Okay, A new installment of Dzl's d̶u̶m̶b̶ basic questions thread (DDQT)

Lately I've been looking into chargers (broadly) and its exposed how shallow my knowledge of charging is.

I know a wall of questions leads to non-answers, please answer 1 or 2 if you can.


1) Is floating at a high state of charge meaningfully worse than resting at a high state of charge. Lets use 3.4 as an example.

2) Is float charging technically just constant voltage charging at a voltage below the max charge voltage?


3) Is there anything actually wrong with applying constant pressure (voltage) to a cell, or is the reason we recommend disconnecting from the charge source once the battery is full related more to holding it at full SOC or at high voltage. For instance, would it be harmful for a cell to be held indefinitely at say 3.35.

4. Overvoltage and undervoltage are bad. But are they bad because they are bad, or because they are a proxy for SOC and over and under SOC are what is really bad?

EDIT: for clarification, LiFePO4 battery chemistry

1) Maybe. Floating at high SoC in a standby application (infrequent cycling) is less desirable than just letting them sit for a bit. If a cyclic application, floating isn't measurably bad. If I were being lazy in a standy application, I would probably still float them, but drop it to 3.30.

Side note: I had charged a bunch of 40Ah CALB cells about a year ago. They retained 95% of their charge since then.

2) yes, presumably at 100%SoC

3) At some point you want a near zero flow of current into the cell. Floating at 3.45V would actually over charge the cell over a long period of time. I conducted some charge experiments and was surprised to be able to get cells charged to 93% SoC @ 3.40V over about 6.5 hours and a cut-off of 0.03C. Would not expect an issue at 3.35.

4) They are both bad. They cause alterations in structure. Above 4.2V, electrolyte actually breaks down.

snoobler said:

1) Maybe. Floating at high SoC in a standby application (infrequent cycling) is less desirable than just letting them sit for a bit. If a cyclic application, floating isn't measurably bad. If I were being lazy in a standy application, I would probably still float them, but drop it to 3.30.

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My use case would probably fall somewhere between the two. But it definitely wouldn't be a 24/7/365 float situation.

I'm thinking about a portable power station that mostly lives in my truck, hooked up to DC, not constantly but I'm forgetful.. Occasionally charged via AC and occasionally charged via solar (with proper 3 stage charging)

snoobler said:

2) yes, presumably at 100%SoC

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snoobler said:

3) At some point you want a near zero flow of current into the cell. Floating at 3.45V would actually over charge the cell over a long period of time. I conducted some charge experiments and was surprised to be able to get cells charged to 93% SoC @ 3.40V over about 6.5 hours and a cut-off of 0.03C. Would not expect an issue at 3.35.

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So if the conditions to be met are (1) near zero current flow (2) below 100% SOC, a simple one (CV) or two (CC CV) stage charger could work if the CV voltage was </= roughly 3.4Vpc.

Note, I am thinking in terms of being held at that voltage for days to a week or two, not months or years.

snoobler said:

4) They are both bad. They cause alterations in structure. Above 4.2V, electrolyte actually breaks down.

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Given the described application, I'd just float at 3.3 and call it done. The cells I recently tested were all at 3.3 after a year.

Since this is in the Beginners Corner, and just for the sake of clarity, from the voltages you state, I take it you are referring to LiFePO4 chemistry, not lead acid.

For lead acid, float charge is a normal part of charging to reduce sulfation.
As I understand it, it is preferable to have no float charge on lithium chemistries. Holding them at a high state of charge is bad for long cell life, and forcing current into a fully charged lithium battery will eventually cause damage.

I'm guessing that you are asking what to do with that setting on a programmable charger?

snoobler said:

Given the described application, I'd just float at 3.3 and call it done. The cells I recently tested were all at 3.3 after a year.

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That would simplify things greatly. I think I can find cheap current limiting DC-DC buck/boost converter for under $30 to accomplish this for the DC portion, and a current limiting Meanwell type power supply could do the AC-DC portion, or an external laptop power brick or something fed through the DC-DC converter.

hankcurt said:

Since this is in the Beginners Corner, and just for the sake of clarity, from the voltages you state, I take it you are referring to LiFePO4 chemistry, not lead acid.

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Good point, yes I am asking in regard to lithium battery chemistries, LFP in particular in this example, I should've stated that.

hankcurt said:

As I understand it, it is preferable to have no float charge on lithium chemistries. Holding them at a high state of charge is bad for long cell life, and forcing current into a fully charged lithium battery will eventually cause damage.

