1400 Ah Lithium from cells

[nebster]

Originally Posted by john61ct

But that SoC point is not something that can be accurately measured in practice, if you never actually hit your "theoretical 100%" without harming the bank.

If your point is that SOC is arbitrary relative to chosen endpoints, I agree with you. For my thought experiment, I was trying to describe a way that I could charge a single cell at two different rates but achieve the same stored energy (and the same resting OCV). One of those two ways would require me to establish those SOC boundaries. I didn't meant to take things wildly off track with that simplification.

Even learning your bank's actual capacity via a true 20-hour load test is not something you'd want to do more than a dozen times in a bank's lifetime.

Agreed, and it's not that interesting to me, really. With a bottom-balance and then a charge or two, I can easily determine where I must stop, because the weakest cell will dictate the stopping point. I can choose to set "100%" somewhere below that if I want, but for all intents and purposes, the string's capacity is defined by the weakest cell.

A. their actual trigger mechanism is voltage, so the corresponding SoC represented depends on the C-rate of dis/charge

No, my LVD is rate-compensated. I can even specify the curve, if I don't like the default. My HVD is not compensated, but it matters less because I have greater control over my charge rate. I just pick a charge time and rate that I KNOW will not bring any cell into HVD. I determine that time and rate empirically, beforehand.

B. you (should) never actually ever reach either point

Darn right! But if I do, I am much more afraid it will be the LVD, so that is why I am biased towards a bottom-balance.

C. they can be adjusted up and down as you get new information

Certainly. I don't even have a strong final decision on what those values will be, but as you say: if I'm willing to be conservative and not use the upper chunk of my pack, which I am by intention and by system design from day one, and if the memory effect doesn't decimate my capacity in a way I can't recover from or in a way that requires constant annoying maintenance events, then I think my strategy works well and minimizes the number of gadgets and gizmos that my pack depends on to stay healthy.

I already have way too many gadgets and gizmos on my RV.

As far as your bank getting very hot, so much as to require active cooling measures, how does that even happen?

Well, I sure hope it doesn't happen often, but the bay the batteries are in is insulated, and if that bay is also in direct sun on a hot afternoon on hot asphalt, I've data that shows it can reach 125F. That's too high for me for long term health of the chemistry.

I have a passive mechanism to cool (trade air with the rest of the interior of the RV), and I have an active mechanism (A/C ducted directly to the bay).

I have other reasons to engage the A/C on the hottest days, even unattended, because I have things in my RV that won't like getting so hot, the same way you probably wouldn't want your home to reach 110F while you're away. Adhesives start to act up, plants wilt, pets become uncomfortable, etc.

I have another set of problems in cold, although there I use a diesel furnace and radiators for warming. The power demand still closes in on 750W average in the coldest conditions, though.

I personally would not want a boat setup so that I couldn't just turn off all systems and walk away for a couple months at a time.

I travel a fair bit for work, but live full time in the RV otherwise. It's not practical (and definitely not appealing) for me to go through an elaborate shutdown process every so often. In winter it's basically untenable, just like you wouldn't let your house reach freezing.

So... yeah. I'm crazy!

[OceanSeaSpray]

Bottom balancing is the absolute best way of making a bank incredibly problematic to charge as it requires controlling according to the highest cell instead of being able to use pack voltage.
Trying to use a charging topology suitable for a top-balanced pack with bottom balanced cells can only result in ruined cells.

[nebster]

Originally Posted by OceanSeaSpray

Bottom balancing is the absolute best way of making a bank incredibly problematic to charge as it requires controlling according to the highest cell instead of being able to use pack voltage.

Not in my experience so far; it's decidedly trivial. But, you have to be willing to charge to no more than 95% of the smallest cell's capacity, and a bit less is even better.

Pack voltage becomes less and less useful as series count increases. A misbehaving cell can hide in the aggregate easier. So you do need cell-level monitoring, but everyone should have some form of that anyway.

