Real world efficiency of LiFePO4

[Jeremiah]

So I'm gearing up to jump in to LiFePO4 and as I calculate and estimate and my spreadsheet gets bigger and bigger, I'm wondering, how real are the charge / discharge efficiencies I read about online?
I'm looking to put together a 48v system; where my battery bank will stay between 25% and 80% SOC for longevity (may have to eat into 25% on rainy days).
What is a realistic % of energy loss going in to the battery bank and coming out I can expect if my charging rate will never exceed .17 C and my discharging rate will be around .26 C or less? I intend to ensure the battery bank stays bellow 25 degrees C.

 

[MisterSandals]

stay between 25% and 80% SOC for longevity (may have to eat into 25% on rainy days).

Isn't that more like eating 45% all the time?

 

[Jeremiah]

Isn't that more like eating 45% all the time?

55% of the time Watching WIll's video on extending longevity convinced me this is the way to go.; if I could get it down to staying in between 25%-75% they'd theoretically retain 90% capacity after 9 years. Batteries have ticked me off for so long on how wasteful they are, especially the lead acid chemistries. I like the idea of still having real usable batteries after a decade+.

 

[MisterSandals]

if I could get it down to staying in between 25%-75% they'd theoretically retain 90%

I plan on using my batteries for 10+ years and i also plan on using (top) 90% of the capacity.
Maybe I am in for a rude awakening somewhere along the way.

And, i think the chances of there being a better battery technology and/or these being dirt cheap in 10 years is pretty high.

 

[Jeremiah]

I plan on using my batteries for 10+ years and i also plan on using (top) 90% of the capacity.
Maybe I am in for a rude awakening somewhere along the way.
And, i think the chances of there being a better battery technology and/or these being dirt cheap in 10 years is pretty high.

I understand they can still function after 10 years of 90% daily charge / discharge, but won't there be large degradation of capacity doing it that way?
In my case my tolerances are fairly low so if that happened it would mean I would have had to buy more batteries along the way to compensate for the loss in capacity.

I'm perfectly happy to use more capacity like you suggest, if I see studies to the effect I can do that! I know people talk about the wonderful good the future may be and we'll all have cold fusion and such, and it's future guy's problem but I want these things to last either way.

Can you comment on my original question? What charge / discharge efficiency are you seeing?
THanks!

 

[MisterSandals]

What charge / discharge efficiency are you seeing?

You mean if i put 100ah into batteries, how many ah I get out?
I "think" almost the same amount. The efficiencies that I worry about
have to do with losses from inverters and the likes (things that generate heat).

The battery monitors (shunt and hall effect types), from my understanding have little drift because batteries are efficient in terms of charge/discharge. Otherwise they would be extremely inaccurate or need to have some fudge factor built it to estimate capacities.

 

[Jeremiah]

You mean if i put 100ah into batteries, how many ah I get out?
I "think" almost the same amount. The efficiencies that I worry about
have to do with losses from inverters and the likes (things that generate heat).

The battery monitors (shunt and hall effect types), from my understanding have little drift because batteries are efficient in terms of charge/discharge. Otherwise they would be extremely inaccurate or need to have some fudge factor built it to estimate capacities.

No, I mean if your charge controller (or charger) pumps out 10kw-Hr into your battery how much of that gets lost (3%, 5%, etc.) and then when you take 10kw-Hr out form the battery bank, how much is actually usable (forget inverter I got that part covered), this is just on the way to the inverter or DC step down converter.
I'd like to know round trip leaving a charger and leaving the battery later how much loss to expect on a higher voltage LiFePO4 system.
2% in and another 2% out assuming low C rates?

 

[upnorthandpersonal]

Quote:

"The round trip energy efficiency (discharge from 100% to 0% and back to 100% charged) of the average lead-acid battery is 80%. The round trip energy efficiency of a LFP battery is 92%."

Source: https://www.victronenergy.com/upload/documents/Datasheet-BMS-12-200-EN.pdf

 

[MisterSandals]

The round trip energy efficiency of a LFP battery is 92%.

Interesting.

So, is the 8% loss factored into battery monitors? So a monitor with a shunt would detect 100ah going in and 92ah coming out and consider the charge and discharge the same?
For example if i charged a battery from 70% to 90% with 100ah. Then discharge from 90% to 70% with 92ah?

 

[upnorthandpersonal]

I don't think a BMS takes that into account (at least not any that I know of - I'd love to be proven wrong). In fact, it's not just round trip time that is not taken into account. There is an interesting paper here discussing these aspects: https://www.aimspress.com/fileOther/PDF/energy/energy-07-02-186.pdf

One interesting bit of information from that paper:

"Furthermore round-trip efficiency, operation of electrochemical reactions and charge acceptance are influenced by the temperature ... Since the battery SOC and ECM parameters are varied according to the temperature, those variations should be taken into account. As one approach, temperature can be considered as an input to the SOC estimation method, but the ambient temperature is not sufficient enough because it is necessary to model the heat generation and heat transfer (conductive, convective and radiation) [31] to accurately represent thermal behavior."

(ECM = Equivalent Circuit Models)
In other words, the BMS we currently have are 'good enough' but are not 'accurate' in the way a lab tool would be.

 

[Ped]

Yes the efficiency is that good. Yes studies show its better to cycle lightly. Yes its ok to charge to 100%. 30% seems like a sweet spot between longevity and price.