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What I'm trying to do is decipher that two in one statement. Your understanding ^ above is the same as mine. But if you look closely there are a few overlapping things being said and its not clear if one or both are the root of the problem:

These ideas are commonly grouped together:
1. Holding at a high state of charge (or high voltage) is bad
2. Leaving at a high state of charge is bad
2. Floating is bad (or maybe more accurately floating at full charge or max voltage is bad)

Often these are all implied in a single sentence, and at first seem like one and the same. But if you look more closely their are 5 or 6 distinct statements/questions (the three bullet points, further broken down by SOC and voltage)

I would like to break them apparent and try to understand each on its own. And what specifically causes the degradation.

Of particular interest to me is float. Is the idea of floating (holding an LFP battery at a specific voltage or SOC) bad. Or is it only bad if it is holding it at a full or near full SOC or max or near max voltage. Is floating (applying constant voltage) at say 70% state of charge, meaningfully worse than letting that same battery just sit at 70% SOC?

My rudimentary understanding of charging is a CV source can't 'force' anything into the battery once they are at the same voltage, it can 'supply' or 'hold' but not force.

hankcurt said:

I'm guessing that you are asking what to do with that setting on a programmable charger?

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I'm more interested in the theoretical considerations right now. But practically, I'm looking at one and two stage power supplies (no float, just a voltage and current limit). And trying to assess the pros-cons.

I set my charge @ 28.8V
Absorb 28.8 for 15 minutes.
Float 27.0
LVD 22.0, HVD 29.0

Inverter runs 7/24/365. Standby consumption is 18w including in house parasitic loads (microwave LED, Coffee Maker LED and such)
Batteries never at 100% Total SOC.
Float only provides a bit extra which the inverter consumes anyway, reducing some of the draw from batteries.
Float below "your" max SOC setting will prevent from adding charge to batteries/cells. You can even put it lower if wanted. What if you have a Buck Converter or other devices on your DC bus, float can service that as opposed to the batteries
Do not have to monkey with Midnite SCC, it's happy to float even if there 2000W on the panels and the batteries are full. (Takes serious fiddling with WinBang settings a PITA) probably easier with some other SCC's.

Optimal storage voltage is 3.2-3.3, remember what your cells were when you got'em ?

There is some debate in regards to charge cut off amps for SCC and what they should be and the application for it. This is obviously further complicated by battery system complexity, which introduces a lot more variables. More Packs = More Variables, even if they are exactly the same.

If you are interested in studies, I ran across this one that was published in 2015.
Degradation Predictions of Lithium Iron Phosphate Battery

The first graph shows their assessment of capacity loss over time at 90% state of charge an at different temperatures.


Then they showed another graph with all of the cells at elevated temperature (45°C), but at different states of charge. I assume they chose the elevated temperature to enhance the difference, but there is a bit of an assumption in applying it to lower temperatures.

Wow the 45*C spec's are pretty staggering... In under 1/2 year, a cell at 45*C will lose over 25% capacity at 90% SOC, and will lose maybe 8% even stored at only 10% SOC ?

Steve_S said:

I set my charge @ 28.8V
Absorb 28.8 for 15 minutes.
Float 27.0
LVD 22.0, HVD 29.0

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How many cells or what is thec per cell equivalent. If I remember your chemistry is LFP?

Tonight, I got to looking at this paper from NREL that has a lot of math I don't understand.

In the paper they are using experimental data to try to create a formula that will predict LiFePO4 cell degradation. They modeled calendar degradation, separate from cycling, and came up with the pretty chart below. The vertical scale is the resulting "stress factor" that represents the damage to the cell from keeping it at those conditions. The bottom two axis are temperature and state of charge conditions.

a) Calendar stress factor with temperature and State of Charge dependence

One of the useful things from the chart above is that, while the increase in stress on the battery with temperature is a smooth rise, the increase in stress with state of charge has distinct transitions where the stress rises. They note it in the text as follows and provide an additional graph.

Evaluating the stress factors over the state of charge, as shownin Figure 4a, leads to plateau regions for SOCs between 37.5% and 62.5%, as well as between 87.5% and 100%.

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That is one cool looking chart! I need a little time to digest it though

Ampster said:

How many cells or what is thec per cell equivalent. If I remember your chemistry is LFP?

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They are all 8S 24V packs. 175AH & 280AH.

Steve_S said:

They are all 8S 24V packs. 175AH & 280AH.

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Thanks. Initially I had a brain fart and couldn't do the math correctly to reconcile those numbers. All is good.

I get more & more "Seniors" moments as time progresses LMAO - it's part of being alive eh !
Doesn't help with the flood of info being absorbed too.