Depth of Discharge Myth?

[kmacdonald]

Originally Posted by ramblinrod

Of the ones I trust, most agree that if you overcharge or undercharge LFPs without very high risk of irreparable damage.

Never heard that one before.

[newhaul]

Originally Posted by ramblinrod

. Of the ones I trust, most agree that if you overcharge or undercharge LFPs without very high risk of irreparable damage.
.

if the ones you trust agree on this. Well that explains your confusion about Lifepo4 .
The facts are thus.
Without a proper battery management system you can do damage to your battery by over charging ( regardless of chemistry)
by allowing your battery to go to total 0 voltage it will do damage to the battery as well ( regardless of chemistry)

[Adelie]

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.



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!

Using this graph and cycling between 10% and 50% Depth of Discharge (DoD) it indicates that you should get 1700 cycles out of the battery.

Cycling between 90% and 10% you should get about 800 cycles.

Lets make the math simple, 100 amp-hour battery and we ignore the periodic charge to 100%.

That means 40 a-hr discharged per cycle going to 50% DoD or 68,000 a-hr withdrawn over the life of the battery.

For the 90% DoD you get 80a-hr per cylce or 64,000 a-hr over the life of the battery.

So the 50% DoD has a 13% advantage in total power withdrawn.

What if you only cylce 10%-30%, you get 2750 cycles which gives you 55,000a-hr and the 50%DoD has a 24% advantage.

40% DoD is 2,000 cycles or 60,000 amp-hr.
60% DoD is 1,400 cycles or 70,000a-hr
70% DoD is 1,200 cycles or 72,000 a-hr
80% DoD is 1,050 cycles or 73,500a-hr

Using the graph from the next post

30% 1,800 36,000
40% 1,500 45,000
50% 1,000 40,000a-hr
60% 800 40,000
70% 650 39,000
80% 540 37,800
90% 440 35,200 a-hr.
So peak life is about 40% DoD for this battery and lab protocol.


Keep in mind these are lab results not field results and the lab protocols are probably different for each lab which explains some of the difference.

So 50% looks like a decent compromise between these 2 graphs.

Also you want to consider the costs to replace the batteries slightly more often, not just the direct costs of replacing the batteries but also the time and expense for transport and misc.

[senormechanico]

Originally Posted by kmacdonald

According to most people, LFP requires a BMS, LA doesn't. From experience of others failures even with a BMS, the BMS's appear to be inadequate at best.

I have a BMS on both the boat and the house (home).
As we speak, I'm equalizing the charge on the home's 700 ah 24 volt system because one of the BMS boards decided to shunt all the time without showing any visual indication (winky blinky) that it was doing so.


I am glad I spend $ 1.77 each for some cheap led voltmeters from eBay, as I noticed the Magnum going to bulk when it was not expected.
It went to 119 amps charge, for about five minutes, then silent.
All cells showed a decent charge except one. 3.09 volts.


I put a 3.6 volt 40 amp charger on it and 440 amp hours later, it hit 3.45 volts.



I'm thinking of ditching the BMS and just going organic visual.
Heck, I look at the cells almost every day as a habit.


Flame away !

[Delfin]

Originally Posted by ramblinrod

As I understand it LFP is more tolerant of PSOC (not
normally an issue if one has adequate wind and solar) but far less tolerant of
over or undercharge, like kill the bank if ever done.

Call me confused....Is it your position that LFP is tolerant of PSoC, meaning undercharging, or will undercharging kill the bank?

[newhaul]

Originally Posted by senormechanico

I have a BMS on both the boat and the house (home).
As we speak, I'm equalizing the charge on the home's 700 ah 24 volt system because one of the BMS boards decided to shunt all the time without showing any visual indication (winky blinky) that it was doing so.


I am glad I spend $ 1.77 each for some cheap led voltmeters from eBay, as I noticed the Magnum going to bulk when it was not expected.
It went to 119 amps charge, for about five minutes, then silent.
All cells showed a decent charge except one. 3.09 volts.


I put a 3.6 volt 40 amp charger on it and 440 amp hours later, it hit 3.45 volts.



I'm thinking of ditching the BMS and just going organic visual.
Heck, I look at the cells almost every day as a habit.


Flame away !

the only reason I'm installing a bms is for the oh Crap my solar controller stuffed it and is overcharging and I don't catch it first.
But I guarantee your bank is a lot larger than mine will be.

[john61ct]

Originally Posted by ramblinrod

if you overcharge or undercharge LFPs without very high risk of irreparable damage

There are things that irreparably damage LFP.

This is why we use protective electronics to protect them.

But there is no such thing as undercharging them, as I clarified above.

Normal overcharging is just a longevity issues, you still get vendor rated cycles, just not many times that.

[john61ct]

Originally Posted by Delfin

Call me confused....Is it your position that LFP is tolerant of PSoC, meaning undercharging, or will undercharging kill the bank?

