RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
Murray over at Design News posted a video of a lithium-ion (Li-Ion) battery going
up in flames, as a reminder of the potential (pun intended) danger posed by the chemistry
used by the vast majority of our portable devices as well as the current fleet of electric cars. In it, a guy whacks
an iPad battery pack with a hammer causing it to immediately burst into flames - an act that could easily go way wrong
and automatically self-nominate the performer
Darwin Award. Just as the Ford Pinto and early model
Mustangs were discovered to include
an unadvertised explode-on-impact feature with trunk-mounted gas tanks, so too have early model
Teslas with inadequately protected Li-Ion battery packs.
As Mr. Murray suggests, good engineering will eventually eliminate or at least significantly reduce the likelihood
of such events. He points out that the
energy density of gasoline is about 12,000 Wh/kg versus about 200 Wh/kg for lithium-ion. "Somehow, automotive
engineers have figured out how to control the high energy of gasoline. There's absolutely no reason why they
can't do the same with electric cars."
news media crave such stories so badly that you might recall when
NBC News rigged a GM pickup
truck with incendiary devices to fake a crash-related explosion. However, there are plenty of unstaged fires involving
Li-Ion batteries. There have been numerous stories of cellphones burning people while being carried around
in a pocket or left charging
under a pillow. The biggest danger seems to be during the charging cycle since that is when the most rapid creation
of gasses occurs and the temperature increases. Fast recharging is particularly risky. Model airplane and car hobby magazines print safety articles
quite often since fast charge modes are frequently used while at the flying field or car track in order to minimize
time between flights and runs, respectively. The most damaging fires usually happen, however, while charging
at home or even in a personal vehicle on the way to the field since there is more likely to be other flammable
objects nearby to catch fire.
thumbnail to the left is a photo I took of one of the Li-Po (lithium-polymer) battery
packs I was using for an R/C airplane (see "Lithium-Polymer
Battery Characteristics"). It arrived that way from the mail order house. I safely disposed of the pack
by soaking it in a brine solution for a week and then throwing it away. A couple months ago another pack needed to
be disposed of for the same reason. I never charge my Li-Pos at a rate greater than 2C and probably never draw from
them at greater than 15C so they have a pretty easy life. In fact, my batteries' biggest threat is lack of use since,
alas, I rarely take time to go flying (more time spent publishing my
Airplanes and Rockets website).
It is a wonder there are not more fires than we see reported.
Note: "C" is the charging or drainage current multiple of the pack's rated capacity. For
instance, a 1,500 mAh pack is being charged at or delivering a current of 1,500 mA (1.5 A) at 1C. 5C
would be 7.5 A, etc.
Thanks to Design News for posting this video
Posted October 28, 2014
Please Support RF Cafe by purchasing
my ridiculously low−priced products, all of which I created.