Can lithium rechargeables over self-discharge??

[1pt21]

Hello everyone, new guy here with a concern...

After reading so many posts about lithium battery safety and explosions I really got thinking about my current battey lineup.. Mostly i've been using Surefire 123 primaries, but just recently got into the rechargeable scene. So far I have a pair of protected TrustFire RCR123's and a pair of TrustFire 18650's.

I got protected sets so I wasn't constantly worrying over-charging or over-discharging, but I don't really use all my lights that much. So that brings me to my newest concern:

Even though they're protected can these cells self discharge enough over time to cause a possible safety concern??

Should I be regularly charging these things even without using them enough to bring them to an almost drained point??

Am I just putting way too much thought into this?

Thanks for any help, if anybody has any tips on safe storage that would be great as well. As of now I just have them sitting in a dresser drawer, and I could just picture these things.... well you know where I'm going here.

--Paul

 

[SilverFox]

Hello Paul,

Welcome to CPF.

Your protected cells will self discharge, but it may take 4 or 5 years to discharge to the point where you would end up in a over discharged state. Hopefully you will use them once or twice in that amount of time. If not, plan on recharging every two years.

Storing Li-Ion cells is best when the cells are kept at around 40 F and only charged about half full. Find a volt meter and half full is around 3.9 volts.

Now, the purpose of having a flashlight is to be able to use it. In my lights that are used less often, I will charge the cell, then turn the light on for 10 minutes or so, then store it. This way you are not storing the battery at it fullest charge, and you still have most of your available capacity and runtime.

Tom

 

[Buck91]

I've heard this about storing li-ion at half charge a few times before. What is the concern with storing them topped off?

 

[SilverFox]

Hello Buck,

Li-Ion cells wear out through oxidation of the electrodes. At full charge, one electrode is effected, and the other is effected at full discharge. They seem to be very stable at about half way charged.

Battery University published a study showing permanent capacity loss when storing a cell at room temperature fully charged. This study is a few years old and chemistry has changed a little, so I don't know if this is still a big problem, or only a little problem.

On the other hand, many people store their cells and battery packs fully charged and report no problems at all.

Just to be on the safe side, I usually try to store them at less than a full charge.

Tom

 

[2xTrinity]

Battery University published a study showing permanent capacity loss when storing a cell at room temperature fully charged. This study is a few years old and chemistry has changed a little, so I don't know if this is still a big problem, or only a little problem.

The way I understand it, "permanent capacity loss" is a bit misleading. From waht I understand, one of the major ways in which LiIon cells degrade is that they gradually develop higher internal resistance -- for low current applications, such as some LED lights, this is not especially significant. However, in high-drain applications, like incandescent lights, or processor intensive computing, this internal resistance translates into a loss of usable capacity.

Several people have told me that LiIon cells become effectively useless after three years from manufacture date, regardless of whether they are used or not. However, I have some seven-year-old laptop cells that I regularly use in my LED flashlights that still have the majority of their capacity. They dont hold up as well in high-drain applications though.

 

[1pt21]

Thanks for the help, and all the useful info.. I can rest easier knowing I don't need to be worrying about these things discharging.

I really had no idea that they discharged at such a slow rate! I guess I am just used to my RC batteries and still in that frame of mind, if I put those things away for a few months fully charged, by the time I pull them out they're near dead.

I put together a light for a buddy of mine at work, and he just gave me a beautiful FLUKE multimeter last night for putting together such a nice light for him!!! So that baby will be getting pleny of usage to keep an eye on my cells.

--Paul

 

[SilverFox]

Hello 2xTrinity,

You are correct for as far as you went. With an increase in internal resistance, there is a progressively higher voltage drop as the load increases. This higher internal resistance also causes the cell to heat up inside.

However, the damage I was referring to has to do with electrode oxidation.

At the end of the charge cycle, the voltage on a Li-Ion cell is held at 4.2 volts as the current drops down. At some point the low current shut off is reached and the cell is supposed to be fully charged.

Now, as you have indicated, a cell with higher internal resistance will not show much of a voltage change under a low current of around 50 mA. Yet, when you disconnect the charger that was holding the cell at 4.2 volts, it rapidly drops down to a lower voltage. If the cells open circuit voltage drops below 4.0 volts in around 15 minutes after taking it off of the charger, it is considered dead.

The cause of this voltage drop under a no load condition is electrode oxidation. It is usually accompanied by an increase in internal resistance, but in order for internal resistance to have an effect, you need to apply a load. Without any load attached, you can watch the voltage drop off.

The reason you should retire the cell when it drops to a lower voltage is because the oxidation process contaminates the electrolyte mixture. This can be a problem during charging.

I also have some cells from laptops that are quite old. They come off the charger a little under 4.1 volts. They aren't completely dead, but they are not strong performers, even at 1C loads.

Tom

 

[45/70]

Now, as you have indicated, a cell with higher internal resistance will not show much of a voltage change under a low current of around 50 mA. Yet, when you disconnect the charger that was holding the cell at 4.2 volts, it rapidly drops down to a lower voltage. If the cells open circuit voltage drops below 4.0 volts in around 15 minutes after taking it off of the charger, it is considered dead.

I have always wondered what indicates a worn out Li-Ion cell. I know that when they reach about 80% of their original capacity, they are considered "used up", but for a lot of us, that's hard to figure out, not having testing equipment etc. This should be in a sticky somewhere.

Dave