Depth of Discharge Myth?

[newhaul]

Ok here is how my granddad explained it to me many years ago ( it was about the gas in his truck but relevant here). Its much easier to keep the top half of the tank full than to fill it from empty.

[Dockhead]

Originally Posted by thinwater

A question and a thought:


a. Then what chart should we believe? I would think someone would have gone to the trouble of testing in what they consider a real world manner.
b. A few of us cruise long term. Most of us--98% at least--cruise for a few weeks at a time at most, perhaps anchoring 30 nights a year at most (some nights will be in marinas). Over 10 years we will rack up a whopping 300 deep cycles. Clearly, the batteries are not dying from cycles, they are dying from corrosion and perhaps insufficient recharge or loosing water. Perhaps they go flat at some point for a reason unrelated to overnight usage (power failure at the marina or charger failure). This has NOTHING to do with cruising practice, it is an accident more likely to happen to boats that are NOT used continuously. But unless we take the batteries below 30% while cruising with some frequency, the batteries are going to die a natural death long before cycling is even a factor. This is no doubt true of most recharageable devices in light duty service. Not relevant to the full-time cruiser, but relevant to most of us.

Yet another table, contradicting the widely-given advice not to discharge below 50%. If you recalculate the data to reflect kWh handled (and corrected for voltage as some have usefully suggested), then there is no disproportionate cost of handing kWh with deeper cycles than 50%.


Your thought is a good one -- there is no data, because our use case is relatively rare. You're probably right about that.

[Dockhead]

Originally Posted by newhaul

Ok here is how my granddad explained it to me many years ago ( it was about the gas in his truck but relevant here). Its much easier to keep the top half of the tank full than to fill it from empty.

Sure, but that's nonsensical, isn't it?



Why would you use only half the capacity of a truck's gas tank, unless you really need to always have half a tank in reserve? You double the "cycles" -- the overhead required for going to the gas station and going through the process of refueling -- compared to waiting for the tank to be empty (or nearly empty) and filling twice the amount, per refueling operation.

[Dockhead]

Originally Posted by wsmurdoch

Let's say you have a single 100Ah battery and you draw out 10A more each day than you put back in.

If you recharge at 50% state of charge, you start the engine and recharge every five days. The chart in the original posting says you will get 1650 cycles out of your battery. That will be 8250 days before your battery fails.

If you recharge at 80% state of charge, you start the engine and recharge every eight days. The chart in the original posting says you will get 1050 cycles out of your battery. That will be 8400 days before you battery fails.

To me, with the given data, the battery lasts longer and the engine runs fewer times (although for longer). What is not to like?

Or, did I miss something?

Bill

It depends on what you're after, and the whole question is whether it's actually harming your batteries, to use more of the capacity of them. Not indeed whether your use case has any need of that capacity -- that's a different question.

[newhaul]

Originally Posted by Dockhead

Sure, but that's nonsensical, isn't it?



Why would you use only half the capacity of a truck's gas tank, unless you really need to always have half a tank in reserve? You double the "cycles" -- the overhead required for going to the gas station and going through the process of refueling -- compared to waiting for the tank to be empty (or nearly empty) and filling twice the amount, per refueling operation.

the fact is its easier and faster to fill from half on a regular basis than to always go close to empty. It always leaves you reserve for the oh shi!!t situation .
Less likely to cause death from psoc

[MarkSF]

Best guess : Trojan flipped the X axis. Easy mistake.

[Dockhead]

Originally Posted by newhaul

the fact is its easier and faster to fill from half on a regular basis than to always go close to empty. It always leaves you reserve for the oh shi!!t situation .
Less likely to cause death from psoc

I understand you're saying -- better not to run them down far in case you need reserve capacity. OK, that's certainly something to consider, but maybe you don't need 50% reserve if you have a generator and main engine with big alternator always at the ready, no?


And "less likely to cause death from PSOC" -- that's really what I'm trying to understand. Any data? We've all heard this, but is it really true, and if so, why?

[kmacdonald]

Originally Posted by Dockhead

I did, and I still couldn't gather anything significant of relevance to my question.



Maybe you (or anyone else) could just answer a couple of straightforward, technical questions:


1. True or false: Discharging a FLA battery below 50% regularly causes it to wear out faster, than keeping it above 50% all or most of the time. "Faster" doesn't mean "number of cycles"; it means kWh of energy handled.

FALSE
2. If the answer to the question above is True, then what is the mechanism, and why is it not reflected in the cycle life charts?



3. If the answer to the first question is False, then why are we advised to avoid discharging our FLA batts below 50%? Is there any good reason for this advice, or not?

The 50% myth is just that, a myth. Batteries will get more cycles if discharged to 50% DOD instead of a greater DOD but the lifetime energy delivered will be slightly more by discharging deeper. With deeper discharge the batteries wont last as long time wise or cycle wise but you would need to purchase less capacity thus saving money and weight.A lot of variables come into play when deciding how you want to use your battery bank. Do you change the batteries yourself or pay to have them changed? Is weight a big factor? Charging capacity, etc.


I don't want a generalized philosophical treatise; I am trying to understand what is the technical truth behind the conflict between the cycle life charts, and the advice we get on usage patterns. It's not that technical but there are a lot of variables between installations and priorities. What's best for one situation won't be best in another. Mainsail recommends no more that 50% DOD but he is being paid by customers that want their batteries to last a long time, not deliver the most energy over time. His recommendation reflects that, and rightfully so.


If you don't know; that's fine. If no one knows, that's fine too. But if someone knows, I'd like to hear about it.

See above Red comments.

[newhaul]

Originally Posted by wsmurdoch

Let's say you have a single 100Ah battery and you draw out 10A more each day than you put back in.

