New battery technology ?

[twistedtree]

TT,

The only operational downside I have seen to LFP is freezing temperatures. While a BMS should take care of that, you have to spend energy to heat the batteries if you have lost shore power for any length of time and the batteries have dropped below freezing. A boat in the water should be fine, but on the hard or in an RV that might be a problem if something doesn’t work as advertised. A frozen flooded battery is a problem but a charged FLA shouldn’t freeze.

Tom

Yes, I agree.

 

[twistedtree]

The thing that makes me hesitate, and might make a lot of people hesitate, is the long term storage problem. Opinions and strategies vary, but most of the consensus is on depleting them to the 40 - 60% range and keeping them there, cycling every 4-6 months. That is not so easy to accomplish.

And many boats are effectively kept in long term storage. A boat that is used on weekends and plugged in during the week is effectively that. Or a boat that is stored over the winter off season. AGMs are very happy with this, just plug in, fully charge, and forget.

I've seen some opinions that this isn't all that important for LFP, maybe you decrease their life by 10%, but most manufacturers recommend against storage fully charged.

Another issue is high current discharge, which the "drop in replacement" batteries tend to prevent. That would be thruster and starter loads. If you are building your own pack with high current disconnects for protection, you can program your way around that. And again, with the drop in replacement BMS, what happens when it charges fully, and disconnects? A purpose built BMS will disconnect the alternator field, but otherwise? And if I keep my AGMs or FLAs for starting, what voltages do I set my charger for?

None of these are much of a problem in the live aboard on the hook situation, for which LFP are ideal. But I can still make a case for AGMs on a boat used only in season, or only on weekends. It isn't that there is nothing to worry about with LFP, but different things to worry about.

I am about to replace the batteries in both boats, different use cases, and I am struggling with it - not a clear decision.

If you charge voltages are set correctly, the BMS will never disconnect. If it does, something is wrong.


And I agree that if your usage doesn't really use your batteries very much, then there isn't much value in good batteries.

 

[DDW]

If you charge voltages are set correctly, the BMS will never disconnect. If it does, something is wrong.


And I agree that if your usage doesn't really use your batteries very much, then there isn't much value in good batteries.

BMS can disconnect due to discharge conditions as well.

It isn't necessarily not using your batteries much, in my case they are used like an at anchor liveaboard 5 - 6 months then stored for 6 - 7 months. What would your strategy be with LPF? 6 months you can probably discharge them partially and disconnect for storage. My boat has unexpectedly been stuck unused for much longer than that in Canada. What would I do with LFPs in that scenario? Having that problem with the dinghy electric outboard now. With current tech you'd need a caretaker with knowledge on how to maintain them. It is a simple problem to solve in the BMS, but none of them do very much of that now. Float charging is a very simple maintenance regimen on AGMs.

 

[David Ess]

Why?

For electric cars, it's all about weight and space relative to amp hours. In most boat applications, weight and space aren't critical. BTW, you do realize that almost all boats sold in the USA with a gas or diesel engine, come with a FLA or AGM battery(s), don't you?

Ted

Thanks for conceding they wont. The electric cars and the best, newest boats use litium ion. Ill bet those Alva Yachts never switch to old fashioned ones, nor all those new electric ferry boats coming online.

 

[catalinajack]

So, David what are the "best" boats? By whose definition of "best"?

Thanks for conceding they wont. The electric cars and the best, newest boats use litium ion. Ill bet those Alva Yachts never switch to old fashioned ones, nor all those new electric ferry boats coming online.

 

[David Ess]

So, David what are the "best" boats? By whose definition of "best"?

I just gave one example, Alva Yachts, here's another....Hinckley. Now you give an example of what you consider best boats, and I'll look them up to see if they use lead acid batteries.
"Hinckley, the builder of America's finest*yachts*for nearly 90 years, continued its legacy of innovation today by unveiling the world's first fully electric luxury*yacht*– Dasher. ... Dasher powers her 28 feet 6 inches with twin 80hp electric motors and dual BMW i3*lithium ion batteries." They COULD HAVE used lead acid..but didnt.

 

[catalinajack]

"I can supply manufacturer supplied data that shows that Battleborn Lithium Batteries compared to Crown FLA batteries are roughly 150% the price per kilowatt hour of energy delivered over their lifecycle."

