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Old 19-08-2018, 12:42   #1
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Depth of Discharge Myth?

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:


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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.



What am I missing??!!


Unless the Trojan chart is just a lie, then these batteries do not contain only 50% of their nominal capacity, as commonly said, they contain more like 90% -- like lithium. Or at least 80%, if you leave off the last 10% which you don't bother with most of the time when you're off, or at least 70%, if you charge at 20% instead of 10%.


This can be seen vividly with the shape of the curve. If it were really true that "You're killing your batteries discharging them below 50%!", then the curve of life cycle vs. DOD would be STEEPER, the further down you get. But it's not! It's on the contrary shallower!
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Old 19-08-2018, 12:46   #2
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Re: Depth of Discharge Myth?

Doesn't speak to your question, but you can also compare your chart to the Lifeline chart on the last page of this:

http://2cw8eb1vmmgg3g5i7jzt6upo.wpen...cal-Manual.pdf

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Old 19-08-2018, 12:49   #3
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Re: Depth of Discharge Myth?

Quote:
Originally Posted by ranger42c View Post
Doesn't speak to your question, but you can also compare your chart to the Lifeline chart on the last page of this:

http://2cw8eb1vmmgg3g5i7jzt6upo.wpen...cal-Manual.pdf

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50% -- 1000 cycles


10% -- 450 cycles




Similar. If you are starting at 90%, it will cost you slightly more per kWh, to cycle to 10%, than it will cost you to cycle twice as often to 50%, but only slightly.





So why do people tell us not to cycle below 50%?




Also, in neither chart, does discharging right down to 0%, result in "killing the battery". No steep falloff in cycle life.


By the way, the Trojan chart is for the T-105's, giving a lot more cycle life than the Lifelines.
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Old 19-08-2018, 13:01   #4
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Re: Depth of Discharge Myth?

the real question is what you they consider a cycle and whether that has much in common to real life use

But I don't see how lithium will ever become as cost effect as golf cart batteries except on a lab spreadsheet. If they were don't you think golf courses would start using them. I got 5 years out the last set and abused them, so they cost me $0.25/day. I probably spill more beer a day than that.
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Old 19-08-2018, 13:07   #5
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Re: Depth of Discharge Myth?

Quote:
Originally Posted by sailorboy1 View Post
the real question is what you they consider a cycle and whether that has much in common to real life use

But I don't see how lithium will ever become as cost effect as golf cart batteries except on a lab spreadsheet. If they were don't you think golf courses would start using them. I got 5 years out the last set and abused them, so they cost me $0.25/day. I probably spill more beer a day than that.

All I can say is that if you spill less beer than that a day, you're not livin' right!




Whether lithium can actually be cheaper or not depends partially on the answer to this question. It is advertised that when you go to lithium, you can halve the size of the bank, since the realistic capacity of lead is only 40% or so of nominal capacity (cycling between 90% and 50% in real life off-grid usage). That is a big advantage. But if we could be safely discharging our banks to say 20%, that's really a game-changer, because that almost halves the number of cycles we have to put them through for a given amount of energy, and almost halves the how often we have to charge, bringing the whole picture more in line with lithium, other than the acceptance rate.



But I'm sure that we have been told this business about 50% for some objective reason -- it just can't be what it looks like on those charts. I think we're missing something.
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Old 19-08-2018, 13:19   #6
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Depth of Discharge Myth?

Your missing lab conditions with each cycle being a full, 100% recharge, plus these charts are extrapolated, I don’t for a minute think any manufacturer has spent years cycling a bank to see what the real world cycle life is.

50% has based on actual use, become a rule of thumb that a bank will give you good service if you stop there, plus go much deeper and I think you have no chance of recharging on Solar, I try to keep my discharge to 25% or so, and then a full recharge isn’t all that likely. Plan on discharging to 25% SOC left and a full recharge is more important, and tougher to get, taking hours longer, so it’s a double edged sword.
Remember these charts assume an honest full charge at each cycle, something that if you go down to say 20% likely takes 8 hours or more to achieve, so your running a generator for 8 hours a day, every day. Will you do that? Unlikely.
In your use, I believe your probably better served in cycling between 40% and 90% SOC, while your out, followed by a equalization when your back in port.
For those that are very seldom on dock power, this would lead to an early death, but I believe you go on vacations and return to work, then go out again? If so then abuse a lead acid bank, replace it when it needs it and forego the science experiment that Lithium is.

Lithium’s biggest advantage, in my opinion is it’s stupid high charge acceptance rate that doesn’t plane out, and the fact that you shouldn’t fully charge each cycle, stop when you want to.
So if you had great big chargers, you really could get by with an hour, maybe two hours of generator run time a day.
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Old 19-08-2018, 13:24   #7
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Re: Depth of Discharge Myth?

The depth discharge you "could" do is kind of meaningless isn't it? I normally don't discharge my T105s below 80% SOC. Why would I pay more to be able to discharge to 20%? This is just marketing, just like that AGMs charge faster (they do but only if SOC is low).

The how often you charge is also sometimes a false thing. If it's sunny out I end up running my generator when at anchor to make hot water and just charge some at the same time. I have estimated that if I upgraded my solar for $500 to not have to run the generator once in a while for charging that it would take over 2 years to pay back. But it would probably take 5 years of running the generator "only"to charge my batteries.

So how often do you run your generator only to charge your batteries? If you run it for hot water, air conditioning, etc. those days you are almost battery charging for free.
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Old 19-08-2018, 13:30   #8
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Re: Depth of Discharge Myth?

