"Floating" LFP batteries

[Maine Sail]

Originally Posted by a64pilot

As a side note, would terminating charge with some really low voltage like 12V effect the battery at all?
Wouldn’t drain the bank, would it?
I’m not talking plugged into a dock and a permanent float, but as a way to terminate charge during the day with Solar, assuming only an hour or so of float before Solar drops off line anyway.

12V is not a charge it is essentially 0% SOC for LFP or at the top of vertical part of the lower knee. There's essentially no usable capacity below 3.0VPC so why even bother taking the cells to 2.5VPC when there is nothing there to go after. Perhaps I am misunderstanding what you are asking..?

[TheOffice]

Please correct me:
LFP batteries are NOT to be stored fully charged. If they are stored at 80% and can safely be discharged to 20%, that means that 60% is useable, so the advantage over lead is only 10% (80-20 vs 100-50), not 30% 100-20 vs 100-50).

[Maine Sail]

Originally Posted by TheOffice

Please correct me:
LFP batteries are NOT to be stored fully charged. If they are stored at 80% and can safely be discharged to 20%, that means that 60% is useable, so the advantage over lead is only 10% (80-20 vs 100-50), not 30% 100-20 vs 100-50).

The key word is stored.. They love to be actively cycled between 100% SOC and 20% SOC, or anywhere in between if you desire. I know some owners who actively cycle to down to 10% SOC...

Storage means just that sitting there doing nothing such as being tied to a dock or a boat stored for the winter. Getting to 100% SOC is a good thing for lead acid storage but not for LFP storage. In these cases they will do best to be in a storage mode well below 100% SOC & not being continually held at 100% SOC.

[a64pilot]

Originally Posted by Maine Sail

12V is not a charge it is essentially 0% SOC for LFP or at the top of vertical part of the lower knee. There's essentially no usable capacity below 3.0VPC so why even bother taking the cells to 2.5VPC when there is nothing there to go after. Perhaps I am misunderstanding what you are asking..?

I’m asking what if you were to charge them to 80% SOC and then let the charger drop to some low voltage, what happens?
I would assume that the pack will be drawn down until that voltage and at that point the charger picks up any additional loads and the pack will be maintained at the charge that the set point of the charger equates to?

I’m trying I guess to determine the logic of charge until full if you will, then discharge until empty, then recharge again.
It would seem that a storage voltage to use your term could be picked and that if nothing else would be a safety net to ensure pack remained at a good storage SOC.
I used to try to shoot for 50% on my scooter and Dive light batteries for instance, and I still have lights that are over a decade old, yet still give what I think is nearly full burn time.

[a64pilot]

It’s not that I’m against having a second lead bank, it’s I guess that I don’t want to go to the trouble of switching charge sources over to that bank, and isolating an LFP bank.
Seems like a lot of work to stay in a Marina for awhile

[Maine Sail]

Originally Posted by a64pilot

I’m asking what if you were to charge them to 80% SOC and then let the charger drop to some low voltage, what happens?
I would assume that the pack will be drawn down until that voltage and at that point the charger picks up any additional loads and the pack will be maintained at the charge that the set point of the charger equates to?


I’m trying I guess to determine the logic of charge until full if you will, then discharge until empty, then recharge again.
It would seem that a storage voltage to use your term could be picked and that if nothing else would be a safety net to ensure pack remained at a good storage SOC.
I used to try to shoot for 50% on my scooter and Dive light batteries for instance, and I still have lights that are over a decade old, yet still give what I think is nearly full burn time.

See post #7 in this thread. If you want a slightly higher "storage SOC" raise that 13.0V / 3.25VPC up a bit.

This evening, when we get aboard, and the bank is at 45% SOC or so, I will fire up the engine and kick on the engine driven refrigeration & alternator while we are getting ready to leave the mooring. By the time we are ready to go we've already replaced nearly a day and a half worth of energy needs. I then run the engine getting out of the harbor at about 80% loading for another 5-10 minutes then shut her down. That short run time on the mooring, from a storage SOC, provides us with enough energy in the bank to go at least 3.5 days with no charging what so ever. If I turn on the PV it can extend this out another day or more.

Since going to LFP I was able to convert the second coil in the holding plate to DC as opposed to AC and once the box is chilled we can now run on DC refrigeration only with no need at all for the engine but once ever three to four days. In Maine, with fickle summer winds, the engine gets run more so we take opportunity charges when we need to but rarely before we hit 70% DOD.. This is totally different from the way we needed to use lead.

[a64pilot]

So it will work? As a temporary measure maybe a few days to a few weeks at a time?

[Maine Sail]

Originally Posted by a64pilot

It’s not that I’m against having a second lead bank, it’s I guess that I don’t want to go to the trouble of switching charge sources over to that bank, and isolating an LFP bank.
Seems like a lot of work to stay in a Marina for awhile

Where we cruise, Maine & Maritimes, there are so few marinas it's a non issue. That said flipping off the LFP and then flipping on the lead bank takes but a few seconds. It's more work getting out the shore power cable & plugging it in. The last time we used a marina we never plugged in at all. With lead I would have jumped at the opportunity to plug in. With LFP I just poured another cocktail and sat back and relaxed.......

[gmakhs]

When I was considering lfp , my plan was no charge at storage at all , no floating .
And for daily use I was planning to separate my bank on 2 one would be in use and the other on charge , and change them with an Arduino automatically or by hand when the service was 20%.
For the dock I already use just the charger , and 220 v.

Anyway lithium is not gonna happen anytime soon on my boat .