And that, folks, is the essence of this discussion. Lithium is not better, just different and it costs a lot more in terms of their ability to deliver energy over their lifetime. Plus, lithium brings with it technical problems that FLA batteries do not have. Lithium has its place among those who spends weeks or months away from shore power and for whom solar is not a solution. That is a very small portion of us, very few. For folks with larger solar arrays, a large FLA battery bank can work just as well as lithium the only difference being that the FLA bank will take up more space but, functionally, there is no difference. For most boaters, 98% is my guess, lithium offers little except more cost and more complexity. On the hook for a few days is not a reason to do lithium. Space or access, but only for FLA, not AGM, can be an issue for some and may be a solution for them.

When I said in an earlier post there seems to be a "blind rush" by some to move to lithium, I stand by that statement. I consider folks who think that lithium is "better", apparently in all use cases, to be among the blind. Lithium is a solution for a few, not many.

That is a blanket statement that encompasses many variables.

it is also frankly false.

I can supply manufacturer supplied data that shows that Battleborn Lithium Batteries compared to Crown FLA batteries are roughly 150% the price per kilowatt hour of energy delivered over their lifecycle.

So... Lithium might be better at some things, but not all things.

 

[catalinajack]

And at what cost for this cutting edge technology? Who among us could afford that? Your example is wildly irrelevant to this discussion.

I just gave one example, Alva Yachts, here's another....Hinckley. Now you give an example of what you consider best boats, and I'll look them up to see if they use lead acid batteries.
"Hinckley, the builder of America's finest*yachts*for nearly 90 years, continued its legacy of innovation today by unveiling the world's first fully electric luxury*yacht*– Dasher. ... Dasher powers her 28 feet 6 inches with twin 80hp electric motors and dual BMW i3*lithium ion batteries." They COULD HAVE used lead acid..but didnt.

 

[twistedtree]

BMS can disconnect due to discharge conditions as well.



It isn't necessarily not using your batteries much, in my case they are used like an at anchor liveaboard 5 - 6 months then stored for 6 - 7 months. What would your strategy be with LPF? 6 months you can probably discharge them partially and disconnect for storage. My boat has unexpectedly been stuck unused for much longer than that in Canada. What would I do with LFPs in that scenario? Having that problem with the dinghy electric outboard now. With current tech you'd need a caretaker with knowledge on how to maintain them. It is a simple problem to solve in the BMS, but none of them do very much of that now. Float charging is a very simple maintenance regimen on AGMs.

The most common approach to idle time, say when plugged into shore power, is to set the charger to float at a voltage below 100% SOC. 3.35 vpc is the most common value which ends up as 13.4V for a 12V system. And most suggest only charging to 3.45vpc in normal charge cycles rather than the 3.6vpc that yields a 100% full battery. In other words, don’t operate the battery in the first place in the zone that causes longer term issues. In practice, you are only forfeiting a very small amount of capacity.

 

[David Ess]

And at what cost for this cutting edge technology? Who among us could afford that? Your example is wildly irrelevant to this discussion.

I never even mentioned cost, just better technology, and that's why Hinckley uses them.

 

[rslifkin]

I never even mentioned cost, just better technology, and that's why Hinckley uses them.

Remember, the vast majority of us don't have unlimited boat budgets. If we upgraded everything to the best we could find regardless of cost, most of us wouldn't be boating at all.

 

[David Ess]

Remember, the vast majority of us don't have unlimited boat budgets. If we upgraded everything to the best we could find regardless of cost, most of us wouldn't be boating at all.

Fair enough, but I never referred to the 'vast majority' either, just the technology.

 

[ksanders]

Lithium has some advantages over FLA technology but it is not the end all do all as some claim.

One great advantage is weight. My 840AH battery bank weighs around 500 pounds. A replacement Lithium bank would be a 600AH bank and would weigh 180 pounds.

Another advantage is physical size. Since Lithium batteries are lighter they can be more easily stacked, reducing the square footage needed.

Another advantage is maintenance. That is a real advantage as FLA technology takes a person some time to actually look at their batteries, and add water as necessary.

The challenge is that on a boat these advantages are not universally valuable.
Some boats yes, some boats not so much.

The things that are often touted regarding lithium technology like reduced generator run time, and lower lifecycle costs are pure numbers manipulation and great salesmanship.

I am happy to demonstrate either of those parameters using real comparisons, and actual manufacturer supplied data.

When doing the comparison what I Will Not do is to compare a legacy, poorly engineered FLA based system with a new, properly engineered LiFeP04 solution. That would be unfair, and that is what the Lithium sales folks do.

I would compare two properly engineered battery systems, one being FLA based and one being LiFeP04 based. When you do that the LiFeP04 "advantages" diminish dramatically or are eliminated.