Quote:
Originally Posted by sailorboy1 View Post
The depth discharge you "could" do is kind of meaningless isn't it? I normally don't discharge my T105s below 80% SOC. Why would I pay more to be able to discharge to 20%? This is just marketing, just like that AGMs charge faster (they do but only if SOC is low).

The how often you charge is also sometimes a false thing. If it's sunny out I end up running my generator when at anchor to make hot water and just charge some at the same time. I have estimated that if I upgraded my solar for $500 to not have to run the generator once in a while for charging that it would take over 2 years to pay back. But it would probably take 5 years of running the generator "only"to charge my batteries.

So how often do you run your generator only to charge your batteries? If you run it for hot water, air conditioning, etc. those days you are almost battery charging for free.

I think this is all sensible, but it does vary a lot according to use case.


Yes, I try to concentrate high load tasks into a couple times during the day -- once in the morning and once in the evening, do them all at once and charge batts at the same time.



But in my particular case, I am forced to charge pretty often and can't rely on charging only when I have other stuff to use the generator for. Nor do I always have AC stuff to do as often as twice a day.


Likewise on long passages under sail, and this is worse because I don't usually have a lot of AC power activities on passage, so running the generator (or main when I don't need it) is driven mostly by battery charging needs. And I hate spoiling a good sail with a generator run.



But in that case, I am anyway discharging my batteries deeper, because I figured it was worth it in any case. Down to 35% or so; haven't dared to go lower than that.
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Old 19-08-2018, 13:38   #9
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Re: Depth of Discharge Myth?

Yes I understand the long sail thing as that being the biggest load times (it's the same for me). But those times just don't add up to enough to become a "need". If you are using this as your reason for changing to lithium it because it is a "want" of convenience to you. In which case you are wasting your sanity looking for a way to justify it other than what you just stated, that you don't want to have to run the generator underway.

There's nothing wrong with using this as justification. It's your money and boat, it's Ok to do something just because you want to!!
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Old 19-08-2018, 13:39   #10
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Re: Depth of Discharge Myth?

Quote:
Originally Posted by sailorboy1 View Post
Yes I understand the long sail thing as that being the biggest load times (it's the same for me). But those times just don't add up to enough to become a "need". If you are using this as your reason for changing to lithium it because it is a "want" of convenience to you. In which case you are wasting your sanity looking for a way to justify it other than what you just stated, that you don't want to have to run the generator underway.

There's nothing wrong with using this as justification. It's your money and boat, it's Ok to do something just because you want to!!

I'm not looking for any justification -- on the contrary, I haven't made the decision, and I want objective information, and not myths!
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Old 19-08-2018, 13:44   #11
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Re: Depth of Discharge Myth?

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I'm not looking for any justification -- on the contrary, I haven't made the decision, and I want objective information, and not myths!
I don't think that the not discharging below 50% is a myth. But it probably doesn't really match Trojans chart. In the "old" days before battery monitors etc. it was the norm to maintain batteries between 50%-90% just because it took too long to charge beyond the 90%. We here would consider than battery abuse, yet cruisers got 4 years or so out of the batteries.

I would bet the only ones who really know how long and how many cycles a T105 will last are golf courses. Maybe there's forum for that.
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Old 19-08-2018, 13:46   #12
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Re: Depth of Discharge Myth?

It's probably related to sulfation when you discharge cells. My understanding is the deeper you discharge them the more the lead sulfate forms on the plates which reduces the surface area of the plates and the capacity of the cells and increases their resistance and reduces power capacity. Making them harder to charge and more likely to get hot, and in doing so more likely to bubble off electrolyte. The more this happens the more crystalline the lead sulfate becomes and the harder it is to remove and crystals can fall to the bottom of the battery. On deep cycle cells with more lead in them you may be able get around this by using a charger that desulfates by pulse charging and utilise that extra capacity, I don't know.
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Old 19-08-2018, 13:51   #13
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Re: Depth of Discharge Myth?

As a 4yrs user of a LiFePO4 battery bank on a live aboard boat...

At this point in the development curve, you are NOT buying Lithium batteries to save money...not a chance. You are doing it for the technical advantages. So if those mean less to you than the money...you wouldn't do it.

Now after having LiFePO4 would I switch back to Lead? No....it would be like going back to Dial-Up internet after having a cable modem. BUT I did just finish installing two new Lead Acid banks on my kids boats...if they want better, they can PAY for it themselves. Dad just helped with the Boats as their Liveaboard College dorms!
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Old 19-08-2018, 13:52   #14
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Re: Depth of Discharge Myth?

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It's probably related to sulfation when you discharge cells. . . .

Well, this could be. If you're not getting them to 100% every time, then discharging them deeper could be causing sulphation problems which are not reflected in the theoretical cycle life tables.


Does anyone know anything about this? Paging MaineSail!
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Old 19-08-2018, 13:55   #15
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Re: Depth of Discharge Myth?

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As a 4yrs user of Lithium.....
At this point in the development curve, you are NOT buying Lithium batteries to save money...not a chance. You are doing it for the technical advantages and if those mean less to you than the money...you wouldn't do it.

Now after having LiFePO4 would I switch back to Lead? No....it would be like going back to Dial-Up internet after having a cable modem. BUT I did just finish installed two new Lead Acid banks on my kids boats...if they want better, they can PAY for it themselves. Dad just helped with the Boats as their Liveaboard College dorms!



To be clear -- this is not primarily about cost, but performance. Cost is not irrelevant by any means, but it's not the only factor under consideration.


Discharging lead deeper means longer charging runs in the efficient bulk phase, less frequent charging, better choice of when to charge -- it dramatically improves performance and usability of the lead bank.


Discharging deeper could EVEN mean greater chance to get the bank up to 100% -- if you're charging only once a day, it's less of a big deal to run the generator longer.
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