[OceanSeaSpray]

Originally Posted by nebster

Not in my experience so far; it's decidedly trivial. But, you have to be willing to charge to no more than 95% of the smallest cell's capacity, and a bit less is even better.

Easily said, not so easily achieved and impossible to achieve 100% reliably without having all individual cell voltages.

Originally Posted by nebster

Pack voltage becomes less and less useful as series count increases. A misbehaving cell can hide in the aggregate easier. So you do need cell-level monitoring, but everyone should have some form of that anyway.

It is just as useful when the pack is top-balanced and individual cell monitoring is a requirement either way. It is just that controlling according to pack voltage can be done readily, controlling to the highest cell is rather challenging short of developing a lot of custom electronics.

Now, all this being said, I hadn't come around CF for several months and this is about all the time I am going to put into this thread. So best of luck.

[john61ct]

Originally Posted by nebster

The inverter alone is 80W at idle.

Yes completely different design philosophies. If I use any AC power, I get the smallest cheapest inverter for that one consumer, and switch the inverter on and off.

You also make incorrect assumptions about both my mobile and S&B homes, all get shut down completely when we're going to be away for more than a few days. If it requires energy unattended I don't own it.

[nebster]

Originally Posted by OceanSeaSpray

Easily said, not so easily achieved and impossible to achieve 100% reliably without having all individual cell voltages.

It is just as useful when the pack is top-balanced and individual cell monitoring is a requirement either way.

You wrote these two sentences back to back, which I find rather strange. Which is it? Do we implement cell-level voltage monitoring and have 100% reliability, or do we violate a "requirement either way" and not have 100% reliability?

It's pretty obvious that either management strategy works well, and so it comes down to deciding which package of benefits and downsides you prefer.

It is just that controlling according to pack voltage can be done readily, controlling to the highest cell is rather challenging short of developing a lot of custom electronics.

When you get back to this thread, please share an example of what you mean by "custom electronics."

In my bottom-balanced pack, and those of others, you control charging with pack voltage. You set a CV threshold and a rate and a stopping condition, and you go. It's exactly the same as with a top-balanced pack. I don't have any custom electronics.

[nebster]

Originally Posted by john61ct

Yes completely different design philosophies. If I use any AC power, I get the smallest cheapest inverter for that one consumer, and switch the inverter on and off.

You also make incorrect assumptions about both my mobile and S&B homes, all get shut down completely when we're going to be away for more than a few days. If it requires energy unattended I don't own it.

Nothing wrong with that at all, and I didn't mean to suggest your specific home worked the way mine does.

I do think maybe you are in the minority, though, as most people I know leave their refrigerator on and don't kill their water supply at the mains when they hop on a plane for a work trip. I usually count on that analogy (to a S&B) to help people who hear "RV" and assume I'm living in a trailer park with a propane refrigerator and cooking over a campfire every night.

[john61ct]

Originally Posted by nebster

"custom electronics."

PLCs, Arduino and Raspberry Pi are common examples, coupled with per-cell monitoring of course.

[nebster]

Originally Posted by john61ct

PLCs, Arduino and Raspberry Pi are common examples, coupled with per-cell monitoring of course.

I love rpis and arduinos, but I confess I have none of those things in my LFP charging system.

Given that everyone here seems to agree that cell-level monitoring is mandatory, I'm not inquiring about that controller, since it applies to any pack balancing strategy.

The idea I am attempting to refute is that bottom-balanced packs require special, additional circuitry, or, more broadly, are difficult to manage.

I can't even imagine what the special circuit would be, even if one wanted to add something. OceanSeaSpray sounds like he might know about something extra/new that I should at least be cognizant of.

[john61ct]

Well your setup is making it impossible to get above, or even to, 3.5Vpc

Most would say that is a dealbreaker, with a decent setup that should be easy, even if you don't choose to routinely go that high.

The reason behind that failing is you have chosen to bottom balance, and apparently the extreme imbalances you see as you near that 3.5V average is an unavoidable result of that choice.

Perhaps a custom regulator designed for per-cell-charging would allow for evening the cells out as you get past averaging say 3.3V and 3.4V, making it easier for you to then continue upward safely.

