li-ion : is a rest needed between charge/discharge?

[tino_ale]

Hi guys,

Couldn't find this information with the search function, don't really know what to look after.

- I would like to know if li-ion cells can be safely used right out of the charger.

- Same question when the cells have just cutoff in a flashlight, can I pop them right after in the charger, or should they rest for a while?

- Last question : is there a max number of cycles per day (or night)? Or can they be charged/discharged/charge/discharged several times is a raw?

I'd like to make them last as long as possible, so I'm looking after the "best practice" advise.
Thanks for help!

 

[matrixshaman]

Not an expert here but I don't see any reason they can't be used right off the charger as long as your light will handle the full max voltage that they come off with. And as far as popping them right out of a light into a charger I would say that's fine too unless they are really hot or very warm - you might want to let them cool down some in that case. But unless you've got a fairly high current charger putting them on is not likely to make them hotter as much as they would just be slower to cool down some.

 

[tino_ale]

Thanks for the reply

I'm asking this because I believe (correct me if I'm wrong) that NiMH chemistry for example need to rest after use and before charging. It is also recommended to avoid more than one cycle charge/discharge per day...

My questions are only about how to handle the batts, I'll make sure everything is fine with the flashlight (max input voltage etc).

 

[winny]

tino_ale,

SilverFox will probably burst in here in moments and tell you how it works but let me try to answer your question in the meantime.

Resting a cell, weather NiMH or li-ion can serve two purposes, voltage drop and temperature drop. Temperature is rarely a problem, except perhaps in extreme hotwires. Voltage can be problems with overdriven incans where freshly charged NiMHs can reach 1.45 V / cell and they drop down to 1.3-1.4 V / cell after a short while. When you are running on the edge, this can be critical.

I would answer your questions like this for any normal operation of the cell:
Yes.
Yes you can pop them right into the charger.
No, as long as the temperature won't exceed the cell's limit.

The trade off is just about always shorter life when you "abuse" your cells. If they have 200 useful cycles to begin with and you charge twice a week, newer and better cells have hit the market after 100 cycles so you are already thinking about replacing them to get those last few minutes of runtime from them. Therefor, I would say that you can threat them pretty hard.
Out of all tricks, hacks and tips regarding batteries, 50 % of them are BS, 40 % are semi-BS but might prolong the life of your cells and the remaining 10 % is what you get from SilverFox, and he is the guy to listen to. :rock:

 

[eluminator]

I'm asking this because I believe (correct me if I'm wrong) that NiMH chemistry for example need to rest after use and before charging. It is also recommended to avoid more than one cycle charge/discharge per day...

I never heard that and I don't believe it. I'm willing to be convinced though.

 

[tino_ale]

I've been reading this advice about NiMH battery packs used for RC electric models. According to winny's comments, it does makes sense about the cooling down, since the batts can be pushed quite hard by powerfull models...

But the "not more than one cycle per day" figure :shrug: I don't know! It is true that it is pretty hard to get reliable info on how to treat batts in general.

I never heard that and I don't believe it. I'm willing to be convinced though.

 

[SilverFox]

Hello Tino Ale,

It looks like Winny beat me to the punch... Let me add this.

Never charge hot batteries or cells. I define "hot" in this case as a wall temperature over 80 F.

The best performance with NiMh cells comes when the cells are heated up to around 140 - 150 F. You loose cycle life because of the heat, but they really perform very well. With Li-Ion cells, the best performance comes when they are at around 100 F.

The one discharge a day advice comes from the RC people. They are pulling 30 amps from their cells, and they get really hot. On top of that they charge at rates up to 8 amps, which further heats the cells up. It takes a long time for a sub C cell to cool off. The people who want to do more than one run a day on a pack utilize cooling tubes. This is a piece of pipe with a fan that blows air over the pack to cool it off rapidly.

If you charge at lower than the maximum charge rates, you may be able to convince yourself that you can start charging before the cells have completely cooled down, but it is best to just wait until they are cool before charging.

Back to your questions...

Hot off the charger will give you the best performance, but you may blow lamps.

Never charge a warm cell.

You can do as many cycles a day as you want as long as you let the cell cool off, or force it to cool off, before charging.

Tom

 

[tino_ale]

Thank you for the complete answer!!
I got all I needed to know

 

[Hallis]

Yah the RC crowd really puts it on the cells. SilverFox's word is pretty much the gospel as it pertains to battery treatment. Especially as it pertains to technology that has been out long enough to have a defined track record like NiMH, Li-ON, etc.

