200a Portable Lifepo + BMS + Balancing

[john61ct]

Originally Posted by tanglewood

5C is a really big charge current compared to others I have seen. Maybe they have some magic in their cell construction.

No, you likely weren't looking at bare large prismatic cells.

Bog standard for LFP IRL.

But you do need to put temp protection on the bank once past 1C, and be really sure of it over 2C.

But as you say, with House banks not a concern IRL

[john61ct]

Originally Posted by rgleason

So, I may need to leave some room for a small computer fan mounted in the old drawer front area with similar area for exhaust, with t'stat activation.
In any event the drawer front should be redesigned allow free air flow.

Do these batteries ever leak acid or anything? I don't think so. So there is not any real need for a full container. Just strap them in well, sideways and tied down.
Is this correct?

No active venting needed unless ambient goes crazy.

Just have overtemp conditions isolate the bank.

[john61ct]

Originally Posted by rgleason

It might be a good idea to get a 140ah alternator so that it could recharge the sea load more quickly (a long hour) if cut back to 120a max with the belt load program reset.
But I should be careful of the electroplating issue and not charge too fast. Not over.0.3C. This appears to be the only reason I have found so far to have a bigger bank.

I'd rather use a smaller cheaper bank and closer to its full practical capacity and recharge more frequently with alt or small solar while on the mooring.

The sea use is less frequent, if I were to charge at say .4C or .45C what would be the result, reduction in capacity or reduction in number of cycles?

No forget that whole theoretical electroplating thing.

Charge as fast as you like, not an issue IRL, below C rates even temp protection is not really required in temperate climates, more a belt and suspenders thing.

[tanglewood]

Originally Posted by john61ct

No forget that whole theoretical electroplating thing.

Charge as fast as you like, not an issue IRL, below C rates even temp protection is not really required in temperate climates, more a belt and suspenders thing.

Good thing we have you to correct all the studies that have tested things under controlled conditions, show empirically what happens, then dissect the batteries to see exactly what happened. I’m glad we can ignore it all.

[nebster]

Originally Posted by john61ct

Charge as fast as you like, not an issue IRL, below C rates even temp protection is not really required in temperate climates, more a belt and suspenders thing.

Many of you know I have a substantial LFP bank in real use on my RV. I am typing these words on a laptop powered by those lithium ions right now, in fact.

While the general spirit of what John writes is true ("charge as fast as you like"), in practice for certain battery banks and charge management strategies, including mine, I have found that the charge rate very much does matter.

Let me give a few reasons why C-rate can matter in real life. These are some ideas that others may want to consider:

1) If you observe manufacturer spec sheets on LFP cells and compare the ratings within the same line of cells, you may find that specified maximum C-rates for both charge and discharge are gradually lowered as you move to the larger cells. I suspect this is conservatism on the part of the manufacturer in light of the fact that it is for some reason harder to build larger, monolithic prismatic cells that stand the test of time. (My guess? Decreased mechanical stability.) To that end, you can even find LFP cells that have 1C limits specified. There is a general, suggestive trend underlying the specs -- which have also gone down over the years, if you read back historically -- that points to lower rates likely promoting better performance. How much better, I can't say.

My guess, though, is that fundamentally this is about heat. We know that heat absolutely encourages degradation. Localized heat or hot-spotting, perhaps due to physical non-uniformities in the cells, may be why they are ratcheting down the suggested levels.

2) It is most definitely the case in my real world system (32kWh, 52V) that charging at 0.25C versus 0.50C yields dramatically different thermal results, both in the cells and in the chargers.

Ohmic heating goes as the square of the current, so charging twice as fast yields four times the heat. In my particular system, on a warm summer day, my battery pack becomes warm enough under a 0.50C charge that it begins to require active cooling. (I do have active cooling from a roof air conditioner as an option.) That same pack subjected to a 0.25C charge stays comfortably under 40C in the same weather.

If your pack is somewhere nice and cool, or your ohmic losses are smaller relative to the thermal mass of your environment, or you have no problem pumping glycol through your pack all the time, then you might be able to ignore this issue. But if you have a sizable pack and are thinking about charging above 0.2C, you need to run the experiments. And if you are considering charging above 0.5C, you almost certainly either need to be in a cold climate or have a plan for how to cool actively.