 

[David Ess]

Ksanders, thanks for conceding to some of the advantages, and to me , such advantages mean ....higher tech. Glad I not one of the guys who claimed any "end all". I agree with Hincnkley, and Alva Yachts and the others that theyre better.

 

[twistedtree]

The most common approach to idle time, say when plugged into shore power, is to set the charger to float at a voltage below 100% SOC. 3.35 vpc is the most common value which ends up as 13.4V for a 12V system. And most suggest only charging to 3.45vpc in normal charge cycles rather than the 3.6vpc that yields a 100% full battery. In other words, don’t operate the battery in the first place in the zone that causes longer term issues. In practice, you are only forfeiting a very small amount of capacity.

If the boat is regularly laid up for 6 months of the year, then I think there are two approaches depending on the expected temps.

If you can completely disconnect the batteries and they will remain within the allowed storage temp range, they will lose pretty much no charge over the storage period. If temps are a problem, then I’d remove them and store them in a more controlled environment.

If the batteries need to remain in service, say to power an alarm and bulge pump, then I’d float them like above provided temps are acceptable. If temps won’t cooperate, then it’s either climate control or back to AGMs.

 

[twistedtree]

Thanks for conceding they wont. The electric cars and the best, newest boats use litium ion. Ill bet those Alva Yachts never switch to old fashioned ones, nor all those new electric ferry boats coming online.

Those are all propulsion applications, with very different requirements vs a house battery bank. You have to pick the solution based on the problem you are trying to solve.

 

[twistedtree]

"I can supply manufacturer supplied data that shows that Battleborn Lithium Batteries compared to Crown FLA batteries are roughly 150% the price per kilowatt hour of energy delivered over their lifecycle."

And that, folks, is the essence of this discussion. Lithium is not better, just different and it costs a lot more in terms of their ability to deliver energy over their lifetime. Plus, lithium brings with it technical problems that FLA batteries do not have. Lithium has its place among those who spends weeks or months away from shore power and for whom solar is not a solution. That is a very small portion of us, very few. For folks with larger solar arrays, a large FLA battery bank can work just as well as lithium the only difference being that the FLA bank will take up more space but, functionally, there is no difference. For most boaters, 98% is my guess, lithium offers little except more cost and more complexity. On the hook for a few days is not a reason to do lithium. Space or access, but only for FLA, not AGM, can be an issue for some and may be a solution for them.

When I said in an earlier post there seems to be a "blind rush" by some to move to lithium, I stand by that statement. I consider folks who think that lithium is "better", apparently in all use cases, to be among the blind. Lithium is a solution for a few, not many.

I don’t know about market percentages for each, but I agree one solution doesn’t fit all use cases. A while ago I talked someone out of LFP because their boat was inherently a “generator on all the time” boat. All the batteries ever do is bridge the gap between generators and shore power.

 

[DDW]

The most common approach to idle time, say when plugged into shore power, is to set the charger to float at a voltage below 100% SOC. 3.35 vpc is the most common value which ends up as 13.4V for a 12V system. And most suggest only charging to 3.45vpc in normal charge cycles rather than the 3.6vpc that yields a 100% full battery. In other words, don’t operate the battery in the first place in the zone that causes longer term issues. In practice, you are only forfeiting a very small amount of capacity.

Have you seen any manufacturers data or independent testing which shows lifetime effect of a 3.35 vpc float? I've seen data supporting the idea of charging only to 3.45 or 3.5, i.e., don't bang the corners of the envelope. AGMs like to live in the upper right corner, LFPs like to live in the middle. Most LFP manufacturers seem to recommend a complete cycle every 6 - 8 months, not just a constant float.

Here for example is a recent post from Rod Collins who stays on top of this stuff. No one seems to know what the effect on life of a CV float is.

I've got two boats both needing batteries. Time constraints dictate that if I go LFP they would have to be drop in replacement style, not cells + external BMS + external disconnects. I can parallel enough to get sufficient peak amps to keep the BMS from disconnecting from the high draws, and I can put catch diodes on the alternator to prevent runaway in the event of an inadvertent disconnect (problem theoretically exists with any battery). That leaves one other problem: what to do about engine start. That requires either a specialized LFP with high enough current, or use the house batteries to start, or stick with AGMs. If the latter, how do they get charged if the charge voltages are all set for LFP? A DC-DC boost converter I guess, but that is not without problems. The devil is in the details and there are a few of them.

