What is the best voltage to charge your cells to? 3.65, 3.55 or 3.45?

SherylinRM

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I have read so many debates on this site and others about this. Finally someone was smart enough to just do the tests.

I extremely, highly recommend watching this video.

In the end it will open your eyes in ways no debate could ever do. :)

Enjoy and learn and have fun.
I hope this helps someone. :)
 

toms

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Good video, charging voltage is always going to be application specific.

Effects on lifespan of battery need to be tested in real time, some very early tests on the effects of charging voltage/tail current that began in 2010 are still ongoing.

At this stage my money is on charging to 3.65V with a 0.05C tail current stop will provide the most kwh over the life of a LiFePO4 cell.

In a stationary application this isn’t the most practical approach though.
 

SherylinRM

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Good video, charging voltage is always going to be application specific.

Effects on lifespan of battery need to be tested in real time, some very early tests on the effects of charging voltage/tail current that began in 2010 are still ongoing.

At this stage my money is on charging to 3.65V with a 0.05C tail current stop will provide the most kwh over the life of a LiFePO4 cell.

In a stationary application this isn’t the most practical approach though.
You did actually watch it right? Because your opening statement makes it sound like you did not do so.
Ok whatever.
 

SherylinRM

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Very interesting. Thank you :)
EDIT: The chart at 24:24 is probably the most interesting.
The fact that it won't even fully charge is interesting as well.
Thanks again for the video.
 
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Frick

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You did actually watch it right? Because your opening statement makes it sound like you did not do so.
Ok whatever.
I think the description of “best” voltage is problematic. Best for what? battery longevity? Best for capacity? Best for fast recovery? Best for safety?
I think @toms is right here, it’s application specific for what is best. In my pack I want to err on the side of longevity, but if it was an electric bicycle I would go for max capacity and possibly fast recharge. Then there is different battery chemistries that play in and some will argue the faster you charge a lifepo4 the better for it.

I think The video is but one piece of a larger ppuzzle, not the end all statement. True testing takes a long time, many years likely, and I would weigh any results obtained that way as much more useful.
 

SherylinRM

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I think the description of “best” voltage is problematic. Best for what? battery longevity? Best for capacity? Best for fast recovery? Best for safety?
I think @toms is right here, it’s application specific for what is best. In my pack I want to err on the side of longevity, but if it was an electric bicycle I would go for max capacity and possibly fast recharge. Then there is different battery chemistries that play in and some will argue the faster you charge a lifepo4 the better for it.

I think The video is but one piece of a larger ppuzzle, not the end all statement. True testing takes a long time, many years likely, and I would weigh any results obtained that way as much more useful.
He explains in the video what he determines to be "best".
And because I watched the video. I am in agreement with him for his version of "best" as well.
And when people watch the whole video they will probably also agree as well with his version of best if they actually watch the video.
It is just simple math and basic facts. Nothing too elaborate here.
 

Mcgivor

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The charging voltage should be what the manufacturer recomends for a given application, in photovoltaic systems it's undesirable to turn off the charging source because it will then prevent loads from using the PV, resulting in drawing from the battery, until a subsequent charge cycle is initiated, to prevent micro cycles a float cycle is introduced to maintain a lower than maximum SOC, but sufficiently high enough to ensure around 90% of capacity is available when the array can no longer provide energy to support the loads.

This is a discussion I had with a well known EV guru, who claimed was it is impossible to use LiFePo4 in solar applications because the charging source needs to be terminated once an individual celll reaches 3.650V. Obviously he was only comfortable within his knowledge base, not knowing the intricacies of charge controllers, but otherwise extremely knowledgeable. Typically in EV bottom balancing is performed to have the cells aligned at the lower end, perhaps to ensure maximum range, but in my opinion it wouldn't make any difference, top or bottom as ultimately the battery capacity as a whole is only as great as the weakest cell.

Using between 20% and 90% SOC is probably a realistic long term strategy, this is something Battleborne dose on their batteries, although they advertise 100% of capacity is available, they back that up by using a larger capacity than advertised to ensure customer satisfaction, a very smart strategy, they are very forthcoming with information, a welcome change to the average vendor.

Included below is the charging recomendations from the manufacturers manual I received with the cells I purchased, note the reference to photovoltaic and float.

