Depth of Discharge Myth?

[Dockhead]

Originally Posted by evm1024

My day job is in a combined Computer Science and Electrical Engineering department of a University.

I fully understand the wording that they used. Their wording gets them off the hook for describing the mechanics of capacity loss.

Right now a bunch of our students are defending their PHd thesis. If they claim outside of their thesis we may just grill them for hours on those claims. In some cases we have had them under the "lights" for 6 hours.

Advice to students, stay on topic, do not make any claim you are not able to defend verbally in your defense.

Well, I also understood the wording -- that was my point.



But more importantly, we now see that you have not only the skills, but also the LIBRARY ACCESS to answer our question You've been hiding your light under a bushel



So -- is there any actual description of the "mechanics of capacity loss" in lead-acid batteries available in the literature anywhere? Stop holding out on us!

Originally Posted by evm1024

Super, I have that thread where I am listing papers and testing results for batteries.

I would like to include the paper/test-results that show where permanent damage begins in FLA PSOC and DOD regimes.

Shoot me the link and I will included it that thread. The thread title says LiFePO4 but FLA can be included as well.

http://www.cruisersforum.com/forums/...ga-206796.html

Some people are not interested in the actual science, or don't have the background to read it, and that's fine.


Unlike you, I don't have access to a good technical library, so I've had to use ResearchGate and DeepDyve, and there is no such paper available through those sources. As far as I can tell, despite the huge installed base of lead-acid batteries in renewable power systems, no one really understands this.



Maybe you should open a separate thread on technical papers on lead and lead-carbon batteries, so that we can collect what there actually is. I could upload a bunch of stuff after this round of study.

[kmacdonald]

Originally Posted by Dockhead

Well, having reserve capacity in a given bank only delays the inevitable, if there is a fault in all your charging sources and you find yourself unable to produce power. That's not a solution.


The solution is to have redundancy in power production. I suppose there is some cruising boat somewhere which has only solar as a charging source, but I don't think I've ever seen one. If you start to run out of power, you simply crank up the generator or main engine and start charging -- that's the standard solution.


If you have only one alternator on one main engine and no other charging source, then of course you are vulnerable to a fault in that single point of failure. But keeping some "reserve capacity" in the batteries is not a solution to that, unless you are simply never too far away from plugging in somewhere that this "reserve capacity" can actually get you home. But even in that case it's a backwards way to engineer the system and a poor substitute for backup power production capacity. A cruising boat which needs electrical power for important or mission-critical purposes (like navigation systems) should be able to rely on being able to PRODUCE power, not just having some power stored.


On my boat, this is not an issue -- I can produce 6.5kW with the generator and 2.5kW from the second alternator on the main. Besides that, there are two more alternators charging two dedicated separated start batteries. An extreme case perhaps, but not all that unusual. So the lower 50% capacity of a lead-acid bank is just "dead lead" (as MaineSail calls it), for me as for everyone with a reasonably well designed power system, of practically no value whatsoever.

My Capri only has solar for charging a lithium battery. I don't cruise it in remote places so redundancy isn't a priority with it. Solar is extremely reliable unless you get bargain basement panels and controllers from some unscrupulous vendor or service provider.

[ramblinrod]

Originally Posted by kmacdonald

Not including the full blow infomercials pushing solar panels...

I don't believe there has been.

It has been pointed out that solar is a valid charging source, and should be considered, in every individual vessel energy needs analysis, especially if one is considering a LFP transition, that may be unnecessary.

Perhaps this has been represented with equal fervor to those who have suggested LFP should be considered as a valid alternative to FLA.

[Dockhead]

Originally Posted by ramblinrod

. . . It has been pointed out that solar is a valid charging source, and should be considered, in every individual vessel energy needs analysis, especially if one is considering a LFP transition, that may be unnecessary.. .

Very sensible! This is quite evolved compared to earlier statements, and I don't think anyone would disagree with this.

[Dockhead]

Originally Posted by kmacdonald

My Capri only has solar for charging a lithium battery. I don't cruise it in remote places so redundancy isn't a priority with it. Solar is extremely reliable unless you get bargain basement panels and controllers from some unscrupulous vendor or service provider.

Solar is extremely reliable except at night!


But I'm joking -- your point of course is correct. Day sailing or cruising within a few hours of a harbor is a different use case again. You are probably not using an electric autopilot and don't have critical nav gear either. So no need to overcomplicate anything for your particular use case.





Did you mention earlier you are using some kind of drop-in lithium? Can you describe it and what kind of luck you've had with it?

[evm1024]

So let's take a balanced system of a set of 4 Trojan T105 giving us about 450 AH capacity. This is in line with the suggested ration of 300 AH for every 100 AH actual daily use. Or about 150 AH use per 24 hours.

We can add in 200 AH solar and an 80 AH alternator.

