I read somewhere that with lithium ion cells one of the main problems is that when they get older air will get in and cause rapid deterioration! So = I wonder if these cells were operated in some kind of stable, non conductive, not inflammable, corrossion resistant liquid, that might be a way to get them to behave safer!? Any idea what liquid may be appropriate? On Tue, May 31, 2011 at 5:06 AM, RussellMc wrote: > > First place I'd look would be on eBay or the Chinese distributors like > DealExtreme. > > Be aware that small low cost cells may not have internal protection > circuitry. > > Note ahead of protection comment: In-cell protection circuitry is not > as crucial in tiny cells if all is down well. The prospect exists with > some spectacular results but usually nothing too nasty. With laptops > and other systems with multiple 18650 cells the energy levels can be > substantial. A 2 Ah 18650 cell contains about 7 Watt.hour of energy > (depending on discharge rate ~=3D 20 kW.s or ~~=3D 30 Horsepower seconds= .. > If that gets out fairly instantaneously or even over a few 10's of > seconds. Add a number of companions getting in on the act and it can > make for a very spectacular result.it can make > > Protection circuits made for inclusion inside the cell packaging > usually limit charging voltage and some or all of minimum voltage. max > discharge current , max charge current, minimum voltage at which full > charge will be accepted. minimum voltage at which ANY charge will be > accepted. Gross over current is usually handled by a fuse* or maybe > polyfuse. *Maybe in the cap / nipple end and not looking much like a > normal fuse. > Minimum voltage at full charge matters as a cell may become unhappy > below this. Pump up from lower voltage may work but there is a voltage > below which a cell may be considered dead (even if actually not). > > Most protection circuits will not cover all these features - many > being left to the charger IC proper. > > Modern LiPoly are generally better behaved than earlier LiIon tubular > BUT "vent with flame" is still a possibility in the large majority of > cases. Protection circuitry is intended to present this or, rather, to > reduce its incidence. Problems that occur within the battery structure > cannot be prevented by external circuitry protection. Some laptop > batteries made by very reputable battery makers and installed in very > well known brand laptops, occasionally ate their laptops for breakfast > when a mechanical shock was applied "just so" to some units. There was > a mechanical clearance issue and the shock could cause battery short > circuit and away it went. Once it gets to the gaseous Lithium stage > you are good to go under almost any external conditions. Adding water > adds to the fun. > > None of this is liable to be an issue with small LiPoly - especially > if your control circuit is suitably benign. But, be aware that an > unprotected cell can be a 'fun' device to manage incorrectly. > > > > Russell McMahon > Applied Technology ltd. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Tobias Gogolin Tel. Movistar (646) 124 32 82 Tel. Telcel (646) 160 58 99 skype: moontogo messenger: usertogo@hotmail.com Blog: http://zeitgeistensenada.blogspot.com/ You develop Sustainable Ranch Technology at http://tech.groups.yahoo.com/group/SURA-TECH an Open Source Electric Motor/Alternator at http://groups.yahoo.com/group/Performance_Axial_Flux and an Open Source Motor Controller at http://groups.yahoo.com/group/GoBox --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .