Good Sam Club Open Roads Forum: LiFePo4 vs Lead Acid...Some points of interest.
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 > LiFePo4 vs Lead Acid...Some points of interest.

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Freep

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Posted: 04/14/21 08:21am Link  |  Quote  |  Print  |  Notify Moderator

3 tons wrote:




“Who was it that told me” ??...It was first hand knowledge, and be advised that I’ve mentioned this before - It was my Xantrex meter... Try as I might, once below about 60’ish %, regardless of what settings I used it would not track LFP SOC with any accuracy, nor does the owners manual specify LFP...However, the Xantrex worked flawlessly for 11 years with my former GC 6v...

3 tons


I think you may have just encountered a bad device. There's nothing I can find that shows the three devices use different techniques to arrive at the SOC. I have two AiLi meters. I realized I'd be pulling a large number of amps and would require a larger shunt so I got the bigger one and just swapped out the shunt and not the head unit because the head units looked identical. It turns out it wasn't and I was not getting consistent readings with the new shunt and the old head unit. Once I replaced the head unit too, everything was fine.


Do you think it would be more accurate if the meters counted Amp hours?


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3 tons

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Posted: 04/14/21 09:08am Link  |  Quote  |  Print  |  Notify Moderator

“ Do you think it would be more accurate if the meters counted Amp hours?”

A few post back I added links to two engineering discussions on this very matter that helped ‘wash my mental windows’ Well worth one’s investment of time to gain a ‘beyond novice’ understanding of the difficulties in determining SOC of LFP’s - trust, not really a ‘one solution fits all’ situation...I offer this mostly to the broader audience...

3 tons

Freep

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Posted: 04/14/21 09:22am Link  |  Quote  |  Print  |  Notify Moderator

3 tons wrote:

“ Do you think it would be more accurate if the meters counted Amp hours?”

A few post back I added links to two engineering discussions on this very matter that helped ‘wash my mental windows’ Well worth one’s investment of time to gain a ‘beyond novice’ understanding of the difficulties in determining SOC of LFP’s - trust, not really a ‘one solution fits all’ situation...I offer this mostly to the broader audience...

3 tons


Yeah but you didn't really answer the question.

Do you think it would be more accurate if the meters counted Amp hours?

3 tons

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Posted: 04/14/21 09:44am Link  |  Quote  |  Print  |  Notify Moderator

Answer: No, Personally I would defer to sage engineering reasoning and avoid speculation - I’m sure that other’s (novices) may go different routes...

3 tons

Freep

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Posted: 04/14/21 09:47am Link  |  Quote  |  Print  |  Notify Moderator

From https://diysolarforum.com/threads/coulom........-strictly-compatible-with-lifepo4.21238/

well, only an old retired engineer here, so I will take a shot at making it "simple" [emoticon] (said absolutely every engineer as they start their presentation)

An amp is just a coulomb/sec
or
1coulomb = 1amp * 1sec.
the rate of flow of the electron does not give a hoot about the voltage so it literaly is the number of electrons that have flowed.
energy is the relationship between voltage and current.

if you know how many amp-seconds worth of electrons you put into something, well, when you have started to pull that many out you are running on empty. Sounds like people are trying to compare "current" and "energy"...they are related but different.

the "energy" is a function of voltage * coulombs used; and therefore the potential energy in the cell is dropping as the voltage drops even with a constant coulomb draw... the flat discharge nature of the lifepo4 means that the window of constant energy draw (consistant voltage and current) is also mostly flat until the end.
Once you have hit the point of rapid voltage drop you ARE getting less "energy" for the same coulomb draw; but the end is near anyway due to the sharp dropoff. Your coulomb counter is still valid, if you want to maintain a specific power output, you will draw more coulombs as the voltage drops...shunts measure coulombs and you will not get out more than you put in(if you do, quick, get a patent!!!).

you can tell how long you have left before dead just by coulomb counting...
If I have 200coulombs left and I am drawing them thru the shunt at some rate, then its easy to solve for 0.
You could probably take the standard discharge curve of the end of the lifepo4 and create a simple two point equation from it once you hit the drop-off to actually estimate the amount of energy left. The voltage will keep changing and coulombs will increase to sustain the energy demand.
Or, you can just count coulombs, when you hit zero, game over hehe

You are using the energy but its really the flow of electrons you are controlling.