LFP benefits from PSOC, and is harmed by sitting anywhere near Full.

[Delfin]

Originally Posted by evm1024

Guys,

I'm sure that Rod mis-typed there and he meant to type: Of the ones I trust, most agree that if you cannot overcharge or undercharge LFPs without a very high risk of irreparable damage.

Mistyping is easy enough to do.

This is as close to saying that he not an expert on LiFePO4 and is just learning about them as we are likely to get. Refreshing.

Let's not beat him up over the typo.

I assumed the typo. What I still don't understand is how you can say that LFP handles PSoC just fine, but are killed with undercharging, which is the definition of PSoC. Perhaps in addition to a typo, a mis-statement?

[evm1024]

Originally Posted by senormechanico

I have a BMS on both the boat and the house (home).
As we speak, I'm equalizing the charge on the home's 700 ah 24 volt system because one of the BMS boards decided to shunt all the time without showing any visual indication (winky blinky) that it was doing so.


I am glad I spend $ 1.77 each for some cheap led voltmeters from eBay, as I noticed the Magnum going to bulk when it was not expected.
It went to 119 amps charge, for about five minutes, then silent.
All cells showed a decent charge except one. 3.09 volts.


I put a 3.6 volt 40 amp charger on it and 440 amp hours later, it hit 3.45 volts.



I'm thinking of ditching the BMS and just going organic visual.
Heck, I look at the cells almost every day as a habit.


Flame away !

I have the house power cell boards and have removed the shunt resistors. I want them to tell me. Not to take things into their own hands.

I'll keep the cell boards hooked up for their per cell indications.

[senormechanico]

Originally Posted by evm1024

I have the house power cell boards and have removed the shunt resistors. I want them to tell me. Not to take things into their own hands.

I'll keep the cell boards hooked up for their per cell indications.

Exactly what I'm going to do. The output transistor which is in series with the shunt resistor is probably shorted.
It's a guess at this point, but your suggestion is going to be my "final solution".


BTW, the boat's 12 volt 200 aH system's BMS is working perfectly. Under normal circumstances, it doesn't do anything at all.
Both are Clean Power Auto systems.


Gosh, I have a 33 foot boat with a 2 foot bowsprit and only a 200 aH bank !!

I have lots of juice eating stuff too !! Horrors !!!!
I must be dreaming and having a nightmare.
Somebody please wake me up !

[ramblinrod]

Originally Posted by newhaul

question where did you get the .3C discharge and max of 1C charging.
The cells I am considering have a 1C charge and discharge rate.
With burst to 3C discharges.
With a cycle rating of 7500 with an 80% DOD and a 10 year warranty

CALB cells.

Please post the make and model of the cells you are referring to and the independent lab that verified the specs.

[ramblinrod]

Originally Posted by Dockhead

I would like to know what LiFePo4 cells you've seen, which are rated like that. None that I've seen. I believe you have mistaken DOD for percentage charge.

No mistake.

Calb CA100FI

Winston -- the most popular ones here, and the ones Victron use in their lithium systems, are rated 3000 cycles at 80% DOD, and 8000 at 55%. MaineSail's (and other) testing suggest that these ratings are very conservative.

Interesting, how interpretation is in the mind of the individual.

What I got from Rod (Collins) is that he doesn't trust anything about Winston cells and their published specs.

And that's very different from lead-acid batteries, which give not more, but far less cycle life than their ratings, if used in any kind of PSOC regime. Typically a few hundred cycles, which is fine for boats which spend 90% of the time in marinas, but much less good for those used much off grid

We won't know real world cycle life until there is sufficient volume that the distributors start collecting returns for recycle.

I've heard that many have trashed their first set of LFP batteries, within weeks. (Few are likely to admit this publicly on CF.)

Don't forget that FLA real world cycle life is influenced by the 80% of boaters who pay little to no attention and abuse their cheap batteries.

Those who invest a wad in batteries are much more likely to take care of them.

So we can expect real world life cycle numbers to be skewed just by the mindset of the folks likely to buy them.

[ramblinrod]

Originally Posted by Delfin

Call me confused....Is it your position that LFP is tolerant of PSoC, meaning undercharging, or will undercharging kill the bank?

LFP is tolerant of being left at PSOC of charge, where FLA isn't.

FLA is tolerant of being held at 100%, where LFP isn't.

Most chargers, alt regulators, and charge controllers, even in "Li" mode, tend to hold the battery at 100%.

These all pretty much need to be changed when switching to LFP, else the entire burden of protecting the bank is left to the BMS.

One little mistake and $$$$$$$.

[ramblinrod]

Originally Posted by evm1024

Not me, That was Rod's type/mis-understanding.

No typo, no misunderstanding on my part.

We all know that LiFePO4 loves being "undercharged" and hates being over dis-charged.

Undercharged can = 0% SOC. Over discharged can be 0% SOC.

Potato / Tomato. ;-)

This shouldn't be so hard for "experts" to figure out.