Overdishcharge of protected/non protected 18650s...?

[R.ticle One]

Hello everyone,

I recently received my Solarforce L2 host (just waiting for the right dropin to become available, now --- and thanks to those who steered me towards it and saved me some cash over buying a 6P instead).

It came with an unprotected 18650, which I've been using as it is, in the light.

I'm thinking of getting a couple of protected Trustfire 18650s, too, in case it's a while til I get a dropin that'll work on another battery type (or if I want to try the single 18650 with an M30 type dropin).

I'm not certain about discharging; is it a risk to use the included unprotected 18650 cell until it simply won't power the light anymore, if I have no intention of recharging it after? (I have the feeling that it would be safe after reading some more, but I'm not a hundred percent sure).

And if I use the protected Trustfires (single battery configuration), does their protection mean that they can also be discharged until the light won't get enough power for them, at which point I could safely recharge them? (I haven't got a charger yet, so if that happened, they might sit around drained for some time). Basically, will their protection circuit ensure that they'll only provide as much energy to the light as is safe for them before they need recharging, at which point they'll stop powering the light?

Many thanks

 

[Marduke]

Discharging an unprotected cell that far is dangerous. Leaving it remain in that state will induce permanent damage.

Doing the same to a protected cell is also damaging.

I recommed getting a charger before running any cells down, and disposing of the unprotected cell.

 

[R.ticle One]

Discharging an unprotected cell that far is dangerous. Leaving it remain in that state will induce permanent damage.

Doing the same to a protected cell is also damaging.

I recommed getting a charger before running any cells down, and disposing of the unprotected cell.

Oh! So a protected cell will not prevent itself from becoming overdischarged? I was under the impression it would...?

Is the danger you speak of particularly related to the cell not being properly chargeable after such a draining, or will it increase the risk of it going ?

At least - assuming that a cell, whether protected or not, was as discharged as it could be in a light, and there was no attempt to recharge it - would there be a risk of explosion, or only that the cell wouldn't have much life left?

Thanks!

 

[Marduke]

A protection circuit will usually protect the cell from becoming dangerous, but frequently draining the cell to the point at which it kicks in is damaging, and leaving it discharged in that state for a long time is further damaging.

Unprotected cells can be discharged to the point at which they actually become dangerous, both in the discharged state, and during recharging with a cheap charger that does not recognize the overdischarged state and adjust the algorithm accordingly.

I highly recommend you read up on Li-Ion safety, and the do's and don'ts.

 

[recDNA]

It's an interesting question. I haven't seen an explanation of what problem a completely discharged unprotected cell may cause (as long as no attempt is made to charge it.) Obviously you won't be able to recharge it but I don't know if it will blow up on you when it is totally discharged. I suspect it could blow up in the flashlight when you are totally discharging it but I haven't seen that in print either.

Keep in mind I'm a newbie like you. Basically I'm just saying you've asked an interesting question. If you find the answer please post it.

 

[R.ticle One]

Aye, that's my main concern (if the battery becomes physically dangerous at the point of major discharge).

So, Marduke, if I understand you, it's usually "safe" (from an explosion and venting perspective) to discharge a protected cell (such as the Trustfires, I hope) to the point at which the circuit kicks in and the light won't function - it's just unwise to do that because the cell's life will be depleted...correct?

And don't completely discharge a non protected cell, is the general rule?

So, how do most people know when to take a cell out of a light (before it's circuit kicks in or it's fatally discharged), and dispose of it or recharge it - are you doing this when the light dims, or checking the voltage of the cell periodically, or...?

Thanking you

 

[Marduke]

1) Correct. While relatively safe, it is damaging to routinely discharge a protected cell to cutoff, and further damaging to leave it uncharged in that state.

2) It is a general rule to not completely discharge any rechargable cell, regardless of chemistry or presence of protection. Dangers aside, shallow discharges are healthier for the cell.

3) When a light and/or cells is new, it takes a bit of trial and error. You learn how much runtime you typically get on a cell with a particular light, and what voltages show you what signs (if any). You may trip a protection circuit a number of times, but it is not that big of a deal as long as you don't do it frequently, and you recharge quickly.

Eventually you get a feel for the light, and how often you need to recharge. Personally, if I know I typically get __day/weeks of service on a given setup, I'll recharge and top up in 1/4 to 1/2 that time, so I know I'll always have no less than half my total battery life available. Also, if I use the light for an extended period of time, I will recharge regardless of schedule and reset the rotation.

With Li-Ion, a quick voltage check at any time will give you a rough idea how close you are to NEEDING to recharge.