Shane

 

[tino_ale]

Tom,

I have another question related to li-ion charging and temperature.

I notice my batts get warm when charging. With my DDM and a thermal probe, I read a surface temp of 108F (42°C) at their maximum.

I wanted to know if it would be usefull/good idea to reduce this temp by placing the charger in a cooler place, like close to an opened window and such. Not talking of forced airflow, but that alone would certainly buy a few degrees of batt temp, and much more during winter.

Thoughs? Would that be of any use? Would that allow the cells to take more capacity?

Thanks again for the good advice

 

[SilverFox]

Hello Tino ale,

I think that is pretty normal. If your charger allows, you can try a lower charging rate. Li-Ion cell manufacturers specify 0.7C as the ideal charge rate, but even at that rate there will be some warming taking place.

I have no idea if cooling the cells will help anything. If you are using them in colder temperatures, they will work better when warm, but I don't think we are pushing things to the limit and we probably won't be aware of this benefit.

Capacity is voltage related and not, within normal working temperatures, temperature related.

Tom

 

[tino_ale]

Thanks, that was just an idea... I doubt it would really make a difference anyway.

:goodjob:

 

[Mike abcd]

Tom,

I have another question related to li-ion charging and temperature.

I notice my batts get warm when charging. With my DDM and a thermal probe, I read a surface temp of 108F (42°C) at their maximum.

I wanted to know if it would be usefull/good idea to reduce this temp by placing the charger in a cooler place, like close to an opened window and such. Not talking of forced airflow, but that alone would certainly buy a few degrees of batt temp, and much more during winter.

Thoughs? Would that be of any use? Would that allow the cells to take more capacity?

Thanks again for the good advice

LiOn charging at 1 C to 4.20 V is over 99% efficient and the cells should have very little heating. Personally I'd start to pay attention at more than 5-6 deg C temperature rise. If your ambient temps are around 20 deg C and you're seeing a 20 deg C rise, IMHO, it's way too much.

Possible causes
- Battery being heated by waste heat from charger. In that case, a fan would help but the low cost LiOn chargers I have are low rate and don't heat the battery much (DSD type).
- Damaged cell from over discharge or age. A drop in capacity increases the C rate if the charge current is kept the same.
-Charging at too high a rate. I usually charge mine at .5-.7 C. The LiOn batteries I've purchased all have overated stated capacity. I also judge acceptabel charge rate by how much the battery voltage increases when the charging current is supplied since it's an indicator of charging impedance.
-Charging to over 4.20 V can also cause heating and is really bad for cycle life.

Mike

 

[tino_ale]

Some info on my case :
- I am using AW AF 139 charger, see here
- Cells are very new and in good health, two 17500 running a MN10 LA (approx 1,1Amps, typically 1C), protected, not abused in any way
- I measured 275mA as a charging rate. Is is even less than what the charger is advertised (450mA) and is equivalent to 0,25C
- the cells are 4,25V out of the charger, but cells are warm well before reaching the end of the charge

The charger do gets hot while charging, but is this heat is not located where the batts are. I really don't know if is is the charger which is heating the batts.

I will try to pay attention next time, but IIRC my other lii-on cells are heating the same way. The ambiant temp is about 23°C.

No so sure what to believe :huh2:
Since I am not abusing the cells and considering the fact that they all do the same (I need to confirm this), I don't think I need to worry...

LiOn charging at 1 C to 4.20 V is over 99% efficient and the cells should have very little heating. Personally I'd start to pay attention at more than 5-6 deg C temperature rise. If your ambient temps are around 20 deg C and you're seeing a 20 deg C rise, IMHO, it's way too much.

Possible causes
- Battery being heated by waste heat from charger. In that case, a fan would help but the low cost LiOn chargers I have are low rate and don't heat the battery much (DSD type).
- Damaged cell from over discharge or age. A drop in capacity increases the C rate if the charge current is kept the same.
-Charging at too high a rate. I usually charge mine at .5-.7 C. The LiOn batteries I've purchased all have overated stated capacity. I also judge acceptabel charge rate by how much the battery voltage increases when the charging current is supplied since it's an indicator of charging impedance.
-Charging to over 4.20 V can also cause heating and is really bad for cycle life.

Mike