That is my interpretation.

[nebster]

Just to note, you've changed the subject here. I am trying to correct factual inaccuracies in other people's posts, which apparently is a monumental effort. I hope that future readers (I was once one of those!) will be able to sift through the chaff to find the wheat.

Originally Posted by john61ct

Well your setup is making it impossible to get above, or even to, 3.5Vpc

Most would say that is a dealbreaker, with a decent setup that should be easy, even if you don't choose to routinely go that high.

Why? This is the question I've asked five or six times in the last four days here. Why is 3.5Vpc so valuable? Is it to clear the charging bump we've been discussing? Is there some other reason you're excited about that number?

Most people get passionate about high voltages because they want their active BMS to activate and balance charges and get them an extra 2% stored energy for huge hassle. But you and OceanSeaSpray -- and no one else, anywhere, on any forum that I can find, mind you -- I believe are suggesting that being unable to erase a memory effect is a big deal.

I'll say it for the sixth or seventh time: the research paper alone does not convince me that an LFP cell becomes untenable after it has experienced the memory effect. If there were real-world empirical evidence that charging becomes impossible and that capacity is diminished in a substantially-meaningful way over time, that would be different and I would gladly eat my words and apologize for guessing incorrectly.

The reason behind that failing is you have chosen to bottom balance, and apparently the extreme imbalances you see as you near that 3.5V average is an unavoidable result of that choice.

I question the premise, kind sir.

Perhaps a custom regulator designed for per-cell-charging would allow for evening the cells out as you get past averaging say 3.3V and 3.4V, making it easier for you to then continue upward safely.

You're right to point out that there are lots of options on the spectrum between doing nothing and having a full-blown active BMS, if one wants to bring lots of cells in to a higher voltage in series. What you describe sounds like another form of charge-equilibration at the top, one much like many other BMSes employ in an automated or semi-automated form.

Another option that's plausible in my scenario would be to unbank a 16s string (since I can take them out of the pack as I like, as long as they are close in series voltage when the string is joined back in), rewire it in parallel, and program the powerlab to run up whatever voltage/cycles are needed to erase the memory bump. Then drain down to LVD for bottom-balance, rewire, charge back, and bring back in. Doing that once every three years sounds like no big deal to me. (I already plan to break strings down occasionally for maintenance anyway: to check individual cells for capacity slippage that may forewarn a failure, to re-torque the bus bars, etc.) If I had to do that every month, on the other hand, that'd be a showstopper!

Someone with a small pack could reasonably take the strongest cells (the weakest one or ones will get above 3.50Vpc regularly) and manually push them over with a little charger directly on the cell in need, and then manually bring it back down.

None of these are daunting tasks.

[arsenelupiga]

Originally Posted by CatNewBee

OK, if you have the same configuration as I have on my L400S2 (from 2013),
there are Crystrec chargers for shore power and battery isolator diodes for the alternator charging installed.

If you have no other sources - besides the Victron MPPT's, that usually can be set up properly - and if you decide to go with the REC BMS with custom programming - this could be your solution:

The REC BMS is custom-programmed to split opto-coupler and relay outputs.

The relays of the BMS switch via an interface to ML-RBS latching relays (or similar relays) all charging sources in case of over-voltage (MAX Cell Voltage - e.g. 3.65V) off and by a second relay in case of low-voltage (MIN Cell Voltage e.g. 2.9V) all loads off. This is what regular BMS should do.

The optocoupler for over voltage are programmed to react on the value END Charging (e.g. set to 3.5V), that can be separately set in the BMS. This will switch over an Interface a relay off and on that controls all incomming current from your Cristec chargers and from the battery isolator / charge distribution diodes.

In fact, the BMS will enforce a LFP charge regime using your legacy gear.
It will turn off this chargers as soon as the condition is met and re-enable them after a hysteresis, that can also be programmed (Charge Hysteresis, usually preset to 0.25V - so charging will be re-enabled below cell voltage of 3.25V, but you can configure any suitable value).