The good news is, you can choose to charge slower. This likely will keep the thermal management much simpler. It is up to you to decide if that tradeoff is worth it. I hate listening to my generator, but not enough that I'm willing to sizzle my batteries to top them off in one hour. (Originally, naively, I thought that that's what I would end up being able to do!)

2b) There is a related issue with thermal management of the charger(s). With my gear, a set of Victron inverter-chargers, I run into their thermal derate fairly quickly on warm days. Like with the battery enclosure, I have a separate enclosure that houses the power components. That enclosure, and the chargers, get hot quickly. Especially when they are called upon to deliver upwards of 15kW of charging at once. On warm days, they simply can't achieve that with the limited environmental conditioning I can provide in that space.

Obviously this side of the problem has nothing to do with LFP per se, except that the massive charge acceptance makes it much easier to start melting stuff on the other end of the wire. I mention it here for completeness, and because it was yet another thing I didn't fully consider at first.

3) Finally, and at least as importantly as the above, there is a dynamic voltage-rise phenomenon that occurs when charging LFP. If you charge at a higher rate, the voltage in the cells will rise faster again than simply the rate at which the charging happens. That means that, if you use the strategy of "CC then stop", at any arbitrary stopping voltage -- say, 3.45V/cell -- your actual SOC can be different. If you use voltage-based charge termination, you must parameterize that decision on both voltage and rate if you want the same end SOC.

For example, if you charge to 3.40V at 0.1C and stop immediately, your cells might reach ~80% SOC. If you instead charge to 3.40V at 0.2C and stop, your cells might only be at about 70% SOC! You would then need to apply a CV stage ("absorb") to continue to put more energy into that cell.

I have seen John dismiss various more complex charging regimes (instead advocating for "just set a voltage point and stop"), advice which on the surface seems reasonable. But in real life, my experience is that that does not quite work out so conveniently and simply. The actual voltage curve depends on stored energy, recent-historical rate, and even temperature (which, itself, depends on rate!).

---

There's no single right answer or one recipe for success here. It's also not "hard" or mysterious, nor should it be seen as daunting despite my wall of text above and the thousands of posts on this forum. At the same time, I don't think it's quite right to suggest that LFP is so straightforward as to accept any old rate up to and including >1C.

If you are reading these threads and thinking of building a lithium battery, and especially if it is larger than a 1p4s pack or if you have one chance to get things right in terms of physical placement and temperature management, you should keep these things in mind.

Cheers all,

Ben

[john61ct]

My "you" was addressed to the OP's stated parameters, fact that a puny 1C was at the high end - which is true for most.

I also stated the required provisos wrt infrastructure and heat protection.

Taking one bit out of context to pretend I over-generalize is deceptive.

[john61ct]

Originally Posted by nebster

if you use the strategy of "CC then stop", at any arbitrary stopping voltage -- say, 3.45V/cell -- your actual SOC can be different

And, what?

There is **no reason** to standardize the top SoC, no reason to go any higher with an Absorb stage.

In daily cycling, if you need to precisely calibrate some defined 100% point, use a .03C endAmps setpoint.

But that is rarely required, for me an occasional gear maintenance procedure.

[john61ct]

Originally Posted by nebster

I don't think it's quite right to suggest that LFP is so straightforward as to accept any old rate up to and including >1C.

As long as the essential provisos are observed, I stand by it.

And with informed caution, 2C 3C and higher are fine as well wrt safety.

Longevity will be affected to some extent, but not as radically as with average DoD, and few advocate limiting that to 50% as they do with lead.

[nebster]

Originally Posted by john61ct

There is **no reason** to standardize the top SoC, no reason to go any higher with an Absorb stage.

Of course there is, John.

Go back and actually read the example I posed. There, a change of rate reduces the usable stored energy by 12.5%. Who wants to throw away 12.5% of their battery just because they elect a different rate?

In daily cycling, if you need to precisely calibrate some defined 100% point, use a .03C endAmps setpoint.