If we were to wake up in the morning with the FLA house bank at 20% SOC we would have the engine in a panicked heartbeat.

That engine with an 80 amp alternator will be feeding the FLA bank somewhere around 72 Ah per hour (80 amps in at a CEF of 90%).

Math shows us that we get to 50% SOC in that system with about 2 hours of engine running time. (50% - 20% = 30%, 30% of 450 AH = 135 AH, 135 AH / 72 A = 1.93 hours)

Now lets contrast that to a LiFePO4 system. I do not promote the idea of a 200 AH bank of any type. 200 AH is too close to the daily usage and is in the boundry conditions of the domain.

But let's try the numbers with a 200 AH bank of LiFePO4.

So we wake up and find that our 200 AH LiFePO4 bank is at 20% state of charge. This means that we have 40 AH of capacity in the bank and we know that because this is a LiFePO4 bank that no damage is being done to the bank. The BMS has not alarmed and we notice that the 200 watts of solar is starting to generate some amps. Shortly after waking we notice that he solar is now generating power equal to loads and the bank is starting to charge.

The sun is warm and life is good so we fire up the expresso machine. The machine takes about 7 AH out of the LiFePO4 + solar capacity leaving around 35 AH in the LiFePO4 bank. As we sip our espresso the sun rises and the 200 watt of solar starts cranking out power. Under idle conditions we might get 15 amps out of the solar but due to the time of day and so on we are only getting 7 amps.

No engine run time, no panic, no PDQ the solar just keeps filling up the LiFePO4 bank. And the espresso was great. No 2 hours of heat from the engine to get the house bank up to 50% SOC so as to minimize the damage.

Time for a swim.

[evm1024]

Originally Posted by ramblinrod

SNIP

Perhaps this has been represented with equal fervor to those who have suggested LFP should be considered as a valid alternative to FLA.

You did state that LFP is a viable alternative to FLA in some cases.

[evm1024]

Originally Posted by Dockhead

Solar is extremely reliable except at night!


SNIP

But, knowing when something will not work is often more valuable then knowing when it will.

Solar is reliable in the sense we can say with a high degree of certainty that we will not get any useful charging from it at night.

(could not help myself)

No wait. I'm wrong. I did once have a slip under some very serious lights. There must have been usable power from those lights.

[kmacdonald]

Originally Posted by Dockhead

Solar is extremely reliable except at night!


But I'm joking -- your point of course is correct. Day sailing or cruising within a few hours of a harbor is a different use case again. You are probably not using an electric autopilot and don't have critical nav gear either. So no need to overcomplicate anything for your particular use case.





Did you mention earlier you are using some kind of drop-in lithium? Can you describe it and what kind of luck you've had with it?

It's a 20Ah GBS lithium battery. I charge it with a 30 watt solar panel. I used to have a 60 watt panel but didn't need one nearly that big so I switched to the smaller panel. I use a Genasun charge controller and manually turn off the charge at about 13.8V. It's plenty of power for a depth finder, cell phone, VHF, laptop, LED lights, water pump. This is the fourth or fifth season with it and I have had no problems with it. Very light. No BMS. I monitor the system with a Victron BMV-700.

[Dockhead]

Originally Posted by evm1024

. . I do not promote the idea of a 200 AH bank of any type. 200 AH is too close to the daily usage and is in the boundry conditions of the domain.. .

Just a minor point -- why do we care? It's just a matter of how the user intends to use his charging sources, isn't it? It is not an objective systems engineering issue, how much time the battery bank should be able to run the boat, is it?


Some people would like to run the boat for a week without charging; others might just want to coast for a couple of hours between generator runs -- there's no objective "correct design" for this.


My boat was designed without battery capacity for long periods without the generator running. She was built with four battery banks -- two start banks, a bank for technical loads (4x 12v batteries for windlass, bow thruster, winches), and a house bank (another 4x 12v batteries). The house bank was only 220 amp/hours x 24v so only enough for a few hours of operation. And no inverter. So the designers specified a heavy duty prime power rated generator which they intended would be running more or less all the time, like the way it's done on many power boats.



I didn't like that so I combined the technical and house banks and added an inverter. But that's just my taste, not something objectively right or wrong. As it is, battery capacity for 24 hours of power would be nice to have, but since I run the generator at least twice a day anyway for other purposes, 24 hours is not really essential.


My own biggest issue with battery capacity is long passages under sail. I don't like to run the generator while heeling and sailing hard upwind, especially in rough sea conditions -- I'm afraid about oil starvation. And I don't like to stop if I'm sailing well. So for this one purpose, more than a half day's capacity would be nice to have. Also, a larger battery bank allows you to capture and store more power produced incidentally, when you're running one engine or another for other purposes, or when you have a lot of sun on your solar system.