 

[rslifkin]

catch diodes on the alternator to prevent runaway in the event of an inadvertent disconnect (problem theoretically exists with any battery). That leaves one other problem: what to do about engine start. That requires either a specialized LFP with high enough current, or use the house batteries to start, or stick with AGMs. If the latter, how do they get charged if the charge voltages are all set for LFP? A DC-DC boost converter I guess, but that is not without problems. The devil is in the details and there are a few of them.

My solution to both would be to have either FLA or AGM start batteries on the engines. Feed the alternators to the start batteries. Use a DC-DC charger that only runs with engines running to charge the LFP house bank. Keeps it easy, safe for the alternators, and not an issue to use with internally regulated alternators either. For shore charging, I'd just use a separate small charger for the engine batteries.

 

[Bacchus]

But at the same time, we have to remember the 2 kinds of better: "technically better" and "will work better in my application". Plenty of things can be technically better, but based on a specific application won't provide any benefit (but would for someone else).

Mant times a third kind...
Makes sense economically.

 

[Bacchus]

If the boat is regularly laid up for 6 months of the year, then I think there are two approaches depending on the expected temps.

If you can completely disconnect the batteries and they will remain within the allowed storage temp range, they will lose pretty much no charge over the storage period. If temps are a problem, then I’d remove them and store them in a more controlled environment.

.

What is "the allowed storage temp range"?

 

[DDW]

My solution to both would be to have either FLA or AGM start batteries on the engines. Feed the alternators to the start batteries. Use a DC-DC charger that only runs with engines running to charge the LFP house bank. Keeps it easy, safe for the alternators, and not an issue to use with internally regulated alternators either. For shore charging, I'd just use a separate small charger for the engine batteries.

This is what I do on my sailboat as the boat is 24V and the engine start battery 12V. A little more difficult if the start battery has to be higher voltage than the house but it can be done with a boost converter. On the trawler, the draw on the engine battery is substantial though, being a common rail engine. It takes about 20 A just to keep it running, so the DC-DC has to be pretty big.

All these little details add up to it being not trivial, especially in a retrofit situation.

 

[twistedtree]

Have you seen any manufacturers data or independent testing which shows lifetime effect of a 3.35 vpc float? I've seen data supporting the idea of charging only to 3.45 or 3.5, i.e., don't bang the corners of the envelope. AGMs like to live in the upper right corner, LFPs like to live in the middle. Most LFP manufacturers seem to recommend a complete cycle every 6 - 8 months, not just a constant float.

Here for example is a recent post from Rod Collins who stays on top of this stuff. No one seems to know what the effect on life of a CV float is.

I've got two boats both needing batteries. Time constraints dictate that if I go LFP they would have to be drop in replacement style, not cells + external BMS + external disconnects. I can parallel enough to get sufficient peak amps to keep the BMS from disconnecting from the high draws, and I can put catch diodes on the alternator to prevent runaway in the event of an inadvertent disconnect (problem theoretically exists with any battery). That leaves one other problem: what to do about engine start. That requires either a specialized LFP with high enough current, or use the house batteries to start, or stick with AGMs. If the latter, how do they get charged if the charge voltages are all set for LFP? A DC-DC boost converter I guess, but that is not without problems. The devil is in the details and there are a few of them.

It will be decades before there is good empirical data on lifespan for lightly cycles LFP batteries. 99% of the research, and there is tons of it, hammers the batteries from full charge to full discharge, all at high charge and discharge rates. In this worst case situation, 2000 cycles have been demonstrated over and over again, so that's the minimum you can expect



But at the same time, the mechanisms that cause loss of capacity in LFP batteries is well understood, and basically comes down to heat, and high C-rate currents. And the heat is typically caused by high C-rate cycling. In a house battery application, the C-rates are comparatively very low in the 0.5C and below. You would have to cycle a cell at that rate continuously for nearly 5 years to reach 10,000 cycles. Rod has a bank he has been cycling for years and don't think he's even hit 2000 cycles.


So the short answer is that know behavior of LFP suggests much longer cycle life for lighter cycling, but I don't think anyone really knows if that means 3000, 5000, 10000, or what.


Regarding start batteries, I kept two banks of AGM start batteries on my current boat serving the main engine, wing engine, and two generators. They are charged via MasterVolt DC/DC chargers from the house bank, so they get a proper AGM charge cycle. My boat is 24V, but there is also a small 12V service that is a Group 31 AGM that is similarly charged by a DC/DC charger.


