Assessing Battery Health Help

How do you know / measure how your battery's health is deteriorating .... obviously they seem not to last as long is it that the capacity drops off and they don't hold as much charge i.e. a 2500 may only take 1000 mah to fully charge from flat?

I know you can also measure the internal resistance ... is there a way to do this if your existing charger doesn't have that functionality or do you need a charger that does and what levels / reading should you expect for both good health and bad health???

I was curious about your question, and I found something on batteryuniversity titled "how to measure internal resistance." They have several methods, but I was surprised that the article started with an explanation of how, for lithium-ion, internal resistance was not particularly useful for determining the state of health of a cell. They suggested a capacity measurement is the best indication. They mentioned the caveat that, at the end of life, IR might fall off a cliff; and, IR is more useful for nickel-based cells.

FWIW, I have a couple chargers that report internal resistance, and they do not have very good repeatability of measurements. That is, get a measurement, remove the battery, immediately replace it and get another measurement, and scratch my head as to why they are pretty different. I found that if you press the spring-loaded charger contacts hard into the battery, I usually get a lower (I think more believable, but still variable measurement-to-measurement) reading for internal resistance than if you just let the chargers' springs push the contact into the cells.

If you want something to measure capacity (and IR), I'd start with this index; there are some at $30 or less with good reviews from forum member HJK: https://lygte-info.dk/info/roundCellChargerIndex Analyzer UK.html

He also has an index of DMM, some of which might be able to help if to execute the DC load method or the "1000 Hz ohm test" (which, evidently, are both valid, and return different numbers....what is going on here, quantum mechanics in my cells?!)

Capacity is the usual yardstick, SoH% defined as a specific current rate held constant, precisely timed from 100% to 0%.

Resistance is much more variable harder to measure apples to apples, has more to do with power density rather than energy density.

But if you create a standard benchmark measure when New, then do the exact same test with the same gear on the same cells as they get worn down, you can see a relationship there and develop a feel for it.

I've read that a general standard as to whether or not a lithium ion battery is considered done with it's usefulness is when the capacity that it can discharge drops below 80%. Is that percentage, below 80% considered a dead battery or can you continue to use the battery to maybe 60% (or less) safely? And as internal resistance builds up over time is there more danger for catastrophic failure? Is there a "rule of thumb" to go by for safety? I have some older li-ions I still use that are probably below 80% capacity. FWIW they do charge up to at least 4.1v.

Cheers for the replies and I've recently got a charger (not used it yet) that can measure IR as well as the amount of capacity going into a battery which will be interesting to see.

My query was triggered as I do quite a bit of cycling and the light's powered by 18650's, the torches I use have some warning of battery level i.e. step down on one that then lasts a while before it then goes lower again.

This has worked in the previous 2 winters but I noticed one of the batteries went to the lower level and couldn't hold that for any period of time before it immediately dimmed again.
I don't have a problem with that as the batteries have lasted the best part of 3 years going unused for months in the summer albeit kept around storage charge but I don't want to be in a situation where I'm 10 miles out in the middle of farm tracks in the pitch dark the battery and any spares all fail.

Thanks again for people's input ... much appreciated !!!

LED Monkey said:

I've read that a general standard as to whether or not a lithium ion battery is considered done with it's usefulness is when the capacity that it can discharge drops below 80%.

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That is a traditional standard used in industry.

Less wealthy consumers willing to risk "unexpected failure" modes may go past 70%

but with dangerous as in fire / boom chemistries, I wouldn't.

I've never heard that "old" lithium-ion batteries become dangerous when their capacity drops below 80% or 60% or whatever. They just have less capacity than when they were new, and likely have higher internal resistance. Just use them in medium drain lights, and not in your pocket rockets such as a D4.

My personal experience is that old lithium-ion cells work fine; they just have lower capacity. Internal resistance may be double, but it's still low enough for drains at 3 amps (which gives about 1000 lumens).

OTOH, I do find that NiMH batteries (specifically Eneloops) don't function very well by the time their capacity drops below 80%. The internal resistance is way too high at that point. But they still work fine in low-drain stuff like remotes or dim lights.

There are dozens of chemistries under the "lithium" umbrella.

And then all the different packaging technologies and form factors.

There is no doubt that those that are dangerous, become more so as they are approaching EoL.

If this community uses specific types that aren't dangerous, perhaps letting overt performance failure dictate time to scrap them is safe, I dunno.

In other use cases, where hundreds of pounds of batteries are mobile and in the same space as humans, I've learned to default on the conservative side.

People use their cell phones, tablets, and laptops until the batteries are so worn out they have little capacity left. The only safety issues we see in the news are from new devices with new batteries (e.g., Samsung), not old ones.

Maybe old batteries are more dangerous than new ones. But they can't be too much more dangerous.

Those are computers which incorporate pretty sophisticated battery management circuitry.

And do very regularly, randomly burst into flames, often enough it isn't worthy of media coverage except when a design flaw is at fault.

I believe as the lithium ion cell is used it starts to form dendrites, branch like structures that grow in the cell, as it ages it will form more dendrites to the point which can eventually lead to a short circuit between the anode and cathode. If the short circuit is severe enough you'll get a thermal runaway event. I really don't know at what point a worn out lithium ion battery is reaching a "unsafe" level. I know batteries are quite safe for the great majority with very few failures that lead to major damage, but is there a rule of thumb, a percentage of capacity perhaps that Joe consumer can go by? As the OP's title says "Assessing Battery Health".

