li-ion basic questions

More specifically, i'm wondering how they compare with NiMH in certain respects. I know about the "using multiple unprotected cells" thing, so no one needs to warn me about that.

Is there a certain no-load voltage they shouldn't be run below so you don't run the risk of shortening the battery's life? For NiMH, .9v is typically stated, and i *think* i've seen 2v for li-ion, but i'm not sure.

Is there a break-in period where you have to run them down to a certain voltage several times before they're fully formed?

Can they take hot/cold better than NiMH?

I thought there was one more i wanted to ask, but am drawing a blank, so feel free to include anything you think it'd be good to know.

Hello Archangel,

Li-Ion cells do not need to "conditioned" before use, but there is a lot of evidence that they work better and last longer if you do. I put cells through 4-5 cycles of 0.5C charge and between 0.5C-1.0C discharge and feel that I get extremely good service from them.

Li-Ion cells loose capacity and cycle life when over discharged. There is some question as to the low voltage cut off value. It ranges from 2.5 volt running to 3.7 volts resting. While the low voltage cut off number is important, it is also important to realize that the time spent at the low voltage state of charge is also important.

Li-Ion cells seem to be unstable at low and high charge states. You can get away with short excursions to the extremes, but when left at these extremes for an extended amount of time, cell damage results.

Li-Ion chemistry seems to be sensitive to temperature. These cells work best when the cell chemistry is between 100-140 F.

Tom

most protection ckts kick in between 2.7 and 2.5V... but i use 3.0.. maybe 2.8... i have noticed that 95% of the energy is sucked out of a LiON if at high-drain at 3.5V... there is not much need for pulling much lower than that.

The studies that test LiON chemistry for longevity show they prefer to be 40% charged and room temperature.. any deviation from that will reduce the WH capacity. I believe that they can handle hot and cold better than NiMH but they can't handle high current as well.. (i.e. a typical high-drain NiMH can output 10-20C... LiON high-drain means 2C).. of course that's the equivalent of 6C to an NiMH.

-awr

OK, another question: Is there an advantage to charging a li-ion at .2c versus .5C? Or maybe a better questions would be, Is there a maximum recommended rate?

Hello Archangel,

The maximum charging rate (for most cells, but it may change in the near future...) is 1C. Most manufacturers recommend 0.7C for better life.

Tom