Your choice of terminology is confusing here, since we are not talking about 100%, but I am glad you are now in agreement that an absorption period is useful in some cases.

But that is rarely required, for me an occasional gear maintenance procedure.

It is required every day on my pack to be able to put 90Ah into my 100Ah cells. And it has nothing to do with "100%" and everything to do with the way the chemistry responds.

At what rate do you charge your cells, John? Can you share some data from your hands-on experience?

[newhaul]

Originally Posted by nebster

Many of you know I have a substantial LFP bank in real use on my RV. I am typing these words on a laptop powered by those lithium ions right now, in fact.

While the general spirit of what John writes is true ("charge as fast as you like"), in practice for certain battery banks and charge management strategies, including mine, I have found that the charge rate very much does matter.

Let me give a few reasons why C-rate can matter in real life. These are some ideas that others may want to consider:

1) If you observe manufacturer spec sheets on LFP cells and compare the ratings within the same line of cells, you may find that specified maximum C-rates for both charge and discharge are gradually lowered as you move to the larger cells. I suspect this is conservatism on the part of the manufacturer in light of the fact that it is for some reason harder to build larger, monolithic prismatic cells that stand the test of time. (My guess? Decreased mechanical stability.) To that end, you can even find LFP cells that have 1C limits specified. There is a general, suggestive trend underlying the specs -- which have also gone down over the years, if you read back historically -- that points to lower rates likely promoting better performance. How much better, I can't say.

My guess, though, is that fundamentally this is about heat. We know that heat absolutely encourages degradation. Localized heat or hot-spotting, perhaps due to physical non-uniformities in the cells, may be why they are ratcheting down the suggested levels.

2) It is most definitely the case in my real world system (32kWh, 52V) that charging at 0.25C versus 0.50C yields dramatically different thermal results, both in the cells and in the chargers.

Ohmic heating goes as the square of the current, so charging twice as fast yields four times the heat. In my particular system, on a warm summer day, my battery pack becomes warm enough under a 0.50C charge that it begins to require active cooling. (I do have active cooling from a roof air conditioner as an option.) That same pack subjected to a 0.25C charge stays comfortably under 40C in the same weather.

If your pack is somewhere nice and cool, or your ohmic losses are smaller relative to the thermal mass of your environment, or you have no problem pumping glycol through your pack all the time, then you might be able to ignore this issue. But if you have a sizable pack and are thinking about charging above 0.2C, you need to run the experiments. And if you are considering charging above 0.5C, you almost certainly either need to be in a cold climate or have a plan for how to cool actively.

The good news is, you can choose to charge slower. This likely will keep the thermal management much simpler. It is up to you to decide if that tradeoff is worth it. I hate listening to my generator, but not enough that I'm willing to sizzle my batteries to top them off in one hour. (Originally, naively, I thought that that's what I would end up being able to do!)

2b) There is a related issue with thermal management of the charger(s). With my gear, a set of Victron inverter-chargers, I run into their thermal derate fairly quickly on warm days. Like with the battery enclosure, I have a separate enclosure that houses the power components. That enclosure, and the chargers, get hot quickly. Especially when they are called upon to deliver upwards of 15kW of charging at once. On warm days, they simply can't achieve that with the limited environmental conditioning I can provide in that space.

Obviously this side of the problem has nothing to do with LFP per se, except that the massive charge acceptance makes it much easier to start melting stuff on the other end of the wire. I mention it here for completeness, and because it was yet another thing I didn't fully consider at first.

3) Finally, and at least as importantly as the above, there is a dynamic voltage-rise phenomenon that occurs when charging LFP. If you charge at a higher rate, the voltage in the cells will rise faster again than simply the rate at which the charging happens. That means that, if you use the strategy of "CC then stop", at any arbitrary stopping voltage -- say, 3.45V/cell -- your actual SOC can be different. If you use voltage-based charge termination, you must parameterize that decision on both voltage and rate if you want the same end SOC.

For example, if you charge to 3.40V at 0.1C and stop immediately, your cells might reach ~80% SOC. If you instead charge to 3.40V at 0.2C and stop, your cells might only be at about 70% SOC! You would then need to apply a CV stage ("absorb") to continue to put more energy into that cell.