As stated above, 80% SoH to be conservative, pushing past 70% risks increase not linearly but geometrically.

And just a quick follow up post here. Batteries can and do fail. I know because it did happen to me in a flashlight that I was using in my hand at the time with 2 lithium Cr123s primaries in series. After a little while boom! I think I may had put a low (spent) battery in the light with a new one by accident, yes my fault! But my point is it can happen. I'm glad I was outside when that happened. This was probably about 6 years ago and I've learned more about batteries since then.

LED Monkey said:

I believe as the lithium ion cell is used it starts to form dendrites, branch like structures that grow in the cell, as it ages it will form more dendrites to the point which can eventually lead to a short circuit between the anode and cathode. If the short circuit is severe enough you'll get a thermal runaway event. I really don't know at what point a worn out lithium ion battery is reaching a "unsafe" level. I know batteries are quite safe for the great majority with very few failures that lead to major damage, but is there a rule of thumb, a percentage of capacity perhaps that Joe consumer can go by? As the OP's title says "Assessing Battery Health".

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Again, laptops are still in use with 10+ year old batteries in them. They're not exploding. I'm not aware of any hard cut-off for battery safety, as long as the cell still charges okay, holds a charge, etc. If you're worried about it, batteries are fairly cheap. Get rid of them if they're 3-5 years old, or are down to 80% of their original capacity. You may be wasting batteries, but at least you'll be using good cells all the time, and the peace-of-mind is quite inexpensive.

Regarding internal resistance: unless you've made a very low resistance connection with the cell, the IR readings are likely to be inconsistent. YMMV but this is where the XTAR Dragon tester may have an advantage. The dedicated probes are first shorted, which zeros out the inline resistance of the wires and internal circuitry, I believe. Those probes have a relatively large contact face which has many sharp contact points that dig into the cell's terminals - making a more repeatable, low resistance connection. Any dirt or film on a charger's regular contacts may add many milliohms of inaccuracy. Contact pressure of the spring-loaded sliders is probably too low for any genuine consistency. The IR feature is still nice for at least a ballpark reading and is sometimes quite accurate.

WalkIntoTheLight said:

Again, laptops are still in use with 10+ year old batteries in them. They're not exploding. I'm not aware of any hard cut-off for battery safety, as long as the cell still charges okay, holds a charge, etc. If you're worried about it, batteries are fairly cheap. Get rid of them if they're 3-5 years old, or are down to 80% of their original capacity. You may be wasting batteries, but at least you'll be using good cells all the time, and the peace-of-mind is quite inexpensive.

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Yes you're 100% correct. I just wanted to see if there is some kind of guideline that is used for li-ion cells. Millions of people use batteries with zero knowledge at all. I'm not worried about it but I would like to know, and without knowing then yes I could be wasting money by throwing out batteries prematurely which I try not to do. I don't want peace-of-mind from just buying new batteries but rather through knowledge. The batteries do charge up OK and they do hold a charge but I can see a loss of capacity and notice they will not sustain a "turbo mode" as much as they used to in some lights. New li-ions are pretty tough and safe. I watched a YT video of some crazy guy smackin the hell out of a li-ion battery with a hammer and no venting happened . Just givin yah a hard time.

Old cells and "battery vampire" lights are a match made in the CPF zone.

LED Monkey said:

Yes you're 100% correct. I just wanted to see if there is some kind of guideline that is used for li-ion cells. Millions of people use batteries with zero knowledge at all. I'm not worried about it but I would like to know, and without knowing then yes I could be wasting money by throwing out batteries prematurely which I try not to do. I don't want peace-of-mind from just buying new batteries but rather through knowledge. The batteries do charge up OK and they do hold a charge but I can see a loss of capacity and notice they will not sustain a "turbo mode" as much as they used to in some lights.

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I think the most important thing you can test, is to see if a cell will hold a charge without dropping much voltage over a week or two. All cells will drop initially, but after a day or so they should be quite stable. If you find the cell drops from something like 4.17v to 4.14v over a week, that indicates a high self-discharge, and that might be a sign of some dendrites forming. Do the test a few days after letting the cell rest, after a full charge to 4.20v. It should stabilize at something above 4.16v.

Also, I wouldn't bother with a cell that won't charge up higher than 4.10v (after it rests). That means there's something wrong, too.

New li-ions are pretty tough and safe. I watched a YT video of some crazy guy smackin the hell out of a li-ion battery with a hammer and no venting happened . Just givin yah a hard time.

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Wow, that guy sounds like a real idiot! Must have a death-wish or something.

That's another good point about the lack of charging over 4.10 volts. I'm surprised that most of my old 16340s (2006 - 2014) are still usable, holding their voltage and much of the original capacity. They are limited to use in moderate drain lights mostly ~400 lumens or less. One a side note: I've some four year old Fenix cells that always had a faster self-discharge down to ~4.0 volts or so, depending how long they sat. Could it be the protection circuitry or the chemistry?

Even though IMR cells have less capacity, they're able to power the Turbo level in high-lumen, single cell, pocket rockets.

jayflash said:

One a side note: I've some four year old Fenix cells that always had a faster self-discharge down to ~4.0 volts or so, depending how long they sat. Could it be the protection circuitry or the chemistry?

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That doesn't sound good. Protection circuitry takes a very small drain, far slower than normal self-discharge. Discharge down to 4.0v sounds like either they never received a full charge, or there's something wrong with the cells. I might expect that with old, worn-out cells, but not with new-ish cells. Perhaps they're fakes? Are you measuring them with a good DMM?