Preventing Li-Ion fires - a summary

[Ken Rice]

Every half-decent laptop since 1997 has Li-Ion batteries, often an
eight-cell pack
of 18560 (185 x 60 mm) cylindrical cells. Every cell phone I have ever opened up
has a single Li-Ion cell in it.

Li-Ion fires are extraordinarily rare, considering the millions of Li-Ion packs
in the field.

When portable, I run my K1 and K2 off Li-Ion packs, but I DON'T  put
Li-Ion packs inside
radios.  Here's a picture of a couple of my homebrew Li-Ion packs.

       http://world.std.com/~rice/ham/projects/K1-GO-KIT/battery-packs.html

There are only a few ways to torch a Lithium Ion battery:

1.  Charging at too high a rate.  Prevent this by using a manufacturer-
     recommended charger and a protective PCB (see below).
2. Continuing to charge a battery that is fully charged.
3. Discharging at too high a rate.
4. Using a battery that has  grossly imbalanced charges across
    its constituent cells, resulting in a self-charge (from the charged
    cells to  the undercharged cell)  that exceeds that cell's charge
     rating.
5. Self-discharge at too high a rate owing to internal conduction paths
    (partial shorts) that result from damage (dings) or manufacturer's
defects.
    Internal shorts are alleged to have caused the handful of laptop fires
    that resulted in over a million recalls in recent months.

You can avoid #1, #2, #3, and #4 by using both:

1.  A slow-blow fuse mounted CLOSE to the battery pack, and rated at
   less than half the manufacturer's maximum discharge rate.
2.  A PCB that (1) limits charge rate (2) limits discharge rate and
     (3) balances the charge across each cell in the pack.

You can avoid most of  #5 (damage-induced internal shorts) by mounting your
pack inside a sturdy, padded container with a rational connector scheme
that prevents  (1) shorted leads and (2) dings to a cell that cause
problem #5, above.

There's NOTHING you can do about manufacturer-induced
internal shorts, but there have been only a handful of such cases
in the last ten years.  It's not entirely clear to me that there has been
even a single case of this problem.

Protective PCBs and battery packs with pre-installed PCBs are
available in various pack configurations here:

   http://www.batteryspace.com/

Here's an example of a PCB for a 11.1 volt pack.

   http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2779

Disclaimer:  I have bought batteries and PCBs from batteryspace.com for several
years, but otherwise I have no relationship with the company.  In particular, I
don't accept discounts from their review-for-discount program which is said to
offer discounts even for negative reviews.

Homebrewing Li-Ion packs is easy if you take the precautions set out above.

You DO have to be careful when designing Li-Ion packs.

Hope this is helpful.

73,

Ken Rice
K3VV
Coopersburg, PA
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