But i don't think you can abstractly specify how much capacity any particular user needs -- one day is completely arbitrary. Why not two? Why not a half?

[Dockhead]

Originally Posted by evm1024

. . . Solar is reliable in the sense we can say with a high degree of certainty that we will not get any useful charging from it at night.. .

Ah, yes, and by that criterion, wind is even more reliable! You can be sure you won't get any useful power out of it, ever!

[newhaul]

Originally Posted by Dockhead

Ah, yes, and by that criterion, wind is even more reliable! You can be sure you won't get any useful power out of it, ever!

just on a lark here 400 watt wind genny in winter I get on average between 8 and 10 amps for at least 10 hours per day mostly at night. Works out to be 80 to 100ah available during the time of year when solar output is low . My Lfp will be more than happy .

[evm1024]

Originally Posted by Dockhead

Just a minor point -- why do we care? It's just a matter of how the user intends to use his charging sources, isn't it? It is not an objective systems engineering issue, how much time the battery bank should be able to run the boat, is it?


Some people would like to run the boat for a week without charging; others might just want to coast for a couple of hours between generator runs -- there's no objective "correct design" for this.


My boat was designed without battery capacity for long periods without the generator running. She was built with four battery banks -- two start banks, a bank for technical loads (4x 12v batteries for windlass, bow thruster, winches), and a house bank (another 4x 12v batteries). The house bank was only 220 amp/hours x 24v so only enough for a few hours of operation. And no inverter. So the designers specified a heavy duty prime power rated generator which they intended would be running more or less all the time, like the way it's done on many power boats.



I didn't like that so I combined the technical and house banks and added an inverter. But that's just my taste, not something objectively right or wrong. As it is, battery capacity for 24 hours of power would be nice to have, but since I run the generator at least twice a day anyway for other purposes, 24 hours is not really essential.


My own biggest issue with battery capacity is long passages under sail. I don't like to run the generator while heeling and sailing hard upwind, especially in rough sea conditions -- I'm afraid about oil starvation. And I don't like to stop if I'm sailing well. So for this one purpose, more than a half day's capacity would be nice to have. Also, a larger battery bank allows you to capture and store more power produced incidentally, when you're running one engine or another for other purposes, or when you have a lot of sun on your solar system.



But i don't think you can abstractly specify how much capacity any particular user needs -- one day is completely arbitrary. Why not two? Why not a half?

I fully agree. there is no magic number in capacity (be it battery, generation, solar etc)

There are tradeoffs to achieve a specific set of parameters.

Using a generator near 100% of the time works for all those power boats and a bunch of Swans. When I was saying that 200 AH was too small ( in the example of 400 AH FLA vs 200 AH LiFePO4 ) was speaking only for myself. I do have 700 AH in my own boat but that is my choice.

I'm likely to change my house bank before we go long term cruising to somewhere closer to 1000 AH in a string of smaller cells than I use now. Just my preference and nothing close to "being there".

[newhaul]

Originally Posted by evm1024

I fully agree. there is no magic number in capacity (be it battery, generation, solar etc)

There are tradeoffs to achieve a specific set of parameters.

Using a generator near 100% of the time works for all those power boats and a bunch of Swans. When I was saying that 200 AH was too small I was speaking only for myself. I do have 700 AH in my own boat but that is my choice.

I'm likely to change my house bank before we go long term cruising to somewhere closer to 1000 AH in a string of smaller cells than I use now. Just my preference and nothing close to "being there".

exactly now for my boat 100ah Lfp will be plenty for now when dad passes and I cast off I will likely add an additional 100ah . My daily ah usage is estimated to be 35ah for me on my boat. For some others that's not even enough power to make their morning coffee.

[Dockhead]

Originally Posted by evm1024

I fully agree. there is no magic number in capacity (be it battery, generation, solar etc)

There are tradeoffs to achieve a specific set of parameters.

Using a generator near 100% of the time works for all those power boats and a bunch of Swans. When I was saying that 200 AH was too small ( in the example of 400 AH FLA vs 200 AH LiFePO4 ) was speaking only for myself. I do have 700 AH in my own boat but that is my choice.

I'm likely to change my house bank before we go long term cruising to somewhere closer to 1000 AH in a string of smaller cells than I use now. Just my preference and nothing close to "being there".

I guess it's like boat size -- the "right size" is always 6 feet longer than what you already have.


Same with me I guess. I have 420 x 24v in lead, which means 210 usable. I guess 300 usable would be nice.


One thing I can't decide about though -- if you go to lithium, do you suddenly really want much more capacity, since you can charge it more efficiently, or on the contrary, will you be satisfied with much less --- for the same (paradoxical) reason?


I don't know the answer and will have to just guess, if I do this. I would probably end up with a hybrid bank with 210 amp/hours of lead (105 usable?) and 260 of lithium, which I think would put me right at around that 300 usable.