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Dave avatar image

Official Depth Of Discharge Recommendations For LiFePO4

Having recently had a 4 x 100 Ah LiFePO4 system installed, I am finding hard to shake off the habits learned from a crappy lead acid system & allowing the state of charge to dip to 50% goes against the instincts learned from past experiences.

What am I safe to use out of these batteries?

Lots of hearsay, not too many hard facts.

Lithium Battery
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4 Answers
Paul B avatar image
Paul B answered ·

Hi @Murph yes they may say that but ie at the end of 2500 Cycles at 80% DOD each time as a example.

They are still not stuffed they have 80% of there orginal capacity.

They dont seem to tell anyone that

we just keep on believing that like a LEAD Acid they are fully stuffed.


BUT NOT SO just degraded by 20%

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It depends on the application and just how much excess capacity was available when the system was new. You can certainly design in a higher capacity, allowing you to continue to get useful life beyond the typical 80%. If you are doing that, it's important to factor in the increased DC internal resistance as they degrade, which reduces the safe maximum current capability (in addition to the reduced energy capacity); i.e. they will generate more heat internally for the same current as they age, and the voltage will sag more heavily under load.

At some point, you can end up in a death spiral where the increased internal heating causes rapid degregation, unless you derate them (e.g. retire them into a less demanding and low current role). The manufacturers give you a cycle life which mostly keeps you clear of these issues, as long as the system design was good (i.e. excess capacity designed in to allow for them dropping to 80% and max current kept within reasonable limits). It's difficult for the manufacturers to make promises about life past that point, as it's highly variable depending on how they were treated up to that point and the demands you will put on them after it.

esteban avatar image
esteban answered ·

That number of 50% DoD for Battleborn does not sound right.

Battleborn says this: "Most lead acid batteries experience significantly reduced cycle life if they are discharged more than 50%, which can result in less than 300 total cycles. Conversely LIFEPO4 (lithium iron phosphate) batteries can be continually discharged to 100% DOD and there is no long term effect. You can expect to get 3000 cycles or more at this depth of discharge. "

I will add that Battleborn has their BMS set to cut off before there is an actual full discharge, but it's also believed that they over engineer the battery so that you can get and use a full 100ah out of a 100ah rated battery. So 100% discharge is 100% of rated AH, not actually draining the cells all the way to the bottom. This is ideal. A 100 ah battery is designed to give you a full 100 ah of power without damaging the cells. Amazing that you can actually use what you were sold and shows how accustomed we are to the deception of being sold a "100 ah" Gel battery that can only give 50 ah of actual power before it's life cycles are seriously affected.

A thought about some tables that show 5000 cycles at 50% DoD vs 3000 at 80%.. I think that their definition of cycle life is that a 50% DoD cycle on a "100ah" battery is only 50 ah so you would expect to get twice as many cycles. There is a reportedly real advantage to shallower DoD, but if it was measured in total AH of usable power and not as smaller 50 ah cycles compared to larger 80 ah cycles, for example, the difference in total life would be smaller - certainly not double.

I also understand, as mentioned, that cramming that last bit of charge into the battery to hit 100% is the biggest stress for the battery. With that in mind, I'll charge to only 95% capacity and also cutoff loads @ 10% on the bottom end, in order to extend life. Most of the time, I don't need anywhere near the 85% of my battery bank.

But it's all there to use. Sometimes it's raining a lot and I need more capacity for a situation, then I'll use 100%. It's only on occasion and nice to be able to have the full 100% without damaging the battery, just maybe dropping the life by a tiny percent since the full discharge is not repeated.

Another point: With my particular RV use, (seasonal) I was thinking about how long it would take me to use 3000 full cycles and I'm not sure I even need my Lithium to last 5000 cycles. By 2030 there will likely be better cheaper batteries! And anyway, the lithium doesn't drop off rapidly at that point like a Lead Acid, it just drops to 80%, then 70% slowly....I could then add just 25% more battery and bring my capacity back up to good as new for 1/4 the cost of full replacement.

A lot of advantages to the LiFePo4 chemistry! I believe that those who complain about the cost have not thought it through and are captured by the comfort of sticking with the cranky old LA chemistry that they know.

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geomz avatar image
geomz answered ·

As with any battery, capacity degrades over time with use. Most manufacturers offer some form of DoD chart.

Battle Born claims 3K cycles at DoD at or below 50%, increasing that to ~5K cycles with shallower DoD; while still retaining 80% capacity.

Life Blue claims 2.5K cycles to 100% DoD, and increases that to 6K cycles if DoD is kept below 50%; also while retaining 80% capacity.

One thing to keep in mind is that charging your lithium batteries to 100% all the time is also not a great idea. Most chemistries do best when charged to no more than 80-90% of capacity and only occasionally run to 100% to resync BMSes and BMVs et al.


HTH,

GM

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murph avatar image
murph answered ·

Check the datasheet for the specific batteries you have, or ask the manufacturer. For example, the datasheet for Victron's LFP-Smart range says:

CYCLE LIFE (capacity ≥ 80% of nominal)
80% DoD 2500 cycles
70% DoD 3000 cycles
50% DoD 5000 cycles
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