Screenshot_2021-08-25-18-27-59.png
 

Horsefly

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It seems like the area of common conclusion of both videos is that there really is nothing to be gained by charging to 3.65V/cell, and because cells are not going to be perfectly balanced at that point in the curve, trying to charge to 3.65V/cell will - more often than not - result in HVD by the BMS. Since both videos showed that charging to 3.5V per cell gets you to the same capacity anyway, I'm not sure why anyone would try to charge much higher. 3.4V seems to work just as well, although it takes a bit longer.

My intent when I get my 8S 230Ah pack installed at our cabin is to set Bulk/Absorb to somewhere close to 28.0V-28.2V (3.5V-3.525V/cell), and hold it there in absorption until the current gets to 0.02C (4.6A). Then I'll go to float at 3.4V.
 

thegoogler

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So ideally: charge at 3.65V in order to charge quickly (since 3.45V takes almost twice as long) but stop charging when the cell hits 3.45V because you're at 99.4% capacity. (Or arguably stop at 3.35V or whatever is 90% SOC... something I'd like to do but haven't quite mastered the Victron settings to actually accomplish this)
 

Horsefly

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So ideally: charge at 3.65V in order to charge quickly (since 3.45V takes almost twice as long) but stop charging when the cell hits 3.45V because you're at 99.4% capacity. (Or arguably stop at 3.35V or whatever is 90% SOC... something I'd like to do but haven't quite mastered the Victron settings to actually accomplish this)
I don't think that is really a good idea. To fully charge, you don't just stop as soon as the cells get to the voltage you are charging to. The reason it takes a long time to charge at 3.45V is because once you get to 3.45V you still need to do absorption, as the current gradually drops down to near zero (maybe 2% of the Ah rating of the cell), indicating the cell is charged. If you stop as soon as it hits 3.45V, you are not doing any absorption, and you are a long way from fully charged. What Andy from the Off Grid Garage YouTube Channel showed is that absorption takes longer when you are charging at a lower voltage, but in the end you get the same energy stored in the cell.
 

Horsefly

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By the way, I don't know much about Victron charge controllers, but most charge controllers either will stay in absorption until the current drops to a defined level, or will stay in absorption for a fixed period of time. Whichever comes first (current drop or time) will cause the charge to drop out of absorption and into float.
 

thegoogler

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I don't think that is really a good idea. To fully charge, you don't just stop as soon as the cells get to the voltage you are charging to. The reason it takes a long time to charge at 3.45V is because once you get to 3.45V you still need to do absorption, as the current gradually drops down to near zero (maybe 2% of the Ah rating of the cell), indicating the cell is charged. If you stop as soon as it hits 3.45V, you are not doing any absorption, and you are a long way from fully charged. What Andy from the Off Grid Garage YouTube Channel showed is that absorption takes longer when you are charging at a lower voltage, but in the end you get the same energy stored in the cell.
Absorption (on LiFePO4) only adds about 10% capacity, at least at 3.65V. His charge graph is a good example, at some point with a higher voltage (3.65V) the current drops off precipitously when the charge switches from CC to CV. At that point he's at least close to 90% charge, if not over. (5.296A x 4 hours = 21.18Ah... and he shows his battery is 24.8Ah max). If your goal is 90% SOC then you should just stop right there. (My goal is only 90% because I want to extend battery life, and so I don't want to be fully charged). If you want 100% charge then yes switch to absorption for the next couple hours to push in the last few amps

1629925325115.png

It's my understanding that absorption is just the tapering off of current, which happens when you get above ~90%. Like at Thanksgiving:
  1. Bulk charging is when you can devour a plate full of food in a short sitting
  2. Absorption is when you start to feel full. You can still "make room" for a slice of pie, but you take your time eating it
  3. Float is the glass of (whatever you drink) that you sip on for hours later
  4. (and Equalize is when you pop your belt another notch a month later to eat just at Christmas dinner)
High voltage is when your aunt Edna keeps telling you that you're too skinny and is putting extra scoops of mashed potatoes and stuffing on your plate (or perhaps when you just eat your food faster because you know your grandmother didn't cook enough for everyone she invited and you're racing to eat your share before your fat uncle Larry takes your helping too). (OK not a perfect analogy).

