LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[john61ct]

Originally Posted by rgleason

how much difference would charging at .5C occasionally, say 25% of the time, make in cycles/life?

As you know already .5C rate of charge will reduce cycle lifetime compared to .3C.

And IMO .3C will do so compared to .2C

The effect will be dramatic if a high percentage of cycles are at the higher rate.

But there is no quantification of the effect yet.

So, if your otherwise coddling-ideal treatment of the bank is already yielding over 8000 cycles, and an occasional high charging C-rate only knocks off say 500 cycles

I think most people would agree with CNB and say NBD.

The fact that hourly fuel and wear&tear costs on your ICE are not cheap is another factor, likely tips the balance economically, even if you highly price maximizing longevity.

Once you have decided to go LFP, then that little bit of solar becomes much more optional, to me only needed then if it will further reduce your ICE runtime.

But for a lead-only setup, solar becomes more necessary, since striving to get to 100% is now required for longevity (Firefly excepted).

Now enough solar to provide most of your energy needs, will lead to a higher % of inputs being at a low C rate - good for LFP, bad for AGM!

BTW, LFA? Is that supposed to mean some subset of lead chemistries, maybe you mean FLA?

[newhaul]

Originally Posted by john61ct

As you know already .5C rate of charge will reduce cycle lifetime compared to .3C.

And IMO .3C will do so compared to .2C

The effect will be dramatic if a high percentage of cycles are at the higher rate.

But there is no quantification of the effect yet.

So, if your otherwise coddling-ideal treatment of the bank is already yielding over 8000 cycles, and an occasional high charging C-rate only knocks off say 500 cycles

I think most people would agree with CNB and say NBD.

The fact that hourly fuel and wear&tear costs on your ICE are not cheap is another factor, likely tips the balance economically, even if you highly price maximizing longevity.

Once you have decided to go LFP, then that little bit of solar becomes much more optional, to me only needed then if it will further reduce your ICE runtime.

But for a lead-only setup, solar becomes more necessary, since striving to get to 100% is now required for longevity (Firefly excepted).

Now enough solar to provide most of your energy needs, will lead to a higher % of inputs being at a low C rate - good for LFP, bad for AGM!

BTW, LFA? Is that supposed to mean some subset of lead chemistries, maybe you mean FLA?

considering mine are rated for 3k cycles at a max charge rate of 5C losing 1 cycle per 1k at .5C (estimated that .5C charge will reap in excess of 10k cycles ) losing that one is well worth it . And the difference in loss between that and lower C rates is not even calculatable. Especially when placed up against the additions maintenance costs of longer ice runs.

Aside from the fact that most banks on boats its almost impossible to have enough charging capability to exceed .5C charge rate.

[john61ct]

Originally Posted by newhaul

And the difference in loss between that and lower C rates is not even calculatable. Especially when placed up against the additions maintenance costs of longer ice runs

Yes, exactly my point above.

Just because the hard data is not available to quantify the effect, does not mean it isn't a lot greater than my hypothetical example.

> Aside from the fact that most banks on boats its almost impossible to have enough charging capability to exceed .5C charge rate.

We weren't discussing most boats. Most boats have no House bank at all, much less thousands invested in LFP.

With a large enough setup, getting to .5C would not be that hard if that is as important a goal as it is to @rgleason.

[john61ct]

The fact that charging at higher C-rates will adversely affect longevity is well established, should not be controversial.

Sure you can choose to go well above 1C if you like, in the world of electric propulsion that is very common. Just don't exceed mfg specs without justification and solid engineering-level caution.

All depends on your use case and priorities.

[newhaul]

Originally Posted by john61ct

Yes, exactly my point above.

Just because the hard data is not available to quantify the effect, does not mean it isn't a lot greater than my hypothetical example.

> Aside from the fact that most banks on boats its almost impossible to have enough charging capability to exceed .5C charge rate.

We weren't discussing most boats. Most boats have no House bank at all, much less thousands invested in LFP.

With a large enough setup, getting to .5C would not be that hard if that is as important a goal as it is to @rgleason.

considering all of your experience is theoretical. I understand your lack of reading comprehension I did not say most boats banks I said banks on most boats.