You can also use the second opto coupler to disable heavy loads - like an inverter earlier by a separate parameter - like I do, but this is optional.

What are the benefits?

1.) Well the disconnect of the alternators is not a problem with our lagoons, because the start batteries remain connected via the second path of the diode isolators, so no harm on this side, the power then goes only to the start battery (AGM)

2.) the disconnect of the Crystec 220V charger is no problem at all. They have 3 separate outputs that can be used to charge up to 3 batteries in parallel, one goes to the start battery, the second to the house battery and the third - optional generator start battery. There are at least two of them - one per engine. It is not critical to just disconnect them from the house LFP.
When the LFP is dropped, they continue to charge the start batteries alone.

3.) The solar charger remain on, they are programmed for LFP, so they do not exceed the allowed voltage, also they are programmed for a very short absorption (to allow some balancing if necessary) and will go to float at a low voltage afterwards. It is low enough, that charging will stop, but they will provide power to the house when loads kick in and battery voltage drops below the float setting, they will try keep up voltage at this level.

About the interface mentioned above - it is pretty simple, I can provide you my solution for this. The reason I add it is, the relay outputs are NO/NC relay contacts that provide a static output. (one side on / the other off and vice-versa). But I need a set / reset impulse for the ML-RBS latching relays, so I added 1 capacitor, 2 resistors and 1 LED per channel to create the impulses and have a visual feedback of the state of the BMS relays and a protection of the BMS relay contacts from over-current. I use latching relays insted of regular solenoids, because they draw zero power after the switching. They are expensive, but have a great advantage over other relays - they can be switched manually on and off. So if something breaks, you always have an option to tutn off the BMS and override manually the bms logic. This is a great safety feature when something happens on high seas and you have no time to figure it out and fix it.

I also have built an interface for the opto-coupler outputs, that switch a small signal relay with potential free contacts, so I can isolate more circuits and protect the opto-coupler outputs from over current (Interface consists of a relay, npn transistor, LED and a resistor that protects the relay contacts from over-current)

The circuitry mentioned above has been tested with my cells and works great.

If you use different relays, you may need a different interface or can contact them directly to the BMS outputs - the relay contacts can switch up to 1A according to the product sheet.

I was thinking about lithium and your proposed solution and this is what I am leaning towards:

Disconnect alternator charging house batteries both engines. I find does not charge much and most of time bulk is done by solar, rest is filled in by generator.

Change setting on solar mppt to lithium

Do nothing with 220 V cristec charging. Already have switch in main panel and turn it on when charging needed like now. My mastervolt shunt will advise when close to full which is when I will turn off generator charging. Now, if I forget to turn it off for whatever reason, BMS should kick in and shut down charging (need to confirm this part). Honda generator is relatively noisy, so would be hard to forget. So I am expecting this error should happen less than 1 x year.

Get lithium batteries in, add BMS including monitoring, and off you go. Simple manual system.

I am not electric whizz, but above is in my powers. Also only new complexity here is lithium battery. 9 out of 10 days, solar will be only charger, at least this is what is my experience so far. What I will gain is extra energy that gets now wasted in post bulk stage of gel algorithm.

What do you say ?

[CatNewBee]

That is OK, I think. Just set your BMS approprietly an make sure it disconnects the charge or discharge path if you forget something. 400Ah is a small battery, so you could use even a cheaper BMS solution than the REC if you not need all the functionality and programmability.

[arsenelupiga]

Originally Posted by CatNewBee

That is OK, I think. Just set your BMS approprietly an make sure it disconnects the charge or discharge path if you forget something. 400Ah is a small battery, so you could use even a cheaper BMS solution than the REC if you not need all the functionality and programmability.

great, will test every bit of setup before putting all together.

HOHO, never thought i could pull something like that together. thanks CatNB for encouragement.

[john61ct]

Originally Posted by CatNewBee

a cheaper BMS solution than the REC if you not need all the functionality and programmability.

Examples, ideally links?