How to prevent over discharge of Li-Ion in flashlights?

[cranphin]

Right

Got my Pila IBC charger, some protected ultrafire RCR123/18650's, and some of AW's underway.
There should be a MRV and a VB-16 incoming too ^_^
I intend to use both with a 2xRCR123 and a 1x18650 setup.

Now my big question, and to my surprise I haven't really found any topics on this yet, even after some digging.
How do I prevent from over discharging the cells ?
That is, dropping the single cell voltage under 3.0V-2.5V. (correct?)

I understand it's really bad for the cell to go below 3.0V, and that the protection might decide to render the cell unusable when this happens. (right?)

So should I pop out the cells and measure them on a voltmeter every 5 minutes?
Can I just look at dimming of the light ? That does not seem a good option for 2xRCR123 on a light that will run on a single 3.6V cell too
Do these (more expensive) lights I bought have anything built in to prevent it ?
Should I use the lights only for a certain amount of time, based on runtime charts or so ?

There's lots of people here who seem to use 18650 or 2xRCR123 set ups.
So I'm sure there's a common and accepted and simple and logical method, but I really wouldn't know what it is. Am I missing something? ^_^

So, how do people do this ?

Thanks!

 

[mudman cj]

I can't comment on the Ultrafire cells (and wouldn't :green, but the protected AW cells are supposed to cut off at 3.0 V under load, 2.45 V at very low current draws. The protection doesn't render the cell unusable, but rather is designed to prevent the cells from becoming unusable. It is not recommended that you use the overdischarge protection as a means of determining when to recharge the cells, since discharging Li-ion cells to less than 3.0 V will reduce their capacity each time.

You are unlikely to notice dimming in most LED lights before the cell needs to be recharged, especially on a light that takes two 3.6 V cells to drive an LED because the 7.2 V from the batteries must be stepped down to around 3.3-3.5 V (varies) for driving the LED, and your cells are not going to discharge to the point that they can't deliver that. The best method is to just recharge often after use. If you need to use the light and run the batteries all the way down, then so be it. The protection will kick in before the cells are ruined, but one second it will be working and the next...nothing.

There are few lights with battery capacity indication, but they do exist. In time, I think you will find that it is not such a bad problem after all.

Cheerio,
C.J.

 

[Curious_character]

As mudman cj says, there's no problem with protected cells -- as the name implies, they protect themselves from excessive discharge. If, on the other hand, you use unprotected cells for some reason (lower cost, higher capacity, smaller size -- some won't fit in some lights -- or salvage from a defunct computer supply), you do have to prevent overdischarge. I have several lights designed to run from either two CR123A cells or a single 18650 or 17670 cell. I've found that in all cases, the supply to the LED becomes unregulated when the cell reaches something like 3.8 - 3.9 volts, which is early in its discharge period when running from a single Li-ion cell (18650 or 17670). From then on, the LED voltage drops as the battery voltage drops. By the time the voltage gets down to 3.0 volts, the LED is very, very dim. And at 2.5 volts, you'd see a glow in a dark room, but that's about it. So you won't have any trouble telling when the voltage is getting too low. Of course, if the light is unattended, you could overdischarge the cell, but it would take an awfully long time, since the current drops exponentially as the voltage drops.

But if you're using two unprotected Li-ion cells (RCR123A), there's no good way to detect the low voltage point, and you'd have to depend on keeping track of how long you've had the light on since the last charge. I'd use protected cells only for this configuration.

c_c

 

[LuxLuthor]

As the two previous posts say. It is simple.

Use protected cells. :thumbsup:

 

[DM51]

... and recharge them frequently, for better cell life. Don't let them run down to empty.

 

[Curious_character]

... and recharge them frequently, for better cell life. Don't let them run down to empty.

I've read several places that this makes no significant difference to Li-ion cell lifetime. Can you point me to a reference which explains this?

According to the manufacturers, Li-ion cell life is best when cells are stored at about 40% charge, and at lowered temperature. Doing either one helps some. I found out the hard way, though, that although freezer temperature of about 0F (-20C) is within the permissible storage temperature range of Li-ion cells according to some manufacturers, that it can kill some cells. I also had failure of two protection circuits from condensation. So now I store unused cells in the refrigerator.

c_c

 

[cranphin]

Well, think that answers my questions mostly

Really in short:
- Protection should kick in before anything bad happens, so don't worry too much.
- It's not that hard to keep them from over discharging, just recharge cells after every use, and don't use it till it goes flat. Li-Ion love regular and partial recharging anyways.

I guess what I misunderstood was that I saw protection as a last line of defense, something you should try hard not to trigger

I'll still do some regular voltage measurements the first times I use the flashlights, just to get a feel for how long I can use them with the cells. I still prefer not to trigger the protection, it could always be broken, even on AW's cells

 

[DM51]

I've read several places that this makes no significant difference to Li-ion cell lifetime. Can you point me to a reference which explains this?

According to the manufacturers, Li-ion cell life is best when cells are stored at about 40% charge, and at lowered temperature. Doing either one helps some. I found out the hard way, though, that although freezer temperature of about 0F (-20C) is within the permissible storage temperature range of Li-ion cells according to some manufacturers, that it can kill some cells. I also had failure of two protection circuits from condensation. So now I store unused cells in the refrigerator.

c_c

Hi again, CC - I remember our previous discussion about cold storage of Li-Ion cells.

This time I am not referring to storage. You are correct in saying 40% charge (~3.8v) is the optimum charge for storage (although there is very little self-discharge or damage if Li-Ions are stored at up to 4.0 - 4.1v).

My point here was aimed at everyday use in general. If you let a cell run down to empty (3.5v resting voltage) or with a protected cell to the point where the LV protection kicks in, it won't last as long as if you charge it up after each use. For example it may read 3.9v after a spell of use, but it is still a good idea to recharge it. Sure there is still plenty of life left in it, but it is just easier on the cell if you top it up before it gets too low.

I repeat, this is for cells that have daily use, not for storage purposes.