Debunk what I "THINK" I know about Lithium Ion cells - Page 1
Forum section: electronics / projects

author: theoldwizard1
author_rank: Regular Contributor
timestamp: August 28, 2016, 02:45:08 pm
content: I have nee doing a lot of research on Google and Wikipedia regarding Lithium Ion cells and battery packs. I am sure some of my notion are skewed or completely wrong and I need some knowledgeable people to correct my misconceptions and add to my personal knowledge base.Lithium iron phosphate (LiFePO?, a.k.a. LFP or LiFe) cells/batteries are "different" !While they are considered "safer" (less likely to ignite) if short-circuited, overheated, overcharged, etc. They have lower nominal voltage (3.2V), lower energy density, Most importantly, they require different charging techniques.There are a lot of different "chemistries" available, but unless you are buying a huge quantity, it probably does not matter ! Buy a battery(cells or packs)that meet your requirements(volts, amp-hours, weight and cost).The "18650" cell is the industry standard size, based on the fact that this is what is used in Tesla electric cars. This is good and bad. Good because many companies make cells of this size and competiton "improves the breed". It is bad because there are so many knock-off and down right "pirated" copies, so buying a "brand name" is not always a guarantee of quality.Many manufacturers out and out LIE about their cell capacity.Buy "name brand"(Panasonic, Sony, LG, etc.)cells from a reputable distributor. The only way to know for sure, is to test each cell, individually.Used laptop battery packs are a good source of cheap 18650 cells. If you are building a pack from reclaimed cells do a discharge, charge, discharge cycle with a good charger/discharger (SkyRC makes many different models), record the the capcaity on the side of the cell and build a pack from similar sized cells.Lithium polymer (LiPo) cells where invented to improve "energy density" (mAh per gram). My placing the battery "guts"(anode, cathode and electrolyte)inside a polymer (non-metallic) envelope weight was reduced compared to cells placed inside of a round metallic cylinder(i.e. 18650). Because they do not have a metallic shell, there are easy to damage and fail in a spectacular manner !Battery packs made of 18650 cells seem to be 1 or 2 cell wide(18.3 or 36.5 mm)and as long as required, but 8 cells is a good number(more later). LiPo cells tend to be flat (but can be almost any shape). The most common size is approximately 25mm wide, 50mm tall and 10mm thick.Li-Ion battery packs should have a "battery sense" lead attached to each positive cell as well as one to the pack negative so the chargers with battery management can do a "balanced' charge.

author: amyk
author_rank: Super Contributor
timestamp: August 28, 2016, 02:57:58 pm
content: Phosphate cells are charged in the same way as the other types, with current and voltage limiting. They just have a different termination voltage (3.6V).A good thread to read:https://www.eevblog.com/forum/projects/diagnosing-lithium-cells/

author: theoldwizard1
author_rank: Regular Contributor
timestamp: August 28, 2016, 11:29:13 pm
content: That is like trying to get enough water for a cup of coffee from a fire hose.No one willing to comment on specific details in my post ?

author: retrolefty
author_rank: Super Contributor
timestamp: August 28, 2016, 11:39:26 pm
content: Well I think you did a pretty good research evaluation. There are lots of myths and bad/unneeded advice on these type of cells and you didn't seem to fall for any of them that I'm aware of.I would only add that I seem to recall reading that if Li-po are not going to be used for long lengths of time it's best to stow them at 50% charge state for longest cell life?

author: theoldwizard1
author_rank: Regular Contributor
timestamp: December 28, 2016, 05:29:22 pm
content: Hold on to your hats boys and girls, the world is about to change. The 18650 (which is 18mm diameter and 65.0 mm long) is about to be superseded !Panasonic and Tesla have announced that the new Gigafactory will make a new "standard" size cell.http://fortune.com/2016/07/27/tesla-bigger-battery-gigafactory/The a new format of lithium-ion batteries called the "21-70", will be 21 millimeters in diameter and 70 millimeters in length.You should also expect that electrolyte, anode and cathode have been tweaked for even more power. SWAG - >5000mAh

author: ConKbot
author_rank: Super Contributor
timestamp: December 28, 2016, 06:33:50 pm
content: Quote from: theoldwizard1 on December 28, 2016, 05:29:22 pmGiven 0.5mm wall thickness, 5mm dedicated to the top seal/ptc/etc and a 3mm core (mandrel the cell gets wound on, etc) available volume goes from 13cc to 19.8cc. A 50% increase, so on high energy density cells (I.e. 3500 mah 18650s) 5000mah should be achievable. Probably about 4000mah on high power density cells. And that's assuming tesla isn't going to roll any special sauce or tricks into it.(Hexagonal case that doesn't hurt stacking density, cell rolled into a circle in the center, and the corners used as a path for vapor phase electrolyte to flow because they invented a new electrolyte that can be used as a heat transfer fluid and are sealing the cells under a partial vacuum?)