That being said many with Lfp are running in excess of 1kwh of banks so having the 1000+ ah would mean 500 amp charging to reach the .5C or 1000 amps chargiing capability for 1C . About the most I have seen on a consistent basis is in the 200 to 250 amp charging range. Aka .2C to .25C

[john61ct]

Yes, just as @rgleason's 200Ah bank would get to stressful current levels well within the output of a single normal alt.

[newhaul]

Originally Posted by john61ct

Yes, just as @rgleason's 200Ah bank would get to stressful current levels well within the output of a single normal alt.

??????? Normal stock alternator??
Most I have dealt with are anywhere from 35amp to 65 amp stock.
With the " stock 65 amp alternator" even at max output is only .325C

And we all know you will never get full rated output for more than a few minutes till it derates itself to about 80% output or 52 amps which is .26C .

What are you hypothetically saying the stock alternator is?

[senormechanico]

Is this now the joke thread??

[john61ct]

I nowhere said stock. Normal as in the sub-250A range, as you just referenced above.

As opposed to the 10+ kW units someone with an unusually large setup might invest in.

[Pete7]

Originally Posted by john61ct

I nowhere said stock. Normal as in the sub-250A range, as you just referenced above.

Why would RG want to invest in that sort of set up? Suggest you go back and read his recent posts were he states his requirement very well. Personally I don't think he will gain much from LFP without spending alot of money on other products to then fast charge the LFP, pointless!

However, add solar to top his LFA up to 100% on a regular basis would be excellent, wouldn't it. Confirm that this is a cost effective solution please J61. Remember its easy to spend someone else's money.

Pete

[CatNewBee]

Originally Posted by john61ct

The fact that charging at higher C-rates will adversely affect longevity is well established, should not be controversial.

Sure you can choose to go well above 1C if you like, in the world of electric propulsion that is very common. Just don't exceed mfg specs without justification and solid engineering-level caution.

All depends on your use case and priorities.

Hi John,

the effect of this "fact" needs to be proven in sub 1C range.

What is even worse, high ambient temperatures have a much bigger effect on life expectations, so I would recommend to cruise sub-polar CLIMATES during the summer months instead of blue waters for the sake of a long battery life. BTW, solar panel efficiency is in colder regions higher, and the sun is shining longer in the summer months. I would plan my travels accordingly if I would care so much about longevity as you do.

Do you actually cruise from time to time, or do you only plan to in theory, as with the LFP setup.[emoji12]
If you stick with the planning only, it is easy to optimize everything's longevity, because you can omit the real world's use pattern.

[john61ct]

Originally Posted by Pete7

Why would RG want to invest in that sort of set up?

Not saying he does, just helping lay out the parameters of different choices, his evolving plans have always included future upgrades.

I've been using solar since the early 80's, have been a rabid anti- carbon tree hugger before Al first got elected in TN.

But on a boat with limited space, and especially where windage and appearance are priorities, I don't indulge in fan-boy blanket recommendations for any one piece of the puzzle.

[Delfin]

Originally Posted by CatNewBee

Hi John,

the effect of this "fact" needs to be proven in sub 1C range.

What is even worse, high ambient temperatures have a much bigger effect on life expectations, so I would recommend to cruise sub-polar CLIMATES during the summer months instead of blue waters for the sake of a long battery life. BTW, solar panel efficiency is in colder regions higher, and the sun is shining longer in the summer months. I would plan my travels accordingly if I would care so much about longevity as you do.

Do you actually cruise from time to time, or do you only plan to in theory, as with the LFP setup.[emoji12]
If you stick with the planning only, it is easy to optimize everything's longevity, because you can omit the real world's use pattern.

Cruising in the Arctic rather than the tropics should be a small price to pay if you can get another 20 cycles out of your LFP bank. I think.

[newhaul]

Originally Posted by Delfin

Cruising in the Arctic rather than the tropics should be a small price to pay if you can get another 20 cycles out of your LFP bank. I think.

I wouldn't recommend arctic cruising in winter without enclosed steering station and a good heating system. BTDT.