If you recharge at 50% state of charge, you start the engine and recharge every five days. The chart in the original posting says you will get 1650 cycles out of your battery. That will be 8250 days before your battery fails.

If you recharge at 80% state of charge, you start the engine and recharge every eight days. The chart in the original posting says you will get 1050 cycles out of your battery. That will be 8400 days before you battery fails.

To me, with the given data, the battery lasts longer and the engine runs fewer times (although for longer). What is not to like?

Or, did I miss something?

Bill

bill do you have your numbers off because if I interpret your post correctly in the second case you are only using 20% of the capacity. Which would require charging more often. If on the other hand you are discharging to 20% or using 80% that's different .
It may be more actual days but its still fewer cycles. So less life for the battery bank.
by your scenario are also subjecting the battery to PSOC Issues.

[kmacdonald]

The difference in total lifetime energy delivered between cycling to 50% or 80% DOD is not significant enough to favor one usage pattern over the other. Other variables will need to be considered to make a rational decision.

[sailorboy1]

Originally Posted by Dockhead

Is it a myth, that it is harmful and unprofitable to discharge your lead acid batteries below 50%?


I'm mulling over these questions again while contemplating switching to lithium.


This table:


Attachment 175751


Seems to imply that operating the battery between 90% and 50% (with appropriate regular 100% charges and equalization, of course), would cost something like 0.0625% of the batteries life, per cycle. Where as cycling it between 90% and 10% would cost only 0.125% per cycle, for a cycle which yields twice the energy. So you are wearing the battery out LESS per kw/h of energy stored, by cycling it down to 10% capacity, rather than charging every time when it reaches 50% as we are trained to do.

I had to go back to the start because I lost track of the question.

I would say that there is no basics for the 50% depth of discharge, it is a myth. You get X amount of cycles X power removed. Taking 30% out 2700 times is about the same as taking 50% out 1700 times, or 70% out 1200 times.

So assuming you fully charged the batteries each cycle the batteries give the same amount of power over their life no matter if discharge to 80%, 50%, or even 30% SOC.

I guess the 50% myth has its' basis in old school cruiser voltage readings and voltage drop. Where people got equipment operational issue it they allowed the SOC to get too low. Also in those days no one charged back to 100% unless they connected to shore power and I bet over time the 50% got written in a book "just because" and we all know how people like to use some boat writter's info as "real".

[Dockhead]

Originally Posted by sailorboy1

I had to go back to the start because I lost track of the question.

I would say that there is no basics for the 50% depth of discharge, it is a myth. You get X amount of cycles X power removed. Taking 30% out 2700 times is about the same as taking 50% out 1700 times, or 70% out 1200 times.

So assuming you fully charged the batteries each cycle the batteries give the same amount of power over their life no matter if discharge to 80%, 50%, or even 30% SOC.

I guess the 50% myth has its' basis in old school cruiser voltage readings and voltage drop. Where people got equipment operational issue it they allowed the SOC to get too low. Also in those days no one charged back to 100% unless they connected to shore power and I bet over time the 50% got written in a book "just because" and we all know how people like to use some boat writter's info as "real".

This is turning into a really interesting discussion.



Some people say that although you can't see it in the cycle life charts, there is an objective reason why you are disproportionately shortening the life of your batteries if you discharge them regularly below 50%, and you shouldn't do it.


Other people say: It's a myth; there is no downside to using more of the batteries' capacity.




Where is the truth in this? And on the basis of what facts and data? Fascinating. I have no idea what the right answer is.




I wonder what Trojan would say about this? They of all people must know the answer. I am unfortunately in a remote place with no easy telephone access, or I would just call them up.

[sailorboy1]

I think the answer from Trojan is going to be:

If you fully recharge after each cycle, depth of discharge per cycle doesn't matter to the total lifetime amount of power you can remove from the batteries.

But if you don't fully recharge after each cycle the sulfication remaining from each cycle will shorten the life cycle and you will get less total power out the battery.

So the depth discharge effects the amount of sulfication and greater depth result in more damage to the battery.

But, this of course is just my myth theory :-)

[thinwater]

Two more thoughts, probably not relevant to everyone and a little off topic:

• Weight. For a multihull guy, wasted weight has a cost. For example, how much more who you pay for a boat that was 500 pounds lighter, assuming strength and durability were equal? If there were no value, we would not have carbon fiber in the beams and core above the water line (and in some cases below the water line). One of my boats was nearly all Kevlar. For example, if you pay $120,000 for an 8500-pound used boat, that was $15/pound. My F-24 is closer to $20/pound, and a new multi could be $100/pound. The value of a pound saved will be greater or less depending on personal wants and where it is located (masthead vs. cabin). Let's say for the sake of argument it is $20/pound. If I save a battery because I am willing to discharge deeper, that's BOTH the cost of the battery ($150) AND the cost of the weight saving bonus ($1500). Obviously, there are major advantages to using light batteries and using them fully. This is a cheaper way to save money that more carbon fiber. Yes, the battery life is reduced, but for the primarily-daysailer, this is a trivial number, since the battery will die in 5-7 years anyway.
• Reserve power. This is the down side of deep discharge. On the other hand, my 34-foot catamaran and my F-24 have outboards that can be pull started. No big deal. I personally find it weird that boats that can be navigated in remote areas do not have an alternate starting means. Of course, with solar, this just means isolating the start battery and waiting a little while.

[kmacdonald]

With a deeper DOD the batteries won't last as long time wise so aging is less of a factor in battery life. You could avoid this question altogether by getting lithium but that's a pretty big leap. I use LA in one boat and lithium in another because of a different set of priorities. Only you can determine what's best in your own case but you need to really understand All the variables at play to do so. Looks like to me you have what it takes to make the right choice for your circumstances.