I prefer this approach of charging the AGMs vs DC/DC vs what a lot of people do charging the LFP via a DC/DC charger. The primary reason is that the start and 12V batteries don't require a big charge rate. It takes very little to recharge a start battery, so the chargers are small and affordable. Then the higher capacity charging devices like alternators, inverter/chargers, and shore power chargers can directly charge the LFP bank and do so at their max charging capacity. If you do it the other way, the DC/DC charger that charges the LFP bank pretty quickly become the limiting factor in how fast you can charge the LFP bank, and defeats one of it's biggest advantages. So my start battery chargers are I think 30A each, and I can dump as much as 1000A directly into the LFP bank if I wanted to crank everything up.

 

[twistedtree]

What is "the allowed storage temp range"?

The spec sheet for my MG Energy batteries says:


0-45C for charging


-20-55C for discharge


-20-45C for storage

 

[DDW]

In fact, Rod measured significant degradation of some LFP cells kept on float and not cycled. Which squares with the manufacturers recommendations to cycle them.

It is more complicated when the LFP and AGM are both 12V. Now you can't use a buck converter, and the boost converter choices are more limited. While it takes very little to charge the start battery, if it is the thing running the engine it takes a fair amount just to keep up with its draw. On the sailboat it was easy, like you a Mastervolt buck converter 24 -> 12V and 10A was more than enough. Since all that is already in place the sailboat conversion is a bit simpler. On the trawler, I'd need maybe a 40 A 12->12 boost converter with 3 stage charge capability. They must exist, I haven't really looked, but you don't pick them up at West Marine on sale. Looks like Victron makes a 30A, might be enough.

 

[twistedtree]

In fact, Rod measured significant degradation of some LFP cells kept on float and not cycled. Which squares with the manufacturers recommendations to cycle them.

"Floating" LFP has become a storm of misunderstanding, and it really comes down to your definition of a "float charge".


In all definitions, a float charge is a charger operating at some pre-set voltage level.


Historically in the lead acid world, that voltage level is set such that there is a small, continuous charge current buy setting the float voltage ABOVE the resting voltage of the battery. This makes lead-acid batteries very happy, but will indeed damage LFP. In a lead battery, the trickle of current generates a small amount of heat and counteracts the self-discharge that is inherent in lead batteries. But LFP doesn't work that way, and if you continue to charge them, even at a low rate, they will be damaged.


Because lots of people managed to wreck a lot of expensive LFP batteries, Rod's mantra had been to charge through the bulk/absorb stage, "then STOP".


But what this means is "stop applying a charge current". It doesn't mean you can't use the float function in a charger, and in fact there are many boat situations where a constant "float" function is extremely useful. A boat on shore power is an excellent example. You want the ongoing loads to be carried by the shore power, not the batteries. Same if you have a big solar array. Otherwise you would be needlessly cycling your batteries all the time.


So the trick is to set the charger's float voltage to a point where no current flows when the batteries are at some less-than-100% SOC. In other words, match the charger's float voltage to the resting voltage of the battery as say 80% SOC. If there is no voltage difference, there will be no current flow, i.e. zero charging action. That's where the 3.35 vpc figure comes from, and is what most manufacturer's recommend. So if you plug into shore power with full batteries, your batteries will continue to carry loads until they get down to 3.35 vpc. Then the charger will start to carry the load, and your battery will be maintained at roughly 80% SOC.


One concept that has been bantered about and that I think is a good idea in concept is to further have a "storage" mode where the float voltage is lowered even more such that the batteries are maintained in the 40-50% SOC range. Typical chargers don't have such a capability, other than reprogramming the float voltage for the duration of storage, then bringing it back up again when you re-activate.

 

[twistedtree]

It is more complicated when the LFP and AGM are both 12V. Now you can't use a buck converter, and the boost converter choices are more limited. While it takes very little to charge the start battery, if it is the thing running the engine it takes a fair amount just to keep up with its draw. On the sailboat it was easy, like you a Mastervolt buck converter 24 -> 12V and 10A was more than enough. Since all that is already in place the sailboat conversion is a bit simpler. On the trawler, I'd need maybe a 40 A 12->12 boost converter with 3 stage charge capability. They must exist, I haven't really looked, but you don't pick them up at West Marine on sale. Looks like Victron makes a 30A, might be enough.

Check out the MasterVolt MAC series of DC/DC chargers. They are available is many combinations of input and output voltages. My start battery chargers are 24V to 24V.