 

[Jeremiah]

@MisterSandals so here are the numbers from the Battery university graph which WIll referenced. I just looked at the chart and extrapolated the numbers. Note All these assume never discharging bellow 25%, the Batt. University guys say these are however lab results and that for unknown reasons the real world performance is worse than these across the board: Cycling no higher than 75% and no lower than 25% at 25 deg. C yields 90% of AH capacity remains after 9 years, 73% after 38 yrs! No higher than 85% and no lower than 25% yields: 90% capacity remains after 5.5 yrs, 87% after 10 yrs, 65% after 38 yrs No more than 75% and no less than 45% yields 90% capacity after 14 yrs (but who wants to only use 30%) Charging a full 100% but not dipping bellow 25% yields 90% capacity after 3yrs, 85% after 6 yrs, 80% after 10 years so yeah, hitting 100% SOC even if only using 75% of the full capacity definitely takes it's toll. If you want to use 75% total of capacity it's not good to hit 100% SOC better to range from not dipping bellow 15% to not going above 85%. That final climb to 100% is quite damaging. Other charts indicate that the internal resistance increases significantly that last 10% from 90%-100% and does some decent permanent damage by means of making some of the extra resistance permanent (I can link to those studies if you want). Here's the link to Will Prowse' video: extending longevity of LiFePO4 batteries, which led me to the batt. university stuff, etc. so that's why I'm only planning to eat about 55% if I can though I want a smart BMS so on extended rainy days I can dip into the lower 25% if I need to. Of course different folks have different world views. I'm just coming from the conservationist, "make your stuff last" philosophy. I spent so much of my life just destroying stuff and not taking care of my things, now that I live in the 3rd world I'm embarrassed to be that way any more.

 

[Ped]

The company offering the industry best warranty (10 years)

we want people to charge to 100%

9:02

 

[Jeremiah]

The company offering the industry best warranty (10 years)
9:02

Well put CEO, "for longevity it's OK to go to the extremes occasionally' (regarding temp.)
Regarding topping it off, I wouldn't be surprised if a company as expensive and with such a great warranty has already reserved some of the extreme regions by their BMS and just don't let the user see it. This is at least the only way to reconcile the scientific data (which indicates topping off is not a good idea). For sure they could bake that into the price; so the take away is if you own his specific product (very expensive battle born batteries), then it's good to "top them off" which probably is not really topping off the battery, just appears to be to the user. Notice he didn't make a blanket statement about LifePo4 batteries; just about his design.
Protecting users from themselves is good engineering practice

 

[Ped]

Well put CEO, "for longevity it's OK to go to the extremes occasionally'

...of operating temperatures

 

[JoeHam]

The company offering the industry best warranty (10 years)

Dakota battery is offering 11 years but I doubt anyone will collect on either. And the risk of a long warranty can be mitigated with insurance etc. so it's probably not the best indicator of quality FWIW.

For real world experience read this long, but very worthwhile piece:

DIY LiFePO4 Batteries On Boats - Marine How To

DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals

marinehowto.com

Now this guy is pulling full capacity AFTER 772 CYCLES! That's good enough for me.

 

[Ped]

My 2011 valence has 800kwh on the clock. Its lost about 12%...

And the risk of a long warranty can be mitigated with insurance etc.

Insurance companies arent in the business to lose money either..

 

[Jeremiah]

Dakota battery is offering 11 years but I doubt anyone will collect on either. And the risk of a long warranty can be mitigated with insurance etc. so it's probably not the best indicator of quality FWIW.
...
Now this guy is pulling full capacity AFTER 772 CYCLES! That's good enough for me.

Thats 2 years right? That's consistent with all the data we've discussed...
Look fella's if the sientific data Will shared is wrong I'm happy to not believe it; just haven't seen anything in this thread that even remotely challenges it so far.
Having a 10 year warranty doesn't say you won't have lost capacity does it? And if it does as I said for that price (you could buy double or triple the capacity elsewhere) why wouldn't they just set the BMS to stay away from bottom and top regions? Much better to undersell your product and the user is stupified "wow why is brand x so much better? I guess the cells they buy from china are just so much better than everyone elses' !"
I'm not seeing the disagreement here; the data is consistent. Perhaps we could add a specifier: If you are going to buy really really really expensive batteries, then probably they have already protected you from even using the upper 10% and lower 10% (or greater) of the battery because that's just industry best practice to make a great product that way... if, however, you are a cheapskate like myself building your own batteries from cheapo China-fakes, then you have to be careful and protect yourself from ruining the battery.
Fair take? Usually lab data is better than real life so my expectations are actually a bit lower than what the data predicts...

Anyway as a total LifePo4 newb with engineering experience in a different field, that's what I'd do if making a quality battery; just make it difficult for the user to ruin it; even treat it like a factor of safety for mission critical systems (which even the lowest rule of thumb is 25% reserved)

 

[Ped]

Are you not aware of how cell balancing works?

cheapo China-fakes

Practically all lithium batteries come from china. Fakes have mostly been from pack assemblers like shunbin, not raw cells.

 

[Jeremiah]

Are you not aware of how cell balancing works?

Practically all lithium batteries come from china. Fakes have mostly been from pack assemblers like shunbin, not raw cells.

Yep.
The data indicates not to charge to 100% if one wants to extend longevity minimizing capacity loss.
I still haven't seen anything to counter that. If I stepped on your toes because it's already been your strategy to go to 100% then fine who am I to challenge that? I'm just a newb. Do what works for you @Ped.
I don't understand why my post asking about the efficiency loss real world round trip is causing such an upset; hehe.

I'm gonna design mine to last long and maintain capacity, and that makes me happy.
Fortunately, someone already answered my question which as op of this thread was supposed to be the subject of this thread.
Thanks @upnorthandpersonal for answering my question:
anwer: "[round trip discharge from 100% to 0%] The round trip energy efficiency of a LFP battery is 92%."
my bad if I stepped on any toes; I'm sure you know a lot more than I do about this; I'm learning and growing so thanks for contributing to my thread I appreciate it (and also the push back and links!)
peace