. Leave it on charge at 14v for a week? You'll reach a full charge condition eventually - even though never rising above 14v - and damage the cells.

OK, but then I have an issue. Initially when setting up the charging settings for my LiFePo4 bank, I had the MPPT drop a volt from "Absorption" to "Float." Problem was, when that happened, the MPPT would go straight to zero current output, and all the loads would run off the battery until its voltage fell down to the new low (5 minutes later). If the voltage was "too high", I would expect it to fall if removed...?

[PNjunction]

Just be careful since like many, one can get obsessed with voltage alone and forget about *time*.

In other words, you can charge a cell to a conservative 3.45v, but if left on charge long enough - even with the voltage limited to 3.45v, you will end up over-charging and damaging the battery. The only reason people don't notice at first is because it takes much longer to achieve full charge at 3.45v vs a higher 3.6v.

From a cell standpoint, *current* decides when you are done, (as long as you use the minimum CV setting of 3.45v) not battery voltage. Typically .005C current. Anything more, and you are oxidizing it.

So - can you overcharge a 12v drop-in if your CV is set to a conservative 14.0 volts? YES, given enough time. This voltage is low enough to allow some leeway if one is cycling it daily so you'll never reach full charge anyway. Leave it on charge at 14v for a week? You'll reach a full charge condition eventually - even though never rising above 14v - and damage the cells.

[NochiLife]

Ditto with my LFP bank; I run it at 3.5vpc tops.

[Ampster]

Yes the stated voltage range of LifePO4 cells is 2.2 to 3.65v but I never exercised mine that much. In fact the OP said he only takes his to 3.5 per cell (pack =14v).

[NochiLife]

I have an 125AH dropin battery I use for camping and charge it at just 14V, 0.3C charge rate. I have done 4 cycles to around 5-10% soc & back to full, after letting sit for 12hours at 60%Soc,
the bluetooth app shows the cell voltages are 3307mV, 3311mV, 3312mV, 3321mV, another few seconds later they are: 3309mV, 3314mV, 3314mV, 3322mV,

Is this fine or too much drift between them? cheers

I wouldn't worry too much about drift less than .1v. Those are actually really close.

The core issue is the voltage range of a LiFePo4 cell (generally given as 2.5 to 3.65v), and that unlike lead-acid batteries, LiFePo4s don't self-balance. When/if the cells get imbalanced (particularly with deep cycling), it is very easy for one cell's voltage to go outside of the safe specification (i.e. 3605, 3599, 3598, 3942). When that happens, it doesn't take too long to irreversibly damage the cell--and then the whole pack becomes rather useless. (Heard of a Prius battery with one bad cell: the car was all but undrivable because the one cell was going negative under acceleration, and way overvoltage when charging. When the one cell was replaced, a perfectly working Prius resulted.)

[Ampster]

What I look for in my Nissan pack is the cell voltage deltas at various SOCs of the pack. In my case I have 6 cells on parallel and if one group has a high delta above the average at close to 100% SOC and then also has a high delta but at a level below the average when the pack is at 50% then I know one or two cells in that group have less capacity. Some people would not call that drift but rather a weaker group of cells exhibiting normal behavior that is symptomatic of less capacity. You could make these observations under load, after charging or at rest.

EDIT
To answer your original question about how much drift is acceptable the answer depends on your application, how good your Battery Management System is and what safety controls you have installed. You and a multimeter can be a very inexpensive BMS. Good BMSs can run as much as $1,000 or more depending on cell count and complexity..I don't worry about a 1/10 volt (0.1 volt) over a SOC range of 30% to 90% because my pack never gets that low or or high and if it does the BMS shuts it down and sets off an alarm. I also have low and high voltage cell cutoffs.

[Jman]

I have an 125AH dropin battery I use for camping and charge it at just 14V, 0.3C charge rate. I have done 4 cycles to around 5-10% soc & back to full, after letting sit for 12hours at 60%Soc,
the bluetooth app shows the cell voltages are 3307mV, 3311mV, 3312mV, 3321mV, another few seconds later they are: 3309mV, 3314mV, 3314mV, 3322mV,

Is this fine or too much drift between them? cheers