LiFePo4 vs Lead Acid...Some points of interest.

Freep, You must be having an argument with a ‘straw man’ - lol! ..,,From learned experience my position is “you usually get what you’re willing to pay for” (Victron $$).. But at least you’ve arrived at the conclusion that there IS the possibility of errors with straight coulomb counting, so we’ve made progress, eh :) , and kudo’s to you Sir if your favorite meter (unknown to me) is meeting all your expectations - sounds like you’ve saved a few bucks too!!... For ‘evidence’ you might try litigation...,


3 tons

3 tons wrote:
“ So 3 Ton, what makes you think the Victron Smart Shunt is doing anything different than the other two meters?”

FWIW, I went with a Victron BMV 12 because my faithful and otherwise perfectly good LinkLite was ‘Lost in Space’ with LFP, regardless of what settings I programmed... You seem determined to impute from a distance that this is due to either a ‘bad meter’ (or I suppose operator error??), but you’d be wrong (meaning out on a speculative limb...)....

And why would I be wrong? I'm OK with having my mind changed. But you just haven't provided any convincing evidence other than your experience with one meter that doesn't work.

3 tons wrote:

Bottom line is, Victron WORKS!!

I don't dispute that. I still don't see any evidence that Victron is determining SOC any differently than the other two meters. A claim you have made and not backed up with evidence. I'm willing and happy to change my mind but it takes evidence.

“ So 3 Ton, what makes you think the Victron Smart Shunt is doing anything different than the other two meters?”

FWIW, I went with a Victron BMV 12 because my faithful and otherwise perfectly good LinkLite was ‘Lost in Space’ with LFP, regardless of what settings I programmed... You seem determined to impute from a distance that this is due to either a ‘bad meter’ (or I suppose operator error??), but you’d be wrong (meaning out on a speculative limb...).... Bottom line is, Victron WORKS!!...BTW, maybe it’s just me but Will Prowse does not impress - I find his presentations invariably leave more questions begging than are answered (I have no idea about his background?) but other than that he seems like a nice fella making a living... I don’t mean to sound dismissive here, as I implied this is just my own opinion period...

So 3 Ton, what makes you think the Victron Smart Shunt is doing anything different than the other two meters?

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...

I don't think you'd be able to tell a difference between 50 and 20 because of the flat discharge curve of LiFePo4 cells. Counting amps is going to be just as accurate as counting amps and taking voltage into account.

I think this is especially true in our RV application. The video you referenced was talking about EVs. That's a much different use case. In an EV application you're going to see drift by counting amps because of the high current and high amperage conditions that we just don't see in RV house battery applications.

It would be interesting to run some experiments on these meters to verify. I'm pretty sure Will Prouse has already done that, I'll have to check.

Here's a picture of the discharge curve at 2C for my battery. Keep in mind that discharging at 2C is not normal and the discharge rate would be much lower and therefor the curve would be even flatter until it falls off. Notice that the voltage doesn't drop below 3.2 volts until 210 AH have been discharged. That's 77% of capacity discharged or 23% remaining. Even then it's still above 3.1 volts until it hits 240 AH or 88% discharged(12%remaining). So in other words, I'd have to already be going too low before the meter would start having the accuracy problem stated in the video you shared.

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 ;)

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?

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

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.

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

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 ;)

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

From https://diysolarforum.com/threads/coulomb-counter-not-strictly-compatible-with-lifepo4.21238/

well, only an old retired engineer here, so I will take a shot at making it "simple" ;) (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.