 

[R.ticle One]

Thanks, Marduke. Main reason I ask is that perhaps I may encounter situations in which I wouldn't have a spare battery, and was running a single 18650 and needed to squeeze all the light I could out of it. Okay, it hasn't happened yet, but who knows...if I had to do so, it's good to know that I wouldn't risk blowing myself up.

I guess it will take a while to get used to how much time brings the battery to what level.

As for checking the cell's voltage, I've never understood how low a cell can safely go. An 18650 is listed as 3.7V, right? But (sorry, totally stupid question), is 3.7V its minimum voltage at which point it would either be dead, or shut off by its protection circuit (as in, a fully charged 12V deep cycle battery is usually around 14 plus volts, as a similar example)?

The battery which came with the L2 shows about 3.89V (though it's an older analog Micronta multimeter, I've no idea if that's affecting it).

Thanks

 

[Marduke]

4.2v is fully charged, most protection circuits cut off ~2.75v, and it's a good idea to stay above 3.2v or so (I think it's 3.2)

 

[R.ticle One]

Thank you for that information! If fully charged is 4.2 volts, why are the batteries listed as 3.7V, if they are also able to be discharged measurably lower than that?

 

[wapkil]

4.2v is fully charged, most protection circuits cut off ~2.75v, and it's a good idea to stay above 3.2v or so (I think it's 3.2)

I think most protection circuits nowadays trigger around or below 2.5V. That's what I measured for AW's cells and TrustFires. 2.3V - 2.5V is also what I saw in a few protection ICs datasheets.

The question to what voltage the battery can be discharged under load is rather complicated - it depends on the current. After the load is removed (e.g. the flashlight is turned off), the battery voltage will raise. Generally it should raise above 3.0V if the battery is not overdischarged. Below 2.8V Li-Ion cells are considered overdischarged by charging ICs. I think that in the area between 2.8V and 3.2V they may or may not be overdischarged, depending on many factors.

 

[flasohollic]

2,3-2,5v there isent realy much juice left to power anything so it may not kicked in at all the divece may stopped working becouse lack of power.
Dischargin them to 2,5-2,9 is not dangerous, but it may damage them in away they not performing well ore shorten lifespan..
The danger is realy when charging and shorts

 

[Marduke]

Thank you for that information! If fully charged is 4.2 volts, why are the batteries listed as 3.7V, if they are also able to be discharged measurably lower than that?

Rechargable cells are marked with nominal voltage, where primaries are usually marked with initial voltage.

 

[wapkil]

Rechargable cells are marked with nominal voltage, where primaries are usually marked with initial voltage.

It is actually an interesting question. Do you know if there exists any formal definition of nominal voltage? It's usually described as "typical voltage during use" which obviously doesn't have any precise meaning. Maybe it is just an arbitrarily selected value used by convention...

 

[Marduke]

I believe it is the average voltage between fully charged and the voltage the chemistry is discharged to in order to obtain the rated capacity under some standard load.

 

[wapkil]

I believe it is the average voltage between fully charged and the voltage the chemistry is discharged to in order to obtain the rated capacity under some standard load.

Wouldn't it make the nominal voltage dependent on particular cell? For example an 2000mAh Eneloop discharged with standard current (0.2C, I think) would need to have a higher nominal voltage than an 2000mAh non-LSD NiMH, yet they are both 1.2V.

 

[Marduke]

That is why I said "chemistry", not "cell". I believe it is some industry standard. Someone more well versed in that industry propably knows the specifics.

 

[Bullzeyebill]

Some good advise given here. :twothumbs What I do is treat every LiIon I own as if it were an unprotected cell, and do periodic monitoring. I use both protected and unprotected cells, mostly unprotected, and it keeps me on my toes about the condition of my cells, and I can wear out a DMM measuring voltage. :naughty:

Bill

 

[wapkil]

That is why I said "chemistry", not "cell". I believe it is some industry standard. Someone more well versed in that industry propably knows the specifics.

I think Eneloops can be treated as a different chemistry (mainly cathode alloy) than non-LSD cells. Anyway, I know that according to the IEC 60050 (the vocabulary used in other battery standards, AFAIK) nominal voltage is "suitable approximate value of the voltage used to designate or identify a cell, a battery or an electrochemical system". That was one of the reasons that made me suspect it may be not used as a precise definition, only a kind of battery type designator.

 

[Mjolnir]

I try to recharge all of my Lithium ion cells after I use them. However, if I am somewhere away from a charger, I would probably use the light until the protection circuit kicks in. It may shorten the battery life, but I can get a new pair of batteries for under 10 dollars, so it isn't all that big of a deal.