I do think he's actually hitting 90% SOC no matter the voltage. If you look at the 3.45V charge graph, you can see that it doesn't get above 3.4V until the battery is nearly full. But it takes a long time to get there.

1629925857706.png

I do think the place where this because especially interesting is when doing the initial top balance. All those folks who say "charge at 3.4V, then let it settle and repeat at 3.5V, then settle and repeat at 3.6V" are spending tons of time charging when they should be able to cut the initial top balance down if they'd just start off set at 3.65V
 

thegoogler

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By the way, I don't know much about Victron charge controllers, but most charge controllers either will stay in absorption until the current drops to a defined level, or will stay in absorption for a fixed period of time. Whichever comes first (current drop or time) will cause the charge to drop out of absorption and into float.
Victron MPPT SmartSolar can be configured to do both of those. The issue I've had is that at any voltage I've tried, even when already full the controller seems to want to go into bulk mode for a while before moving into the variable/configurable absorption phase. I can drain 1A off my 280Ah battery overnight but in the morning whether my controller is set to 13.6V or 14.4V it wants to charge in bulk mode for a few hours before switching over. That feels wrong since the battery is >99% already. I *think* that's an artifact of starting the day fully charged and whatever algorithm all of the SCC's I've tried use... the behavior might more closely mirror what he graphed if I drained off 50A, but I haven't spent enough time in the camper with the new Victron to really see.

His video though and the graphs kind of indicate to me that I *should* use a high charging voltage (3.65V) but then basically disable the absorption phase so that I stop at ~90% SOC. Right now I have a much lower charging voltage configured (3.4V) in an attempt to trick the controller into giving up bulk mode sooner, but it doesn't actually seem to make a difference.
 

Horsefly

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Absorption (on LiFePO4) only adds about 10% capacity, at least at 3.65V.
Yep, at 3.65V. Absorption when charging to 3.65V also doesn't last very long. You were talking about charging to 3.4V, and not doing any absorption. Absorption when charging to 3.4V takes lots longer, and yes it gives you another 10% capacity. When you are staying well under 3.65V, I can't imagine a reason to NOT want that additional 10%.

I don't really agree with all the turkey dinner analogy, but to each his own.
 

Horsefly

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Victron MPPT SmartSolar can be configured to do both of those. The issue I've had is that at any voltage I've tried, even when already full the controller seems to want to go into bulk mode for a while before moving into the variable/configurable absorption phase. I can drain 1A off my 280Ah battery overnight but in the morning whether my controller is set to 13.6V or 14.4V it wants to charge in bulk mode for a few hours before switching over. That feels wrong since the battery is >99% already. I *think* that's an artifact of starting the day fully charged and whatever algorithm all of the SCC's I've tried use... the behavior might more closely mirror what he graphed if I drained off 50A, but I haven't spent enough time in the camper with the new Victron to really see.
Remember that bulk and absorption are really targeting the same voltage. Bulk is just constant current until it gets to the target voltage, then it switches to absorption, with a constant voltage. I wouldn't worry about it being in bulk for a bit. The charge controller has to decide that the target voltage has indeed been reached, and it's not just a fluctuation. A few hours doesn't sound right, but it really should not cause a problem, as long as your bulk / absorption voltage is reached.

And your statement that the battery is >99% is probably not right. If you drained 1A overnight, I'm assuming that is for 8-10 hours. That's 8Ah-10Ah, which is more like 5% of the capacity.
 

thegoogler

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When you are staying well under 3.65V, I can't imagine a reason to NOT want that additional 10%.

Battery life. CATL claims 4500 cycles if I cycle my battery within 10-90% SOC. The number is lower if I charge to 100% (the docs I've found say 2500 cycles). 2500 cycles is roughly 7 years, whereas 4500 is 13.
 

thegoogler

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Remember that bulk and absorption are really targeting the same voltage. Bulk is just constant current until it gets to the target voltage, then it switches to absorption, with a constant voltage. I wouldn't worry about it being in bulk for a bit. The charge controller has to decide that the target voltage has indeed been reached, and it's not just a fluctuation. A few hours doesn't sound right, but it really should not cause a problem, as long as your bulk / absorption voltage is reached.