Good point about the power consumption of the engine's control system, and a reminder that I need to go check that. I'm actually quite curious to see what the actual power draw is for my engines while running, including the throttle and gear controls. They are all powered from the start batteries, so the DC/DC charger needs to carry those loads, plus recharge the battery. My wing engine and generators have alternators that also charge whatever start bank they are connected to, but my main engine does not. Both main engine alternators are directed to the house bank.

 

[DDW]

So the trick is to set the charger's float voltage to a point where no current flows when the batteries are at some less-than-100% SOC. In other words, match the charger's float voltage to the resting voltage of the battery as say 80% SOC. If there is no voltage difference, there will be no current flow, i.e. zero charging action. That's where the 3.35 vpc figure comes from, and is what most manufacturer's recommend. So if you plug into shore power with full batteries, your batteries will continue to carry loads until they get down to 3.35 vpc. Then the charger will start to carry the load, and your battery will be maintained at roughly 80% SOC.

This is the thing Rod tested, floating long term at lowered voltage and no cycling. Results were mixed, with some cells losing significant capacity in less than a year. The only scholarly papers I can find talk about degradation of the electrodes due to static charge collection if not cycling. I can find no manufacturer that suggests that a static float is a good idea. Almost all say the battery should be cycled a full or partial cycle at least every 6 months. It would be no problem for a BMS to do this, but most don't. All of the info you find about float and float voltages for LFP are opinion - perhaps it is reasonable opinion - but not backed by any data.

The LFP supplied with my eProp outboard does state that after 20 days at full charge, it will self discharge (purposely) to 60%. What is needed is a program that automatically cycles the batteries, say down to 30% and then up to 70% every few months if left on float.

Check out the MasterVolt MAC series of DC/DC chargers. They are available is many combinations of input and output voltages. My start battery chargers are 24V to 24V.


Good point about the power consumption of the engine's control system, and a reminder that I need to go check that.

I have 3 of the Mastervolt Mac 24/12V chargers on the sailboat. Less thrilled with the newer ones which have Faston rather than screw terminals (at least in the 10A versions I use). On the other hand I have had only one failure in 12 years, and that was the screw terminal block unsoldering itself - probably because the screws became loose? But I despise Fastons for any high reliability connection.

I've measured the engine consumption on the trawler, a Cummins QSB with electric shift. It is always in excess of 15A and can be quite a bit higher if the lift pump is running or the preheat is running or other unknown reasons (though these are short duration). Furthermore, it will complain loudly if the voltage sags much at all, below 12.5V alarms go off after a short delay. They assume it is being actively charged when running. The 75 hp Volvo on the sailboat is mechanical, and draws virtually no power while running.

There is another capacity issue I'm wrestling with: on the trawler, I usually anchor for one night, sometimes two, rarely three. I figure -150 AH overnight. AGMs seem to match this use. If I have 600 AH (looking at four 6CT Lifelines), most nights they'd be down 25%, some nights 50%, and rarely 75%. This is ideal for them. Most nights I could get by with 200 AH of LFP, but have to buy 450 AH for the occasional three-nighter. So the cost discount of needing fewer AH in LFP is destroyed to some extent. In name brand drop-in type replacements, LFP is a bit over $3000 compared to $1600 for the Lifelines for the same 450 AH deep discharge. If every cycle was 50% (-300 AH) then the LFP are $2000 and make more sense. Or if I had a huge charger and could fill the LFP in a short genset run. Since I run almost every day the AGMs would get charged. The sailboat is quite a different case, absolutely want to minimize charge time and I have a very big alternator.

 

[twistedtree]

6 months is a long time. If I were laying up for that long, I'd try to completely disconnect the batteries and store them around 40% SOC.

 

[rslifkin]

I prefer this approach of charging the AGMs vs DC/DC vs what a lot of people do charging the LFP via a DC/DC charger. The primary reason is that the start and 12V batteries don't require a big charge rate. It takes very little to recharge a start battery, so the chargers are small and affordable. Then the higher capacity charging devices like alternators, inverter/chargers, and shore power chargers can directly charge the LFP bank and do so at their max charging capacity. If you do it the other way, the DC/DC charger that charges the LFP bank pretty quickly become the limiting factor in how fast you can charge the LFP bank, and defeats one of it's biggest advantages. So my start battery chargers are I think 30A each, and I can dump as much as 1000A directly into the LFP bank if I wanted to crank everything up.

This makes sense to me. I definitely agree with putting solar, big chargers, etc. right into the house bank. The only thing I might do differently is alternators, but practicality of feeding the engine battery first, then a DC-DC to the house would depend on the size of the alternator. The bigger it gets, the less practical that is.