[OceanSeaSpray]

Originally Posted by nebster

I’ve read the literature on this effect (in part, thanks to you bringing it to my attention), and I still can’t really marry up what they observe with the practical impact on a real battery pack. Maybe it is so subtle that I can’t understand it, or maybe I am dense. But I will try again.

Which part of "unable to recharge past 70% SOC" is so subtle??

Originally Posted by nebster

I’m not personally that concerned about lifetime beyond about 1500 cycles, because for me that is ten years, and that’s way longer than I’ll need.

It just happens that not everybody is fortunate enough to operate in conditions where charging can be carried out whenever desired. This bank was cycled daily and was recharged up to whatever energy availability allowed each day. For several months each year, it meant that it almost never even reached absorption stage.
When it was possible to recharge "fully", then the bank was charged to 14.0V and absorbed until the current dropped to a conservative 0.05C. Too conservative in hindsight... but this has not been an uncommon approach.

Originally Posted by nebster

... if you are always charging to 14.0V, that is definitely not a partial charge.

And here we go again... completely wrong. There is NO RELATION WHATSOEVER between the end of charge voltage and the SOC achieved. This is undissociable from the charge rate and the absorption time.

Originally Posted by nebster

...it seems to me that an LFP cell that has just experienced normal degradation might exhibit behavior identical to what you describe; namely, charge to a “full voltage” and only measure 70% of the original energy going in. How do you distinguish between normal deterioration and this special effect?

There is no such thing as "full charge voltage".

Now, in this instance the current tapered off to the termination point after 70% of the nominal capacity had been absorbed by the battery. However, the immediately following stabilised OCV in discharge also confirmed a SOC of about 70%. In other words, the electrochemical state of the cells in terms of the distribution of the charge carriers also indicates that there is plenty of capacity left over and above the termination point. It is a matter of going there to get it.

Originally Posted by nebster

Would take, or does take? I’m guessing you have tried it, and it does work? And how many hours and hours does it take? Is it, say, 0.015C for 30% = ~20 hours? (And is that holding CV at 3.5Vpc?)

If I had done it, I imagine that I would have said so.

Originally Posted by nebster

Another question I have is, why did you decide to run your experiment to 70%? (If that is what you decided; it’s not quite clear, but I am presuming.) 70% is an unusual number, because the data suggest that something like 90% is nearly as good... way more storage, fewer cycles per total energy used, and not much more heating. I would expect most people would pick 85%, or 90%, or 95%. I chose 92%

You don't "pick" your final SOC. It just varies according to what the charge parameters were: absorption voltage, absorption time and C-rate.

I never decided on any "value". I always attempted to recharge the battery properly, by using 3.5/V cell at termination and charge absorption when this was feasible. It often wasn't, because there was not enough solar energy available.
When the cells were new and fresh, they easily recharged close to 100% SOC whenever they got a chance and the absorption times were short. After 4.5 years of cycling randomly taking advantage of the energy available when it was available and never overcharging, the battery now won't recharge beyond 70% SOC when using a normal charging method. And unless I somehow sort it out now, this figure will be lower next year, because this is what happens. So this pack is screwed up and will stay so until I somehow recover it.

A few years ago, a poster explained how he had to "recover" all his installations (and that was a lot of them!) after about 3 years only because they had no capacity left. Root cause: "weak" improper charging without absorption. He was commercial and these few most interesting posts "disappeared" shortly afterwards. However, what he had done was raising the absorption voltage to 3.7V and keep going before discharging and repeating again until things came right. And they seemingly did.

What I want to study now is what I need to do to recover it, because it will allow developing remediation methods that can be implemented in a BMS for batteries that have cycled for long periods without being returned to full. It is a matter of managing long term battery health.
What I already know is that charging to the normal "fully absorbed" termination point fails to push that upper limit back. Getting the missing capacity back is going to require more absorption to get into the upper 30%. It may become easier once I "break" through that 70% limit or not. I don't know.

That is the next part of the experiment, but I have more urgent things to do right now and the problem is not going anywhere. The bank is still perfectly operable.