And your statement that the battery is >99% is probably not right. If you drained 1A overnight, I'm assuming that is for 8-10 hours. That's 8Ah-10Ah, which is more like 5% of the capacity.
No I'm actually only consuming about 1A per day. It's just parasitic draw from the BMS, SCC, SOC meter, gas detector, and stereo when I'm not in the camper. Total draw is maybe 75mA, but the SCC handles that when it is in float mode so it's only roughly 0.075A x 12.

When I use the camper (one or two weekends per month, plus a couple week or multi-week trips) I'll routinely draw it down anywhere from 10A to 40A per day or more. Sometimes I'm parked in the shade for multiple days and depending on temperature I might draw 100-120A over 4-5 days before I have a chance to recharge
 

Horsefly

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When you are staying well under 3.65V, I can't imagine a reason to NOT want that additional 10%.

Battery life. CATL claims 4500 cycles if I cycle my battery within 10-90% SOC. The number is lower if I charge to 100% (the docs I've found say 2500 cycles). 2500 cycles is roughly 7 years, whereas 4500 is 13.
Well, I would be willing to bet they are talking about charging to 3.65V as reducing the life. It harms the battery's life if you repeatedly go up to 3.65V or down to 2.5V. I have not seen anything that says it harms it to charge at 3.5V or 3.4V and hold a absorption until the cell stops taking much current. That wouldn't make sense.

No I'm actually only consuming about 1A per day. It's just parasitic draw from the BMS, SCC, SOC meter, gas detector, and stereo when I'm not in the camper. Total draw is maybe 75mA, but the SCC handles that when it is in float mode so it's only roughly 0.075A x 12.

When I use the camper (one or two weekends per month, plus a couple week or multi-week trips) I'll routinely draw it down anywhere from 10A to 40A per day or more. Sometimes I'm parked in the shade for multiple days and depending on temperature I might draw 100-120A over 4-5 days before I have a chance to recharge
You need to learn the nomenclature. 1A is a measure of current. 1Ah is a measure of Coulomb, or energy at a given voltage. So saying that you consume 1A per day, or 10A per day, or 40A per day doesn't make sense.

Anyway, I think you are convinced of what you are convinced. It won't cause any harm for you to cut off your absorption prematurely, so go ahead. You just won't be getting what you paid for from your cells.
 

thegoogler

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Well, I would be willing to bet they are talking about charging to 3.65V as reducing the life. It harms the battery's life if you repeatedly go up to 3.65V or down to 2.5V. I have not seen anything that says it harms it to charge at 3.5V or 3.4V and hold a absorption until the cell stops taking much current. That wouldn't make sense.

I'm not a battery engineer, but if 3.45V is 99% SOC (according to the LiFePO4 SOC charts floating around on this forum) then I would think it would in fact harm the battery. At least to your point below, I'm convinced of this until a battery manufacturer or someone who is a chemical engineer can confirm that it's true. I dunno, I'm just a hobbyist.

You need to learn the nomenclature. 1A is a measure of current. 1Ah is a measure of Coulomb, or energy at a given voltage. So saying that you consume 1A per day, or 10A per day, or 40A per day doesn't make sense.
Meh, sorry "Amp hours". Yes I know the difference. I'm not writing a scientific paper that's getting peer-reviewed and published and gonna change the world here, I'm asking a question on the internet while sitting at my desk trying to multi-task between this and 2 other things that I'm actually supposed to be focused on :p

Anyway, I think you are convinced of what you are convinced. It won't cause any harm for you to cut off your absorption prematurely, so go ahead. You just won't be getting what you paid for from your cells.
Depends on your POV I guess. I'd say I am. I paid for a battery that provides either 271Ah for 2500 cycles (100% DoD) or 216Ah for 4500 cycles (80% DoD at 10-90% SOC). I can pick which of those batteries I want.

So yeah I'm not getting the full Ah (capacity) from my battery by cutting off charging early, but I am (supposedly) extending the battery life considerably. I bought more capacity than I need because the incremental $/Ah (or $/kwh if you prefer) was small, but the cost for me to replace my battery in 6-7 years is considerable. I expect to have my camper until my kids are out of the house, which is 9-10 years, so if I can manage to make the battery perform well for 13 years I'll be thrilled to sell the camper a decade from now with a functioning battery rather than dropping several hundred bucks on a replacement a few years before I trade down to something smaller.
 
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