That said, its way simpler to just accept that once you drop under 3volts, you are done. If you have been counting coulombs all along, you know how many amp-seconds you have left if you want to predict exactly how many watt-seconds remain.

Freep

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Posted: 04/14/21 10:20am Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:


Testing an alternator vs LFP on a bench is very different than in an RV. Unless of course you have a converted bench as an RV [emoticon]



True. But if you don't use a DC-DC converter you probably should do something to avoid this.

3 tons

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Posted: 04/14/21 10:20am Link  |  Quote  |  Print  |  Notify Moderator

Well I find it kinda interesting that you’d choose to withhold that dissenting point of view ‘pending’ whatever my response was??...Having worked professionally with numerous engineers of various disciplines and opinions myself (not unlike expert Economist lol!...) while at the manufacture of Hydrogen and hydrocracking, I’ll easily stick by my previous judgement and let the free-lancers do what free-lancers do..,JMO

I will add that the engineer you cite is focused primarily on only one aspect, that of coulomb counting... The previous links I provided dispel this as an overly simplistic approach, and why I suggested a review...

Critical thinking requires all sides (dimensions)
of the story...

3 tons

* This post was edited 04/14/21 10:28am by 3 tons *

Freep

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Posted: 04/14/21 10:32am Link  |  Quote  |  Print  |  Notify Moderator

3 tons,
If you look at that thread I referenced you will see some interesting opinions on this particular issue you bring up.

For example, the problem the engineer in the video you posted is talking about is a problem that doesn't really exist in our application. Also the charge/discharge curve he's using as an example is not nearly as flat as a LiFePo4 curve. The drop off in current is going to be less than the 20% mark where you shouldn't be discharging anyway.

So while he is technically correct, it is an irrelevant detail in this application.

This is similar to the fact that Newtonian physics was proven wrong by Relativity. Sure it's wrong, but calculus is still very useful in applications where relativity has a negligible effect.

Again, I'm not seeing anything that shows the Victron is doing anything different to report on state of charge than the other meters mentioned.

3 tons

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Posted: 04/14/21 11:03am Link  |  Quote  |  Print  |  Notify Moderator

Freep wrote:

3 tons,
If you look at that thread I referenced you will see some interesting opinions on this particular issue you bring up.

For example, the problem the engineer in the video you posted is talking about is a problem that doesn't really exist in our application. Also the charge/discharge curve he's using as an example is not nearly as flat as a LiFePo4 curve. The drop off in current is going to be less than the 20% mark where you shouldn't be discharging anyway.

So while he is technically correct, it is an irrelevant detail in this application.

This is similar to the fact that Newtonian physics was proven wrong by Relativity. Sure it's wrong, but calculus is still very useful in applications where relativity has a negligible effect.


Again, I'm not seeing anything that shows the Victron is doing anything different to report on state of charge than the other meters mentioned.


If I accept (say for argument...) your idea that 20% is the cut-off point, then as was pointed out in both of the FLP discussions I pointed to, it’s the region below 50% SOC where coulomb counting alone begins to fall short (thus, 50-20’ish %) and where accuracy becomes a more driving concern...I take note of your position that this has only a negligible effect...

3 tons

time2roll

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Posted: 04/14/21 11:31am Link  |  Quote  |  Print  |  Notify Moderator

Freep wrote:

time2roll wrote:


Testing an alternator vs LFP on a bench is very different than in an RV. Unless of course you have a converted bench as an RV [emoticon]

True. But if you don't use a DC-DC converter you probably should do something to avoid this.
Yes that is the bench I am talking about. No effort was made to replicate an RV electrical system.
Maybe start with a fuse?


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