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 28, 2016, 06:42:09 pm
content: I will add that "I think I know" that state of charge on LiFe cells is more difficult to assess due to a flatter voltage/charge curve. This has a number of secondary effects in multi-cell packs related to balancing charge on the cells.

author: IanB
author_rank: Super Contributor
timestamp: December 28, 2016, 07:51:11 pm
content: Quote from: CatalinaWOW on December 28, 2016, 06:42:09 pmI'm not absolutely sure that it's really "flatter". If you scaled up a LiFePO4 discharge curve to be on the same voltage scale as a LiCoO2 cell I bet they would look very similar.

author: IanB
author_rank: Super Contributor
timestamp: December 28, 2016, 07:54:43 pm
content: Quote from: theoldwizard1 on August 28, 2016, 02:45:08 pmThis is the one point that is not quite right. LiFe cells are charged with the same CC/CV method as other lithium ion cells, the only difference being a lower termination voltage.

author: Seekonk
author_rank: Super Contributor
timestamp: December 28, 2016, 09:09:28 pm
content: On a similar quest. I've collected 18650 laptop batteries for a while and haven't used them for much more than LED lamps. Now I have a need for a 150 wattish battery, either a 4 or 8 string and I programmed up a little tester to read out amp minutes. A little obscure rating system, but results so far are pretty disappointing on these used batteries. Not using up a lot of minutes and current draw over 1/4A seems to greatly shorten capacity. Maybe just one cell went bad in these packs, but the still working ones seem to be near the end of their life. These were stored for a while and still had a decent charge on them. Interested in the experience others have had with these old laptop batteries.

author: theoldwizard1
author_rank: Regular Contributor
timestamp: March 30, 2017, 03:19:51 am
content: Quote from: theoldwizard1 on December 28, 2016, 05:29:22 pmWell, Tesla's "gigafactory" has been cranking out 2170 cells for a few months, but that is for "internal" consumption (Tesla vehicles)Now the Pansonic (Sanyo) version of the same (?) cells has hit the market. They are being sold as Sanyo(Panasonic bought Sanyo's battery business a few years ago)NCR20700B. The "B" version is rated at 4250mAh. The "A" version is rated at 3000 mAh. They are selling for $8.50-$10US on eBay.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2017, 05:44:23 pm
content: Quote from: theoldwizard1 on August 28, 2016, 02:45:08 pmNo, not the envelope. Li-Ion is liquid electrolyte, thus a rigid shell is needed. Li-Polymer is a gel or solid polymer electrolyte:https://en.wikipedia.org/wiki/Lithium_polymer_batteryTim

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2017, 01:04:31 am
content: Quote from: T3sl4co1l on March 30, 2017, 05:44:23 pmUnfortunately the term "lithium polymer battery" is used for both batteries which use an actual polymer (solid) electrolyte and lithium batteries in a polymer pouch. As I have discovered, if you take one of the common later batteries apart, the electrolyte is a liquid. Also, it is a bad idea to get the liquid electrolyte on your skin.

author: helius
author_rank: Super Contributor
timestamp: March 31, 2017, 01:21:24 am
content: Li-ion cells contain acetonitrile as a solvent, which is acutely toxic if absorbed in large amounts. Luckily it does not, as far as we know, have long-term effects.

author: amyk
author_rank: Super Contributor
timestamp: March 31, 2017, 12:05:53 pm
content: Quote from: helius on March 31, 2017, 01:21:24 amI think you're referring to LiSO2 cells, as those are the only ones I could find whose MSDS mentions it. Regular lions do not contain acetonitrile.

author: helius
author_rank: Super Contributor
timestamp: March 31, 2017, 07:05:25 pm
content: MSDS is only required to disclose the substances that could be hazardous if spilled. Batteries contain small volumes of electrolyte and it is very unlikely that a large volume could spill out, even from a large battery shipment. So listing the contents of batteries, like other manufactured articles, is not required.A supplier of acetonitrile mentions Lithium-Ion batteries prominently:http://www.parchem.com/news-articles/Parchem-is-the-Premier-Supplier-of-Acetonitrile-N000142.aspxYou can also find it discussed in most academic papers on the subject. Do manufacturers want to disclose that they use it? Probably not, since it only takes one Buzzfeed journalist writing that there's something scary called "methyl cyanide" in your child's toy to cause large intangible losses.