I have seen John dismiss various more complex charging regimes (instead advocating for "just set a voltage point and stop"), advice which on the surface seems reasonable. But in real life, my experience is that that does not quite work out so conveniently and simply. The actual voltage curve depends on stored energy, recent-historical rate, and even temperature (which, itself, depends on rate!).

---

There's no single right answer or one recipe for success here. It's also not "hard" or mysterious, nor should it be seen as daunting despite my wall of text above and the thousands of posts on this forum. At the same time, I don't think it's quite right to suggest that LFP is so straightforward as to accept any old rate up to and including >1C.

If you are reading these threads and thinking of building a lithium battery, and especially if it is larger than a 1p4s pack or if you have one chance to get things right in terms of physical placement and temperature management, you should keep these things in mind.

Cheers all,

Ben

I'm trying to learn here so tech paper references would be extremely helpful to backup your assertions
I am building a 4p4s bank the man specs say up to a 5C charge rate
( never will that even be possible on my boat) is there a real reason to suspect the manufacturers spec sheets on all of their prismatic cells?

[john61ct]

Originally Posted by nebster

Your choice of terminology is confusing here, since we are not talking about 100%, but I am glad you are now in agreement that an absorption period is useful in some cases.

Only for occasional calibration / testing purposes. 100% is defined by the owner, the arbitrary vendor setpoint has no relevance to real-world usage.


> It is required every day on my pack to be able to put 90Ah into my 100Ah cells

What, why? Is your daily usage / DoD pattern really that consistent?

Most setups vary from one day to the next. I rarely use more than 40-60%, build in some reserve capacity, especially in mostly solar setups without convenient ICE charging on demand.


> There, a change of rate reduces the usable stored energy by 12.5%. Who wants to throw away 12.5% of their battery just because they elect a different rate?

If that is true, just use a different voltage for the different sources to get to the same SoC.

Whether 1C or .1C, adding Absorb time at a given voltage does not result in actually raising SoC significantly, the extra AH are getting dissipated as heat and, more importantly, likely reducing lifespan if over 3.45Vpc

MS recently posted very detailed test results proving exactly that, differences were usually 1-3% at most.


CV / Absorb stage in daily cycling just adds complexity and risk, without any real-world benefit.

Lead thinking.

[nebster]

Originally Posted by newhaul

I'm trying to learn here so tech paper references would be extremely helpful to backup your assertions

Well, I'm not asserting much, I'm just relating my own experience on my setup. I have a sample size of 1, so you have to take that for what it is (with a large grain of salt). You also have to decide if the things I've run into apply in your situation. I'm not sure how I could justify that my batteries get hotter when I charge them faster, except to say "my batteries get hotter when I charge them faster."

I'm not aware of any "technical papers" on the practical aspects of implementing a LFP bank, if you mean academic ones. But you can find some good web pages where people have detailed their builds. You might look at Maine Sail's site, and if you go to the main lithium battery thread here you can find some links to a few other boat installs as well. I learned a lot from those, although none of them were thorough enough to cover all of issues, and most mostly because the documented installs were much smaller.

I am building a 4p4s bank the man specs say up to a 5C charge rate
( never will that even be possible on my boat) is there a real reason to suspect the manufacturers spec sheets on all of their prismatic cells?

I'd be extremely suspect of 5C charging on any available prismatic LFP cell today. As I wrote above, the trend is clearly towards lower and lower rates. Many cells you can buy today specify 1C charge maxima.

The good news is that the chemistry is the chemistry. If you adopt the conservative, battle-tested parameters that some of us espouse in these threads, you will be fine with whatever cells you use.

[newhaul]

4

Originally Posted by nebster

Well, I'm not asserting much, I'm just relating my own experience on my setup. I have a sample size of 1, so you have to take that for what it is (with a large grain of salt). You also have to decide if the things I've run into apply in your situation. I'm not sure how I could justify that my batteries get hotter when I charge them faster, except to say "my batteries get hotter when I charge them faster."

