Lithium Ion battery state of charge

[Grumps]

So, if someone gave you a battery and asked how much charge is left, can you buy a meter that does this?

[Theo]

You can tell something from the cell voltage. A full lithium ion cell is

4.2v, but it falls rapidly to about 3.8v, and then gradually declines as it gets discharged to 3.5v, from where it falls rapidly again (and hopefully something will prevent it falling dangerously low).

If you have a voltmeter you can judge roughly where you are on this curve, but most of the capacity lies between about 3.5v and 3.8v so it needs to be sensitive, and the curve isn't linear (although you could just ignore the end phases and define outside these ranges to be 'full' or 'empty'). If you have a pack with multiple cells, multiply these voltages by the number of cells in the pack.

You can buy LED bargraph meters that attempt to do this:

who knows how accurate they are.

Personally, I'd prefer a basic voltmeter:

than a bargraph whose behaviour is unknown.

A better kind of meter is one that counts how much charge went into the battery and how much has been taken out:

that needs to be wired in the charging and discharging path, so you can't measure a standalone battery (although it also contains a voltmeter you could use for the above).

Theo

(none of these are recommendations of particular products, just examples to point at)

[Harry Bloomfield Esq]

Not with any certainty. Voltage is an indication, but only an indication. Where it is so far as charge is concerned, depends upon what it measured when it started fully charged.

[The Natural Philosopher]

Dangerously incorrect. Lithium cells discharge very linearly from 4.2v to around 3.7V (off load). Beyond 3.7V off load you are damaging the cell

State of charge can be read by monitoring off load voltage and interpolating

[The Natural Philosopher]

In the case of Li-ion, with great certainty. And indeed most batteries, if in good condition have a stable and predictable V versus charge state curve/

[Theo]

It depends on the exact cell chemistry ('lithium ion' is a basket of related chemistries), but 3.7v is well within the normal operation range.

exact voltage changes depend upon load, which is why the voltage curve is not a great indicator.

On the first graph there, if you stopped at 3.7v you'd only get half or less of the cell capacity. Those cells have 'end of discharge' cutoff at 3.0v, although the final dropoff is rapid (if you are monitoring this by hand and don't have a protection circuit, you want to catch it well before it drops into the danger zone).

If your discharge current is significant, the voltage declines faster in the

4.2 to 4.0v region. Using that region as a SoC indicator is not recommended, because many uses (power tools for example) take big gulps of current that will make the SoC look worse than it actually is.

That's a paper from 2005, which uses a Sony US18500G3 cell. Those cells were brought to market over 20 years ago and aren't representative of modern lithium ion chemistries. For one thing, energy density is about half what we get today.

Theo

[Grumps]

OK, I think. So which of these curves is more accurate:

The 1st link shows that you can't tell the remaining charge just by measuring the voltage. It depends upon the discharge rate prior to removing the battery from the equipment.

[Rob Morley]

No - you need to run a charge and discharge cycle to discover the cell characteristics (the data sheet will probably give an indication but you don't know how the cells have aged). You'll also want to know what the power requirements are for the application, both current draw and acceptable voltage (and also the battery management system parameters if any).

[Harry Bloomfield Esq]

Exactly the point I tried to make.

[The Natural Philosopher]

Not really, the voltage at no load, is what you measure, to remove the effect of battery internal resistance

[Brian Gaff]

No, nut unless you know how its wired or what circuit the system uses and how much the load takes it was designed for. Brian

[Fakir Neejit]

Seem that this bloke is just as nuts as you.

An extract from:

Third-party characterizations of Lindzen

An April 30, 2012 article in The New York Times included the comments of several other experts. Christopher S. Bretherton, an atmospheric researcher at the University of Washington, said Lindzen is "feeding upon an audience that wants to hear a certain message, and wants to hear it put forth by people with enough scientific reputation that it can be sustained for a while, even if it's wrong science. I don't think it's intellectually honest at all." Kerry A. Emanuel, another M.I.T. scientist, said of Lindzen's views "Even if there were no political implications, it just seems deeply unprofessional and irresponsible to look at this and say, 'We're sure it's not a problem.' It's a special kind of risk, because it's a risk to the collective civilization."[71]

A 1996 article in The New York Times included the comments of several other experts. Jerry D. Mahlman, director of the Geophysical Fluid Dynamics Laboratory, did not accept Lindzen's assessment of the science, and said that Lindzen had "sacrificed his luminosity by taking a stand that most of us feel is scientifically unsound." Mahlman did, however, admit that Lindzen was a "formidable opponent". William Gray of Colorado State University basically agreed with Lindzen, describing him as "courageous". He said, "A lot of my older colleagues are very skeptical on the global warming thing". He added that while he regarded some of Lindzen's views as flawed, he said that, "across the board he's generally very good". John Wallace of the University of Washington agreed with Lindzen that progress in climate change science had been exaggerated, but said there are "relatively few scientists who are as skeptical of the whole thing as Dick [Lindzen] is".[3]

The November 10, 2004 online version of Reason magazine reported that Lindzen is "willing to take bets that global average temperatures in

20 years will in fact be lower than they are now".[80] However, on June 8, 2005 they reported that Lindzen insisted that he had been misquoted, after James Annan contacted Lindzen to make the bet but claimed that "Lindzen would take only 50 to 1 odds".[81]

The Guardian reported in June 2016 that Lindzen has been a beneficiary of Peabody Energy, a coal company that has funded multiple groups contesting the climate consensus.[82]

Lindzen has been called a contrarian, in relation to climate change and other issues.[83][84][85] Lindzen's graduate students describe him as "fiercely intelligent, with a deep contrarian streak."[86]

The characterization of Lindzen as a contrarian has been reinforced by reports that he claims that lung cancer has only been weakly linked to smoking.[87][88] When asked about this during an interview as part of an Australian Broadcasting Corporation documentary, Lindzen said that while "the case for second-hand tobacco is not very good ... the World Health Organization also said that” (referencing a 1998 study by the International Agency for Research on Cancer (IARC) on environmental tobacco smoke (ETS)[89]), on the other hand "With first-hand smoke it's a more interesting issue ... The case for lung cancer is very good but it also ignores the fact that there are differences in people's susceptibilities which the Japanese studies have pointed to."[90] Again, when asked to clarify his position Lindzen wrote "there was a reasonable case for the role of cigarette smoking in lung cancer, but that the case was not so strong that one should rule that any questions were out of order ... the much, much weaker case against second hand smoke [is] also being treated as dogma."[91] A

[Fredxx]

Is that your contribution to this thread?

[Fakir Neejit]

Turnip quoted Richard Lindzen.

Just read the extract. It should be obvious to anybody, even you, that Lindzen is a dodgy character that among other things, is in the pocket of a coal company that has funded multiple groups contesting the climate consensus.

Just the type Turnip admires.

[Fredxx]

Please do find the quote for "Not really, the voltage at no load, is what you measure, to remove the effect of battery internal resistance". Shame you snipped this bit.

[Fakir Neejit]

Turnip is in the habit of putting daft signatures at the end of his posts that are irrelevant to the subject. These normally don't elicit a response. This was an attempt to highlight such stupid signatures.

[Fredxx]

Then I recommend you quote the signature rather than looking the fool.

[Fakir Neejit]

So you didn't read Turnip's ridiculous signature? It's there for everyone to see, except perhaps the dim-witted amongst us..

[Paul]

Paul