[a64pilot]

Originally Posted by Maine Sail

Where we cruise, Maine & Maritimes, there are so few marinas it's a non issue. That said flipping off the LFP and then flipping on the lead bank takes but a few seconds. It's more work getting out the shore power cable & plugging it in. The last time we used a marina we never plugged in at all. With lead I would have jumped at the opportunity to plug in. With LFP I just poured another cocktail and sat back and relaxed.......

I assume then you have separate chargers?
I can see that as an answer, change from one bank to the other with the A, B battery switch and don’t turn on the LFP chargers, just the lead one, drive the Solar voltage to some low voltage.
Or are you connecting chargers into the back of the battery isolation switch or similar?
We are full time and will travel on land occasionally and possibly travel home on occasion.
I learned a hard lesson last year, sometimes things can happen that can make you abandon the Boat for months at a time, best have a strategy / Plan on how to do that. Thankfully we were in Vero Beach’s mooring field and not at anchor somewhere.

[tanglewood]

Originally Posted by gmakhs

When I was considering lfp , my plan was no charge at storage at all , no floating .
And for daily use I was planning to separate my bank on 2 one would be in use and the other on charge , and change them with an Arduino automatically or by hand when the service was 20%.
For the dock I already use just the charger , and 220 v.

Anyway lithium is not gonna happen anytime soon on my boat .

At the risk of belaboring the point of this thread, "float", and "continuing to charger" are not necessarily the same thing. It all depends on the float voltage. By setting an appropriate float voltage, you are just providing standby power while the batteries sit idle.

[tanglewood]

Originally Posted by a64pilot

It’s not that I’m against having a second lead bank, it’s I guess that I don’t want to go to the trouble of switching charge sources over to that bank, and isolating an LFP bank.
Seems like a lot of work to stay in a Marina for awhile

Exactly. I think that with a "float" voltage set appropriately, you get standby power without having to isolate you LFP bank.

[a64pilot]

Originally Posted by tanglewood

Exactly. I think that with a "float" voltage set appropriately, you get standby power without having to isolate you LFP bank.

Only I believe after you discharge your LFP bank to the voltage your floating at.
I think. So before you leave it would behoove you to recharge the LFP bank if I’m understanding.

[tanglewood]

Originally Posted by a64pilot

Only I believe after you discharge your LFP bank to the voltage your floating at.
I think. So before you leave it would behoove you to recharge the LFP bank if I’m understanding.

I think the idea is to float at the voltage (and SOC) you are comfortable leaving the battery at. On one hand, you would want that to be full charge so you have max available power in the batteries. But on the other hand you want to store at mid charge for better battery life. So you need to find a happy median.


But it's not clear to me that anyone actually knows how the stored-SOC time/level continuum actually maps out. How does life vary based on stored SOC? And how much does it depend on the length of time a battery stands at a particular SOC? Does a week at 100% SOC reduce life, or does it make no difference if you then fully cycle the battery? What about a month? Or a year? I don't necessarily disbelieve the general assertions we hear, but I only really believe them when there is some scientific data to back them up.

[Delfin]

Originally Posted by tanglewood

Thanks. It does appear to hinge on what you mean by "float". I like your term "standby voltage" to distinguish between a voltage that continues to create current flow into the batteries from a voltage that does not.


My concern is that "don't float" has become LFP dogma with people not understanding what's behind it. So nobody (or very few people) operates with chargers at a standby voltage to carry loads rather than cycling the batteries. This standby voltage is accomplished by carefully programming the "float" voltage and trigger conditions in a charger. But the use of that nasty word, "float" scares people away.


The broken english in the CALB manual is pretty priceless, and often very difficult to figure out what they are saying. I took that quoted section to mean the float voltage in applications where there is an on-going power source, unlike an EV or portable device. So precisely what we are now calling a standby voltage.


But you are saying it's a different charge protocol? And I'm not sure what you mean by "If you are using these cells in a standby system then they want you to charge to 3.4VPC / 13.6V...". Is it charge to 3.4 vpc and hold there? And if so, what's the "/ 3.6 vpc" part mean?


I think another factor in this is how full you charge the cells in the first place. Charging to 3.6 vpc would be full charge, so holding them there with a standby voltage would essentially be storing them at full charge which is discouraged. Charging to 3.4 vpc and holding there would be charging to what, around 70% SOC and holding there? That seems OK, and would be consistent with the described standby power applications. My assumption is that on a boat, batteries are operated roughly 20%-80% SOC, so a charge to say 3.45 vpc, followed by a standby voltage of 3.30 vpc. That would charge to about 80%, then drain down to maybe 70% and hold at that point.


The alternate strategy of disconnecting the LFP bank and switching in a small LA buffer bank of course works, but I'm hoping to eliminate as much switching and switching logic as possible. Plus I'd like to not be cycling my batteries every day when on shore power while my boat sits for a month or two or three.


Oh, and there is another reason I'd like to keep the LFP bank connected. My inverters have a boost function where they will draw battery power to supplement shore or gen power for short term overloads. I think this is an excellent way to live on smaller shore power connections that you might otherwise need, and to run a generator that is sized more to the average power load rather than the peak power load. None of this may matter to a typical sailing vessel, but for other types of boats it's a big benefit.

For what it's worth, I sat at the dock this last winter for at least three months with the LFP bank disconnected, the LA starter bank hooked to the charger and I don't think the LFP bank declined more than from 26.3 to 26.2 volts over that month. It was stored around 50% SoC.

So, no cycling. Just go away and return a few months later to a happy LFP bank.