I'm not aware of any "technical papers" on the practical aspects of implementing a LFP bank, if you mean academic ones. But you can find some good web pages where people have detailed their builds. You might look at Maine Sail's site, and if you go to the main lithium battery thread here you can find some links to a few other boat installs as well. I learned a lot from those, although none of them were thorough enough to cover all of issues, and most mostly because the documented installs were much smaller.



I'd be extremely suspect of 5C charging on any available prismatic LFP cell today. As I wrote above, the trend is clearly towards lower and lower rates. Many cells you can buy today specify 1C charge maxima.

The good news is that the chemistry is the chemistry. If you adopt the conservative, battle-tested parameters that some of us espouse in these threads, you will be fine with whatever cells you use.

the 5C top fast charge and 10C discharge is what camel battery places on their cells ( they produce the cells for the Nissan ev market)
they also a .5C standard charge as well .
( I have posted the specs see post 55 here . )

[nebster]

Originally Posted by john61ct

Only for occasional calibration / testing purposes. 100% is defined by the owner, the arbitrary vendor setpoint has no relevance to real-world usage.

Your way of thinking about this is weird, but the real problem is that it causes what you WRITE to be confusing to everyone else.

The vendor designs the cells to hold a certain amount of energy when charged to a certain voltage. It's not arbitrary, it's the whole basis for how you buy a battery: by a specified capacity. I can't believe you're filling up this thread talking about this.

> It is required every day on my pack to be able to put 90Ah into my 100Ah cells

What, why? Is your daily usage / DoD pattern really that consistent?

Why do I want to put 90Ah into my 100Ah pack? Because I paid for 100Ah and would like to use as much of it as I can. Does that seems strange to you?

Of course my DoD varies day to day. But when it's time to charge, to get back to 90Ah (because you seem so hung up on "arbitrary setpoints," I'm not using SOC% here), what I said earlier is what I have to do.

Most setups vary from one day to the next. I rarely use more than 40-60%, build in some reserve capacity, especially in mostly solar setups without convenient ICE charging on demand.

So what do you have set up? What are your cells? What charger do you use? What PV do you have installed?

> There, a change of rate reduces the usable stored energy by 12.5%. Who wants to throw away 12.5% of their battery just because they elect a different rate?

If that is true, just use a different voltage for the different sources to get to the same SoC.

That would be an option if the charging were at a perfect, constant rate. In the real world, the charger is sharing its shore supply with other loads in the cabin, and so the available energy can fluctuate. It's not possible to prescribe a single voltage in my system.

Whether 1C or .1C, adding Absorb time at a given voltage does not result in actually raising SoC significantly, the extra AH are getting dissipated as heat and, more importantly, likely reducing lifespan if over 3.45Vpc

Here's another of your sentences that is objectively false. Go look at a voltage charge curve for LFP parameterized on rate. Oh, shoot, I'll just do it for you:



Pick a voltage, V, you like on the left and follow it across. The difference on the x-axis between the blue and green curves is how different the SOC will be if you stop after CC when you reach V when you charge at 0.1C compared to charging at 0.2C. It is a substantial difference.

I also wrote the same thing in the post you are quoting, but you apparently didn't read it or understand it. Maybe the picture will help you.

MS recently posted very detailed test results proving exactly that, differences were usually 1-3% at most.

MS's experiments were all conducted under the same charge rate. We are talking about charging at different rates, and you asserted (incorrectly) that the rate doesn't matter and that absorption charging is pointless.

What rate do you use for your own pack, John?

CV / Absorb stage in daily cycling just adds complexity and risk, without any real-world benefit.

Yeah, you've really nailed it.

[nebster]

Originally Posted by newhaul

4

the 5C top fast charge and 10C discharge is what camel battery places on their cells ( they produce the cells for the Nissan ev market)
they also a .5C standard charge as well .
( I have posted the specs see post 55 here . )

Yeah I mean no one has anything close to a 5C charge in real life in any production-scale lithium chemistry that's available today, to my knowledge. The fastest I've seen is a few phones that'll do 2C for a portion of the charge, and we know how tortured and short-lived phone batteries are.

My (conservative) take is to stay away from anything over 0.5C. If you have a charging source that can exceed 0.5C, one way to "solve" this is to buy twice as many cells.